Journal of Thermal Science Vol.15, No.4 371―376

Received: March 2006 Wen ZENG: Dr. Eng.

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DOI: 10.1007/s11630-006-0371-5

Multi-dimensional Modeling of the Application of Catalytic Combustion to Homogeneous Charge Compression Ignition Engine

Wen Zeng* MaoZhao Xie

Department of Power Engineering, Dalian University of Technology, Dalian 116024, China

The detailed surface reaction mechanism of methane on rhodium catalyst was analyzed. Comparisons between numerical simulation and experiments showed a basic agreement. The combustion process of homogeneous charge compression ignition (HCCI) engine whose piston surface has been coated with catalyst (rhodium and platinum) was numerically investigated. A multi-dimensional model with detailed chemical kinetics was built. The effects of catalytic combustion on the ignition timing, the temperature and CO concentration fields, and HC, CO and NOx emissions of the HCCI engine were discussed. The results showed the ignition timing of the HCCI engine was advanced and the emissions of HC and CO were decreased by the catalysis.

Keywords: catalytic combustion, homogeneous charge compression ignition(HCCI), CFD, methane, detailed chemical kinetics. CLC number: TK401 Document code: A Article ID: 1003-2169(2006)04-0371-06

Introduction

With the rapid development of world economy, the problems of energy sources crisis and environment pollu-tion become more and more seriously. As one of the main energy consumers and the main source of environment pollution, the automobile gets comprehensive attentions of worldwide researchers. Because of its high efficiency, very low nitrogen oxide (NOx) and particulate matter emissions, recently, the homogeneous charge compres-sion ignition (HCCI) engine has become the focus of engine research. However, the HCCI engine has still some problems to be solved, which include high hydro-carbon (HC) and carbon monoxide (CO) emissions, control of ignition timing and limited operating range, etc[1 ― 6]. Furthermore, because of the lower exhaust temperature, dealing with the emissions by after-treat- ment becomes difficult. However, using catalysis in the after-treatment process is a conventional means to de-crease the emissions of the conventional internal com-

bustion engine. So we must find new means to resolve these problems.

Comparing with the traditional combustion mode, catalytic combustion is a different process in which there is no flame propagation, and its ignition temperature is very low. For this reason, catalyst coatings on the internal cylinder head, pre-combustion chamber, piston surface and piston crown, can be used to modify the combustion process and the combustion characteristics, decrease HC, CO, NOx emissions and extend the operating range of the internal combustion engine. Gaffney et al.(1980)[7] re-ported that Pt coatings decreased soot emission from a diesel engine by ~40 percent, although the coatings was lost within 8hrs of engine operation presumably because of poor adhesion caused by carbon and iron oxide depos-its on the piston surface. A “catalytic engine” was de-scribed by Thring (1980)[8] which consisted of a metha-nol-burning, modified indirect injection diesel engine where the pre-combustion chamber was fitted with both a glow-plug and Pt mesh catalyst. The results showed that

372 Journal of Thermal Science, Vol.15, No.4, 2006

this engine had light-load fuel economy as good as a diesel, and the HC emissions lower than either petrol or diesel engine. Hu and Ladommatos (1995, 1996)[9―10] reported that unburned HC emissions were reduced by 20% by the deposition of a catalytic Pt-Rh coatings on the top and side surfaces of the piston in an SI laboratory engine. All these are experiments for the application of catalytic combustion to the conventional internal com-bustion engine. Due to the lack of detailed surface reac-tion mechanism and the limitation of computer capability, the numerical simulation of the effects of catalytic com-bustion on the combustion characteristics of the conven-tional internal combustion engine has not been reported in the past. The reasons are that catalytic combustion is a complicated physical and chemical process which in-cludes surface reaction kinetics, gas reaction kinetics and transport phenomena of mass, momentum and energy. Furthermore, the combustion process and the flow proc-ess in the conventional internal combustion engine are also very complicated. Simulations of this combustion process need vast computer resources.

Recently, with the computer capability advancing, new methods of mathematical simulation developed, detailed reaction mechanisms built, and specially new models of combustion (HCCI) provided, the mathematical simula-tion of this process has become possible.

In this paper, by coupling the DETCHEM[11] and CHEMKIN[12] chemical kinetics package and the KIVA- 3V package[13], the combustion process of the HCCI en-gine whose piston surface has been coated with catalyst (rhodium and platinum) was numerically investigated, and a multidimensional CFD model with detailed chemical kinetics was built. The effects of catalytic combustion on the ignition timing, the temperature and CO concentration fields and HC, CO and NOx emissions of the HCCI engines were analyzed through this model when the inhomogeneity of temperature and species concentration in the cylinder was considered. Some aca-demic proofs were provided for the application of cata-lytic combustion to the HCCI engine to decrease the emissions of HC、CO and extend the operating range of the engine.

Chemical kinetics model

Surface reaction kinetics model Detailed surface reaction mechanisms of methane

were adopted in the catalytic combustion simulation. The

surface reaction mechanism on rhodium catalyst consists of 17 gas species, 12 surface species and 38 surface reac-tions[14], and consists of 17 gas species, 12 surface spe-cies and 26 surface reactions[15] on platinum catalyst. The two reaction mechanisms have been verified by results of extensive simulations and experiments[14,16]. Good agreements between the results of simulation and ex-periments have been obtained.

