Troels Dyhr Pedersen.indd - Solid Mechanics

Notes on pressure and heat release curves
The cylinder pressure curves illustrated are drawn
from data which has been smoothed by moving
average as described on the previous page. The
cylinder pressure data recorded would otherwise
have displayed super imposed pressure oscillations
of varying amplitudes, ranging from merely visible
at low loads to amplitudes of more than 20 Bars.
The quantity of heat release is not very precise, as
the derivation of the heat release is based on a
moving average of the cylinder pressure. The
peaks of heat release are therefore also lower than
if untreated data was used. This damping effect is
highest at curves for 3000 RPM, where the average
of 30 values has been used. By comparison with
untreated data it is found that peaks are lowered
about 30-40 percent at 3000 RPM.
Observed effects of engine speed
In general, increasing the engine speed seems to
lower the rate of heat release significantly. But
taking into account the effect of the data treatment
that was necessary to produce the curves in the
first place, the decrease in reaction rate may in fact
be more modest.
It is observed that CA 50 at lambda 2.5 is slightly
advanced with engine speed, which may be a result
of higher compression pressure and higher incylinder
temperatures. This seemingly counteracts
the decrease in time for the reactions effectively at
this equivalence ratio. At the leaner mixtures
however, CA50 is delayed with higher engine
speeds, indicating that the increased compression
pressure and heat flux resulting from the higher
engine speeds do not adequately compensate for
the reduced time for reactions to finish.
Another observation is that knocking amplitude is
strongly enlarged with higher engine speeds at
lambda 2.5. Running with lambda 2.5 at engine
speeds of 2000 and 3000 RPM produces
unacceptable levels of knock with any compression
ratio other than the lowest possible. Thus, operation
with lambda 2.5 requires the lowest possible
compression ratio and a low engine speed to avoid
knock.
At lambda 3 and 4, knocking amplitude is also
higher with the higher speeds, but the amplitude is
closer to being acceptable especially near the lower
limit of usable compression ratio. The values for
lambda 3 at 3000 RPM are however beyond
acceptable limits.
The LTR is not affected in a negative way by engine
speed. On the contrary it is seen that LTR occurs
earlier with higher engine speed.
Higher engine speeds made it generally more
difficult to initiate self-sustaining combustion. This
may be the result of inadequate cylinder
temperatures and inadequate time for the reaction
to initiate.
Observed effects of compression ratio
As expected, LTR, HTR and therefore also CA50
was generally advanced with increase in
compression ratio.
At 1000 RPM for all equivalence ratios, knocking
amplitude seems largely unaffected by changes to
the compression ratio even though CA50 is
advanced. At 3000 RPM, knocking amplitude is
however increased with compression ratio. It is
therefore obvious that the lowest possible
compression ratio should be used if heavy knock is
to be avoided at these speeds.
It should be expected that the best performance is
achieved with the lowest possible CR that could
sustain the combustion, as the major part of the
combustion would then be positioned after TDC.
However, it was found that increasing the CR
beyond the point where the combustion of a given
amount of fuel is self sustaining does not
consistently reduce the IMEP. This may be the
results of a more complete combustion, e.g. a
higher conversion of CO to CO2 and a better
penetration of the thermal boundary layer that
quenches the combustion.
Observed limitations of the used equivalence ratios
Lambda 2.5 will operate at all speeds with
reasonable levels of knock with a compression ratio
of 9.0. Operating at 3000 RPM produces excessive
knock in any case and operation at 3000 RPM is
therefore not an option with this equivalence ratio.
Lambda 3 requires a compression ratio of at least
9.2 to initiate combustion at all engine speeds.
Operating at 3000 RPM is however not
recommended, as knock exceeds 5 bars
disregarding the compression ratio.
Lambda 4 requires a compression ratio higher than
11.0 to initiate combustion at all engine speeds.
Knock is very limited except at 3000 RPM where
knock approaches 5 bars of amplitude. The general
impression was however that knock is acceptable
under all conditions at lambda 4.
IMEP
Figures 28-30 display the calculated IMEP for all
points of operation.
The values of IMEP achieved with lambda 2.5 show
that the highest output should be expected with a
compression ratio around 9. Higher compression
ratios will only lower the performance, as
combustion is advanced and engine knock
becomes unacceptable.
For lambda 3, peak values of IMEP are found at a
compression ratio of 9.2, except at 1000 RPM
where peak IMEP is found at a compression ratio of
8.8. As compression ratio is increased, IMEP is
lowered but remains high throughout the tested
interval.