Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
If the scavenge ratio, SRV, exceeds unity, and clearly SEV cannot do so, then:
if Vas>Vcy
then Vta = Vcy (3.1.11)
consequently, SEV =1 (3.1.12)
TF - SE v - l
and v " SR~ ~SR~ (3 ' L13)
3.1.2 Perfect mixing scavenging
This was Hopkinson's second concept. In this scenario the entering air has no perfect
displacement characteristic, but upon arrival in the cylinder proceeds to mix "perfectly" with
the exhaust gas. The perfect displacement zone in Fig. 3.1 does not exist and Vpcj is zero. The
resulting increment of exhaust gas effluent, dVe, is composed solely of the mixed cylinder
charge at that particular instant. As Vpcj is zero, then the volume of trapped air is composed of
Vam less that which has been lost to the exhaust system. To analyze this process, consider the
situation at some point in time where the instantaneous values of air supplied have been Vas,
and the scavenge ratio and scavenging efficiency values to date are SRV and SEV, respectively.
Upon the supply of a further increment of air, dVas, this will induce an exhaust flow of equal
volume, dVe, containing an increment of air, dVae. The volume of air retained in the cylinder,
dVta, due to this flow increment is given by:
dVta = dVas-dVae
or dVta = dVas-dVenm
However, in this idealized concept, as the cylinder purity is numerically equal to the
scavenging efficiency, then:
dVta = dVas-dVeSEv
Differentiation of Eq. 3.1.3 for scavenging efficiency gives:
(3.1.14)
= —— (3.1.15)
v cy
Substituting dVta from Eq. 3.1.15 into Eq. 3.1.14, and taking into account that dVex is
numerically equal to dVas, produces:
VcydSE = d Vas - dVasSEv
214
(3.1.16)