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Effects of Combustion Phasing, Combustion Duration, and Their Cyclic Variations on Spark-Ignition (SI) Engine Efficiency Fanhua Ma,* Yu Wang, Junjun Wang, Shangfen Ding, Yefu Wang, and Shuli Zhao State Key Laboratory of Automobile Safety and Energy, Tsinghua UniVersity, Beijing 100084, People’s Republic of China ReceiVed May 1, 2008. ReVised Manuscript ReceiVed June 18, 2008
A zero-dimensional two-zone model was employed to investigate the influence of the combustion process of the spark-ignition (SI) engine on its thermal efficiency. Attention was mainly paid to the effects of combustion phasing, combustion duration, and their cyclic variations. These combustion parameters were varied by changing spark timing and the hydrogen/natural gas blending ratio (hydrogen and natural gas mixtures were used as the fuel). The results show that there always exists an optimized combustion phasing for best engine thermal efficiency. Combustion phasing that deviates from this optimized value would decrease thermal efficiency. The fact that heat transfer loss increases with the advance of combustion phasing was thought to be the reason why best efficiency thermal efficiency does not occur under the condition when the combustion rate peaks at TDC. It is also found unexpectedly that although reduction in combustion duration can increase the degree of constant volume combustion, it does not have obviously positive effects on the final thermal efficiency. Finally, with regard to the cyclic variations, it is concluded that cyclic variations in the combustion process exert a heavy effect on thermal efficiency. The higher the variations, the larger the negative effect.
1. Introduction The quest for improved fuel efficiency has always been in the mind of engine researchers and designers since the first internal combustion (IC) engine was built centuries ago. In the context of recent concerns about energy shortages and increasing global warming effects, increasing fuel efficiency will continue to be a major topic in the auto industry.1,2 After a quick review of recently published efficiency related literature, one would find that most efficiency-improving technologies achieved their functionality through improving the combustion process.2 This is not hard to understand considering combustion is the core operating process in IC engines. Many of the newly developed techniques that help to keep down fuel consumption achieve their functionality through optimizing the combustion process, for example, increasing flow turbulence, employment of nonconventional ignition devices3 and combustion modes,4,5 and use of fuel additives 6,7 all manifest their advantages in the improvement of combustion. * To whom correspondence should be addressed: State Key Laboratory of Automobile Safety and Energy, Tsinghua University, Beijing 100084, People’s Republic of China. Telephone: +86-10-62782352 ext. 11. Fax: +86-10-62796002. E-mail:
[email protected]. (1) Ayala, F. A.; Gerty, M. D.; Heywood, J. B. Effects of combustion phasing, relative air-fuel ratio, compression ratio, and load on SI engine efficiency. SAE Paper 2006-01-0229, 2006. (2) Ayala, F. A. Combustion lean limits fundamentals and their application to a SI hydrogen-enriched engine concept. Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, MA, 2006. (3) Czekala, M.; Johnston, B.; Morganti, C.; et al. Matching ignition system multi-spark calibration to the burn-rate of an engine to extend ignitability limits. SAE Paper 981046, 1998. (4) Kimura, S.; Aoki, O.; Ogawa, H.; et al. New combustion concepts for ultra-clean and high-efficiency small DI diesel engines. SAE Paper 199901-3681, 1999. (5) Soylu, S. Examination of combustion characteristics and phasing strategies of a natural gas HCCI engine. Energy ConVers. Manage. 2005, 46, 101–119.
