Troels Dyhr Pedersen.indd - Solid Mechanics

The highest IMEP measured (approx. 500 kPa) is obtained with the ring shaped piston crown at 2 CAD ATDC.
It appears that this piston crown can result in a higher IMEP than the flat piston crown at a later combustion
phasing than TDC, although the ring shape results in more noise.
The flat piston crown has an advantage by having the smallest surface area and a small crevice volume as well.
The result is the highest IMEP at any given combustion phasing.
The diesel bowl piston crown has a large crevice volume of 7 % of the total compression volume. In addition, a
large part of the unburned charge on top of the piston will also be compressed into the crevice volume during
combustion in the bowl which will further decrease combustion efficiency. The IMEP of this piston crown type
is therefore lower than the flat and ring shaped piston crowns.
The remaining 4 piston crowns have equally poor performances in terms of IMEP. The primary reason for the
lower IMEP of these piston crowns may be attributed to increased heat losses, as well as the large crevice
volumes with the internal cavity piston crowns.
Since heat transfer may be expected to be considerably higher when surface area is increased, the temperature
of the piston crown can be expected to be increase as well. No problems related to advanced ignition or thermal
damage was however detected in this experiment. Hot spots occurring at sharp edges could function as local
ignition points, but there is no indication that such ignition affects the overall reaction timing.
Frequency distribution
Figures 15-21 show the Fast Fourier Transform (FFT) analysis of the external peak sound pressure. The peak
sound pressure is computed as an average of the peaks in 20 consecutive cycles, since some variation does
occur in the sound pressure level. The peak pressure has been chosen since this was considered most relevant in
terms of annoyance from transient acoustic noise. A short term average (5 ms) will typically be 5-6 dB lower
than the peak SPL. The SP value on the y-axis is 1 Pa in all graphs, corresponding to a SPL of 94 dB.
It was found that the damping of the emitted noise after the combustion was quite similar with all piston
crowns. The high frequency noise from directly transmitted frequencies disappeared within 10 ms, while the
engine resonance would be more slowly attenuated.
The three lines present in each graph below were obtained at three different inlet temperatures of 10, 0 and -10
C. The different temperatures result in different timing and inlet density, but as can be seen from the figures, the
effect of temperature on sound pressure level is not very strong.
Figure 14 reveals that the heat losses and increased crevice volumes caused by the alternative piston crown
shapes reduce IMEP significantly.
Page 15 of 21