Design and Simulation of Two Stroke Engines

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Design and Simulation of Two-Stroke Engines Fig. 5.2(a) Exhaust control valve in fully retracted position. Fig. 5.2(b) Exhaust control valve lowered increases effective trapped stroke. 364
Chapter 5 - Computer Modeling of Engines can never seal the port to the extent that a piston can accomplish within the cylinder bore, the design of the valve can be such that it closely follows the piston profile over the width of the exhaust port(s). There are many practical designs for such valves, ranging from the cylindrical in section [7.4] to guillotine designs and the lever type shown in Fig. 5.2. If the simulation model is to incorporate such a timing edge control valve, then several factors must be taken into account. The first is the altered port profile which can be incorporated into the execution of the analytic process described above. Even the "leakage" profile past the valve can be accounted for by assigning an appropriate width to the port to simulate the area of the leakage path. However, if such a valve is found to seal the cylinder at a timing point other than the fully open position, the simulation model must take into account the change of trapped compression ratio, CRf. This is the consequence of the fact that the clearance volume of the engine will have been deduced from the trapped compression ratio at the full height of the exhaust port. Hence the modified trapped compression ratio with the timing valve lowered is: CR t modified = ( CR t ~ l) -0 " + 1 (5.2.7) x ts The use of a disc valve for the intake system This has been a very popular intake system for many years, particularly for single-cylinder engines, and especially in motorcycles. It is very difficult to incorporate into a multicylinder design. A sketch of the significant dimensions of such a valve is shown in Fig. 5.3 and a photograph of one fitted to an engine is shown in Plate 1.8. Further pertinent information and discussion on disc valve design is seen in Figs. 6.28 and 6.29 and in Sec. 6.4, where the nomenclature is common with this section and the sketches provide further aid to understanding of the analysis below. The total opening period of such a valve, {j)max. is readily seen as the combined angles subtended by the disc and the port: max = p + d (5.2.8) Thus, the total opening period is distributed around the tdc position in an asymmetrical manner, as presented before in Fig. 1.8. The maximum area of the port can be shown to be a segment of an annulus between two circles of radius rmax and rmjn, less the corner radii, rp. A max = ^TTT^max " r min) " r p(4 " *) (5.2.9) The instantaneous area, Ae, is found by a similar theoretical approach to that seen in Eqs. 5.2.1-5.2.4, except that the term for the movement of the disc with respect to the angular crankshaft movement is now more simply deduced in that it is linear. For example, if the disc has fully uncovered the corner radii at opening, the instantaneous area at a juncture of 0° from the opening position is given by: 0 / 2 2 \ 2l ~ 7t l 8 = ^ 360 ^ max " fmin ^ " H iJ (5 ' 2 " 10) 365
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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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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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^ s C£ Q 2^ E Q. Q_ z' o ISSI HCEM
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o > z~ Q w CO UJ w g x O z o z o m
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0.35 • 0.34 LU ° 0.33 - c 03 DC
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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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INLET FOR r AIR AND FUEL Chapter 7
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o Q_ Ld < CD 15-, 10- Chapter 7 - R
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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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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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