The heat release rate, as the most influential factor in the combustion process, plays an extremely important role in engine efficiency. The Otto cycle, which assumes that all of the chemical energy of the fuel is released at constant volume combustion without heat transfer loss, is an ideal model used to analyze qualitatively the working cycle of spark-ignition (SI) engines.8 However, this model is ineffective in reflecting the actual working cycle of the engine. The main problem is simply that a real engine cannot conduct combustion at constant volume combustion because the burning process takes some time during which the piston will keep moving and the cylinder volume will keep changing. This deviation from constant volume combustion makes the efficiency of the engine much less than that of an ideal Otto cycle.9 Heat transfer from the cylinder charge to the outside environment during combustion also influences the actual power output and efficiency of the engine. In an effort to maximize SI engine thermal efficiency, one should both tune the heat release rate as much as possible to make it resemble the Otto cycle and keep down heat transfer loss.10 Considering the importance of the combustion process on efficiency, we decided to conduct this research to add to the body of knowledge on whether, how, and why the combustion rate (heat release rate) could affect engine thermal efficiency on a relatively basic level. Because heat transfer loss in a real (6) Huang, Z. H.; Wang, J. H.; Liu, B.; et al. Combustion characteristics of a direct-injection engine fueled with natural gas-hydrogen blends under various injection timings. Energy Fuels 2006, 20 (4), 540–546. (7) Ma, F. H.; Wang, Y.; Liu, H. Q.; et al. Experimental study on thermal efficiency and emission characteristics of a lean burn hydrogen enriched natural gas engine. Int. J. Hydrogen Energy 2007, 32, 5067–5075. (8) Heywood, J. B. Internal Combustion Engine Fundamentals; McGrawHill: New York, 1988. (9) Abd Alla, G. H. Computer simulation of a four stroke spark ignition engine. Energy ConVers. Manage. 2002, 43, 1043–1061. (10) Shudo, T. Improving thermal efficiency by reducing cooling losses in hydrogen combustion engines. Int. J. Hydrogen Energy 2007, doi: 10.1016/j.ijhydene.2007.06.002.
10.1021/ef8003027 CCC: $40.75 2008 American Chemical Society Published on Web 07/22/2008
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engine is also influenced by the combustion rate and it in turn may affect engine efficiency, heat transfer is as well taken into consideration. Furthermore, considering cyclic variations in combustion would also become a significant problem influencing efficiency; thus, we also discuss the effects of cyclic variations on the combustion rate. The analytical tool used, as will be detailed in the Research Method, is a zero-dimensional two-zone thermodynamic model. The reason for our choosing this kind of model is that it satisfies the timing requirement and the level of sophistication required to conduct this study, the core work of which is to derive the heat release rate profile from the experimental cylinder pressure data and to predict cylinder pressure by use of the already known (arbitrarily specified) heat release data. Earlier research showed that the zero-dimensional model could do the above work quite well.11–13 Of course, a multidimensional model can give us some deeper information, such as the details of the cylinder flow or the cylinder temperature.14 However, considering that this kind of detailed information is not necessary for this study and that a multidimensional model would cost a substantially longer time to compute, we finally chose the simpler zero-dimensional model. Of note is that some conclusions drawn in this work may not be new. However, the authors think the most important contribution of this work is the efficiency analysis method presented, which is how to separate the coupled effects of combustion phasing, combustion duration, and heat transfer on efficiency and analyze them one by one. 2. Research Method 2.1. General Description. The main goal of this work is to examine the effects of different combustion rates on SI engine efficiency. Different combustion rates here mean different combustion-cylinder volume phasing and different combustion duration. The combustion rate data used are experimentally derived. Combustion-cylinder volume phasing is varied through adjusting the spark timing, while combustion duration is varied through adding different amounts of hydrogen into natural gas, which can vary the flame propagation speed. A zero-dimensional two-zone diagnostic model and a “predictive” model were employed in this research. The diagnostic model was used to derive the combustion rate profile from the experimental cylinder pressure. By employing the empirically based Woschni heat transfer formulation,15,16 the crank-angle resolved heat transfer data could also be determined. Then, the experimentally derived combustion rate data (mass fraction burned data) were used as an input into the “predictive” model, which reproduced cylinder pressure and calculated the corresponding work output and indicated thermal efficiency. The reason for introducing the “predictive” model can be explained as follows: as the main topic of effects of combustion phasing, combustion duration, and their cyclic variations (11) Soylu, S. Simple modeling of