Gas reaction kinetics model A detailed gas reaction mechanism of methane is

adopted. It consists of 53 species and 325 reactions. Through validations by substantive researchers, good agreements between the results of simulation and ex-periments have been obtained[17].

Physical model

The engine used for this investigation is derived from a Cummings B-series production diesel engine[18]. The engine is equipped with a custom HCCI piston as shown in figure 1, and the specifications of the Cummings B-series production diesel engine are listed in Table 1.

Fig. 1. Schematic of the piston

Computational model

Because ignition in the HCCI engine occurs at multi-ple sites simultaneously across the combustion chamber, once the ignition occurs, the gas mixture is consumed quickly even without discernable flame propagation, so the detailed chemical kinetics is usually used to simulate

Table 1. Specifications of the Cummings B-series engine Bore Stroke Connecting rod length Engine speed Compression ratio

102/mm 120/mm 192/mm 1200/r.m−1 17.0 Intake temperature Intake pressure Air excess ratio Wall temperature Fuel

470/K 1.0/bar 3.0 400/K CH4

Wen ZENG et al. Modeling of the Application of Catalytic Combustion to Homogeneous Charge C I Engine 373

HCCI combustion. At the same time, the flow field in the cylinder has some effects on the rates of chemical reac-tions and the combustion process. So in this paper, the CHEMKIN chemistry solver and the DETCHEM surface reaction solver are integrated into the KIVA-3V code for solving the chemistry in coupling with multidimensional engine simulations. The KIVA code is used to compute the flow field, including the distributions of mass and temperature in the cylinder, the CHEMKIN solver is used to simulate the gas reactions, and the DETCHEM solver is used to simulate the surface reactions. The detailed computing flow chart is shown in Figure 2.

At first, because the KIVA code is short of the ther-modynamic information of species, the CHEMKIN and DETCHEM interpreters read the mechanisms of gas and surface reactions, and the thermodynamic information of species. At the same time, these two interpreters translate the thermodynamic information of species from NASA format to JANAF format for KIVA code. In the comput-ing course of flow field and combustion, the KIVA code provides CHEMKIN and DETCHEM with the informa-tion of species, temperature and pressure of each compu- tational cell, the CHEMKIN and DETCHEM solvers compute the species and heat release after t time, and Δ

back to KIVA to compute the flow field. The computation starts from 25° CA-deg after bottom

dead center(ABDC) to 60° CA-deg before exhaust valve open(EVO), and the distributions of temperature, pres-sure and species are assumed to be homogenous after intake valve closure (IVC). The computing mesh at 25° CA-deg after IVC is shown in Figure 3. Because the catalyst coatings are on the piston surface, in order to simulate the effects of catalytic combustion on HCCI combustion process, surface reactions occur only in the meshes adjoining the piston surface.

Computational results

The effects of catalytic combustion on the ignition timing of the HCCI engine

As shown in Figure 4, the ignition timing is advanced with catalyst coatings on the piston surface. The ignition timing is 1° CA-deg before top dead center (BTDC) without catalyst coatings, while is 4°/5° CA-deg BTDC with catalyst Pt/Rh coatings on the piston surface. With the ignition timing advancing, the peak pressure in the cylinder is elevated. The peak pressure is 71 MPa with-

Fig. 2 The combine of KIVA code, CHEMKIN and DETCHEM solvers

374 Journal of Thermal Science, Vol.15, No.4, 2006

Fig. 3 The computing mesh of 25° ABDC

Fig. 4 Effects of catalytic combustion on pressure in cylinder

out catalyst coatings, and is 73 MPa with catalyst Rh coatings on the piston surface. The peak pressure is ele-vated by 2 MPa.

For the HCCI engine, one of the most important prob-lems is the control of the ignition timing. Some control

mechanisms such as alterations of the initial pressure and temperature, variable compression ratio and exhaust gas re-circulation (EGR) have potentials for controlling combustion timing[6]. However, with the ignition timing advanced or delayed slightly, the peak temperature and pressure are elevated or fallen greatly by these means. If the peak temperature in the cylinder elevated greatly, engine knock may be produced to mangle the engine and the emissions of NOx would be enhanced. On the other hand, if the peak temperature in the cylinder fallen greatly, incomplete combustion will happen, which would enhance the HC emissions. As shown in Figure 4, with the ignition timing advancing greatly, the peak pressure is elevated slightly when the piston surface has catalyst coatings. So in the aspect of controlling the igni-tion timing, catalytic combustion has special advantages over other means.

