Design and Simulation of Two Stroke Engines

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CO E 50 40 - O O _i Hi > I CO 30 Z) o CO 20 SQUISH VELOCITY Chapter 4 - Combustion in Two-Stroke Engines esq KEsq KINETIC ENERGY —r~ 10 20 30 TRAPPED COMPRESSION RATIO • 80 - /o . -60 . - 50 -40 Fig. 4.17 Effect of compression ratio on squish behavior. So that the designer may assess all of these factors, as well as be able to produce the relevant data for the prediction of squish velocity by Prog.4.1, there are six design programs included in the Appendix Listing of Computer Programs to cover the main possibilities usually presented to the designer as combustion chamber options. The physical geometry of these six combustion chambers is shown in Fig. 4.18, and all but one are intended primarily for use in loop-scavenged engines. Clearly Prog.4.7 is specifically for use for QUB-type deflector piston engines. The names appended to each type of combustion chamber, in most cases being the jargon applied to that particular shape, are also the names of the computer programs involved. To be pedantic, the name of computer program, Prog.4.4, is BATHTUB CHAMBER. Actually, the "bowl in piston" combustion system would also be found in uniflowscavenged engines, as well as loop-scavenged engines, particularly if they are diesel power units, or even gasoline-fueled but with direct in-cyUnder fuel injection. An example of the calculation is given in Fig. 4.19 for the hemisphere combustion chamber, a type which is probably the most common for loop-scavenged two-stroke engines. A photograph of this type of chamber appears in Plate 4.1, where the bowl and the squish band are evident. A useful feature of this calculation is the data insertion of separate spherical radii for the piston crown and the squish band. A blending radius between the bowl and the squish band is also included as data. The output reflects the tapered nature of the squish band, but it is within the designer's control to taper the squish band or have it as a parallel passage. In the calculation for squish velocity, a tapered squish clearance would be represented by the mean of the two data output values; in the case shown it would be the mean of 1.6 and 2.1 mm, or 1.85 mm. Similarly, the designer can tailor the geometry of any of the central types of chamber to have no squish band, i.e., a zero value for the squish area ratio, should that quiescent format be desirable in some particular circumstance. 335 30 s E V" CD DC UJ 2 LU o h- UJ ~ZL *L Q UJ X CO Z5 O CO
Design and Simulation of Two-Stroke Engines Prog .4.2 HEMI-SPHERE CHAMBER Prog .4.! TOTAL OFFSET CHAMBER Prog .4.3 HEMI-FLAT CHAMBER Prog.4.6 BOWL IN PISTON Prog .4.4 BATHTUB CHAMBER Prog .4.7 QUB DEFLECTOR Fig. 4.18 Combustion chamber types which can be designed using the programs. As with most of the programs associated with this book, the sketch on the computer screen is drawn to scale so that you may estimate by eye, as well as by number, the progress to a final solution. In the operation of any of these six programs, the computer screen will produce an actual example upon start-up, just like that in Fig. 4.19, and invite you to change any data value until the required geometry is ultimately produced. Finally, the printer output is precisely what you see on the screen, apart from the title you typed in. 4.7 Some general views on combustion chambers for particular applications The various figures in this chapter, such as Figs. 4.13, 4.18 and 4.19 and Plates 4.1-4.3, show most of the combustion chambers employed in spark-ignition two-stroke engines. This represents a plethora of choices for the designer and not all are universally applicable for both homogeneous and stratified charge combustion. The following views should help to narrow that choice for the designer in a particular design situation. 336
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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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Design and Simulation ofTwo-Stroke
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Design and Simulation ofTwo'Stroke
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Chapter 4 Combustion in Two-Stroke
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Chapter 4 - Combustion in Two-Strok
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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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• • / . • 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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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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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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