combustion for natural gas engines. SAE Paper 2002-01-2733, 2002. (12) Catania, A. E.; Misul, D.; Mittica, A.; et al. A refined two-zone heat release model for combustion analysis in SI engines. The Fifth International Symposium on Diagnostics and Modeling of Combustion in Internal Combustion Engines (COMODIA), Nagoya, Japan, July 1-4, 2001. (13) Hajireza, S.; Mauss, F.; Sunden, B. Two-zone model of gas thermodynamic state in SI engines with relevance for knock. COMODIA 98, 1998; pp 203-208. (14) Dober, G. G.; Watson, H. C. Quasi-dimensional and CFD modelling of turbulent and chemical flame enhancement in an ultra lean burn SI engine. SAE Paper 2000-01-1263, 2000. (15) Woschni, G. A universally applicable equation for the instantaneous heat transfer coefficient in the internal combustion engine. SAE Paper 670931, 1967. (16) Guezennec, Y. G.; Hamama, W. Two-zone heat release analysis of combustion data and calibration of heat transfer correlation in an IC engine. SAE Paper 1999-01-0218, 1999.
on engine efficiency, we need to change these parameters and examine their influence on efficiency. The change in efficiency can be easily examined by studying the change in cylinder pressure. As mentioned in the paper, sometimes to separate the effects of the influencing factors, we need to know the cylinder pressure and thermal efficiency data, which cannot be obtained by experimental measurement (for example, cylinder pressures at different combustion durations with fixed heat transfer loss). To meet this requirement, a predictive model is needed, which can predict cylinder pressure by use of the specified heat release and heat transfer data. We mean “predictive” here only because it can derive cylinder pressure. Actually, the basic control equations in this model are equivalent to the diagnostic model, the only difference being that the diagnostic model was used to derive heat release and heat transfer profile from known cylinder pressure data and the “predictive” was used to derive cylinder pressure from known (specified) heat release and heat transfer data. Further, there are additional reasons why the predicted pressure data were chosen over the experimental ones for calculation and comparison of efficiency and these can be summarized as follows: (1) Experimental efficiency data are very sensitive to the measured fuel flow rate, the error of which may cause a large deviation in the experimentally calculated efficiency and thus make the comparison meaningless. While in predictive models, the determination of the fuel flow rate is no longer a problem because it can be arbitrarily set and serve as an input into the model. (2) Numerical prediction makes it possible to separate the effects of the combustion rate and heat transfer loss on engine efficiency. In the model, we can keep heat transfer unchanged while changing heat release rate data to examine the pure effect of the heat release rate on efficiency or vice versa, because both of them can be treated as already-known variables and input into the predictive model. By doing this, the magnitude of the contribution of each effect to the overall efficiency could be determined, thus providing a more deep insight of how efficiency is influenced, which is not achievable through a pure experimental method. (3) The effects of cyclic variations on efficiency can be examined by numerical models. In a lean-burn SI engine, cyclic variations in combustion may become a significant efficiency limiting factor. This is mainly caused by the difference cycle by cycle in combustion rate and pattern, which makes most cycles develop with a non-optimum combustion process. Previously, studies have indicated that generally a 6% improvement in fuel economy could be achieved if all cycles burned at the optimum rate.17 In the prediction, we can either use the experimental-derived cycle-based combustion rate data to simulate a statistically meaningful number of cycles (less or equal to the number of cycles measured at a certain operating condition) or use a fixed optimum heat release data. Thus, the effect of cyclic variations on the heat release rate can be investigated by comparing the two prediction results. 2.2. Experimental Setup and Procedure. The test engine was a six-cylinder natural gas SI engine with a compression ratio of 10.5 (see Table 1 for specifications). In-cylinder pressure was measured by a kistler 6117B piezoelectric high-pressure transducer with the resolution of 1 crank angle (CA) degree. Other measuring instruments, such as flow meter, lambda sensor, etc., can be found in ref 7. The aim of varying engine operating conditions is to obtain different heat release data, for comparison purposes. During the tests, the engine speed and intake manifold absolute pressure (MAP) were kept unchanged at 1600 rpm and 70 kPa, respectively. Spark advance was changed from MBT+10 to MBT-10, and the hydrogen enhancement level was changed from 0 to 50% (volume fraction) to vary heat release phasing and burn duration, as mentioned previously (MBT is spark timing for maximum brake (17) Lyon, D. Knock and cyclic dispersion in a spark ignition engines. Proceedings of ImechE, International conference on petroleum based fuels and automotive applications, Nov 25-26, 1986; pp 105-116. (18) Erickson, W. D.; Prabhu, R. K. Rapid computation of chemical equilibrium composition: An application to hydrocarbon combustion. AIChe J. 1986, 32.
