MOTOR BAKAR

( 3 SKS)

Sub processes of mixture formation and combustion in diesel engines

Schematic representation of spray dispersion

Composition of spray packets

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Crank Angle (deg)

Combustion Pressure (M

pa)

0% EGR 12% EGR 30% EGR 30% EGR w ith ESP

Injection Timing 19o

BTDC

Ignition Timing

Peak Pressure

Combustion PressureCombustion Pressure

Physical and chemical processes in ignition delay The important physical processes - atomization of fuel, - evaporation and mixing of fuel vapor and air

- the formation of an ignitable mixture The chemical processes- pre-reactions in the mixture - autoignition, which ensue at a local air ratio of 0.5 < < 0.7.

Local ignition ranges contingent on the time after the start of injection and the distance from the nozzle orifice

Combustion

Phase I: premixed combustion

Phase II: diffusion combustion

Phase III: post-combustion

Combustion Phase I: premixed combustion- The fuel injected during the ignition delay mixes with air in the combustion chamber and forms a nearly homogeneous and reaction-capable mixture.

- After the ignition delay period, this mixture burns very quickly (premixed peak).

- The combustion noise typical for the diesel engine is caused by the high pressure increase speed dp / dϕ at combustion start.

Combustion Phase II: diffusion combustion- Mixture formation processes continue during the main combustion phase and decisively influence both the combustion course itself as well as pollutant formation.

- The chemistry of this phase is very rapid- the combustion process is controlled by the mixing rate

- The end of the main combustion phase is characterized by the attainment of the maximum temperature in the combustion chamber.

Combustion Phase III: Post - combustion- Towards the end of combustion, pressure and temperature in the flame front have decreased so much that chemical reactions become slow in comparison with the simultaneously progressing mixture processes.

- This last combustion phase is thus decisive especially for the oxidation of previously formed soot.

- Over 90 % of the total soot produced is broken down again in the combustion chamber during this phase.

Rate of Heat Release

ddVP

vcpcpc

ddPV

vcpcvc

dRQ

QR : heat releaseP : pressureV : volumecv, cp : specific heats : crank angledV/d is calculated by equationVd : displacement volume of cylinderR : ratio of connecting rod and crank radius

2/122 sincos1sin2

RV

ddV d

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Crank Angle (deg)

Combu

stion Pressure (M

pa)

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Rate of H

eat R

elease (J/deg)

Pressure ROHR