Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
screen or line-printer output without graphics, so Table 6.2 is a precis of the use of Prog.6.3
for the analysis of the actual chainsaw engine and the Grand Prix racing geometries discussed
in Chapter 5.
Table 6.2 Measured time areas for a chainsaw and a GP racing engine
Specific time area,
s/m
65 cm 3 chainsaw
125 cm 3 racer
Asve
0.0071
0.0164
ASvb
0.00015
0.00124
Asvt
0.0067
0.0096
Asvj
0.00110
reed valve
It can be seen that the specific time area values for the actual engines are very similar to
those from Table 6.1, predicted as being necessary to produce the required performance characteristics.
In other words, the specific time areas predicted as requirements, and found as
examples, are sufficiently close to give some confidence in the predictive tool as an initial
design step. There are some differences which will be debated below, together with the
designer's options and methodologies when tackling the initial steps in the design process.
The topic of specific time area is not mentioned very frequently in technical papers.
However, a useful reference in this context, which would agree with much of the above discussion,
is that by Naitoh and Nomura [6.5].
6.1.3 The effect of changes of specific time area on a chainsaw
Consider the 65 cm 3 chainsaw engine as the example and let an engine simulation of it
provide the performance characteristics for the debate. The raw dimensions of the porting are
shown in Fig. 6.2 and the remaining data are as given in Sec. 5.5.1. The "standard" exhaust
port timing is an opening timing of 108° atdc.
A change of exhaust port timing with others constant
Let all other data values in the simulation stay constant, but change the exhaust port
timing to 104°, 106° and 110° and compare the ensuing performance characteristics with each
other and the "target" specific time area values seen in Table 6.1. The Asv values that emanate
from these timing changes are summarized in Table 6.3.
Table 6.3 Time areas for various exhaust timings for a chainsaw
Exhaust timings
104° atdc
106° atdc
108° atdc (std)
110° atdc
target values
Asve
0.00827
0.00765
0.00705
0.00648
0.0095
426
ASvb
0.00027
0.00021
0.00015
0.00011
0.00027
ASvt
0.0067
0.0067
0.0067
0.0067
0.0066