computational study of homogeneous and …...computational study of homogeneous and stratified...

Computational Study of Homogeneous and Stratified Combustion in a Compressed Natural Gas Direct Injection Engine

S. ABDULLAH, W.H. KURNIAWAN, M.A. AL-RAWI, Y. ALI AND T.I. MOHAMAD

Department of Mechanical and Materials Engineering, National University of Malaysia 43600 UKM Bangi, Selangor, Malaysia

[email protected]

Abstract: - In recent years, the type of combustion occurred within engine cylinder plays an important role determining the performance and emissions. In the present study, the computational investigation was performed in order to compare characteristics of homogeneous and stratified combustion in a compressed natural gas direct injection engine. The numerical simulation was performed for single cylinder of the 1.6-liter engine running at wide open throttle at 2000 rpm and 4000 rpm, accounting for medium and high loads for light-duty vehicle. The results of Computational Fluid Dynamic (CFD) shows that calculated data of homogeneous combustion are in good agreement with the experiment data obtained from previous work single-cylinder engine. Parametric studies are then conducted to address effect of some parameters on engine performance. Important parameters analysed include cylinder pressure, engine power, mass fraction burned, CO, CO2 and NO emissions. It is found that homogeneous combustion has an advantage in producing more performance compared to the stratified condition with drawback on higher emissions level, while stratified combustion has ability in reducing the exhaust emissions with less performance. Key-Words: - Homogeneous combustion, Stratified combustion, Compressed natural gas, Direct injection, Exhaust emissions 1 Introduction Over recent past years, natural gas has been seen as an ideal replacement instead of crude oil based fuels in internal combustion (IC) engine for automotive use. The advantages of natural gas as has high thermal efficiency and low exhaust emissions including CO2 due to the higher octane level and lower ration of carbon and hydrogen ratio, respectively. For light-duty vehicles, direct injection (DI) of compressed natural gas (CNG) provides potentials for high thermal efficiency as compared to those of high compression ratio and unthrottled diesel engines, while maintaining the smoke-free operation of spark ignition (SI) engines as well as producing slightly higher NOx emissions with the proper operating condition [1].

Computational fluid dynamics (CFD) has become an essential tool in analysing the complex in-cylinder phenomena due to its capability to produce the detailed descriptions and characterizations of the turbulent velocity fields and chemical reacting flows. Thus, the amount of testing and measurement can be reduced significantly. Such tools had been used by automotive engineers in order to investigate behaviours of or homogeneous and stratified combustion in an IC engine. Previous works on combustion of natural gas engine in the homogeneous charge regime was Zhang and Frankel [2], El Hassaneen, et al. [3], Ben et al. [4], Yossefi et al [5], Agarwal and Assanis [6], Zheng et al. [7]. On the other hand, the stratified combustion in a natural gas

engine for either experimental or numerical work has been started and studied by Huang et al. [8], Huang et al. [9], and Choi et al. [10].

In this paper, the comparison of homogeneous and stratified combustion occurred in compressed natural gas direct injection (CNG-DI) engine is highlighted. Some important engine parameters will be analysed and optimised for better engine performance and exhaust emission. For engine performance, parameters to be evaluated are in-cylinder average pressure, in-cylinder average temperature, indicated power resulted from combustion process that obtained from the pressure-volume (P-V) diagram, heat release rate inside engine cylinder and mass fraction burned during combustion. In term of exhaust emissions produced from the combustion process, typical exhaust gases of carbon monoxide (CO) nitrogen oxide (N) and combustion product of carbon dioxide (CO2) are assessed for both homogeneous and stratified combustion. 2 Engine Specification and Model A single cylinder engine was based on the 1.6 liter 4-stroke 4-cylinder Proton CamPro engine which was modified to run on CNG as monofuel. In addition, the engine was equipped with the DI system, and an acronym CNGDI was used as a specific reference to this engine. The engine was operated at wide open throttle condition with a compression ratio of 14:1. The main

Proceedings of the 4th IASME / WSEAS International Conference on ENERGY & ENVIRONMENT (EE'09)

ISSN: 1790-5095 377 ISBN: 978-960-474-055-0

engine specifications of the engine were as given in Table 1 and the geometrical detail is depicted in Fig. 1, which shows the position of the intake and exhaust ports, intake and exhaust valves positions, CNG injector, spark plug positions and combustion chamber with the shape of stratified piston.

Table 1. Specification of CNGDI engine.