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torque). Two excess air ratios were employed, 1.2 and 1.5, for the sweep of spark timings and hydrogen enhancement levels. The choice of the excess air ratio of 1.5 is because cyclic variations in the burn rate under this condition were much more obvious, thus making the examination of the effects of cyclic variations on efficiency easier. 2.3. Description of the Thermodynamic Model. Both the diagnostic and “predictive” models are zero-dimensional two-zone thermodynamic models. The following major assumptions are made in the development of the models: (1) The charge in the cylinder during combustion is assumed to be divided into two zones, consisting of the burned products and unburned reactants, respectively. (2) Charges in both zones behave as ideal gases with variable uniform local properties. (3) The pressure at any time is uniform throughout the cylinder. (4) Heat transfer takes place only through external surfaces of the combustion chamber. (5) Flame thickness, crevice effects, and leakage from the cylinder are negligible. (6) Chemical composition of the unburned charge is assumed to be frozen, while that of the burned charge is determined through the calculation of chemical equilibrium.19 Through applying the first law of thermodynamics to both the unburned and burned zones and employment of the abovementioned assumptions, also after some mathematical manipulations, the following equations can be derived (detailed procedure of derivation can be found in refs 19 and 20):
Vu dP dTu 1 dQu ) + dθ muCpu dθ muCpu dθ
[
(1)
]
Ru dQu V dP + PCpu dθ P dθ
[
]
(2)
[ dP ) dθ Cvu
[
(
Cpu
[ ] [ ] Cvb
P
Cvb dV dV - 1+ P dθ Rb dθ
Rb CvbRu dQu Cpu RbCpu dθ
] }
Cvu
Vu -
-1 CvbRu CpuRb
Vu +
) [
Cvb Rb
1
(
(ub - uu) - Cvb Tb -
]
V
{[ ] 1+
(
Cvb Rb
P
)
Ru T Rb u
Ru dQu dQb dmb + + + (ub - uu) - Cvb Tb - Tu dθ dθ Rb dθ Cvu CvbRu dQu Cpu RbCpu dθ
[
(3)
dV dθ
)]
Dongfeng Motor Co., Ltd. in-line six cylinders, SI 6.234 10.5 105 120 192 IVO: 18° 21′ BTDC IVC: 37° 39′ ABDC EVO: 56° 21′ BBDC EVC: 11° 39′ ATDC
exhaust valve timing
the fraction of the cylinder volume that is occupied by the corresponding zone.21 In the diagnostic model, P and dP/dθ are known from experimentally recorded cylinder pressure data; thus, by solving eqs 1–3, we can get the mass burned rate (dmb/dθ) and normalized mass burned rate (dmb/dθ divided by total charge mass) profile. In the predictive model, however, we should use eqs 1, 2, and 4 and the known normalized mass burned rate data to reproduce the cylinder pressure profile during the combustion process. While for the compression and expansion processes, where there is no combustion, the two-zone model reduces to a single-zone model, and the following equations can be used for pressure prediction:
(
[
) ]
dP 1 R dQ R dV ) - 1+ P dθ V Cv dθ Cv dθ
(5)
1 dV 1 dP dT )T + dθ V dθ P dθ
(6)
]
Note that the prediction is restricted to the closed valve period (IVC-EVO) and the initial conditions are obtained from experimental data.
