Troels Dyhr Pedersen.indd - Solid Mechanics

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Frequency response [dB]
-60
-80
-100
-120
-140
-160
-180
0 5000 10000 15000 20000
Frequency
Figure 22: Frequency response of engine to internal sound pressure
As indicated by the figure, lower frequencies are more efficiently transmitted to the surroundings. This may be
part of the explanation why the diesel chamber is very silent compared to the other geometries. The diesel
chamber resonance frequency was measured to be approx. 10 kHz, and this frequency is attenuated 10 dB more
than frequencies around 5 kHz. The piston crowns producing lower frequencies than 5 kHz were also found to
result in more noise, which is also part due to the frequency response of the engine.
In figure 22, the rise in frequency response from 0-3 kHz is primarily due to this region being the engine’s
resonance frequency. This frequency band is exited shortly after the combustion although the cylinder does not
emit any vibrations at frequencies lower than the first mode, which is approx. 5.8 kHz. The frequency band
from 3 kHz and above indicates a somewhat linear decrease in response at increasing frequencies.
Engine acceleration level
The coherence between the cylinder SPL and the engine acceleration level is illustrated in figure 23. The figure
is a plot of the acceleration level and the acoustic and cylinder SPL for one complete cycle of the engine in
knocking combustion with the ring shaped chamber. The increased acoustic SPL and acceleration levels after -
360 CAD are caused by the diesel combustion event. The HCCI combustion occurs at -10 CAD and causes a
rapid increase in cylinder and acoustic SPL as well as the acceleration level. This is due to acoustic energy from
the cylinder being transmitted directly to the surface during combustion [11]. The SPL and the AL decrease at
the same rate, while the acoustic SPL decreases at a slower rate due to the engine resonance which is not
measured by the accelerometer.
SPL and AL [dB]
160
120
80
Acceleration level
40
Cylinder SPL Acoustic SPL
0
-360 -270 -180 -90 0
CAD
90 180 270 360
Figure 23: Cylinder SPL, acoustic SPL and acceleration level