Design and Simulation of Two Stroke Engines

Chapter 1 - Introduction to the Two-Stroke Engine
Fig. 1.5 Two methods ofuniflow scavenging the two-stroke engine.
lubrication. As the lubrication is of the total-loss type, and some 10-30% of the fuel charge is
short-circuited to the exhaust duct along with the air, the resulting exhaust plume is rich in
unburned hydrocarbons and lubricant, some partially burned and some totally unburned, and
is consequently visible as smoke. This is ecologically unacceptable in the latter part of the
20th Century and so the manufacturers of motorcycles and outboards have introduced separate
oil-pumping devices to reduce the oil consumption rate, and hence the oil deposition rate
to the atmosphere, be it directly to the air or via water. Such systems can reduce the effective
oil-to-petrol ratio to as little as 200 or 300 and approach the oil consumption rate of fourstroke
cycle engines. Even so, any visible exhaust smoke is always unacceptable and so, for
future designs, as has always been the case for the marine and automotive two-stroke diesel
engine, a crankshaft lubrication system based on pressure-fed plain bearings with a wet or dry
sump may be employed. One of the successful compression-ignition engine designs of this
type is the Detroit Diesel engine shown in Plate 1.7.
By definition, this means that the crankcase can no longer be used as the air-pumping
device and so an external air pump will be utilized. This can be either a positive displacement
blower of the Roots type, or a centrifugal blower driven from the crankshaft. Clearly, it would
13