Design and Simulation of Two Stroke Engines

O
01
III
cr
cc
LU
Q.
LU
h-
LU
Z
o
N
Q
LU
Z
cc
3
900 -,
880 -
860
840 •
820 -
800 -
780
760
Chapter 7 - Reduction of Fuel Consumption and Exhaust Emissions
CHAINSAW ENGINE, 9600 rpm
75% MASS
FRACTION
BURNED ~
—i -
10
TCR 7.5
20
—i -
30
CRANK ANGLE, °atdc
—i
40
Fig. A7.4 Effect of compression ratio on unburned zone.
at the same instant in each simulation. It is observed that the end zone temperature at this
instant, at a trapped compression ratio of 8.0, is above the peak value for a compression ratio
of 6.5. The end zone contains air and vaporized fuel at these state conditions of temperature
and pressure. If the temperature in this end zone, or in smaller localized pockets of this end
zone, is above the auto-ignition temperature of any fuel in that pocket, then auto-ignition can
occur and detonation, or "knocking," will be experienced.
It is reported by Bosch [7.56] that detonation can occur by auto-ignition in gasoline-fuel
vapor mixtures at temperatures at and above 880°C. It is observed that this condition is met by
the chainsaw simulation for the entire unburned zone at 12° atdc at a trapped compression
ratio of 8.0, where it is 882°C. Detonation will assuredly occur at this point.
This opinion is reinforced by the data from Kalghatgi [7.57] who shows measured temperature
data for the combustion of iso-octane in a four-stroke cycle engine at 11° atdc. At a
temperature of 830°C, knocking or detonation was not present. At a temperature of 887°C at
the same crankshaft position, knocking was present. The fine limits between the presence of
detonation, or its absence, are clearly to be found within this publication.
The peak temperature in the unburned zone at a CRt of 7.5 is 873°C. Detonation may not
occur at this point, but my more cynical view, already mooted above, is that Murphy, whose
Law applies always and everywhere, will have arranged already to have localized pockets of
air and fuel meet the auto-ignition criterion at a trapped compression ratio of less than 8.0, but
maybe not as low as 7.5.
In terms of the GPB simulation techniques used here, it can be reasonably safely assumed
that detonation will not occur if the unburned zone temperatures are detected not to exceed
870°C, at any simulated speed or load within the operating range of the engine.
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