Troels Dyhr Pedersen.indd - Solid Mechanics

Figure 2: Variable compression
volume piston arrangement
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Figure 3: Pictures displaying the insertion of
variable piston in cylinder head
It was found that the compression ratio necessary to obtain the optimum combustion
timing depends on mainly the engine load. Another factor which affects combustion
timing is engine speed, but the effect was shown to be of less significance than the engine
load. This may be explained by the fact that heat losses are lower at higher engine speeds
and hence the temperature near TDC increases with engine speed.
It was found that high engine loads require the compression ratio to be around
9.5 – 10, while low engine loads require a higher compression ratio of 11 – 13 to
complete the combustion. It was however concluded that a fixed compression ratio of 10
was sufficient to cover the load range of practical interest, which is the medium to high
load.
In the experiments with combustion acoustics, it was necessary to place pressure
transducers around the perimeter of the cylinder liner. Rather than making irreversible
changes to the engine block, it was decided to lift the cylinder head and place the
transducers in a 10 mm steel plate between the engine block and the cylinder head. This
modification is shown in figure 4.
Figure 4: Insertion of steel plate between engine block and cylinder head