Heavy-Duty Spark-Ignited Single Cylinder Engine Fueling ...

Heavy-Duty Spark-Ignited Single Cylinder Engine

Fueling System

Rohan Sharad Kittur

Master of Science Thesis MMK TRITA-ITM-EX 2018:512

KTH Industrial Engineering and Management

Machine Design

SE-100 44 STOCKHOLM

i

Examensarbete MMK TRITA-ITM-EX 2018:512

Bränslesystem för encylindrig motor

Rohan Sharad Kittur

Godkänt

2018-06-26

Examinator

Dr. Andreas Cronhjort

Handledare

Dr. Ola Stenlåås

Uppdragsgivare

Scania CV AB, Södertälje

Kontaktperson

Dr. Ola Stenlåås

Sammanfattning

Forskning inom motorutveckling bedrivs för att möta kommande emissionskrav och samtidigt

minska bränsleförbrukningen. Kommande förbud mot dieseldrivna fordon planeras i flera städer

runt om i världen. Alternativa bränsle som exempelvis naturgas ses som en lovande ersättning

även för tunga fordon. Metan som är huvudkomponenten av naturgas har en fördelaktigt

förhållande mellan väte och kol vilket gör den attraktiv för CO2-reducering. Hur som helst,

bränslets låga cetantal och den höga aktiveringsenergin som krävs för att tända naturgas förutsätter

tändstiftsantändning.

En fördel av att använda en encylindrig motor inom forskning är möjligheten att studera fenomen

utan negativa gasväxlingsinteraktioner från intilliggande cylindrar. Jämfört med en fullmotor

möjliggörs även ett snabbare utbyte av motordelar samt lägre bränsleförbrukning.

Fokus för detta examensarbete var genomförandet av ett flexibelt bränslesystem för en

tändstiftsantänd encylindrig motor. Motorn är en tändstiftsantänd Scania 9 liters som modifieras

för encylinder körning. Flexibilitet som t.ex. laddningshomogenitet, selektiv fyllning av

inloppsporter och förberedelser för direktinsprutning av flytande bränsle realiserades. För enkel

användning är motorn styrd av en eftermarknadsmotorstyrenhet som använder ett användarvänligt

grafiskt gränssnitt för ändring av driftsparametrar. Säkerhetshänsyn vid blandning av gasformiga

bränsle och luft långt innan inloppsporterna har implementerats.

Nyckelord: bränslesystem, tunga fordon, tändstiftsantändning, encylinder motor,

eftermarknadsmotorstyrenhet, bränsle-luft blandningssäkerhet

iii

Master of Science Thesis MMK TRITA-ITM-EX 2018:512

Heavy-Duty Spark-Ignited Single Cylinder Engine

Fueling System

Rohan Sharad Kittur

Approved

2018-06-26

Examiner

Dr. Andreas Cronhjort

Supervisor

Dr. Ola Stenlåås

Commissioner

Scania CV AB, Södertälje

Contact person

Dr. Ola Stenlåås

Abstract

Most of the fundamental research in internal combustion engines is driven by the ever-increasing

stringency of emissions regulations along with the need for increased fuel economy. The proposed

ban on diesel vehicles in several cities around the world combined with extensive availability, has

made natural gas a promising substitute even for heavy-duty applications. The high

hydrogen-to-carbon ratio of methane, the major component of natural gas, makes it attractive from

an emissions reduction perspective. CO2 emissions from natural gas combustion are particularly

low. However, the low cetane number and high activation energy required to ignite natural gas,

requires spark-ignition.

In a research setting, it is often advantageous to have a single cylinder engine. The main benefit is

the ability to study phenomena without adverse interactions which multi-cylinder operation may

cause. This is especially important for gas-exchange studies. Quicker replacement of parts and

lower fuel consumption are secondary benefits.

The focus of this thesis was the implementation of a flexible fueling system for a single cylinder

spark-ignited engine. The engine is a Scania 9-liter spark-ignited engine modified for single

cylinder operation. Flexibility in terms of charge homogeneity, selective intake port filling and

provisions for liquid fuel direct injection have been provided. For ease of use, the engine is

controlled by an aftermarket engine control unit with a graphical user interface for configuration.

Safety considerations when mixing gaseous fuels and air well upstream of the intake ports have

been implemented.

Keywords: Fueling system, heavy-duty, spark-ignited, single cylinder engine, aftermarket engine

control unit, fuel-air mixture safety.

v

FOREWORD

I would like to express my gratitude to all the people who have offered their time and support to

help me see this project to the end.

Foremost, I thank Dr. Ola Stenlåås (Scania CV AB, Södertälje) for motivating me to step outside

my comfort zone so that I could learn something. I also thank him for helping improve my

technical writing skills. My thanks to Dr. Andreas Cronhjort as well for being a constant source of

support.

It has been a pleasure working with Richard Adolfsson (NMEG). I am extremely grateful to him

for sharing his vast practical knowledge of engines. For being accommodating of my lack of

experience and patiently teaching me practical techniques to gain the required experience, I thank

him very much.

I would like to thank the entire Otto engine pre-development and optimization teams for accepting

me as their colleague and for tirelessly answering each of my questions. My particular gratitude

goes to Arne Burger and Mats Janzon (NMEG) for their kind nature, encouragement and

enlightening discussions over lunch.

I extend my special thanks to Tomas Karlberg (NLRD) for patiently answering all my questions

related to the single cylinder test cells. Pål Axelsson and Björn Lind (KETE) have been extremely

helpful with the flame arrestor for which I thank them. I also thank Ludvig Adlercreutz

(AVL, Södertälje) for his support.

My heartfelt thanks go to Philip Thörhammar (KETL), Peter Olsson (KETL) and Timo Lohilahti

(NLWM) for sharing their practical knowledge and tools. I am also grateful to all the other

workshop personnel for their patience and assistance.

My fellow students of the IC engines program at KTH deserve special mention as well, for all the

fun times we had over the course of two years. Particular thanks to Andreas Lius for allowing me

to learn more about engines by assisting him with his thesis.

Lastly, I thank my parents for believing in me and being my pillars during all of life’s highs and

lows.

Rohan Sharad Kittur

Stockholm, June 2018

vii

NOMENCLATURE

The notations and abbreviations used in this report are as follows.

Notations

Symbol Description

bar(a) Absolute pressure

bar(g) Gauge pressure

Abbreviations

BDC Bottom Dead Center

CFD Computational Fluid Dynamics

CI Compression-ignition/ignited

CNG Compressed Natural Gas

ECU Electronic Engine Control Unit

FAME Fatty Acid Methyl Ester

HVO Hydrogenated Vegetable Oil

Hythane Hydrogen-methane blends (% composition may vary)

ICE Internal Combustion Engine

LNG Liquefied Natural Gas

PWM Pulse Width Modulation

RON Research Octane Number

SI Spark-ignition/ignited

TDC Top Dead Center

ix

TABLE OF CONTENTS

Sammanfattning ............................................................................................................................... i

Abstract .......................................................................................................................................... iii

Foreword ......................................................................................................................................... v

Nomenclature ................................................................................................................................ vii

Table of contents ............................................................................................................................ ix

List of figures ............................................................................................................................... xiii

List of tables .................................................................................................................................. xv

1 INTRODUCTION ....................................................................................................................... 1

1.1 Internal Combustion Engines – A Brief Overview ............................................................... 1

1.1.1 Commonly used fueling methods: .................................................................................. 1

1.1.2 CNG as a fuel .................................................................................................................. 3

1.2 Purpose .................................................................................................................................. 3

1.3 Task Definition ...................................................................................................................... 4

1.3.1 Research objectives ......................................................................................................... 4

1.3.2 Project outline ................................................................................................................. 4

1.4 Deliverables and delimitations .............................................................................................. 5

1.4.1 Deliverables .................................................................................................................... 5

1.4.2 Delimitations ................................................................................................................... 5

2 ENGINE SELECTION ............................................................................................................... 7

2.1 Single cylinder research engine ............................................................................................. 7

2.2 Requirement specification and alternatives ........................................................................... 7

2.3 Implementation ...................................................................................................................... 7

3 FUELING SYSTEM ................................................................................................................... 9

3.1 Fuel-air mixture formation .................................................................................................... 9

3.1.1 Gaseous fuel-air mixture ................................................................................................. 9

3.1.2 Effects of fuel injector location and injection timing ..................................................... 9

x

3.2 Charge homogeneity variation ............................................................................................. 10

3.3 Laboratory fueling setup ...................................................................................................... 11

3.3.1 Gas panel ....................................................................................................................... 12

3.4 Installation assembly ........................................................................................................... 14

3.5 Liquid fuel direct injection .................................................................................................. 15

3.5.1 Assignment ................................................................................................................... 15

3.5.2 Implementation ............................................................................................................. 16

3.5.3 Modifications ................................................................................................................ 16

4 ELECTRONIC ENGINE CONTROL ...................................................................................... 19

4.1 Engine control unit .............................................................................................................. 19

4.1.1 Assignment ................................................................................................................... 19

4.1.2 ECU selection criteria ................................................................................................... 19

4.1.3 ECU selection and installation ...................................................................................... 20

4.2 Fueling control ..................................................................................................................... 20

4.2.1 Types of CNG injectors ................................................................................................ 21

4.2.2 Installation ..................................................................................................................... 22

4.2.3 Verification ................................................................................................................... 22

4.3 Lambda control .................................................................................................................... 22