Engine parameters Value Unit Displacement volume 1596 cm3 Bore 78 mm Stroke 84 mm Connecting rod length 131 mm Crank radius 44 mm Compression ratio 14 -

For the focus of this study, two different piston

shapes were selected for the purpose of investigation in order to determine combustion characteristics for homogeneous and stratified charges. Piston A has a bowl at the centre of its crown and represents the occurrence of homogeneous combustion due to symmetry crown between intake and exhaust side. In contrast, piston B has the deeper bowl volume but eccentricity was introduced for the bowl location which depends on the location of the fuel injector. The crowns of piston A and piston B can be illustrated in Fig. 2.

In order to perform appropriate CFD simulations for an IC process, analysis was carried out by using the unsteady (transient) mode, moving meshes and boundaries, high compressible Reynolds number, high fluid dynamics characteristics (turbulence intensity), momentum, heat and mass transfer and complex geometries model and chemical-thermal dependent as well [11]. A customised user-defined grid generation subroutine has been developed which produced hexahedral cells for the computational domain of the engine model, involving intake ports and valves, cylinder head and piston crown as depicted in Fig. 3. In this simulation, a 4-CPU high performance computing (HPC) facility had been utilised in order to speed-up the simulation figure, and different CPUs are represented by different in colours in the figure.

The number of cells is about 43,398 at top dead centre (TDC) position and around 163,110 cells at bottom dead centre (BDC) position, where about the half of the cells used to generate the mesh at the cylinder head and piston bowl in the case of considering the grid sensitivity and reasonable computer run time. In this work, the required CPU time to simulate the combustion process at the engine speed of 2000 rpm for both homogeneous and stratified combustion is around 60-70 hours with 4-CPU on SGI Origin 300 server.

Fig. 1 The geometrical details of CNGDI engine

with stratified combustion.

(a) (b)

Fig. 2 Piston crown for (a) homogeneous combustion and (b) stratified combustion

Fig. 3 Surface model (a) and computational model (b)

3 Methodology The numerical simulation was performed using a CFD code equipped with the moving mesh and boundary algorithms to replicate real engine operating conditions, such as the valves and piston movement. In this section, the chemistry model and reaction mechanism used for the CFD simulation of the combustion process are described below.

The governing equations of the flow of an ideal perfect gas was utilised in order to simulate the combustion process. These equations consist of the mass, momentum and energy equations as well as the gas state equations. Conservations of mass, momentum and energy used that can be referred as the following governing equations, described in vector notation: Mass equation:

( ) 0=⋅∇+∂∂ uρρ

t (1)

Momentum equation:

( )

( )[ ] ( ){ } guuu

uuu

ρλμ

ρρ

+⋅∇+∇+∇⋅∇+∇−

=∇⋅+∂∂

Tpt (2)


ISSN: 1790-5095 378 ISBN: 978-960-474-055-0

Energy equation:

( ) ( )

( ) ( ) ( )[ ]T2 uuuu

uu

∇+∇⋅∇+⋅∇+∇∇+

⋅∇−=∇⋅+∂∂

λ

ρρ

Tk

pTctTc vv (3)

Combustion modelling strategy adopted in this simulation is the three-step global reaction of EBU model, in which the reactions can be written as follows:

OH2O50HCOO50CO

H2COO50CH

222

22

224

→+→+

+→+

...

(4)

The equilibrium composition for the cylinder charge depends on pressure, temperature and equivalence ratio. Chemical species investigated during the analysis were O2, CO2, H2O, N2, H2, CO and NO, whereby natural gas is assumed to be of 100% CH4. Detail mathematical formulation can be referred to Abdullah et al. [12]. 4 Results and Discussions 4.1 In-cylinder Pressure and Temperature The contours of in-cylinder pressure for the homogeneous and stratified combustion process at 2000 rpm and 4000 rpm are shown in Fig. 4 for several degrees of crank angle (701 at ignition event, 720, 730 and 740). The in-cylinder pressure rises almost instantly when the ignition began and reached the maximum around 10º-20º after top dead centre (TDC) position before decreasing gradually until exhaust valve opening.

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Fig. 4 In-cylinder pressure for homogeneous and stratified combustion

As can be seen in the same figure, the pressure

contour on the surface of cylinder head for homogeneous and stratified combustion is totally different with the blue colour representing the maximum value, i.e. the maximum value for the homogeneous combustion process 6.74% higher than stratified combustion at 2000 rpm and 12.12% at 4000 rpm, respectively.

In terms of the cylinder pressure for homogeneous and stratified combustion, the CFD results showed that the homogeneous combustion is able to generate higher cylinder pressure than that of the stratified combustion. Consequently, the homogeneous combustion also produced higher temperature (i.e. about 11%) than that of the stratified combustion for both engine speeds as shown in Fig. 5. In addition, the in-cylinder pressure and the combustion temperature for the both combustion process also are increased with engine speed.

