B URNING V ELOCITY
4.5. SUMMARY
This chapter concentrated on the kinds of engine design modifications that can be made to enable practical “lean-burn” operation in the context of a traditional spark- ignited engine. Lean combustion in a spark-ignition engine is advantageous because of the benefit associated with the increase in the ratio of specific heats, g, which lean mixtures of air and fuel provide. A reduction in the equivalence ratio from stoichiomet- ric conditions withF¼1:0 to near the lean limit withF¼0:7, provides an increase of nearly 10% in the theoretical thermal efficiency. Although, actual engine efficiency is only approximately one-half the theoretical air-standard value, there is nevertheless a significant increase in thermal efficiency with decreasing equivalence ratio. Another major benefit of lean operation is the accompanying reduction in combustion tempera- tures provided by the excess air, which leads directly to a significant reduction in the generation of NO, one of the most problematic exhaust emissions. Lean operation can therefore be used both to increase thermal efficiency, and reduce exhaust emissions.
Ultimately, however, there is a lean limit of operation, beyond which it is impossible to maintain reliable ignition and combustion, resulting in an increased cyclic variation in combustion pressure, and misfires. An extension of the lean limit of operation through improved combustion chamber design is one way to further improve efficiency and reduce emissions. This chapter has summarized the results of two different techniques that have been proposed to extend the lean limit of operation of spark-ignited internal combustion engines.
Combustion chamber geometry has been shown to be an important factor in deter- mining the performance and exhaust emissions of a lean-burn spark-ignition natural gas
engine. The optimum spark advance, specific fuel consumption and specific emission levels are greatly affected by the burning rate in the combustion chamber, which in turn is controlled by both the intensity and scale of the mixture turbulence just prior to ignition and during the early combustion process. The“squish-jet”turbulence-generat- ing chamber design was tested in a spark-ignited research engine, and was compared to a conventional bowl-in-piston design. The squish-jet chamber exhibited a significantly faster burning rate than the conventional chamber, as indicated by the reduction in MBT spark advance required. The faster burning rates led to a 5% reduction in BSFC, compared to the case with a conventional diesel type of quiescent combustion chamber.
A comparison of two versions of the squish-jet combustion chamber and a conventional chamber design in a Cummins L-10 engine, both with and without enhanced swirl motion, has shown the squish-jet configuration to be more effective in increasing thermal efficiency than the use of high swirl levels.
A new design of PSC combustion system was also compared to a homogeneous charge configuration in single-cylinder research engine. Preliminary measurements of engine performance have shown that the stratified-charge design is effective in extending the lean limit of combustion and in significantly reducing the BSFC during lean operation. The use of the stratified-charge design should enable a spark-ignition engine to operate with reduced pumping losses and significantly higher thermal effi- ciency over a complete engine driving cycle. Further research is required to fully determine the effect of the new design in reducing exhaust emissions.
REFERENCES
Andrews, G.E., Bradley, D., and Lwakabamba, S.B. (1976). Turbulence and Turbulent Flame Propagation A Critical Appraisal.Combust. Flame24, 285 304.
Benson, R. and Whitehouse, N. (1979).Internal Combustion Engines. Pergamon Press, London.
Blaszczyk, J. (1990). UBC Ricardo Hydra Engine Test Facility, UBC Alternative Fuels Laboratory Report AFL-90-15.
Campbell, A. (1979).Thermodynamic Analysis of Combustion Engines. John Wiley & Sons, New York.
Dymala-Dolesky, R. (1986). The effects of turbulence enhancements on the performance of a spark-ignition engine. M.A.Sc. thesis, University of British Columbia.
Evans, R.L. (1986). Internal Combustion Engine Squish Jet Combustion Chamber, US Patent No. 4,572,123.
Evans, R.L. (1991). Improved Squish-Jet Combustion Chamber, US Patent No. 5,065,715.
Evans, R.L. (2000). A Control Method for Spark-Ignition Engines, US Patent No. 6,032,640.
Evans, R.L. and Blaszczyk, J. (1993). The effects of combustion chamber design on exhaust emissions from spark-ignition engines.Proceedings of the IMechE Conference on Worldwide Engine Emission Standards and How to Meet Them, London.
Evans, R.L. and Cameron, C. (1986). A New Combustion Chamber for Fast Burn Applications, SAE Paper 860319.
Evans, R.L. and Tippett, E.C. (1990). The effects of squish motion on the burn-rate and performance of a spark-ignition engine.SAE Technical Paper Series, Future Transportation Technology Conference and Exposition, San Diego, CA.
Evans, R.L., Blaszczyk, J., Gambino, M., Iannaccone, S., and Unich, A. (1996). High efficiency natural-gas engines.Conference on Energy and the Environment, Capri, Italy.
Goetz, W., Evans, R.L., and Duggal, V. (1993). Fast-burn combustion chamber development for natural gas engines.Proceedings of the Windsor Workshop on Alternative Fuels, pp. 577 601.
Gruden, D.O. (1981). Combustion Chamber Layout for Modern Otto Engines, SAE Paper 811231.
Heywood, J.B. (1988).Internal Combustion Engine Fundamentals. McGraw-hill, New York.
Mawle, C.D. (1989). The Effects of Turbulence and Combustion Chamber Geometry on Combustion in a Spark Ignition Engine, UBC Alternative Fuels Laboratory Report AFL-89-02.
Nakamura, H.et al. (1979) Development of a New Combustion System (MCA-Jet) in Gasoline Engine, SAE Paper 790016.
Overington, M.T. and Thring, R.H. (1981). Gasoline Engine Combustion Turbulence and the Combustion Chamber, SAE Paper 810017.
Reynolds, C. and Evans, R.L. (2004). Improving emissions and performance characteristics of lean burn natural gas engines through partial-stratification.Int. J. Engine Res.5(1), 105 114.
Semenov, E.S. (1963). Studies of turbulent gas flow in piston engines.NASA Tech. Trans.F97.
Young, M.B. (1980). Cyclic Dispersion Some Quantitative Cause-and-Effect Relationships, SAE Paper 800459.
Lean Combustion in Gas Turbines
Vince McDonell
Nomenclature
APU Auxiliary power unit
Bg Blockage ratio of bluff body to open flow channel CO Carbon monoxide
Dc Bluff body characteristic length Dp Mass ofNOxin grams
DF2 Diesel Fuel #2
Foo Engine maximum takeoff thrust in kN GE General Electric
HSCT High-speed civil transport
ICAO International civil aviation organization LDI Lean direct injection
LPP Lean premixed prevaporized LTO Landing-TakeOff
NASA National Aeronautics and Space Administration OEM Original Equipment Manufacturer
P Pressure
RQL Rich burn quick mix, lean burn
s Entropy
SCR Selective catalytic reduction
T Temperature
To Initial temperature Tu0 Turbulence intensity U Free stream velocity v specific volume t Autoignition time
t0eb Characteristic evaporation time
t0fi Characteristics time associated with fuel injection t0hc Characteristic burning time for the fuel
tsl Characteristic time associated with flame holder length scales fLBO Lean blow out equivalence ratio
Lean Combustion: Technology and Control
Copyright#2008 by Elsevier, Inc. All rights of reproduction in any form reserved. 121