4.3.1 Stoichiometric combustion and the three-way catalyst ................................................ 22

4.3.2 Exhaust gas oxygen sensors/Lambda sensors ............................................................... 23

4.3.3 Implementation ............................................................................................................. 25

5 GAS EXCHANGE SYSTEM ................................................................................................... 27

5.1 Intake damping/mixing tank ................................................................................................ 27

5.1.1 Purpose .......................................................................................................................... 27

5.1.2 Alternative designs ........................................................................................................ 27

5.1.3 Design choice and implementation ................................................................................... 28

5.2 Throttle valve ....................................................................................................................... 29

5.2.1 Purpose ............................................................................................................................. 29

xi

5.2.2 Throttle sizing ................................................................................................................... 29

5.2.3 Implementation and verification ....................................................................................... 29

5.3 Intake manifold modifications and implementation ............................................................ 29

5.4 Variable intake port filling .................................................................................................. 30

5.4.1 Assignment ................................................................................................................... 30

5.4.2 Alternative solutions ..................................................................................................... 31

5.4.3 Implementation ............................................................................................................. 32

5.5 Exhaust side damping tank .................................................................................................. 32

5.5.1 Purpose .......................................................................................................................... 32

5.5.2 Implementation ............................................................................................................. 32

6 SAFETY DEVICES .................................................................................................................. 35

6.1 Flame arrestor ...................................................................................................................... 35

6.1.1 Working principle ......................................................................................................... 35

6.1.2 Motivation ..................................................................................................................... 36

6.1.3 Implementation ............................................................................................................. 36

6.2 Damping/mixing tank overpressure safety .......................................................................... 37

6.2.1 Motivation ..................................................................................................................... 37

6.2.2 Implementation ............................................................................................................. 38

7 CONCLUSION ......................................................................................................................... 41

7.1 Summary .............................................................................................................................. 41

7.1.1 Fueling system .............................................................................................................. 41

7.1.2 Electronic engine control unit ....................................................................................... 41

7.1.3 Safety devices ............................................................................................................... 41

7.2 Conclusions ......................................................................................................................... 42

8 RECOMMENDATIONS AND FUTURE WORK ................................................................... 43

8.1 Recommendations ............................................................................................................... 43

8.2 Future work.......................................................................................................................... 43

9 REFERENCES .......................................................................................................................... 45

xii

APPENDIX A: DRAWINGS .......................................................................................................... 1

xiii

LIST OF FIGURES

Figure 1. Top view of the engine illustrating reasons for choosing to retain cylinder 1 as the

active cylinder ............................................................................................................................ 8

Figure 2. Laboratory gas supply schematic .............................................................................. 11

Figure 3. Gas panel for CNG engines ...................................................................................... 13

Figure 4. Illustration of the recommended gaseous fueling system layout .............................. 14

Figure 5. (a) Proposed orientation of the high-pressure fuel rail, needing 45 mm spacers between

the manifold and cylinder head; (b) Proposed orientation of the high-pressure fuel rail

demonstrating the use of the existing high-pressure connector without changing any bend radii

.................................................................................................................................................. 17

Figure 6. Typical solenoid current build-up, injector valve lift and corresponding fuel flow rate

for (a) voltage-controlled injector and (b) current-controlled injector. .................................... 21

Figure 7. Lambda window for highest TWC conversion efficiency ........................................ 23

Figure 8. Typical narrow-band O2 sensor output voltage as a function of lambda .................. 24

Figure 9. (a) Construction and (b) characteristic curve of a typical wideband lambda sensor. 25

Figure 10. Illustration of intake damping tank (a) Alternative 1 with baffle; (b) Alternative 2

with larger volume ................................................................................................................... 28

Figure 11. Illustration of the method used to adapt the intake manifold for single cylinder

operation ................................................................................................................................... 30

Figure 12. Illustration of alternative 2 for variable intake port filling; (a) flow directed towards

swirl port and (b) flow directed towards filling port ................................................................ 31

Figure 13. Illustration of the exhaust side damping tank with rupture disc for overpressure

protection. ................................................................................................................................. 33

Figure 14. Crimped metal-ribbon flame arresting element ...................................................... 35

Figure 15. Crimped metal-ribbon flame arrestor (a) element and (b) assembly with housing 37

Figure 16. Schematic drawing of the rupture disc with holder and installation flanges .......... 38

Figure 17. Intake side damping/mixing tank with overpressure rupture disc installed ............ 39

xiv

Figure A-1. Spacer between the intake manifold and cylinder head with half the stock filling

port area .................................................................................................................................. A-1

Figure A-2. Spacer between the intake manifold and cylinder head with half the stock swirl port

area ......................................................................................................................................... A-2

Figure A-3. Spacer between the intake manifold and cylinder head with blocked filling port

................................................................................................................................................ A-3

Figure A-4. Spacer between the intake manifold and cylinder head with blocked swirl port A-4

Figure A-5. Spacers with stock port areas for unused cylinders ............................................ A-5

xv

LIST OF TABLES

Table 1. Common SI engine fueling methods ............................................................................ 2

Table 2. Alternative methods for well-homogenized premixture of gaseous fuel and air ....... 10

Table 3. Components of the laboratory fueling setup from tanks to the engine ...................... 11

Table 4. Gas panel components and their respective functions ............................................... 12

Table 5. Gas panel pressure regulator preset pressure settings ................................................ 13

Table 6. Components of the fueling system assembly within the test cell ............................... 15

Table 7. XPI system modifications for use with alcohols ........................................................ 16

Table 8. Comparison metrics for ECU selection ...................................................................... 19

Table 9. Sensors necessary for basic engine operation ............................................................ 20

Table 10. Alternative damping tank designs ............................................................................ 27

Table 11. Intake manifold modifications ................................................................................. 30

Table 12. Alternatives for varying port filling ......................................................................... 31

1

1 INTRODUCTION

This chapter gives a brief introduction to internal combustion engines, the purpose of this

project followed by the deliverables and delimitations.

1.1 Internal Combustion Engines – A Brief Overview

Internal combustion engines (ICEs) date back to the late 19th century when Nikolaus Otto

developed the spark-ignition (SI) engine and Rudolf Diesel invented the compression-ignition

(CI) engine. ICEs have played a dominant role in the fields of energy and propulsion. However,

the last three or so decades have seen vast developments in engine research and development

to tackle the issues of pollution, fuel cost and market competitiveness (Heywood, 1988).

Engines can be classified in a variety of different ways, but for the purposes of this report, they

shall only be classified as being either of the SI type or the CI type. This classification is based

on the method used to provide the activation energy required to initiate the combustion process.

Both methods are applicable to fuels possessing different qualities. Spark ignition is used for

fuels that have high auto-ignition temperatures. As the name suggests, the ignition is initiated

by a spark plug which provides a high voltage electric arc. On the other hand, certain fuels can

be ignited by simply compressing them in a quasi-adiabatic process (Heywood, 1988).

A majority of ICEs operate on what is known as the four-stroke cycle. Each of the four strokes

takes place during half a rotation of the crank or 180°. Thus, one complete cycle requires that

the crank rotate through a total of 720°. The four strokes that make up the cycle are as follows

(Heywood, 1988):

1. The intake stroke which starts at Top Dead Center (TDC) and ends at Bottom Dead Center

(BDC) during which the charge is inducted into the engine via the intake valves. The

exhaust valves remain closed.

2. The compression stroke during which both the exhaust and intake valves are closed. The

charge is compressed to a fraction of its initial volume by the upward motion of the piston

form BDC to TDC. Combustion is initiated toward the end of this stroke and there is a

further increase in the cylinder pressure.

3. The expansion stroke results from the expanding high-temperature gases which are

products of the combustion. This is the only stroke during which work is extracted from

the fuel. The expansion causes the piston to move from TDC to BDC. The exhaust valves

open toward the end of this stroke.

4. The exhaust stroke during which the exhaust valve is fully open and the upward motion

of the piston from BDC to TDC driven by inertia, causes the exhaust gases to be evacuated

from the cylinder. The intake valves open toward the end of this stroke to allow fresh

charge to be inducted for the next cycle.

1.1.1 Commonly used fueling methods:

SI engines employ four main fueling methods (see Table 1) (Heywood, 1988):

2

Table 1. Common SI engine fueling methods

Item

number

Method Advantages Disadvantages

1 Carburetion – makes

use of the Venturi

effect to draw fine

droplets of fuel into

the intake air stream

• Well-mixed charge

induction ⟹ no large

rich/lean pockets in the

cylinders and associated

impact on emissions

• Can cause imbalance

in cylinder filling

depending on the

design of the inlet

manifold. In case of a

log-style manifold,

the cylinder closest to

the throttle could run

rich while the farthest

cylinder could run

leaner.

• Slower response to

load changes due to

the large volume

between the point of

mixing and the

cylinders.

• Result in the intake

manifold being filled

with an inflammable

mixture which is

undesirable in the

event of a backfire.