3. Results and Discussion
CvbRu Cvb dmb dQu dQb dP Cvu ) + Vu Vu + V dθ dθ dθ dθ Cpu CpuRb Rb 1+
value
engine make engine type displacement volume (L) compression ratio bore (mm) stroke (mm) connecting rod length (mm) intake valve timing
[
dTb P dV (RbTb - RuTu) dmb RuVu dP ) dθ mbRb dθ P dθ PCpu dθ
{
Table 1. Engine Specifications item
] }
(4)
where P, T, V, m, Cv, Cp, R, u, and Q represent cylinder pressure, zone temperature, volume, charge mass, constant volume specific heats, constant pressure specific heats, gas constant, specific internal energy, and heat transfer, respectively. The subscript “u” refers to the unburned zone, and the subscript “b” refers to the burned zone. Also note eqs 3 and 4 can be derived from each other. Heat transfer rates, that is dQu/dθ for the unburned zone and dQb/dθ for the burned zone, are determined by Woschni heat transfer correlation.15 The heat transfer area for each zone is obtained by multiplying the area of the combustion chamber with (19) Karim, G. A.; Gao, J. Prediction of the performance of spark ignition gas engines including knock. SAE Paper 932823, 1993.
3.1. Effects of Combustion Phasing on Efficiency. The importance of heat release-cylinder volume phasing (combustion phasing) is evident from the existence of the so-called MBT spark timing. If the release of chemical energy of the fuel starts too early, the work transfer from the piston to the cylinder charge at the end of the compression stroke will be too large, thus reducing work output and thermal efficiency. On the other hand, late combustion could reduce the peak cylinder pressure as well as the volume ratio and temperature ratio through which the gases expand, also resulting in decreases in work output and thermal efficiency. Therefore, there exists an optimum heat release-cylinder volume phasing that gives maximum work output and efficiency.8,9 Combustion phasing is closely related to spark timing in SI engines; thus, the most convenient way to vary it is through changing spark timing, with all other operating conditions unchanged. To give an example, the experimentally obtained variation of the mass fraction burned rate (heat release rate divided by the lower heating value of the fuel) with spark timing is shown in Figure 1. For clarity, data are plotted for only four different spark timings. By inputting these experimentally derived combustion rate data into the “predictive” model, the variation of thermal efficiency for different combustion phasing could be obtained, which are further shown in Figure 2. Clearly, for each type of fuel, there always exists an optimum spark advance (MBT) that gives best efficiency. As spark timing deviates from MBT, (20) Hamori, F. Exploring the limits of hydrogen assisted jet ignition. Ph.D Thesis, The University of Melbourne, Melbourne, Australia, 2006. (21) Soylu, S.; Gerpen, J. V. Development of empirically based burning rate sub-models for a natural gas engine. Energy ConVers. Manage. 2004, 45, 467–481.
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Energy & Fuels, Vol. 22, No. 5, 2008 3025
Figure 1. Rate of mass fraction burned under different spark timings. Figure 4. Variations of the ratio of heat transfer loss to total fuel energy versus spark timing.
Figure 2. Variations of thermal efficiency versus spark advance. Figure 5. Variations of thermal efficiency versus spark advance at fixed heat transfer data.
