Troels Dyhr Pedersen.indd - Solid Mechanics

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11 Engine acoustics
The third paper in appendix deals with the effect of combustion chamber shape on the
combustion noise. Pictures of the piston crowns are found in this paper, as well as the
relevant findings. To evaluate the efforts, it was required to measure the sound emitted by
the engine accurately, as well as using objective means of evaluating the recorded sound.
This section presents the setup used to obtain the data, the data treatment and the
evaluation of the data.
11.1 Measurements
The cylinder pressure and the acoustic noise were measured to determine the coherence
and evaluate the noise level produced by the engine. This section describes the
considerations for these measurements, the data treatment and the presentation of the
data.
11.2 Choice of sampling frequency
In general, the highest possible sampling frequency should be applied in any case, to
obtain the best possible resolution of the signal and to cover as high a frequency range as
possible. If the frequency range is well known, the sampling rate may be set to a value
that satisfies the Nyquist criterion of at least two times the highest frequency present.
The cylinder pressure may be sampled based either on an external or an internal sample
clock. If the data are collected for a heat release analysis, it required that a crankshaft
angle encoder is used as an external sample clock. HCCI combustion is a very fast event
and high frequencies are present in the chamber after the combustion. To capture the
combustion event and avoid aliasing from the resonant frequencies in the chamber, the
sample rate must satisfy the Nyquist criterion. A crankshaft encoder with 3600 pulses per
revolution was chosen for this task. The highest detectable frequency then becomes:
1
= f
2
f Nyquist sample
=
1
2
N
[ rev min]
⋅ 3600[
samples rev]
60[
sec min]
1
= N ⋅ 60
2
[ samples sec]
so that at 1200 rpm, the sample rate becomes 72 kHz and the Nyquist frequency becomes
36 kHz. This is considered to be fully satisfactory under normal circumstances, since the
frequencies generated are rarely detectable above 20 kHz
Frequencies higher than the Nyquist frequency must however be considered as a potential
source of error. When the sampling frequency is inadequate a false frequency is produced
in the sampling process, which is called an alias. In cases of strong combustion knock the
resonance frequency of the diaphragm of the transducer may become a source of aliasing.
The diaphragm resonance frequency is approx. 160 kHz for the Kistler sensor, and 120
kHz for the Optrand sensor. To prevent aliasing an analogue low pass filter must be
applied before sampling, to block these resonance frequencies.