2 Throttle-body

injection using a fuel

injector array

upstream of the

throttle valve in place

of a carburettor

• Well-mixed charge

induction ⟹ no large

rich/lean pockets in the

cylinders and associated

impact on emissions

• More precise fuel

metering than

carburetion

• Same as those for

carburetion

3 Port fuel injection • Eliminates the issue of

uneven charge

distribution

• Can premix fuel and air

sufficiently well

depending on runner

design and injection

timing

• Slight loss in

volumetric efficiency

when using gaseous

fuels since part of the

air in the ports and

runners is displaced

by fuel

• Port and runner length

becomes important to

avoid spill-over of

injected fuel into

runners of adjacent

cylinders

3

• Effects of wall-

wetting need to be

accounted for when

using liquid fuels

4 Direct injection – uses

one injector per

cylinder located in the

cylinder heads

• Eliminates the problem

of reduction in

volumetric efficiency

inherent to port injection

of gaseous fuels

• High injection

pressures are needed

• In-cylinder fuel and

air mixing problems

could arise due to low

penetration of

gaseous fuels in air

1.1.2 CNG as a fuel

Natural gas is a hydrocarbon fuel, the majority composition of which is methane. The methane

composition of natural gas can range from 75% to over 95% depending on the region of origin

(Min et. al., 2002). Compressed Natural Gas (CNG), as the name suggests, is natural gas

compressed to 200 to 250 bar. The simple chemical composition of methane, consisting of only

one carbon atom per molecule, results in over 20% reduction in CO2 emissions compared to

other hydrocarbon fuels. Experiments have shown an average decrease in developed power of

9% while running on CNG and it has a molar heating value that is 4.6 times less than that of

conventional gasoline (Douailler et. al., 2011). However, the sources of natural gas are more

plentiful than the sources of oil (Einewall, 2005).

Natural gas cooled cryogenically to around -160 °C is called Liquefied Natural Gas (LNG).

Liquefying natural gas and storing it at 10 bar (approx.) increases its volumetric energy density

to nearly two and a half times that of CNG at 200 bar. However, LNG has its own drawbacks

that prevent it from being as economical as CNG for commercial use. Firstly, the energy

required to liquefy natural gas is much more than that required to compress it to 200 or 250 bar

at room temperature. It also needs special insulated equipment such as containers for

transportation and bulk storage, pumps for refueling vehicles and insulated on-board fuel tanks

(Kumar et. al., 2011).

CNG is also widely available from sources that are not related to food in any way. Other fuels

such as ethanol and vegetable oils used to create fatty acid methyl esters (FAME) or

hydrogenated vegetable oils (HVO) either use edible matter directly, or are farmed on land

which could be utilized for food production instead.

Ever increasing restrictions on emissions, scarcity of fossil fuels, questionable renewability of

certain alternate fuels combined with low emissions has made CNG an attractive alternative to

conventional oil based fuels.

1.2 Purpose

The ever-increasing demand from automakers to reduce vehicular emissions has made

Compressed Natural Gas (CNG) one of the more promising options. The 4:1

hydrogen-to-carbon ratio of methane compared to the higher 2:1 (approx..) of typical diesel or

petrol is advantageous for reduced CO2 emissions (Reif and Dietsche, 2011). High quality CNG

4

with a high percentage of methane also provides a virtually soot-free operation (Reif and

Dietsche, 2011). Moreover, CNG has an Research Octane Number (RON) of approximately

130 which makes it more knock-resistant (Reif and Dietsche, 2011). This makes it possible to

use higher compression ratios compared to typical gasoline engines. The higher

knock-resistance of CNG also means that the engines running on CNG can be turbocharged to

higher pressures for improved indicated efficiency. The decrease in volumetric efficiency due

to displacement of air by fuel can also be addressed through forced induction.

In a research setting where one intends to study various phenomena concerning engine

operation, it is often desirable to eliminate or isolate as many variables as possible in a

controlled manner. In a multi-cylinder engine, flow and/or electrical phenomena occurring in

one cylinder could possibly affect those in another cylinder. This makes it difficult to perform

a controlled study of the phenomena of interest. Nevertheless, a single cylinder engine permits

the study of these phenomena of interest without disturbances due to adjacent cylinders.

1.3 Task definition

The project aims at implementing flexible fueling of a series production multi-cylinder

heavy-duty CNG fueled engine modified for single cylinder operation using an aftermarket

electronic engine control unit (ECU). The modified engine is intended for further research and

will be installed in one of the test cells at the Internal Combustion Engines laboratory at KTH

Royal Institute of Technology, Stockholm. This thesis is part of the bigger Project CLOCK

(Closed Loop Otto Combustion and Knock control). The participating partners are Scania CV

AB, KTH Royal Institute of Technology, Sweden, and the Swedish Energy Agency. Although

the first customer of the engine once installed at KTH will be Project CLOCK, future projects

could potentially involve research on different fuels and fueling methods.

1.3.1 Research objectives

The main research objectives intended to be fulfilled through this project are:

1. Evaluating the best aftermarket ECU taking the extent of controllability offered into

account while remaining easy to use in an academic setting

2. Making provisions for variable homogeneity of gaseous fuel-air mixture with due

consideration to safety

3. Making provisions for selective intake port usage for future studies on its effect on

in-cylinder flows

4. Making hardware provisions for implementation of liquid fuel direct injection

1.3.2 Project outline

The project involves the following steps/stages:

1. Selection of a suitable engine for modification

2. Selection of a suitable aftermarket ECU to control the fuel system

5

3. Selection of required sensors and actuators needed to implement the control

4. Making required mechanical modifications on the engine from a fueling system

perspective

5. Installing the control hardware and configuring the control system as required

6. Delivering a working engine

The engine uses a port-injected CNG fueling system. The parameters of the fueling system that

are expected to be controlled are: Mass flow rate of fuel, injection pressure and start of injection.

1.4 Deliverables and delimitations

This thesis project spanned over a duration of 20 weeks, was limited in the deliverables. The

following deliverables and delimitations were formulated after mutual agreement between the

author and the project commissioner.

1.4.1 Deliverables

1. The fuel injectors will be controlled with respect to start/end of injection and mass flow

rate of CNG.

2. A conventional speed-density based open loop and lambda sensor-based closed loop

control of the injectors will be implemented.

3. Additional injector(s) (capable of injecting a different gaseous fuel) will be installed in

the intake air path thus varying homogeneity (and composition) of the resulting charge.

4. Provisions will be made for higher or lower flow rate through either intake port.

5. The selected aftermarket ECU should be capable of liquid fuel direct injection, if possible.

6. The ECU should be able to control Exhaust Gas Recirculation (EGR).

7. Safety devices such as flame arrestors and vent valves will be installed at appropriate

locations

8. Documentation for future users will be created for all the non-standard auxiliaries

mounted on the engine.

1.4.2 Delimitations

1. Mechanical modifications for conversion to single cylinder if using a multi-cylinder

engine will not be the main focus although assistance for the same will be provided.

2. Cycle-to-cycle injector control will not be carried out.

3. The ECU will not be programmed at machine level but only via the bundled software

GUI.

6

4. Ignition control remains outside the scope of this project. It shall be the focus of a

concurrent thesis project carried out by a fellow student.

5. Controlling the liquid fuel direct injection fueling will be excluded.

6. Further components in the intake air path upstream of the damping tanks were considered

to belong to the test cell and were excluded from the scope of this project.

Mechanical modifications to the engine for single cylinder operation were carried out as part of

a separate thesis project concurrent to this one. The author has assisted in this work.

7

2 ENGINE SELECTION

The first section of this chapter presents the rationale behind using a single cylinder engine for

research. The final choice of engine based on certain requirements has also been discussed.

2.1 Single cylinder research engine

Internal combustion engines used for research can be either single cylinder or multi-cylinder

based on the phenomenon of interest being studied. When the phenomena of interest are

combustion, combustion chamber designs, in-cylinder flows, flows across valves etc., it is

prudent to use a single cylinder engine.

A single cylinder engine has the inherent advantage of not being affected by the adverse flow

interactions due to other cylinders. It is also logical to use a single cylinder engine to reduce

fuel consumption. The ability to facilitate easy replacement of internal components suitable for

the type of tests intended to be run is another advantage.

2.2 Requirement specification and alternatives

The most important requirement regarding the engine was the availability of spare parts in the

future. The engine was a contribution from Scania CV AB towards Project CLOCK. At the

beginning of the project, there were three tentative engine options to choose from:

1. A custom-built single cylinder engine based on the 13-liter Scania DC13 engine

2. A 5-cylinder Scania OC09 9-liter Otto engine

3. A 6-cylinder Scania OC13 13-liter Otto engine

Unlike options 2 and 3, the custom-built single cylinder engine would not require mechanical

modifications to run it as single cylinder. However, most of the parts for such an engine were

custom-made. From the perspective of ready availability of spare parts, it was decided that the

series-produced multi-cylinder engines would be a more practical choice. Additionally, the

multi-cylinder engines were already balanced and would not vibrate as much even when only

one cylinder was fired.

2.3 Implementation

The availability of a run-in and tested 5-cylinder, 9-liter engine, made it the default choice. All

requisite modifications for conversion to single cylinder operation were done by another

student, assisted by the author. The details of these modifications can be found in a separate

report (Lius, 2018).

It was decided to use cylinder 1 (first cylinder from the front of the engine) as the active cylinder

because of the following considerations:

1. The open end of the intake manifold lies adjacent to cylinder 1, meaning that the existing

flange on the manifold could be used without subjecting the manifold to machining

8

2. Future researchers would want to use the engine for optical studies as well, so cylinder 1

provides better access for the optical diagnostic instruments

3. Setting up ignition timing for cylinder 1 would be easier

Figure 1. Top view of the engine illustrating reasons for choosing to retain cylinder 1 as the active cylinder

The selected engine in its unaltered state, was a port fuel-injected spark-ignited natural gas

engine with two intake and two exhaust ports.