Figure 3. Rate of mass fraction burned at MBT spark timing.
engine thermal efficiency would be undermined. In addition, the penalty in efficiency reduction is more obvious when the spark timing is retarded from MBT than when it is advanced from MBT. The inner reason why efficiency is affected by spark advance, as mentioned before, is the phasing of the combustion process. Figure 3 gives the rate of mass fraction burned corresponding to MBT spark timing shown in Figure 2. Interestingly, under MBT spark timing, the maximum burn rate always occurs at about 9° after top dead center (TDC) regardless of the fuel type. Actually, apart from this study, several other researches have also found that thermal efficiency could be maximized if the peak combustion rate occurs at around 9 °CA after TDC.22 However, because the precision level of the pressure transducers and the methods used to determine the combustion rate may differ in each research, it is still hard to
say if this conclusion can be universally applied. The 9 °CA is of course not an exact value but an approximation, and in our tests, we found that the deviation of this approximation value was always within 1 °CA. Also of note is that Figure 3 shows that the better part of the chemical energy of the fuel is actually released after TDC, in the expansion stroke. However, by analyzing an ideal Otto cycle, we could know that better efficiency is to be achieved when a higher fraction of heat released is at TDC. Therefore, here arises the question: why, in a real engine, does the mass fraction burned rate (combustion rate) under MBT timing dose not peak at TDC? Heat transfer loss, which does not exist in ideal Otto cycles, makes this difference. Figure 4 plots the ratio of the heat transfer loss to the total fuel energy as a function of spark timing. As can be seen, heat transfer loss continues to rise as spark timing advances, and it is believed that, if spark timing is further advanced from MBT, then the penalty in efficiency caused by increased heat transfer loss will outweigh the advantage brought by more heat release at TDC. If we exclude the effects of heat transfer, the best efficiency will occur at the spark timing at which more heat is released around TDC and, thus, at which the curve of mass fraction burned centers itself at TDC. This is proven by Figures 5 and 6. Note that the heat transfer data are fixed with spark timing for the calculation of each efficiency point shown. Specifically, the heat transfer data used are those corresponding to MBT spark timing for each type of fuel. (22) Zhu, G. G.; Daniels, C. F.; Winkelman, J. MBT timing detection and its closed-loop control using in-cylinder pressure signal. SAE Paper 2003-01-3266, 2003.
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Figure 6. Rate of mass fraction burned at maximum efficiency spark timing under the condition of fixed heat transfer. Figure 7. Variations of indicated thermal efficiency versus combustion duration.
In Figure 2, it seems that at MBT spark timing the case of 70% CNG plus 30% H2 shows the worst efficiency. This trend means that the thermal efficiency of the engine does not always increase (decrease) monotonously with the increase of the combustion rate (the case with a higher hydrogen fraction will have a higher combustion rate). As will be detailed in the following section, whether or not an increase in combustion rate could improve thermal efficiency is dependent upon the counter-balancing of the following two effects: on one hand, an increase in the combustion rate would increase the degree of constant volume combustion, which is beneficial to thermal efficiency; on the other hand, it could also increase the heat transfer loss, which is detrimental to efficiency. 3.2. Effects of Combustion Duration on Efficiency. The combustion process can be generally characterized by two parameters. One is the previously discussed combustion phasing, and the other is combustion duration, which represents the immediateness of the combustion process. Just as mentioned before, the Otto cycle assumes that the heat release is conducted instantaneously at TDC. However, this is not possible in a real SI engine because the combustion reaction always takes some time. One may take it for granted that reduction in combustion duration would lead to an increase in thermal efficiency because that reduction increases the immediateness of heat release and makes the real cycle resemble an Otto cycle better. However, according to our results, this is not necessarily the case. To obtain the combustion rate data with different combustion duration for the comparison study, various amounts of hydrogen were introduced into natural gas (NG) to speed up combustion and thus decrease combustion duration (hydrogen volumetric fraction varied from 0 to 50%, with a 10% interval). To avoid the effects of combustion phasing, the spark timing used in each set of data is that which makes the peak combustion rate occur at the same crank angle. Then, by inputting these combustion rate data, which have different combustion duration and identical peak heat release rate phasing into the predicative model, the effects of combustion duration on engine efficiency can be examined. Note that the working charge used in the predictive model in this section is NG, regardless of the heat release data used. Figure 7 gives the variations of indicated thermal efficiency versus combustion duration (defined as spark to 90% mass burned duration). Points that are in the same curve mean that they have identical combustion phasing, which is characterized by the crank angle where peak combustion rate occurs. As can be seen, the curves shown are rather flat, which mean that combustion duration has no obvious effects on the thermal
Figure 8. Variations of heat transfer loss versus combustion duration.
