Design and Simulation of Two Stroke Engines

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Design and Simulation of Two-Stroke Engines 2 and 5, then this section will permit a better initial selection of the data for use within such a model. 6.4 Empirical design of disc valves for two-stroke engines This subject is one which I rarely discuss without a certain feeling of nostalgia, because it was the motorcycle racing engines from MZ in East Germany, with their disc valve inlet systems and expansion chamber exhaust pipes, which appeared on the world's racing circuits during my undergraduate student days. They set new standards of specific power performance and, up to the present day, relegated the then all-conquering four-stroke power units into second place. It was, literally, a triumph of intellect and professional engineering over lack of finance, resources, materials and facilities. For many years thereafter, the disc valve intake system was acknowledged to be the superior method of induction control for high specific output engines. However, it was not long before it was learned how to produce equal, if not superior, performance from piston control of the intake system. The current technical position is that the reed valve has supplanted the piston-controlled intake port for racing engines and is probably the most popular intake control method for all engines from the cheapest 2 hp brushcutter to the most expensive 300 hp V8 outboard motor. Nevertheless, the disc-valved engine, in the flat form made popular by MZ, and identical to the Rotax design shown in Plate 1.8, is still very successful on the racing circuits. Racing engine design has a fashion element, as well as engineering logic, at work; and fashion always comes full circle. Therefore, it is important that a design procedure for disc valve induction be among the designer's options and that engine modeling programs are available to predict the behavior of the disc-valved engine. This enables direct comparisons to be made of engine performance when fitted with all of the possible alternatives for the induction process. As with other data required for engine modeling programs, the designer needs to be able to establish empirical factors so that a minimum of guesswork is required when faced with the too-numerous data bank of a major computer program or when rapid optimization is required for the design. The following discussion sets out a logic procedure which by now should be familiar, for it repeats the same basic empirical methodology used for the piston-controlled intake and the reed valve induction systems. 6.4.1 Specific time area analysis of disc valve systems Figs. 1.7(a), Plate 1.8, and Figs. 6.28 and 6.29 show the mechanical positioning, porting control and timing of the disc valve intake device. The earlier explanations in this chapter regarding the influence of the specific time area on the bmep attainable are just as relevant here, as the disc valve merely gives an asymmetrical element to the inlet port area diagrams of Fig. 6.1 for piston-controlled ports. Consequently, Fig. 6.28 shows that it is possible to (i) open the intake port early, thereby taking full advantage of induction over the period when the crankcase experiences a sub-atmospheric pressure, and (ii) shut it early before the piston forces out any trapped air charge on the crankcase compression stroke. Actually, this is an important feature, for the intake pressure wave is weaker by virtue of taking place over a longer time period, and so the ramming wave is not so strong as to provide the vigorous 456
Chapter 6 - Empirical Assistance for the Designer ramming effect observed for the piston-controlled induction system. In Fig. 5.3 and Fig. 6.28, the angle required to open the port into the engine is represented by (j)p in degrees of crankshaft rotation. The total cutaway angle on the disc is shown as §&. If the disc valve opens at 9dvo°btdc and closes at 0dVc° a tdc, then the total opening period is 9max> where: Gmax = 9dvo + ^d vc (6.4.1) It is also straightforward geometry to show that the relationship between 0max, p and $
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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 4 Combustion in Two-Stroke
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Chapter 4 - Combustion in Two-Strok
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a stratilpl o tions to ivior. If m
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CD 30 -, W 20 - LU CO iS _J LU OC 1
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181 also x3 = — = 0.905 x6 = 2.17
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to be cur .3.20) .3.21) .3.22) for
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3.23) com- :on is hhas has a /eng-
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stroke .3.29) mean me to 3land ssio
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a LU z: en 3 CO a LU z EC D m O ho
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CD 1.2 -i 1.0 - di „„ LU 0.8 cc
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CO —> 111" cc LU < LU _J LU CC £
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Combustion in compression-ignition
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stroke ari more often lseful lental
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lat the ssi ;tribu- ELEVATION VIEWS
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CYLINDER HEAD TYPE IS CENTRAL BORE,
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CO E 50 40 - O O _i Hi > I CO 30 Z)
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CURRENT INPUT DATA FOR 2-STROKE 'SP
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Cylin- PP £ Pa- tics of Paper Chap
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vfitro- ;titi >tants 1970. i(In- Ch
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£2 = Pi P2 I Pi J Chapter 4 - Comb
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CHAINSAW AT 9600 rpm. Chapter 4- Co
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LLI •z. O N -z. 0.21 -, 0.20 DC 0
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o Q_ Ld < CD 15-, 10- Chapter 7 - R
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Chapter 7 • Reduction of Fuel Con
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O 01 III cr cc LU Q. LU h- LU Z o N
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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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Roots blower in turbocharged/superc
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Scavenging (continued) scavenging e
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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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