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Fig. 5 Cylinder temperature for homogeneous and stratified combustion.

4.2 Heat Release Rate and Indicated Power Through this analysis, the heat release rate produced in engine cylinder can be computed. Theoretically, heat release rate represents the rate of the chemical energy from fuel released through the combustion processes, and was is extracted from the graph of the cylinder pressure versus the degree of crank angle. From the numerical simulation, the heat release could be obtained directly from heat release of species involved in the reactions.


ISSN: 1790-5095 379 ISBN: 978-960-474-055-0

As shown in Fig. 6, the heat release rate for the homogeneous combustion has a higher value than that for the stratified combustion for the both of engine speeds. This means that homogeneous piston is able to unleash a very high chemical energy during occurrence of combustion process. At 2000 rpm, during the combustion process, the heat release rate from the homogeneous combustion is higher than stratified combustion i.e. around 11.48%.

However, at 4000 rpm, the heat release rate for homogeneous combustion is lower than stratified combustion i.e. 69% due to a more complete combustion of CNG combustion process.

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-50

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t Rel

ease

Rat

e (J

/deg

)



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Fig. 6 Heat release rate for homogeneous and stratified combustion.

In addition to in-cylinder pressure, temperature and heat release rate, one important factor to determine engine performance is the indicated power. Theoretically, indicated power is obtained from integration of in-cylinder pressure over the cylinder volume in a P-V diagram and known as indicated work.

Fig. 7 shows the P-V diagram for the homogeneous and stratified combustions for the engine speed of 2000 rpm and 4000 rpm. Through integration of the closed loop, the indicated work for homogeneous combustion was found to be 5.42 kW, and for stratified combustion, it was 3.9 kW. As a consequence to a lower cylinder pressure, the stratified combustion produced smaller power as indicated by the numerical CFD result.

4.3 CO, NO and CO2 Emissions From the CFD simulation, concentration emissions, namely CO, NO and CO2, within engine cylinder produced as a result of the combustion process can be predicted. CO is an intermediate combustion product and it is present in low concentration in the region where combustion does not take place at all.

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Fig. 7 P-V diagram for homogeneous and stratified combustion.

Fig. 8 shows CO level concentration of homogeneous

and stratified combustion for certain crank angles during and after combustion at engine speed of 2000 rpm. It can be seen that most of the CO concentration is located near the cylinder liner wall and inside the piston bowl, which is a result of incomplete combustion, which can lead to oxidation process from CO to CO2 in the core gas and the exhaust pipes.

Fig. 8 Contour of CO emission for homogeneous and stratified

combustion at 2000 rpm

Fig. 9 illustrates the contours of NO concentration inside engine cylinder for the homogeneous and


ISSN: 1790-5095 380 ISBN: 978-960-474-055-0

stratified combustions at 2000 rpm. NO is a species generated during combustion because of the reaction of O and N atoms and exists in the region of higher temperature. Most of NO formation resides in the region around spark plug and is a drawback from higher temperature produced during ignition. In this figures, it is visible that existence of bowl geometry at the piston crown can contain the heat since NO is only formed around the bowl. This in return reduces the overall NO formation.

Fig. 9 Contour of NO emission for homogeneous

and stratified combustion at 2000 rpm.

In addition to temperature, the amount of NO generated also depends on the CNG pressure, air-fuel ratio and combustion time within the cylinder. The highest concentration is formed around the spark plug area, where the highest temperature rise occurred. The concentration level of CO, NO and CO2 versus degree of crank angle is plotted in Figs. 10, 11 and 12 respectively for the engine speeds of 2000 rpm and 4000 rpm. As shown, the homogeneous piston has an advantage of reducing CO emission lowers than the stratified piston. Eventually, in the stratified combustion, the emissions level for NO and CO2 was dropped to about 92.2% and 26.6%, respectively, as compared with the homogeneous combustion. However, for the CO emission level, the numerical simulation shows a higher value as compared to the homogeneous combustion, which is about 10% for the both of engine speeds. It may be caused by the combustion process under the homogeneous charge

regime which is more complete than the stratified combustion since the homogeneous combustion can lead to a uniform distribution of flame propagation as the air-fuel mixture burns.

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Fig. 10 The emission level for CO during homogeneous and stratified combustion.

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For NO emission, the higher pressure and

temperature within engine cylinder resulted in a higher NO level for the homogeneous combustion. On the other hand, the stratified combustion provides the decrease in


ISSN: 1790-5095 381 ISBN: 978-960-474-055-0

References: the NO emission concentration which is as low as 200 ppm. This situation occurred because combustion temperature for the stratified combustion was below than 1400 K and thus was not able to generate significant concentration of NO.