efficiency of the engine. This is an unexpected result because several existing studies in the literature have claimed that reduction in combustion duration would be beneficial to thermal efficiency because of the accompanied improvements in the degree of constant volume combustion.23 However, one should still notice that reduction in combustion duration not only brings about the improvements of the degree of constant volume combustion but results in higher pressure rise, higher cylinder temperature, and hence, higher heat transfer loss, the latter of which is of course detrimental to thermal efficiency. Figure 8 further confirms that the heat transfer loss increases as the combustion duration decreases. Therefore, after considering the effects of combustion duration on heat transfer loss, the behaviors of the curves shown in Figure 7 may be more understandable. The potential for increasing thermal efficiency brought by the improvement in the degree of constant volume combustion may be just canceled out by the increase in heat transfer loss as the combustion duration decreases. One should also bear in mind that the above analysis only concerns the indicated thermal efficiency, which does not take the friction work into consideration. In fact, peak cylinder pressure and the rate of pressure rise would be both increased as combustion phasing advances or/and combustion duration decreases, and this would in turn increase the friction work. (23) Han, Y. Q.; Zhao, J. J.; Zhang, L.; et al. Effects of the heat release on the performance of vehicle engines and its optimization. Proceedings of the Symposium on Combustion and Emissions, Chinese Society of Internal Combustion Engine, 2007.
SI Engine Efficiency
Thus, when it comes to the brake thermal efficiency, this would become another factor influencing efficiency adversely besides the increased heat transfer loss. The results presented in this work may differ from others. For example, Yusuf observed that increased flame speed, which allows engine operation closer to constant volume combustion and could increase engine efficiency.24 Therefore, it is thought that the results are engine-dependent (the heat release rate and heat transfer data used in this work are derived experimentally from the measured cylinder pressure). As mentioned above, whether reduction in combustion duration could improve thermal efficiency is dependent upon the counter-balancing effects of a higher degree of constant volume combustion and higher heat transfer loss. Of course, different engines have different heat transfer characteristics. Therefore, to one engine, a certain reduction in combustion duration may cause a significant increase in heat transfer loss, which finally makes the overall effects on efficiency negative; however, to another engine, the increase in heat transfer loss may be not that much and the final effects on efficiency may still be positive. Nevertheless, the efficiency analysis method is, the authors think, universally applicable. 3.3. Effects of Combustion Cyclic Variations on Efficiency. Lean burn has been generally accepted as an effective approach to simultaneously reduce SI engine exhaust emissions and improve its efficiency. Many newly developed engine techniques, such as homogeneous charge compression ignition (HCCI) and stratified charge combustion, basically gained their edges from lean burn. However, in conventional SI engines, lean-burn operation may bring about a series of problems, among which excessive cyclic combustion variations is the most prominent one. As mentioned before, there is always an optimized combustion phasing for best efficiency, which is determined from the average cycle; therefore, this constant situation may not adapt for each individual cycle. Thus, cyclic combustion variations, which make each cycle burn in a different way, would undermine fuel economy.25 Figures 9 and 10 give the combustion phasing (characterized by the crank angle at which peak combustion rate occurs) and spark to 90% mass fraction burned (MFB) combustion duration for 300 consecutive cycles (experimentally obtained) under three operating conditions that differ in the magnitude of cyclic variation (characterized by the coefficient of variation in indicated mean effective pressure). As can be seen, in all cases, especially those with higher cyclic variation, both combustion phasing and combustion duration fluctuated significantly. A previous discussion proved that, for best efficiency, the peak combustion rate occurs at about 9 °CA after TDC. The data shown in Figure 9, which were obtained at MBT spark timing, also showed peak efficiency at around 9°. Thus, the fluctuation that makes the combustion phasing of many cycles deviate from its optimum value would definitely affect thermal efficiency. To examine the effects of this fluctuation on engine efficiency quantitatively, heat release data obtained through analyzing each individual cycle were input into the two-zone predictive model and then the highest thermal efficiency derived was picked out to compare to that derived using averaged heat release data. Table 2 gives the thermal efficiency data for the abovementioned three different operating conditions calculated from (24) Yusuf, M. J. Lean burn natural gas fueled engine: Engine modification versus hydrogen blending. Ph.D Thesis, University of Miami, Coral Gables, FL, 1993. (25) Robinet, C.; Andrzejewski, J.; Higelin, P. Cycle-to-cycle variation study of an SI engine fired by spark plus and a non-conventional device. SAE Paper 972986, 1997.