9

3 FUELING SYSTEM

The following chapter is a discussion on the fueling of the engine, the challenges associated

with gaseous fueling and final implementation.

3.1 Fuel-air mixture formation

This section discusses the main challenge involved in mixing two fluids of comparable

densities. The fluids of interest to this project are CNG and air. Researchers have reported

turbulence, volume and residence time available for mixing affect the homogeneity of the

mixture. Methods to promote the turbulence intensity and to vary the residence time to achieve

varying mixture homogeneity were implemented.

3.1.1 Gaseous fuel-air mixture

Turbulence caused by the transfer of momentum from the periphery of a jet to the surrounding

fluid provides the necessary shear forces to generate eddies and vortices that promote mixing.

The appreciable difference in the densities of liquid fuel droplets and air, create significant

turbulence at the jet-air interface. Vaporization of the droplets creates a vapor trail behind them

which, combined with the momentum induced small-scale vortices, accelerates the diffusion

rate. This process helps create a more uniformly distributed mixture (Khan et. al., 2009).

However, the shear forces between a gas jet injected into another quiescent gas, are low and

mixture formation may become diffusion dominated, which is a relatively slow process. The

comparable densities of gaseous fuels and air, do not allow the utilization of jet penetration for

large scale mixing throughout the volume. Low penetration is characteristic of gas jets

irrespective of injection pressure. Research has shown that mixing can be improved by inducing

large-scale turbulence (Herrera et. al., 2012; Khan et. al., 2009).

3.1.2 Effects of fuel injector location and injection timing

The location of the fuel injectors determines the residence time available for the fuel to form a

homogeneous mixture. Researchers have reported that injecting fuel just downstream of the

throttle body provides a greater time for homogeneous mixture formation owing to the length

of the intake pipe from the injector(s) to the cylinders. The turbulence prevalent after the throttle

valve also serves to promote mixing (Soanes and Koopmans, 2012; Puzinauskas et. al., 2000).

In the case of port fuel injection, variation in mixture homogeneity was reported to be sensitive

to the injection timing. The decrease in mixing length makes the mixing dependent on the inlet

valve opening and pulsations in the manifold (Soanes and Koopmans, 2012). It is believed that

injecting against a closed inlet valve can provide enough residence time for mixture formation.

However, port injection into an open inlet valve was reported to aid turbulence effects and

promote in cylinder mixing. Residual mixture in the port from the previous injection cycle was

observed to reduce the effect of any stratification in the cylinder. However, this strategy was

not found to produce any prominent effects at high loads (Czerwinski et. al., 1999).

10

3.2 Charge homogeneity variation

As discussed in Section 3.1.2, the location of the injectors determines the homogeneity of the

charge. To be able to induct a well-homogenized mixture into the cylinder, it was necessary to

provide as much turbulence and residence time as possible.

The following table provides a comparison of the various methods for creating

well-homogenized mixtures (see Table 2):

Table 2. Alternative methods for well-homogenized premixture of gaseous fuel and air

Item

number


1 Rapidly pulsing one or

more injectors installed

well upstream of the

intake manifold

• Potential to create

evenly spaced fuel

and air pockets of

small volume if

executed properly

• Needs long length

of pipe before the

manifold to provide

highest degree of

homogeneity

• Injector solenoids

could overheat due

to rapid pulsing

2 Using MAN gas-mixer • Good mixture

formation

• MAN spare part

availability and

lead time uncertain

• No performance

testing done by

Scania, hence no

assurance on

performance from

Scania engineers

3 Using the Scania Euro

5 gas-mixer • Excellent mixture

formation assured

by Scania

engineers

• Standard Scania

part still in

production ⟹

quick replacement

possible if needed

• Bulkier compared

to the MAN design

For studies such as the effects of in-cylinder flows on homogeneity, less homogeneous mixtures

were needed.

11

3.3 Laboratory fueling setup

The fueling system in previous projects on CNG engines at Scania have used the following

setup (see Figure 2) (Scania, 2018):

Figure 2. Laboratory gas supply schematic (Scania, 2018)

The various components comprising the laboratory fueling setup are as listed in Table 3:

Table 3. Components of the laboratory fueling setup from tanks to the engine

Item

number

Component Function description

1 Fuel tanks with electronic and manual

shut-off valves for safety

To store CNG under a pressure of up to

250 bar.

2 Primary fuel pressure regulator fed by

high pressure lines

Expands the fuel from the feed pressure

to 50 bar. Fuel is supplied to the various

test cells at 50 bar.

Remark: The expansion process causes

a considerable temperature-drop in the

regulator. Freezing is prevented by

electrically heating the regulator unit.

3 Secondary fuel pressure regulator Expands the fuel from 50 bar to 10 bar.

4 Gas panel with electronic and manual

shut-off valves for safety

Maintains the supply pressure such that

there is a constant 7 bar pressure

difference across the injectors.

12

5 Fuel rail Temporary fuel storage before

injection.

Remark: Another function of the fuel

rail especially in a multi-cylinder setup,

is to ensure that each injector receives

fuel at the same pressure and that none

of the injectors are starved.

6 Fuel injectors connected to the fuel

rail

Injecting fuel in the intake ports to

induct a premixed charge into the

cylinders

Safety devices such as flame traps and shut-off valves (both manual and automatic) are located

at various points in the circuit

3.3.1 Gas panel

Installed within the test cell between the secondary pressure regulator and the fuel rail, is a gas

panel which consists of the following components (see Table 4 and Figure 3):

Table 4. Gas panel components and their respective functions

Item

number

Component Function

1 Reference pressure line to

intake manifold

To maintain a constant pressure difference of 7 bar

across the injector irrespective of intake manifold

pressure

2 Pressure limiting valve To prevent fuel overpressure which could impact

injector opening time and current

3 Pressure regulator To reduce the gas from tank pressure (~ 200 bar) to

injection pressure (~ 7.3 bar)

4 Pressure sensor For diagnostics

5 Solenoid valve For automatic gas supply switching on/off on the

low-pressure side when the power is switched

on/off

6 Manometer For manual inspection of supply pressure from the

tank

7 Fuel filter To remove oil droplets possibly introduced during

compression, and other impurities from the gas

8 Manual shut-off valve To manually turn on/off low-pressure gas supply

13

9 Coolant in/out To prevent freezing of the pressure regulator due to

expansion of the gas from tank pressure to injection

pressure

Figure 3. Gas panel for CNG engines (Scania, 2018)

The gas pressure regulator has the following preset pressure settings (see Table 5):

Table 5. Gas panel pressure regulator preset pressure settings

Outlet pressure 7.3 bar(g)

Inlet pressure 17.2 bar(g)

Pressure relief valve range 10.3 to 12 bar(g)

The regulator has an operating temperature range of - 40° C to 120° C. This gas panel is also a

standard assembly on production trucks.

14

3.4 Installation assembly

A fueling system like the one described in Section 3.3 was recommended to be installed at

KTH. The gaseous fuel-air path inside the test cell was designed as a modular unit as shown in

Figure 4.

Figure 4. Illustration of the recommended gaseous fueling system layout

15

The components of the fueling system along the intake air path are as follows (see Table 6):

Table 6. Components of the fueling system assembly within the test cell

Item

number

Component Function

1 Damping/mixing tank

with overpressure

rupture disc

• The baffle in the tank provides damping of

pressure pulses propagating from the engine;

enables more accurate air mass flow measurement

• Mixing chamber for gaseous fuel and air when the

gas-mixer is installed upstream

2 Gas-mixer (Scania Euro

5 design) • To generate turbulence and introduce the gaseous

fuel into the air stream for good mixing

3 Throttle • To control the flow of air/charge mixture into the

engine

4 Flame arrestor • To prevent flame propagation upstream from the

intake manifold in case of backfire

5 Flexible pipe • To minimize the vibrations due to single cylinder

operation from oscillating the upstream

components, particularly the heavy flame arrestor

assembly

6 Fuel injectors upstream

the intake manifold • To be able to induct a semi-homogeneous charge

mixture into the engine if future studies call for it

7 Port fuel injectors • Stock setup to inject gaseous fuels into the air

stream in the intake runners

As described in item 1 of Table 6, the damping tank can be used to create the most well-mixed

charge possible if the gas-mixer is installed upstream of the tank. The turbulence created by the

mixer combined with the greater residence time creates a very homogeneous mixture.

The flame arrestor and the overpressure rupture disc to vent the damping/mixing tanks on the

intake and exhaust sides have been discussed in detail in Chapter 6.

3.5 Liquid fuel direct injection

Liquid fuel direct injection was decided to be included in the fueling system for the engine with

the aim to offer fueling flexibility.

3.5.1 Assignment

The engine would eventually replace the existing engine in Test Cell 2 at KTH. The existing

engine is based on a Scania 12-liter, 6-cylinder diesel engine and has been modified to provide

optical access on cylinder 1. The new engine would be more beneficial for future projects if

16

provisions are made for flexible fueling. A requirement from the commissioner was to be able

to directly inject liquid fuels such as diesel and alcohols.

3.5.2 Implementation

To fulfill this requirement, it was decided to use the Scania XPI direct injection fueling system.

