Troels Dyhr Pedersen.indd - Solid Mechanics

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diesel type chamber
3 ATDC
1 BTDC
3 BTDC
1000 3000 5000 f (Hz) 7000 9000
Figure 21: Frequency distribution with the diesel bowl chamber piston
The common feature of all the FFT analysis result is that the largest contribution to the total SPL is found in the
lower part of the frequency band from 1-3 kHz. These frequencies cannot exist inside the cylinder, since the
lowest resonance frequency is 5-6 kHz in all cases except for the ring shaped geometry, which has its primary
resonance frequency at 4 kHz. The 4 kHz frequency is clearly present in the analysis for this piston crown (fig.
16).
Engine block resonance will however occur as a result of higher frequencies being transmitted through the
structure. The connection between structure born noise and emitted noise was investigated by Andreae et al
[10]. It was found that the general source of noise was not that which was directly transmitted, but that of
engine structure resonance being exited by the transmitted noise.
A great reduction in the noise level was achieved by moving the chambers away from the cylinder walls and
into the piston. The total surface area and therefore heat transfer was however increased. The crevice volume
with this and the following pistons was very large since it was not possible to fit piston rings. This resulted in a
reduction of combustion efficiency. In terms of noise reduction the performance was similar. The crevice
volume was however unchanged, so the combustion efficiency was still reduced
Frequency response
The frequency response of the engine is here defined by the ratio of the sound pressure measured outside the
engine to the sound pressure generated in the cylinder. The response is calculated for the range of frequencies in
the FFT. A short term average of 5 ms following the peak heat release was used to allow all established modes
of vibration to be included.
The response is converted to dB. Since -100 dB corresponds to an attenuation of 100,000, a SP of 100 kPa in
the cylinder will result in a SP of 1 Pa outside the engine.
Figure 22 is an example of the frequency response of the flat chamber. The response of the other piston crowns
was found to follow this pattern. The response of the flat chamber was selected because it produces a wide
range of frequencies in the cylinder and thus provides a good signal-to-noise ratio for the response curve.
Sound pressure (Pa)
1
0.8
0.6
0.4
0.2
0