Design and Simulation of Two Stroke Engines

Chapter 1 - Introduction to the Two-Stroke Engine
The values for bore-stroke ratio and piston speed, which are typical of the engines listed
as types A-H, are shown in Fig. 1.20. It will be observed that the values of piston speed are
normally in a common band from 12 to 14 m/s for most spark-ignition engines, and those with
values about 20 m/s are for engines for racing or competition purposes which would have a
relatively short lifespan. The values typical of diesel engines are slightly lower, reflecting not
only the heavier cylinder components required to withstand the greater cylinder pressures but
also the reducing combustion efficiency of the diesel cycle at higher engine speeds and the
longer lifespan expected of this type of power unit. It will be observed that the bore-stroke
ratios for petrol engines vary from "square" at 1.0 to "oversquare" at 1.3. The diesel engine,
on the other hand, has bore-stroke ratios which range in the opposite direction to "undersquare,"
reflecting the necessity for suitable proportioning of the smaller combustion chamber of that
higher compression ratio power unit.
1.7.2 Influence of engine type on power output
With the theory developed in Eqs. 1.7.5 or 1.7.6, it becomes possible by the application of
the bmep, bore-stroke ratio and piston speed criteria to predict the potential power output of
various types of engines. This type of calculation would be the opening gambit of theoretical
consideration by a designer attempting to meet a required target. Naturally, the statistical
information available would be of a more extensive nature than the broad bands indicated in
Fig. 1.20, and would form what would be termed today as an "expert system." As an example
of the use of such a calculation, three engines are examined by the application of this theory
and the results shown in Fig. 1.21.
Engine Type
Input Data
power, kW
piston speed, m/s
bore/stroke ratio
bmep, bar
number of cylinders
Output Data
bore, mm
stroke, mm
swept volume, cm 3
engine speed, rpm
Type A
Chainsaw
5.2
12
1.3
4.5
1
49.5
38.0
73.8
9440
TypeC
Racing Motor
46.2
20
1
10
2
54.0
54.0
250.9
11,080
TypeG
Truck Diesel
186.0
10
0.9
7
6
106.0
118.0
6300
2545
Fig. 1.21 Calculation output predicting potential engine performance.
The engines are very diverse in character such as a small chainsaw, a racing motorcycle
engine, and a truck diesel powerplant. The input and output data for the calculation are
declared in Fig. 1.21 and are culled from those applicable to the type of engine postulated in
Fig. 1.20. The target power output in the data table are in kW units, but in horsepower values
45