Apart from the extremely high pressure (~ 1500 bar) fuel injection capability of the system, the

cylinder heads are designed to accommodate the XPI injectors. The engine also had the

gear-train to drive the high-pressure fuel pump pre-installed. These features necessitate no

extensive machining to be able to implement liquid fuel direct injection.

3.5.3 Modifications

Some modifications needed to the XPI system are listed in Table 7:

Table 7. XPI system modifications for use with alcohols

Item

number

Modification Purpose Method

1 Re-calibrating the

high-pressure fuel

pump output

The pump needs to supply fuel

only to one injector in a single

cylinder engine

One of the two pumping

barrels of the pump need

to be plugged shut

2 Plugging of unused

outlets on the high-

pressure fuel rail

The stock fuel rail is meant for

a five/six-cylinder engine and

will have four/five unused

outlets respectively when used

with a single cylinder engine

Plug the unused outlet

ports with standard plugs

The high-pressure line from the fuel rail to the high-pressure connector of injector number 1

needed to be re-oriented. This was necessary since the intake manifold of the stock natural gas

engine differs in design from that of the diesel engine. On the natural gas engine, the intake

manifold occupies the location of the diesel fuel rail and hence the need for the re orientation.

Additionally, the high-pressure fuel delivery lines between the high-pressure pump and the fuel

rail need to be elongated to connect with the rail in its modified location.

17

(a)

(b)

Figure 5. (a) Proposed orientation of the high-pressure fuel rail, needing 45 mm spacers between the manifold

and cylinder head; (b) Proposed orientation of the high-pressure fuel rail demonstrating the use of the existing

high-pressure connector without changing any bend radii

The control of the XPI system is intended to be done via the Scania S8 diesel ECU, again due

to existing compatibility.

19

4 ELECTRONIC ENGINE CONTROL

In this chapter, a discussion of the electronic engine control unit used for this project has been

presented. Also included, are the most important electronic fueling system components – the

injectors and the lambda control.

4.1 Engine control unit

This section provides a brief description of the requirements set on the engine control unit in

terms of controllability, followed by selection and implementation of the control unit.

4.1.1 Assignment

The production version of the original engine is controlled by a Bosch EGC 4 engine

management system specific to natural gas engines. However, this control unit contains

proprietary programming from Bosch and is not open for modification. In order to set up the

engine for the intended tests, a user-configurable ECU was found necessary. Aftermarket ECUs

which usually have a Graphical User Interface (GUI) were found suitable. These ECUs, while

not programmable at the code level, still offered more flexible configurability compared to the

Bosch system. Deciding the most important comparison metrics for selection of the ECU was

undertaken as part of this project.

4.1.2 ECU selection criteria

The main criteria for choice of aftermarket ECU for the project were as follows (see Table 8):

Table 8. Comparison metrics for ECU selection

Item

number

Criterion Description

1 Configurability and ease of use The chosen unit should be easy to

configure, preferably on-the-fly through a

Graphical User Interface (GUI). A Swedish

manufacturer is preferred so that requests

for feature addition/modification can be

done quickly.

2 Cost The cost should be as low as possible since

it is possible that the new Scania Otto ECU

being developed by NESI could replace the

unit eventually.

3 Number of inputs and outputs The ECU should be able to read and control

as many parameters as possible.

Inputs: Crank and cam position, intake air

temperature and pressure, oil pressure,

coolant temperature, exhaust temperature

20

shall be the bare minimum parameters to be

read.

Outputs: Electronic throttle control, drive

for at least six low impedance injectors,

EGR valve control (if possible) shall be the

bare minimum outputs.

4.1.3 ECU selection and installation

The criteria mentioned in the previous section resulted in a choice between either the MaxxECU

Race or the NIRA i7x. Recommendations from Scania engineers with previous positive

experience with MaxxECU were strongly influential in the decision. The NIRA i7x was also

found to lack driver circuits for low impedance injectors. Thus, the MaxxECU Race was opted

for and installed.

The main sensors needed for engine operations (see Table 9) were connected to the ECU via

the accompanying wiring harness.

Table 9. Sensors necessary for basic engine operation

Item

number

Sensor Description

1 Engine speed sensor Senses rotational speed and position of the

crankshaft to determine injection and

ignition timing.

2 Camshaft position sensor Used in conjunction with the engine speed

sensor to determine which of the pistons is

towards the end of the compression stroke.

3 Intake manifold combined pressure

and temperature sensor

Determines the air mass flow rate into the

engine via the Speed-Density algorithm.

4 Wideband lambda sensor Determines the stoichiometry of the

previous combustion event and used for

feedback control.

5 Knock sensor Senses the occurrence of engine knock so

that ignition timing can be retarded.

4.2 Fueling control

The following section describes the hardware and considerations that need to be accounted for

to control the fueling system.

21

4.2.1 Types of CNG injectors

There exist two types of injectors which are suitable for CNG applications namely, the

voltage-controlled and the current-controlled types. Both types of injectors have been used in

all kinds of injection methods such as throttle-body injection, multi-point injection (sequential

and continuous) as well as direct injection. Both types of injectors have electromagnetic

solenoids which can be opened and closed by energizing and de-energizing the coils

(Reif, 2015).

As the name suggests, the voltage-controlled injectors are supplied with the operating voltage

by the ECU injector driver circuit. A corresponding current builds up the magnetic field needed

to retract the injector valve and hold it open against the rail pressure (see Figure 6). The finite

time needed for the build-up of the magnetic field usually results in slower response of these

type of injectors.

(a) (b)

Figure 6. Typical solenoid current build-up, injector valve lift and corresponding fuel flow rate for (a) voltage-

controlled injector and (b) current-controlled injector (Reif, 2015).

One situation where this limitation is highlighted is during engine idling. Idling requires that

the least amount of fuel be injected to just keep the engine running and engine management

systems are usually programmed to lean out the mixture to conserve fuel. The slower response

time of this type of injectors inevitably results in richer idling which is not desirable.

The current-controlled injectors on the other hand, are supplied with a peak current which lifts

the valve from its seat. Once the valve lifts from its seat, the driver then reduces the current to

22

the amount needed to hold the valve open. This current reduction is done to prevent the higher

peak current from potentially burning the coils (Reif, 2015).

4.2.2 Installation

The injectors used for this application are the standard Bosch NGI2 gas injectors with a nominal

impedance of 8.5 Ω. The ECU has eight injector driver outputs. It can supply 8 A of peak

current and 4 A of hold current at 12 V per driver output (MaxxECU, 2018). The injectors

require 1.3 A of peak current and 0.35 A of hold current. At normal operating conditions, the

injectors are restricted to 30 mins at 100% duty cycle. It was therefore decided to install two

injectors in parallel per driver output. Reducing the on-time per injector for a given quantity of

fuel was the rationale behind this decision.

4.2.3 Verification

The ECU was set up for sequential injection every 720° CAD. However, it was not possible to

test the synchronization. Recommendations for testing the injectors and injection timing have

been made in Chapter 8. These tests will be carried out once the engine has been installed in

the test cell where it can at least be cranked using the dynamometer if not under its own power.

4.3 Lambda control

This section discusses the motivation behind maintaining unit lambda, associated hardware

namely the lambda sensor and implementation of lambda control.

4.3.1 Stoichiometric combustion and the three-way catalyst

Lean pre-mixed combustion can reduce the combustion temperature, but prolonged operation

in this mode results in an increase in NOx emissions. Extremely lean mixtures beyond the

flammability limit result in flame extinction and misfire. Experiments have found that lean burn

engines produce very low emissions and lower fuel consumption under low loads. However, at

higher loads, the high temperatures and excess air lead to an increase especially in the NOx

component of the emissions (Saanum et. al., 2007; Verhelst et. al., 2006). Stoichiometric

combustion on the other hand, produce higher NOx, CO and HC emissions and have slightly

higher fuel consumption at low loads. The three-way catalytic converter however, can treat

these emissions as long as the combustion is stoichiometric (Saanum et. al., 2007;

Verhelst et. al., 2006).

Einewall et. al. (2005) also report a decrease in NOx emissions by over 99% and hydrocarbon

emissions by over 90% by operating stoichiometrically compared to lean burn. However, it was

found that EGR inclusion resulted in significantly higher CO emissions along with a slight drop

in efficiency. It must also be noted that the trade-off window between NOx reduction and CO

oxidation in the catalyst is of the order of 0.01 in terms of lambda (see Figure 7) which requires

the lambda control to be very accurate.

23

Figure 7. Lambda window for highest TWC conversion efficiency (Reif, 2015; Heywood, 1988)

4.3.2 Exhaust gas oxygen sensors/Lambda sensors

Measurement of oxygen concentration in the exhaust stream is done by exhaust gas oxygen

sensors. There are two types of O2 sensors namely, the narrow-band and the wideband sensor.

Both types of sensors are essentially galvanic concentration cells (or Nernst cells) consisting of

two half-cells. The composition of these half-cells is identical except for the concentration of

the electrolyte. This concentration difference is the motive force behind the transport of ions.

Ions are charged particles and their motion is indicative of the concentration of the electrolyte

in one half-cell provided the electrolyte in the other half-cell has a known concentration

(Marek et. al., 2003; Reif, 2015).

One type of O2 sensors is called switching type or narrow band O2 sensor. Such sensors only

sense a deviation in mixture quality from stoichiometric. They do not indicate the extent of the

deviation. The narrow-band O2 sensor compares the residual oxygen content in the exhaust gas

with the oxygen content in the reference atmosphere. A ceramic element, usually made of

zirconia is coated with a noble metal such as platinum on either side, which act as electrodes.

