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Lean SI Engines: The role of combustion variability in defining lean limits
ISSN: 0148-7191, e-ISSN: 2688-3627
Published September 16, 2007 by Consiglio Nazionale delle Ricerche in Italy
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Previous research has shown the potential benefits of running an engine with excess air. The challenges of running lean have also been identified, but not all of them have been fundamentally explained. Under high dilution levels, a lean limit is reached where combustion becomes unstable, significantly deteriorating drivability and engine efficiency, thus limiting the full potential of lean combustion. This paper expands the understanding of lean combustion by explaining the fundamentals behind this rapid rise in combustion variability and how this instability can be reduced.
A flame entrainment combustion model was used to explain the fundamentals behind the observed combustion behavior in a comprehensive set of lean gasoline and hydrogen-enhanced cylinder pressure data in an SI engine. The data covered a wide range of operating conditions including different compression ratios, loads, types of dilution, fuels including levels of hydrogen enhancement, and levels of turbulence. The model used captured the underlying physics of the combustion process, accurately predicting the data and the basic trends. The model showed that the rapid increase in combustion variability near the lean limit is due to the inverse dependence of the burning time of the turbulent mixture eddies on the laminar flame speed. This relationship causes the eddy-burning time to grow, initially slowly and then rapidly, with decreasing laminar flame speed amplifying the normal random variability associated with the flame initiation process. Due to the effect of initial conditions on combustion phasing, this rapidly increasing variability during flame initiation leads to asymmetrical variability in the main part of the combustion process.
This modeling study, together with previous research, shows how by reducing the eddy-burning time, the full burn duration curve can be shortened increasing the lean relative air/fuel ratio at peak efficiency and the lean combustion variability limit. This can be done by increasing turbulence levels, effectively decreasing its microscale structure, or by increasing the laminar flame speed, for example, through hydrogen enhancement. Hydrogen enhancement using hydrocarbon fuel reformate shows diminishing returns at high compression ratios and at high loads, due to the detrimental effect of high pressures on laminar flame speed. Reducing the engine's baseline combustion variability during flame initiation can also extend the lean limit. These conclusions are confirmed through experimental results.
CitationAyala, F. and Heywood, J., "Lean SI Engines: The role of combustion variability in defining lean limits," SAE Technical Paper 2007-24-0030, 2007, https://doi.org/10.4271/2007-24-0030.
- 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 2006 World Congress Detroit, MI SAE paper 2006-01-0229
- Blizard N.C. Keck J. C. “Experimental and Theoretical Investigation of Turbulent Burning Model for Internal Combustion Engines, ” SAE Paper 740191 , SAE Automotive Engineering Congress February 25 March 1 1974 SAE Transactions 83 1974
- Tabaczynski R. J. Trinker F. H. Shannon B. A. S. “Further refinement and validation of a turbulent flame propagation model for spark-ignition engines, ” Combustion and Flame 29 2 October 1980 111 121
- Heywood J.B. Internal Combustion Engine Fundamentals McGraw Hill New York 1988
- Poulos S.G. “The effect of combustion chamber geometry on S.I. engine combustion rats: a modeling study, ” Masters Thesis Department of Mechanical Engineering MIT 1982
- Poulos S. G. Heywood J.B. “The Effect of Chamber Geometry on Spark Ignition Engine.” SAE Paper 830587 1983
- Wong V.W. Hoult D.P. “Rapid Distortion Theory Applied to Turbulent Combustion, ” SAE paper 790357 , SAE Congress & Exposition Detroit, MI February 26 March 2 1979 SAE Transactions 88 1979
- Keck J.C. “Turbulent Flame Structure and Speed in Spark-Ignition Engines, ” Proceedings of Nineteenth Symposium (Int'l) on Combustion The Combustion Institute 1451 1466 1982
- Tennekes H. “A first course in turbulence, ” Cambridge, Mass MIT Press 1972
- Tabaczynski R.J. Ferguson C.R. Radhakrishnan K. “A turbulent Entrainment Model for Spark-Ignition Engine Combustion, ” SAE paper 770647
- Hires S.D. Tabaczynski R.J. Novak J.M. “The prediction of ignition delay and combustion intervals for a homogeneous charge, spark ignition engine, ” SAE Paper 780232 , SAE Transactions 87 1978
- Rhodes D.B. Keck J.C. “Laminar Burning Speed Measurements of Indolene-Air-Diluent Mixtures at High Pressures and Temperatures, ” SAE paper 850047 , International Congress & Exposition Detroit, MI Feb. 25 Mar. 1 SAE Trans. 95 1985
- Ivanic Z. Ayala F. Goldwitz J. Heywood J. “Effects of Hydrogen Enhancement on Efficinecy and NOx emissions of Lean and EGR-Diluted mixtures in a SI Engine.” SAE 2005-01-0253
- Ayala F. A. Combustion Lean Limit Fundamentals and Their Application to a SI Hydrogen-Enhanced Engine Concept Doctoral Thesis, MIT 2006
- Fox J.W. Cheng W.K. Heywood J.B. “A Model for Predicting Residual Gas Fraction in Spark-Ignition Engines, ” SAE paper 931025 , SAE International Congress and Exposition Detroit, MI March 1-5 SAE Trans. 102 1993
- Notes from Professor James Keck 2006
- Smith J.R. “The Influence of Turbulence on Flame Structure in an Engine, ” Flows in Internal Combustion Engines, The Winter Annual Meeting of The American Society of Mechanical Engineers Phoenix Arizona November 14-19 1982
- Pischinger S. Heywood J.B. “A Model for Flame Kernel Development in a Spark Ignition Engine, ” Twenty-Third Symposium (International) on Combustion The Combustion Institute 1033 1040 1990
- Pischinger S. Heywood J.B. “How Heat Losses to the Spark Plug Affect Flame Kernel Development in an SI_Engine, ” SAE paper 900021 1990
- Hill P.G. “Cyclic Variations and turbulence Structure in Spark-Ignition Engines, ” Combustion and Flame 72 73 89 1988
- Hill P.G. Kapil A. “The Relationship Between Cyclic Variations in Spark-Ignition Engines and the Small Structures of Turbulence, ” Combustion and Flame 78 237 247 1989
- Smaling R.M. “System architecture selection in a multi-disciplinary system design optimization framework, ” Masters Thesis Technology and Policy Program MIT 2003
- Goldwitz J. Heywood J. “Combustion Optimization in a Hydrogen-Enhanced Lean Burn SI Engine.” SAE 2005-01-0251