Design and Simulation of Two Stroke Engines

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Chapter 4 - Combustion in Two-Stroke Engines theoretical approach, the use of empirically determined coefficients, particularly for factors relating to turbulence, has increased greatly over the earlier proposal of a simple heat release model to simulate the combustion process. It is somewhat questionable if the overall accuracy of the calculation has been greatly improved, although the results presented by Reid [4.29- 4.31] are impressive. There is no doubt that valuable understanding is gained, in that the user obtains data from the computer calculation on such important factors as exhaust gas emissions and the flame duration. However, this type of calculation is probably more logical when applied in three-dimensional form and allied to a more general CFD calculation for the gas behavior throughout the cylinder leading up to the point of ignition. This is discussed briefly in the next section. 4.4.4 Three-dimensional combustion model for spark-ignition engines From the previous comments it is clearly necessary that reliance on empirically determined factors for heat transfer and turbulence behavior, which refer to the combustion chamber as a whole, will have to be exchanged for a more microscopic examination of the entire system if calculation accuracy is to be enhanced. This is possible by the use of a combustion model in conjunction with a computational fluid dynamics model of the gas flow behavior within the chamber. Computational fluid dynamics, or CFD, was introduced in Chapter 3, where it was shown to be a powerful tool to illuminate the understanding of scavenge flow within the cylinder. That the technology is moving toward providing the microscopic in-cylinder gas-dynamic and thermodynamic information is seen in the paper by Ahmadi-Befrui etal. [4.21]. Fig. 4.10 is taken directly from that paper and it shows the in-cylinder velocities, but the calculation holds all of the thermodynamic properties of the charge as well, at a point just before ignition. This means that the prediction of heat transfer effects at each time step in the calculation will take place at the individual calculation mesh level, rather than by empiricism for the chamber as a whole, as was the case in the preceding sections. For example, should any one surface or side of the combustion bowl be hotter than another, the calculation will predict the heat transfer in this microscopic manner giving new values and directions for the motion of the cylinder charge. This will affect the resulting combustion behavior. This calculation can be extended to include the chemistry of the subsequent combustion process. Examples of this have been published by Amsden et al. [4.20] and Fig. 4.11 is an example of their theoretical predictions for a direct injection, stratified charge, spark-ignition engine. Fig. 4.11 illustrates a section through the combustion bowl, the flat-topped piston and cylinder head. Reading across from top to bottom, at 28° btdc, the figure shows the spray droplets, gas particle velocity vectors, isotherms, turbulent kinetic energy contours, equivalence ratio contours, and the octane mass fraction contours. The paper [4.20] goes on to show the ensuing combustion of the charge. This form of combustion calculation is preferred over any of those models previously discussed, as the combustion process is now being theoretically examined at the correct level. However much computer time such calculations require, they will become the normal design practice in the future, for computers are becoming ever more powerful, ever more compact, faster and less costly with the passage of time. 323
Design and Simulation of Two-Stroke Engines K»33 REFERhNCE VECTOR * CS.9 M/S K = 23 KsU Fig. 4.10 CFD calculations of velocities in the combustion chamber prior to ignition (from Ref [4.21]). &PP5 •+
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Design and Simulation of Two-Stroke
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A Second Mulled Toast When as a stu
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Acknowledgments As explained in the
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Table of Contents Nomenclature xv C
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Table of Contents 2.10.1 Flow at pi
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Table of Contents 3.5.3.1 The use o
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Table of Contents 6.1.3 The effect
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Nomenclature NAME SYMBOL UNIT (SI)
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speed of rotation speed of rotation
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attenuation or transmission loss wa
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Design and Simulation ofTwo-Stroke
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Design and Simulation ofTwo'Stroke
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Chapter 7 Reduction of Fuel Consump
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Chapter 7 • Reduction of Fuel Con
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Chapter 7 - Reduction of Fuel Consu
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§ 2000
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y, ,6 Chapter 7 - Reduction of Fuel
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Chapter 7- Reduction of Fuel Consum
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Chapter 8 Reduction of Noise Emissi
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Chapter 8 • Reduction of Noise Em
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Chapter 8 - Reduction of Noise Emis
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• • / . • Appendix Listing of
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Active radical (AR) combustion and
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initial conditions, assumptions reg
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in two-stroke engine (schematic dia
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exhaust timing control valve, effec
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I Exhaust emissions/exhaust gas ana
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Exhaust systems (continued) untuned
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gas constant/universal gas constant
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in crankcase heat transfer analysis
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Mercury Marine air-assisted fuel in
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I PISTON POSITION (computer program
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Pressure wave reflection (in pipes)
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local acoustic velocity, 60 local M
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Simulation, engine See Computer mod
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temperature vs. crankshaft angle (c
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work per thermodynamic (Otto) cycle
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Design and Simulation of Two Stroke Engines

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