The ceramic element is conductive for oxygen ions. The different concentrations of oxygen on

either side of the element generates an electrical voltage across the metal electrodes

(Marek et. al., 2003; Reif, 2015). Such a Nernst cell produces a voltage of around 450 mV

across its electrodes when there exists no concentration difference across the cell. A rich

mixture results in a voltage between 450 mV and around 900 mV, whereas a lean mixture results

in a voltage between 450 mV and close to 0 mV (see Figure 8). The noble metal coating also

serves to catalytically convert other species in the exhaust gas stream such that only an

equilibrium reaction involving oxygen takes place in the cell (Marek et. al., 2003; Reif, 2015)

24

Figure 8. Typical narrow-band O2 sensor output voltage as a function of lambda (Reif, 2015)

The change in oxygen concentration for a 0.02-unit lambda span, can range from 9×10-15 % vol.

on the rich side to 0.2% vol. on the lean side (Reif, 2015). As a result, the ECU constantly hunts

around the stoichiometric point, trying to achieve it but inevitably ending up with an overshoot

in either direction. Cycle-to-cycle control of the air-fuel ratio would require the use of a

predictive model for the amount of air that will be inducted into the cylinders (Reif, 2015).

The other type of O2 sensor is called the wideband lambda sensor. Such sensors not only

indicate deviations from stoichiometry but can also determine to what extent the mixture

deviates. This allows for more precise control of the air-fuel ratio without the ECU hunting.

Knowing the extent of the deviation, the ECU can more accurately determine the amount of

fuel to inject as a corrective measure.

Wideband O2 sensors consist of two electrochemical cells (see Figure 9(a)) with a sensing gap

in between them, where the measurement takes place. The first is a regular Nernst cell which

produces a voltage based on whether the sample of exhaust gas being measured is rich, lean, or

stoichiometric. The second, is a pumping cell. It is so named, since a voltage can be applied to

this cell by a control circuit to enable pumping oxygen ions into or out of the sensing gap (Reif,

2015).

25

(a)

(b)

Figure 9. (a) Construction and (b) characteristic curve of a typical wideband lambda sensor (Marek et. al., 2003).

If the exhaust gas is rich, the Nernst cell which has one of its electrodes exposed to reference

air and the other to the gas sample, puts out a voltage higher than the reference voltage of 450

mV. The controller compares the output to the reference supplies a voltage to the pumping cell

electrodes such that oxygen ions are pumped from the exhaust stream into the sensing gap. The

oxygen ions pumped into the sensing gap result in the Nernst cell voltage returning to 450 mV

(Marek et. al., 2003; Reif, 2015). In case of a lean exhaust gas sample, the pumping voltage is

reversed such that oxygen ions are pumped out of the sensing gap to return the Nernst voltage

to 450 mV (see Figure 9(b)) (Marek et. al., 2003; Reif, 2015). In either case, the current set up

in the pumping cell due to the movement of ions provides the ECU with a quantitative measure

of the mixture quality. Stoichiometric mixtures on the other hand, yield zero pumping current

(Marek et. al., 2003; Reif, 2015).

The O2 sensors are usually located in the exhaust stream between the turbocharger and the

catalytic converter. The choice of location of the sensor is dictated mainly by the temperature

restrictions of the sensor. This requires the exhaust gases to traverse a certain volume between

the exhaust valve and the sensor. The farther away the sensor is, the larger the volume that the

exhaust gases need to traverse before reaching the sensor. This condition results in a limitation

on the response time of the feedback control loop. For the quickest response times, the lambda

sensor ideally needs to be located in the exhaust port. However, since this is not possible, there

will always be a delay between when the sensor senses the oxygen concentration and when the

mixture correction is implemented by the injectors. Lambda sensors can also be used to

compensate for variations in CNG composition which is usually unknown to the end users of

the vehicles.


The ECU is equipped with its own wideband lambda controller including an inbuilt PWM

heater. The heater is used to heat up the sensor to its optimum operating temperature of over

350°C. The supplied sensor, a Bosch LSU 4.2 wideband lambda sensor, was installed in the

exhaust gas stream. Temperature restrictions on the sensor were considered in order to

26

determine the installation location. The sensor was delivered pre-calibrated. However, the

closed-loop lambda control could not be tested as the sensor would remain exposed to ambient

air until installed in the test cell with the associated piping in place.

The ECU also features a proprietary Speed-Density algorithm for feed-forward lambda control.

The ECU consists of an inbuilt reference pressure port to which a pressure line was connected.

The other end of this line was connected to the intake manifold. Nevertheless, the ECU does

not contain pre-programmed fuel maps for CNG. Therefore, for the open loop control to

function, a manual fuel map needs to be programmed into the ECU. For initial engine startup

and idle running, it was decided to use a modified gasoline fuel map. The gasoline equivalent

CNG mass flow rate was determined assuming an average calorific value of gasoline and CNG.

This fuel mass flow rate has been used to set the injection duration but if this method results in

unit lambda, has not been tested.

27

5 GAS EXCHANGE SYSTEM

A majority of the discussion presented in this chapter pertains to the intake air handling system.

A short discussion has also been presented on the exhaust side damping tank.

5.1 Intake damping/mixing tank

The layout of the air intake system has been illustrated in Section 3.4 (see Figure 4). The engine

will be supplied with boosted charge-air from an external compressor belonging to the

laboratory. The first component in the air flow path is the damping tank which can also be used

for premixing a gaseous fuel and air.

5.1.1 Purpose

The tank has multiple purposes, namely:

1. Dampen pulsations in the air path enabling more accurate air mass flow measurement

2. Preparing well homogenized mixtures of air and gaseous fuels before induction into the

engine

3. For mixing EGR into the intake air stream

5.1.2 Alternative designs

Two alternative designs were considered for the tank and have been described below

(see Table 10 and Figure 10).

Table 10. Alternative damping tank designs

Item

number

Description Features

1 Alternative 1 • 12 liters in volume

• Contains a baffle in the middle for damping pulses

• Not been used for mixing gaseous fuel and air

2 Alternative 2 • Approximately 40 liters in volume

• The large volume itself is responsible for damping

• Has provisions for injecting fuel into the tank for

premixed charge formation

• Used to mix EGR with the intake air

• Turbulence induced by the throttle valve installed

upstream of the tank is utilized for improved mixing

28

(a) (b)

Figure 10. Illustration of intake damping tank (a) Alternative 1 with baffle; (b) Alternative 2 with larger volume

5.1.3 Design choice and implementation

The decision to replicate the Scania design was foremost based on space restrictions in the KTH

test cells. A simulation study by Scania has recommended that the volume of the tank be

increased to 18 liters instead of the current 12 liters (Scania, 2016). The study states a drop in

volumetric efficiency of the engine at high speed and high load during simulations as the reason

for the recommendation.

The recommendation to use an 18-liter tank was agreed upon with the commissioner. However,

from a functionality perspective, the tank was also intended to be used for well-homogenized

29

charge mixture formation. At present, no provisions have been made for introduction of EGR

into the tank. If needed however, the modification to the tank would be minor.

5.2 Throttle valve

This section discusses the purpose of the throttle valve, choice of size based on control concerns

and its implementation.

5.2.1 Purpose

The aim of a throttle valve is to control the amount of air which is inducted into the engine. A

throttle valve is of more importance in a stoichiometric SI engine since the amount of air

inducted determines the quantity of fuel to be injected. The precision of the injected fuel

quantity is critical from an exhaust after-treatment perspective due to limitations of the

three-way catalyst.

5.2.2 Throttle sizing

The full engine uses a 57 mm electronically actuated Bosch throttle valve matched to the 9-liter

displaced volume. Issues with air mass flow control using a large throttle valve for single

cylinder operation were expected. It was expected that the steep initial opening ramp of the

throttle plate, could resulting in larger throttle angles than desired at low air mass flow rates.

However, Scania engineers reported that previous tests run in their test cells have used the same

throttle. Furthermore, it was reported that the engine runs on wide-open throttle above 40%

load. It is only at lower speeds and loads that the throttle has any function at all. It was therefore

decided to retain the throttle from the full engine.

5.2.3 Implementation and verification

The ECU supports a single electronic throttle with two motor control outputs and two throttle

position sensors. The two outputs are Pulse-width Modulated (PWM) and the position sensor

characteristics were as provided by the manufacturer. The position sensors are potentiometers

and moving the throttle plate manually could cause a shift in the zero set-point. This in turn,

could cause a difference in the clockwise and counter-clockwise rotation angles of the throttle

plate. Throughout the project, care was taken not to move the throttle plate manually. Therefore,

the position sensors did not need to be re-calibrated. The diagnostic function of the ECU was

used to test the throttle actuation. Also, the throttle is a normally-closed valve and was found to

shut-off after power supply was cut. However, no tests have been carried out scoping the

position sensor output against the demanded position. Such a test will only be possible after the

engine has been connected to a dynamometer with throttle control or at least a throttle pedal to

determine the demand.

5.3 Intake manifold modifications and implementation

The intake manifold required some modification due to single cylinder operation and liquid fuel

direct injection (see Table 11).

30

Table 11. Intake manifold modifications

Item

number

Modification Reason Method

1 Decreasing the

volume

The other cylinders were

deactivated by removing the

intake valves. The intake

manifold was retained to

prevent foreign objects from

entering the cylinders.

