Design and Simulation of Two Stroke Engines

y, ,6
Chapter 7 - Reduction of Fuel Consumption and Exhaust Emissions
SILENCER
Fig. 7.28 Positioning of catalysts in expansion chamber suggested by Laimbock [7.21].
the fuel economy and hydrocarbon emissions of the engine are significantly improved. The
fundamental requirement in design terms is shown in Fig. 7.29. Somewhere in the cylinder
head or cylinder wall is placed a "device" which will supply fuel, or a mixture of fuel and air,
into the cylinder in such a manner that none of the fuel is lost into the exhaust duct during the
open cycle period. Although the sketch shows a two-stroke engine with crankcase scavenging,
this is purely pictorial. The fundamental principle would apply equally well to an engine
with a more conventional automotive type of crankshaft with pressure oil-fed plain bearings
and the scavenge air supplied by a pump or a blower; in short, an engine as sketched in Fig.
1.6, described initially in Sec. 1.2.4, simulated in Sec. 5.4.3, or as designed by Thornhill
[5.23].
The simplest idea which immediately comes to mind as a design solution is to use the
diesel engine type of liquid injection system to spray the fuel into the cylinder after the exhaust
port is closed. This straightforward approach is sketched in Fig. 7.30. Naturally, the fuel
injection system is not limited to that generally employed for diesel engines and several other
types have been designed and tested, such as those proposed by Beck [7.16], Schlunke [7.26],
Stan [7.28] or Heimberg [7.54]. Not many analytical or experimental studies have been carried
out on the direct injection of gasoline; however, the papers by Emerson [7.43], Ikeda
[7.50] or Sinnamon [7.55] deserve study.
However, it is possible that this elegantly simple solution to the problem is not as obviously
effective as it might seem. In Chapter 4, the combustion process by spark ignition of a
fuel and air mixture is detailed as being between a homogeneous mixture of fuel vapor and
air. It is conceivable that a liquid fuel injected in even the smallest droplets, such as between
10 and 15 urn, may still take too long to mix thoroughly with the air and evaporate completely
495