Design and Simulation of Two Stroke Engines

PORT SECTION THROUGH
INCLINED TRANSFER PORT
"UP - AT UP=
^£
UP2
PORT WIDTH IS THE CHORD
AT 902 TO THE GAS FLOW DIRECTION
THE EFFECTIVE WIDTH IS THE
PLAN CHORDAL WIDTH*COSINE(UP).
TRANSFER PORT WIDTH RATIOS ARE
TOTAL EFFECTIVE WIDTH+CVLINDER BORE.
Chapter 3 - Scavenging the Two-Stroke Engine
Fig. 3.39 Second page of output from Prog.3.4, giving port edge machining locations.
3.5.6 Loop scavenging design for external scavenging
The design of engines for the direct injection of fuel, be they compression-ignition or
spark-ignition units, is often carried out by pressure charging the cylinder [3.47-3.49]. This
has already been discussed in a preliminary manner in Sec. 1.2.4. In other words, the supply
of fresh air for the scavenge process is by a supercharger or by a turbocharger directly to the
scavenge (transfer) ports without the use of the crankcase as an air pumping system. Naturally,
if a turbocharger is employed, which is driven by exhaust gas energy entering the turbine
part of the device then the engine has no self-starting capability. Irrespective of how the
engine is started, the air is supplied directly to the scavenge ports by an external air pump. If
it is a supercharger, often referred to as a "blower," it can be of the Roots or Eaton type, a
screw type, or a centrifugal design. If it is a turbocharger, then for the automotive engine this
is conventionally an inward radial flow design for the turbine and an outward radial flow type
for the compressor. For the marine engine, axial flow turbines and compressors are the most
common units.
Externally scavenged engines are usually of a multi-cylinder design, in which case the
trapping of the cylinder charge is of paramount importance as an excess of air supply leads to
high pumping losses in the form of the power required to drive the supercharger. Obviously
this is less vital for the turbocharged case where the air is supplied by exhaust gas energy to
the turbine. Nevertheless, the engine performance is characterized by the cylinder trapping
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