Design and Simulation of Two Stroke Engines

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)
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