6 The Simulation of Engine Structures
In order to gain an understanding of the effect of different
designs of engine on the vibratory and acoustic properties, the Noise
Refinement Centre at Ricardo embarked on building and testing engine-like
structures which are known as crankcase simulation rigs (CSRs).
These structures had all the main features of a heavy duty diesel
engine crankcase / cylinder block / cylinder head assembly, but with the
minimum of irregular details which characterise a real engine design.
The CSRs are slab-sided in-line engine structures. Two different forms
were constructed - the symmetric CSR (CSRSYM) and the asymmetric
CSR (CSRASYM). The structures were tested and measurements of the
excitation forces and acoustic and vibratory properties were taken.
These measurements are useful in the analysis of the
relationships between these properties. The data from these
experiments is also useful for the verification of
results from the application of computational methods.
For further details on the CSR project, see
Croker (1987a,b) and Croker and Tyrrell (1989).
In order to take measurements of the acoustic and vibratory properties,
the CSRs were suspended on elastic cords in a semi-anechoic test
cell. Surface vibration response to internal excitation was measured
at some 250 points using a miniature accelerometer. The sound pressure
was measured at 10 microphone points on an imaginary hemisphere of radius 1.4m
and at 8 microphone points on an imaginary cylinder of the same radius
which joins onto the hemisphere.
The sound power was determined by calculating a numerical approximation
based on an integral over the imaginary surface. The surface
acceleration, the sound pressures, the sound powers, and the cylinder
pressure were converted into 10Hz narrow band frequency spectra.
For more details on this see Croker (1987a,b).