Design and Simulation of Two Stroke Engines

Chapter 3 - Scavenging the Two-Stroke Engine
ports in Fig. 3.31. The twelve ports are configured at a swirl angle, p, of some 10° or 15°.
The port guiding edge is a tangent to a swirl circle of radius, rp. A typical value for rp is
related to the bore radius, rcy, and would be in the range:
r
cy
r
cy
2 p 1.25
The port width, xp, is governed by the permissible value of the inter-port bar, xt,. This
latter value is determined by the mechanical strength of the liner, the stiffness of the piston
rings in being able to withstand flexure into the ports, and the strong possibility of being able
to dispense with pegging the piston rings. Piston rings are normally pegged in a two-stroke
engine to ensure that the ends of the ring do not catch in any port during rotation. Because of
the multiplicity of small ports in the uniflow case, the potential for the elimination of pegging
greatly reduces the tendency for the piston rings to stick, as will tend to be the case if they are
held to a fixed position or if they are exposed to hot exhaust gas during passage over the
exhaust ports. The piston ring pegs can be eliminated if the angle subtended by the ports is
less than a critical value, 0crit- Although this critical port angle qcrjt is affected by the radial
stiffness of the piston rings, nevertheless it is possible to state that it must be less than 23° for
Fig. 3.31 Scavenge port plan layout for uniflow scavenging.
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