However, it was also not

desirable to fill the unused

cylinders with premixed fuel

and air.

A small section of the stock

manifold between the swirl port

runner of cylinder 1 and filling

port runner of cylinder 2 was

sawed off. A solid aluminium

plate was welded in place,

thereby separating the manifold

internally while retaining stock

external form (see Figure 10).

2 Moving the

manifold away

from the

cylinder heads

Reorientation of the

high-pressure fuel rail (see

Section 3.5.2) required the

manifold to be moved away

from the cylinder heads by at

least 45 mm.

Aluminium spacers with holes

matching the port holes

(see Appendix A). If each

spacer cannot be made from a

single piece, two 25 mm thick

pieces may be used per cylinder

with gaskets in between.

Figure 11. Illustration of the method used to adapt the intake manifold for single cylinder operation

5.4 Variable intake port filling

This section describes the need for selective filling of the intake ports, the alternatives

considered to achieve the objective and final choice of method.

5.4.1 Assignment

With potential future projects in mind, a method to vary the amount of charge flowing into

either of the two intake ports was requested by the commissioner. The current cylinder head

31

has a swirl port and a filling port. Increasing the amount of charge flowing into either port

strongly affects in-cylinder flows. While more flow through the swirl port promotes swirl,

increased flow through the filling port promotes tumble.

5.4.2 Alternative solutions

Three alternative solutions were considered to fulfill the assignment described in Section 5.4.1

(see Table 12).

Table 12. Alternatives for varying port filling


1. Using separate throttle

valves of appropriate

dimension for each port

• Possibility for

electronic control

⟹ good precision

and no manual work

needed

• Finding appropriately

sized throttles

• Modifications to the intake

runners with a complex

geometry

2. Installing baffles in the

manifold to direct the

flow towards the desired

intake port, see Figure. 11

• Simpler than

individual throttles

• Need for extensive

machining of the manifold

• Requires at least three

sections of manifold; One

stock and one each for

directing flow towards

each port

3. Installing spacers

between the manifold and

the cylinder head with

either port partially

closed, see Appendix A.

• Even simpler than

baffles to implement

• Requires the manifold to

be removed and re-fitted to

change the port being

restricted

(a) (b)

Figure 12. Illustration of alternative 2 for variable intake port filling; (a) flow directed towards swirl port and (b)

flow directed towards filling port

32


The simplicity advantage of Method number 3 mentioned in Table 12 (see Section 5.4.2) made

it the choice for implementation. Drawings for the spacers (see Appendix A) were made with

the following variations:

1. The filling port area was reduced by half while the swirl port area was retained unchanged

and vice-versa

2. The filling port was completely blocked while retaining the swirl port fully open and

vice-versa

3. All ports were retained fully open

The variations mentioned in items 1 and 2 are meant only for the active cylinder. Variation as

in item 3 is needed for all cylinders since the completer intake manifold has been retained.

5.5 Exhaust side damping tank

A damping tank as on the intake side, was also decided to be installed on the exhaust side.

Details of the same have been presented in this section.

5.5.1 Purpose

As the name suggests, the tank is meant to dampen the extremely pulsatile nature of the exhaust

from the single cylinder engine. This is necessary especially when employing long-route EGR.

Long-route EGR on a single cylinder engine usually must be done by means of a positive

displacement circulation device such as a supercharger. For better EGR cooling and to not

starve the circulation device, the exhaust gases need to be supplied continuously without

pulsations. The exhaust side damping tank is intended for this purpose in anticipation of

long-route EGR operation in the future.


The design for the exhaust damping tank is the same as that used in the Scania single cylinder

test cells. The only modification to the design was the addition of an overpressure rupture disc

type vent (see Figure 12). This safety device was deemed necessary in anticipation of misfire

which could cause violent pressure peaks in the exhaust stream. The details of the safety devices

used in this project have been presented in Chapter 6.

33

Figure 13. Illustration of the exhaust side damping tank with rupture disc for overpressure protection.

35

6 SAFETY DEVICES

This chapter describes the safety devices used in the combined fueling-air handling system.

These devices are meant to provide safety to the users as well as components of the test cell in

the event of backfire. Safety was considered paramount in view of the fuels planned to be tested.

6.1 Flame arrestor

This section discusses the working principle and motivation behind using a flame arrestor.

6.1.1 Working principle

It is common knowledge that a flame can be created, sustained, and propagated if there is fuel,

an oxidizer and sufficiently high temperature. Depriving the flame of one or more of these

ingredients results in extinction of the flame. The basic function of a flame arrestor is to quench

the flame. This is done by absorbing thermal energy from the products of the exothermic

combustion reaction. From elementary chemical kinetics, it is known that the removal of energy

slows down the rate of reaction. If the rate of reaction is decelerated enough, the combustion

reaction can no longer support itself and the flame ceases.

A flame arrestor which works on the above principle, usually consists of a corrugated steel strip

wound together with a flat strip to form triangular prismatic passages (see Figure 13). The

passage gap needs to be smaller than the Maximum Experimental Safe Gap (MESG). In its

simplest form, MESG is defined as the largest gap between parallel surfaces which prevents

flame on one side of the flame path formed by these surfaces from igniting the fuel-air mixture

under test on the other side (Dickie and Lunn, 1983). Flame arresting elements can be metallic

or can contain certain liquids which can quench the flame through a reduction in temperature

(Rogowski, 1980).

Figure 14. Crimped metal-ribbon flame arresting element

36

6.1.2 Motivation

Using a gas-mixer upstream of the intake ports causes the piping between the intake port and

the mixer to be filled with well-mixed fuel and air. In the event of a backfire, it is highly

probable that this volume of mixture could ignite and cause an explosion. Such an occurrence

is not desirable from a safety perspective. Flame arrestors like the ones shown in figures

14 and 15(a) are usually used to stop the flame from propagating.


The Technical Development department for electrical and performance systems at Scania

Engines recommended KITO Armaturen GmbH for supply of the flame arrestor. The high boost

pressures (2 to 3 bar(g)) intended for the engine, and the need to keep the pressure loss across

the arresting element as low as possible, resulted in the selection of a crimped stainless-steel

ribbon arresting element. However, since it was also mentioned to KITO during correspondence

that hythane and pure hydrogen would also be used as fuels in the future, it was suggested to

use multiple elements in series as there were no dedicated arrestors for hydrogen flames.

Furthermore, all standard flame arrestors manufactured by KITO included cast-steel housings.

This the entire assembly weighed approximately 40 kg. The anticipated vibrations due to single

cylinder operation made the heavy assembly undesirable. A custom-made aluminium housing

used by Scania Engines reduced the assembly weight to approximately 10 kg (see Figure 15(b)).

(a)

37

(b)

Figure 15. Crimped metal-ribbon flame arrestor (a) element and (b) assembly with housing

The gap size of the arresting element used by Scania Engines is 0.7 mm and is meant for CNG,

while the theoretical MESG for hydrogen is around 0.08 mm (Grossel, 2002). However,

preliminary communication with KITO revealed that there exist no exclusive arresting elements

for hydrogen flames especially at the boost pressures intended to be run on the engine. Further

study needs to be done on this claim as it is possible that sintered metal flame arrestors could

be used for hydrogen flames.

6.2 Damping/mixing tank overpressure safety

The following section pertains to the safety consideration accorded to the intake

damping/mixing tank as well as the exhaust side damping tank. These tanks are basically

pressure vessels and needed to be protected against explosion due to backfire or misfire

6.2.1 Motivation

The damping tank on the intake side also serves for fuel-air mixing if the gas-mixer is installed

upstream. The same hazard as described in Section 4.1.2 holds true for this tank as well. The

38

consolidated volume of the tank makes any potential explosion more dangerous. As a

precaution, it was decided to vent this tank automatically above a certain pressure.


Fagerberg AB, a Swedish supplier of industrial pressure relief solutions was consulted.

Fagerberg AB recommended not using a reusable vent valve since there is always a slight

leakage associated with such valves. It was instead suggested to use disc-style vents which

rupture due to overpressure (see Figure 16) with an additional cost benefit compared to a

reusable valve.

Figure 16. Schematic drawing of the rupture disc with holder and installation flanges

The ISO DN 200 pipe segments and formed cups used for the fabrication of the tank are

available with a minimum nominal pressure rating of 16 bar. It was therefore suggested that the

discs rupture at 10 bar(a). To achieve this, rupture discs rated at 14.5 bar(a) burst pressure were

selected, inclusive of a safety factor of 1.33. The discs have a rupture pressure tolerance of

± 5%.

According to recommendation from Fagerberg AB, the vent diameter of DN 50 was selected.

The large diameter vent was suggested in view of hydrogen being used as one of the fuels in

the future. It was also recommended that the vent be installed at the top of the damping tank

(see Figure 17) The same arrangement was replicated on the exhaust-side damping tank to

relieve overpressure, should misfire occur (see Section 5.5.2, Figure 13).

39

Figure 17. Intake side damping/mixing tank with overpressure rupture disc installed

41

7 CONCLUSION

This chapter provides a summary of the entire thesis work.

7.1 Summary

This thesis project was carried out concurrently with the work of another student. The focus of

this project was a heavy-duty single cylinder SI engine.

7.1.1 Fueling system

Three requirements set on the fueling system were implemented:

1. Variation in the premixed charge homogeneity was implemented. Turbulence induced by

the gas-mixer was utilized for well-mixed charge formation. The residence time offered

for diffusive mixing by the damping/mixing tank and the length of pipe between the gas-

mixer and the intake ports is believed to be sufficient for good mixture uniformity.

