Design and Simulation of Two Stroke Engines

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Chapter 3 - Scavenging the Two-Stroke Engine
DESCRIPTION DEFLECTION RATIO Ca
A QUB RADIAL 1.44
B QUB RADIAL 1.10
C CLASSIC CROSS 1.65
D CLASSIC CROSS 1.15
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0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
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SCAVENGE RATIO, SRv
Fig. 3.33 Effect of deflection ratio on scavenging of cross-scavenged engines.
The scavenge port width ratio, Cpb, for this type of engine is usually somewhat less than
unity, particularly in the straight-drilled port design configuration shown in Fig. 3.32(a). In
Fig. 3.32(a) the numerical value is approximately 0.80, which is typical of outboard motor
practice.
3.5.3 Unconventional cross scavenging
Fig. 3.32(b) illustrates an alternative design which provides some significant advantages
over the conventional design approach. The unconventional design is characterized by:
(a) A deflector which fills the cylinder bore in exactly the same fashion as the QUB type
discussed in the Sec. 3.5.4. This permits the deflector to run cooler because heat
transfer takes place to the cylinder walls by conduction and from a deflector which is
sufficiently wide so that a significant amount of cooling by convection to the crankcase
gases can occur on the underside of the piston. With no side clearance on the
deflector, the deflection of air charge is more positive and short-circuiting to the
exhaust ports from that source is eliminated. While experiments on the single-cycle
gas scavenge rig show that a deflector side clearance does not adversely affect the
scavenge characteristics at high scavenge ratios, it is at low scavenge ratios that the
effect is significant in that the trapping efficiency can be reduced from a "perfect"
100% to a "near perfect" 95 to 96%. This may sound like pedantry, but if the aim of
an engine design at low load is zero emission of hydrocarbons at low scavenge ratios
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