The effects of catalytic combustion on the tem-perature field of the HCCI engine

As shown in Figures 5a1-2, at 5° CA-deg BTDC, there are a high and a low temperature region in the core re-gion and in the crevices respectively, and the mixed gas in the whole cylinder can not ignited when the piston surface has no catalyst and coated with Pt catalyst. However, when the piston surface coated with Rh catalyst, as shown in Figure 5a3, the temperature of the region that near the piston surface is elevated to 1800K, and the mixed gas is ignited in other regions. At 0° CA-deg BTDC, as shown in Figure 5b1, when the piston surface has no catalyst, there is a big temperature gradient in the cylinder, which means that high and low temperature re-gions are observed in core region and in crevice region and boundary layer respectively. The temperature field in the cylinder is not very homogeneous. As shown in Fig-ure 5b2-3,when the piston surface coated with catalyst Pt

Fig. 5 Effects of catalytic combustion on the temperature field (K) in the cylinder

(a1-3: with no catalyst、Pt catalyst、Rh catalyst on the piston surface at 5ºCA-deg BTDC b1-3: with no catalyst、Pt catalyst、Rh catalyst on the piston surface at 0ºCA-deg BTDC)

Wen ZENG et al. Modeling of the Application of Catalytic Combustion to Homogeneous Charge C I Engine 375

led discussed in next section).

and Rh, the temperature field in the cylinder is very homogeneous and the temperatures of boundary layer and crevice region are elevated. Compare with conventional SI, one of the advantages of the HCCI engine is that the mixed gas ignited at multiple locations simultaneously in the cylinder and the temperature field is homogeneous. However, as shown in Figure 5b1, because of the effects of flow field, there is also a big temperature gradient in the cylinder. When the piston surface coated with catalyst, as shown in Figure 5b2-3, the homogeneity of temperature field is facilitated by catalytic combustion, so the feasibility of engine knock is deminished and the emissions of HC and CO are decreased (which will be detai

The effects of catalytic combustion on the CO con-centration field of the HCCI engine

As shown in Figure 6a1, at 0° CA-deg BTDC, when the piston surface has no catalyst, the core temperatures are high for combustion to go to completion, and most of the mixed gas in the core region react to form CO2. Because of low temperature, the mixed gas in the crev- ices cannot react and only HC is produced. So the CO concentrations of these two regions are low. The tem- perature of the boundary layer is not very high, CO that was produced by the oxidation of mixed gas cannot be oxidized completely to CO2, and so the CO concentra- tion in the boundary layer is high. As shown in Figure 6a2-3, when the piston surface coated with catalyst, the

CO concentration of the whole cylinder is low and the CO concentration field is homogeneous because of the homogeneity of temperature field and the high tem-perature. At 120° CA-deg after top dead center(ATDC), as shown in Figure 6b1-3, although there are little dif-ference of the CO concentration fields among the above three cases, there are also some difference of the peak CO concentration. The maximum peak CO concentra-tion is the case with no catalyst on the piston surface, and the case with catalyst Pt on the piston surface is the minimum. When the piston surface coated with Rh catalyst, the peak CO concentration is between the above two cases.

The effects of catalytic combustion on the emissions of the HCCI engine

As shown in Figure 7, at the same working conditions, comparing with no catalyst coatings, the emissions of HC and CO are decreased by 4% and 9% respectively with catalyst Pt on the piston surface. With catalyst Rh on the piston surface, the emissions of HC and CO are decreased by 8% and 5% respectively. The NOx emissions of the HCCI engine are decreased more substantially than that of the ordinary CI and SI engines, so when the piston surface has catalyst coatings the increasing of NOx emis- sions does not affect the advantages of HCCI engine, and if with catalyst coatings on the piston surface, the emis-sions of HC, and CO are decreased, so the main disad-vantage that the HCCI engine has higher HC and CO

Fig. 6 Effects of catalytic combustion on the CO concentration field (mass fraction) in the cylinder

(a1-3: with no catalyst、Pt catalyst、Rh catalyst on the piston surface at 0ºCA-deg BTDC b1-3: with no catalyst、Pt catalyst、Rh catalyst on the piston surface at 120ºCA-deg ATDC)

376 Journal of Thermal Science, Vol.15, No.4, 2006

Fig. 7 Emissions of HC, CO and NOx (comparisons between

without catalyst coatings and with catalyst coatings) emissions can be solved to some extent.

Conclusions

By coupling the KIVA-3V package, CHEMKIN and DETCHEM chemical kinetics package, in this paper, a multidimensional CFD model with detailed chemical kinetics was built. The effects of catalytic combustion on the ignition timing and HC, CO and NOx emissions of the HCCI engine were analyzed taking into account the in-homogeneity of temperature and species concentration in the cylinder. The study obtained the following results:

(1) In case the piston surface has catalyst coatings, the ignition timing was advanced, the peak temperature and peak pressure in the cylinder were elevated slightly. Us-ing catalytic combustion to control the ignition timing of the HCCI engine has some advantages over other means.

(2) With catalyst coatings on the piston surface, the HC and CO emissions were decreased but the NOx emis-sions were increased.

(3) Comparing with catalyst Pt, with catalyst Rh on the piston surface the HC emissions were lower, but the emissions of NOx and CO were higher. In the practical applications, we can use Pt or Rh catalyst based on our aims.

Acknowledgments This research is supported by the National Key Basic

Research Project of China (No.2001CB209201).

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