Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
1.5.2 Scavenging efficiency and purity
In Chapter 3 it will be shown that for a perfect scavenge process, the very best which
could be hoped for is that the scavenging efficiency, SE, would be equal to the scavenge ratio,
SR. The scavenging efficiency is defined as the mass of delivered air that has been trapped,
mtas, by comparison with the total mass of charge, mtr, that is retained at exhaust closure. The
trapped charge is composed only of fresh charge trapped, mtas, and exhaust gas, meX, and any
air remaining unburned from the previous cycle, mar, where:
m tr = m tas + m ex + m ar
(L5,8)
Hence, scavenging efficiency, SE, defines the effectiveness of the scavenging process, as
can be seen from the following statement:
SE = mtas = m tas (1.5.9)
mtr
mtas + mex + mar
However, the ensuing combustion process will take place between all of the air in the
cylinder with all of the fuel supplied to that cylinder, and it is important to define the purity of
the trapped charge in its entirety. The purity of the trapped charge, n, is defined as the ratio of
air trapped in the cylinder before combustion, mta, to the total mass of cylinder charge, where:
m ta =m tas +m ar (1.5.10)
n = - m ^- (1.5.H)
mtr
In many technical papers and textbooks on two-stroke engines, the words "scavenging
efficiency" and "purity" are somewhat carelessly interchanged by the authors, assuming prior
knowledge by the readers. They assume that the value of mar is zero, which is generally true
for most spark-ignition engines and particularly when the combustion process is rich of
stoichiometric, but it would not be true for two-stroke diesel engines where the air is never
totally consumed in the combustion process, and it would not be true for similar reasons for a
stratified combustion process in a gasoline-fueled spark-ignition engine. More is written on
this subject in Sees. 1.5.5 and 1.6.3 where stoichiometry and trapping efficiency measurements
are debated, respectively.
1.5.3 Trapping efficiency
Definitions are also to be found in the literature [1.24] for trapping efficiency, TE. Trapping
efficiency is the capture ratio of mass of delivered air that has been trapped, mtas, to that
supplied, mas, or:
TE = -^1 (1.5.12)
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mQC