2. Injectors installed further upstream of the intake ports are expected to create

inhomogeneous mixtures. However, no quantification of the mixture uniformity was done

numerically or experimentally.

3. The ability to influence in-cylinder flow through selective intake port usage was

implemented via spacers as described in Section 5.4.3.

4. The Scania XPI common-rail direct injection fuel system was chosen for direct injection

of liquid fuels. Necessary modifications as enumerated in Section 3.5.3 have been agreed

to after consulting Scania engineers.

7.1.2 Electronic engine control unit

Selection of a suitable engine control unit was carried out. The main comparison metrics were

ease of use, cost and the number of inputs and outputs provided.

7.1.3 Safety devices

1. A flame arrestor with MESG corresponding to CNG was decided to be installed close to

the intake manifold as described in Section 6.1. The pressure of the boosted charge-air

meant that no exclusive arresting elements were available for hydrogen and hythane fuels.

Arresting elements are available with a MESG of 0.5 mm which is claimed to be

applicable to hydrogen and hythane as well.

2. Rupture disc type overpressure vents were decided to be installed on the intake and

exhaust damping tanks as described in Section 6.2. The supplier, Fagerberg AB has stated

that test pieces are destructively tested, and a certificate provided for the burst pressure.

42

7.2 Conclusions

Through this project, a flexible fueling system for a single cylinder heavy-duty, spark-ignited

engine has been implemented in theory, pending physical fabrication and installation. No

simulation studies or experimental investigations have been done on the charge mixture

uniformity resulting from the various points of mixing. Selective port filling to influence

in-cylinder flows was carried out using spacers with cut-outs of different areas. Safety was

given high importance especially since gaseous fuels are involved. Hardware for liquid fuel

direct injection and corresponding modifications have been agreed to by the Scania high-

pressure fuel systems department with agreed delivery during Autumn, 2018.

43

8 RECOMMENDATIONS AND FUTURE WORK

In this chapter, recommendations on more detailed solutions and future work in this field are

presented.

8.1 Recommendations

The uniformity of the premixed charge may be determined through CFD studies or appropriate

experimental methods, if quantification of homogeneity is required.

Once the engine is installed in the test cell, the injectors of the gas-mixer, the ones for

semi-homogeneous mixture formation as well as the port injectors need to be configured for

injection timing and duration.

The injector manufacturer limits the boost pressure at which the injectors can operate. The

current value is set to 2.5 bar(a). The constant 7 bar pressure difference across the injector needs

to be maintained such that there is sonic flow at the injector nozzle. Increasing the boost

pressure above 2.5 bar(a) will cause the pressure difference to drop. This in turn, will result in

sub-critical flow. The author speculates that this effect can be negated to some extent by

increasing the outlet pressure of the gas pressure regulator to a suitable value. It is recommended

that detailed calculations for these values be made before increasing the boost pressure.

The flame arresting method for hydrogen and hythane using multiple arresting elements as

recommended by KITO, should be tested on an appropriate test rig before installation in the

test cell. However, the single arresting element with a gap size of 0.7 mm has been tested for

CNG operation by Scania Engines and deemed safe.

To ensure that the intake air path is evacuated of gas-air mixture, it is recommended to

switch-off gas supply and motor the engine for a few minutes before turning it off.

The Scania service manual ought to be referred to when re-fastening the intake manifold bolts

after installing/removing the spacers meant to distribute the charge to the desired intake port.

The same applies to all high-tension bolts such as those for the cylinder heads. The number of

times such bolts can be used needs to be followed as per the service manual to avoid catastrophic

failures. It is also important to follow the tightening torques and order for all bolts and screwed

fasteners.

8.2 Future work

Prior to commissioning the engine for research, the damping/mixing on the intake and exhaust

side need to be fabricated with the inclusion of the rupture disc style overpressure vents during

welding.

The high-pressure gas supply system needs to be installed and connected to the gas-panel. The

pressure relief outlet of the gas regulator on the panel needs to be vented via a suitable method

such as a flexible hose. Care should be taken not to accidentally block the relief outlet.

44

The stainless-steel rupture discs need to be shielded by a metallic umbrella which permits visual

inspection before each experimental run while containing any shrapnel during rupture.

A study on sintered-metal flame arrestor may be undertaken for use with hydrogen and hythane.

This type of flame arrestor could prove to be an alternative to the crimped metal-ribbon arresting

element.

Installation of piping for charge-air and exhaust gases within the test cell needs to be carried

out with the engine installed in the cell. This would aid precise measurement of pipe lengths.

From the fueling system perspective, the modified XPI system for liquid fuel direct injection

needs to be ordered and installed on the engine. The system also needs to be equipped with the

Scania S8 ECU for control.

The throttle valve position sensor outputs need to be tested against demands created by the

dynamometer throttle control or using a pre-calibrated throttle pedal.

The prototype cylinder head with provision for installing a flush-mounted cylinder pressure

sensor needs to be ordered and machined as required. If tests are intended to be run using liquid

fuel direct injection along with spark ignition, another such prototype head needs to be ordered.

The cylinder pressure sensor needs to be installed via a hole through the cooling water jacket

and the spark plug needs to be installed through the hole meant for the flush-mounted cylinder

pressure sensor.

Laboratory grade sensors may be installed where needed. These additional sensors need to be

connected to a separate data acquisition system since all inputs on the ECU have been occupied

by sensors needed for engine operation.

45

9 REFERENCES

Douailler B., Ravet F., Delpech V., Soleri D., Reveille B., Kumar R., "Direct Injection of CNG

on High Compression Ratio Spark Ignition Engine: Numerical and Experimental

Investigation", SAE Technical Paper 2011-01-0923, 2011

Herrera J., Toohey J., Jin B., Rogers T. and Koopmans L., “Effects of Different Port Injection

CNG System Configurations on a 3.8L V6 Engine”, Sustainable Automotive Technologies

2012, Springer-Verlag Berlin Heidelberg, pp. 103-110.

Heywood J., “Internal combustion engine fundamentals”, McGraw-Hill, New York, 1988.

Khan M., Watson H. and Baker P., “Mixture Preparation Effects on Gaseous Fuel Combustion

in SI Engines”, SAE International. Journal of Engines, Vol. 2, Issue 1, 2009, pp.230-245.

Kumar S., Kwon H-T., Choi K-H., Lim W., Cho J.H., Tak K., Moon I., “LNG: An eco-friendly

cryogenic fuel for sustainable development”, Applied Energy, Vol. 88, Issue 12, 2011,

pp. 4264-4273

Lius A., “Single cylinder research engine multi-spark ignition system”, M.Sc. Thesis, KTH

Royal Institute of Techonology, 2018.

MaxxECU, “MaxxECU RACE Quick-start Guide”

Min B.H., Chung J.T., Kim H.Y., Park S., “Effects of gas composition on the performance and

emissions of compressed natural gas engines”, KSME International Journal, Vol. 16, Issue 2,

2002, pp. 219-226.

Puzinauskas P., Willson B., and Evans K., “Optimization of Natural Gas Combustion in Spark-

Ignited Engines Through Manipulation of Intake-Flow Configuration”, SAE Technical Paper

2000-01-1948, 2000.

Reif K. and Dietsche K., “Automotive Handbook (8th ed.)”, Robert Bosch GmbH, Plochingen:

Robert Bosch, 2011.

Reif K., “Automotive Mechatronics: Automotive Networking, Driving Stability System,

Electronics (1st ed.)”, Bosch Professional Automotive Information, 2015.

Reif K., “Gasoline engine management: Systems and components”, Bosch Professional

Automotive Information, 2015.

Saanum I., Bysveen M., Tunestål P., Johansson B., “Lean burn versus stoichiometric operation

with EGR and 3-way catalyst of an engine fueled with natural gas and hydrogen enriched

natural gas”, SAE Transactions Journal of Engines, Vol. 116, 3rd ed. Society of Automotive

Engineers, 2007.

Soanes C. and Koopmans L., “On the Application of Compressed Natural Gas for an SI Internal

Combustion Engine and Two Different Injector Positions”, Sustainable Automotive

Technologies 2012, Springer-Verlag Berlin Heidelberg, pp. 111-118.

Verhelst S., Verstraetenb S., Sierensa R., “A Comparison Between Lean Burn Operation and

Stoichiometric Operation with EGR for a Hydrogen Fuelled IC Engine”, 16th World Hydrogen

Energy Conference Proceedings, Lyon, 2006.

A-1

APPENDIX A: DRAWINGS

This appendix consists of drawings for the spacers used to implement variable port filling. The drawings presented have been used for water-jet

machining of the port and bolt holes.

Figure A-1. Spacer between the intake manifold and cylinder head with half the stock filling port area

A-2

Figure A-2. Spacer between the intake manifold and cylinder head with half the stock swirl port area

A-3

Figure A-3. Spacer between the intake manifold and cylinder head with blocked filling port

A-4

Figure A-4. Spacer between the intake manifold and cylinder head with blocked swirl port

A-5

Figure A-5. Spacers with stock port areas for unused cylinders

Heavy-Duty Spark-Ignited Single Cylinder Engine Fueling ...

Documents

Transcript of Heavy-Duty Spark-Ignited Single Cylinder Engine Fueling ...