[1] Zeng, K. Huang, Z., Liu, B., Liu, L., Jiang, D., Ren, Y. and Wang, J. Combustion Characteristics of a Direct-Injection Natural Gas Engine Under Various Fuel Injection Timings. Applied Thermal Engineering, 26: 806-813 (2006). [2] Zhang, D., & Frankel, S.H. A Numerical Study of Natural Gas Combustion in a Lean Burn Engine. Fuel, 77(12): 1339-1347 (1998)

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ion

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el (%

)



[3] El Hassaneen, A., Varde, K.S., Bawady, A., and Morgan, A.A. A Study of the Flame Development and Rapid Burn Durations in Lean-Burn, Fuel-Injected Natural Gas S.I. Engine. SAE Paper no. l 981384 (1998). [4] Ben, L., Charnay, G., Ducros, N.R. and Truquet, R. Influnence of Air/Fuel Ration on Cyclic Variation and Exhaust Emission in Natural Gas SI Engine. SAE Paper no. 1999-01-2901 (1999). [5] Yossefi, D., Belmont, M.R., Ashcroft, S.J., and Maskell, S.J. A Comparison Of The Relative Effects of Fuel Composition and Ignition Energy on the Early Stages of Combustion in a Natural Gas Spark Ignition Engine using Simulation. Proc. IMechE Part D, 214(D) 383-393: (2000).

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[6] Agarwal, A., and Assanis, D. Multi-Dimensional Modeling of Ignition, Combustion and Nitric Oxide Formation in Direct Injection Natural Gas Engines. SAE Paper no. 2000-01-1839 (2000). [7] Zheng, Q.P., Zhang, H.M. and Zhang, D.F. A Computational Study of Combustion in Compression Ignition Natural Gas Engine with Separated Chamber. Fuel, 84: 1515-1523 ( 2005).

(b) Fig. 12 The emission level for CO2 during

homogeneous and stratified combustion process at (a) 2000 rpm and (b) 4000 rpm.

[8] Huang, Z., Shiga, S., Ueda, T., Nakamura, H., Ishima, T., Obokata, T., Tsue, M. and Kono, M. NO/NO2 Concentration of Direct Injection Stratified Combustion Under Constant Volume Condition Fuelled by Compressed Natural Gas and Gasoline. Proc. IMechE Part D, 217(10): 935-941 (2003).

As for CO2, a complete combustion of the

homogeneous charge led to a higher CO2 level. Therefore, some strategies can be employed in order to intelligently control this greenhouse gas should it be reduced. This aspect will be addressed in future works.

[9] Huang, Z., Shiga, S., Ueda, T., Jingu, N., Nakamura, H., Ishima, T., Obokata, T., Tsue, M. and Kono, M. Feasibility of CNG DI Stratified Combustion using a Spark-Ignited Rapid Compression Machine. Fifth International Symposium on Diagnostics and Modeling of Combustion in Internal Combustion Engines, Nagoya, 2001, pp. 365-373Choi,

4 Conclusion [10] S.H., Jeon, C.H. and Chang, Y.J. Combustion

Characteristics of Stratified Mixture in a CNG Direct Injection Combustion Bomb using 2-Stage Injection. Fifth International Symposium on Diagnostics and Modeling of Combustion in Internal Combustion Engines, Yokohama, 2004, pp. 401-407

Comparison between the homogeneous and stratified combustions in a CNGDI engine was carried out in this work. This was made by analysing several important engine parameters and concentration selected exhaust emissions. Thus, the following items can be concluded as the conclusions of this study:

[11] W.H. Kurniawan & S. Abdullah, 2008, Numerical analysis of the combustion process in a four-stroke compressed natural gas engine with direct injection system, Journal of Mechanical Science and Technology, 22: 1937-1944.

1. Homogeneous charge regime produces more power than stratified combustion as the results of higher cylinder pressure and heat release rate. [12] S. Abdullah, W.H. Kurniawan & A. Shamsudeen. 2008.

Numerical Analysis of the Combustion Process in a Compressed Natural Gas Direct Injection Engine. Journal on Applied Fluid Mechanics, 1(2): 65-86.

2. Homogeneous combustion generates lower CO level with higher CO2 generation, while stratified combustion yields much lower NO emissions.

3. Homogeneous charge can be selected for enhanced engine performance with compromise on higher NO level. On the other hand, stratified combustion can be selected for reduced emissions with drawback on lower performance.


ISSN: 1790-5095 382 ISBN: 978-960-474-055-0

computational study of homogeneous and …...computational study of homogeneous and stratified...

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