Design and Simulation of Two Stroke Engines

Chapter 3 - Scavenging the Two-Stroke Engine
SE„ = Vta
this simplifies to: v v (3.1.3)
cy
The relationship for trapping efficiency, TE, denoted as TEV in this ideal concept, follows
fromEq. 1.5.13 as:
y
TE v=77 5 - (3.1.4)
v as
The expression for the ideal charging efficiency by volume, CEV, follows from Eq. 1.5.15 as:
CE„ = ^ta
V
v
cy
(3-1.5)
In this ideal scavenge process, it is clear from manipulation of the above equations that
the charging and scavenging efficiencies are identical:
CEV
= SEV
. SE V
(3.1.6)
TEV
and
SRV
1E v - — (3.1.7)
3.1.1 Perfect displacement scavenging
In the perfect displacement process from Hopkinson [3.1], all fresh charge entering the
cylinder is retained and "perfectly displaces" the exhaust gas. This air enters the perfect
scavenge volume, Vpd, shown in Fig. 3.1. Corresponding to the present theoretical presumption
of perfect displacement scavenging the value of the short-circuit proportion, a, is zero,
and the mixing volume illustrated contains only exhaust gas. In other words, the volume of air
in the mixing zone is Vam and is zero; the quantity of air entering the mixing zone is dVam and
is zero; the quantity of air entering the displacement zone at any instant is dVpd and is equal
to dVas; the quantity of air entering the exhaust at any instant is dVae and is equal to zero. In
short, in perfect displacement scavenging the air can fill the cylinder until it is filled completely,
in which case it then spills into the exhaust pipe.
Therefore, if all entering volumes of fresh charge, Vas, are less than the cylinder volume,
Vcy, or:
if Vas