Energy & Fuels, Vol. 22, No. 5, 2008 3027
Figure 9. Variations of crank angle for the peak combustion rate versus cycle number.
Figure 10. Variations of spark to 90% MFB burn duration versus cycle number.
both averaged heat release data and an individual data point that resulted in the highest efficiency. Clearly, if cyclic variations are eliminated, all cycles can develop in the best way and thermal efficiency will be increased. Also note that higher cyclic variations (larger value of CoVimep) will exert a heavier effect on decreasing engine efficiency. For example, if cyclic variations are eliminated, thermal efficiency in the case with CoVimep of 1.99% can be increased by 3.62%, while in the case with CoVimep of 5.2%, can be increased by 9.51%, which is nearly 3 times larger. 4. Conclusion A zero-dimensional two-zone thermodynamic predictive model was employed to examine the effects of combustion phasing, combustion duration, and cyclic variations on SI engine thermal efficiency. The main results of this study can be summarized as follows: (1) There always exists an optimized combustion phasing for best engine thermal efficiency. Specif-
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Table 2. Effects of Cyclic Variations on Engine Thermal Efficiency operating condition: n/MAP/λ/fuel
CoVimep (%)
efficiency from averaged heat release rate data (%)
efficiency from best heat release rate data (%)
relative increase (%)
1600/70/1.45/100% CNG 1600/70/1.45/70% CNG 1600/70/1.45/50% CNG
5.2 2.89 1.99
0.323 0.347 0.356
0.354 0.363 0.369
9.51 4.51 3.62
ically, with other conditions being equal, the best efficiency will be reached when the peak combustion rate occurs at about 9° after TDC. Combustion phasing that deviates from this optimized value would decrease thermal efficiency. (2) If heat transfer is neglected, the best efficiency will be reached when the peak combustion rate occurs at TDC. However, in a real engine, advancing combustion phasing would cause the heat transfer loss to rise, which is detrimental to efficiency. It is concluded that the rise of heat transfer loss as combustion phasing advances is the very reason why the combustion rate does not peak at TDC when best efficiency is reached. (3) Although the reduction in combustion duration could increase the degree of constant volume combustion, its effects on the final thermal efficiency are not quite obvious. This is also thought to be caused by the rise of heat transfer as combustion duration reduces. (4) Cyclic variations in the combustion process exert a heavy effect on thermal efficiency. The higher the variations, the larger the negative effects.
Acknowledgment. This study was supported by the National 863 Project for Energy Efficient and New Energy Vehicle (2006AA11A1B7). The author wants to acknowledge all of the teachers and students in our group for their help with the experiments and their great advice during the preparation of the manuscript.
Nomenclature SI ) spark ignition CNG ) compressed natural gas MAP ) manifold absolute pressure EVC ) exhaust valve open MFB ) mass fraction burned TDC ) top dead center MBT ) maximum brake torque IVC ) intake valve closed IC ) internal combustion EF8003027
