Troels Dyhr Pedersen.indd - Solid Mechanics

chambers which often have this flat structure. The advantage of this structure is that the crevice volume is low
and that the flame (in SI engines) is not quenched due to narrow gaps such as with diesel pistons. In HCCI
engines this piston is however less advantageous, since the large open chamber is ideal for pressure waves and
resonance.
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Figure 2: Flat piston crown
The diesel bowl type piston (fig. 3) is relevant to DI CI engines which can operate in HCCI mode as well. The
advantage of this piston is a low heat loss, while the disadvantage is a large volume which is quenched between
cylinder head and piston.
Figure 3: Diesel bowl piston crown
The diesel bowl type piston tested here has a larger combustion bowl than normal to obtain a compression ratio
of 10. The chamber diameter is smaller than the flat piston which gives a higher frequency at the first
circumferential mode. The frequency will decrease as the piston moves down after TDC and the full diameter of
the cylinder becomes dominating, thus shifting the frequency towards that of the flat piston. The acoustic
properties of the bowl type piston has been studied by Broatch et al. [9] who made a numerical study on the
subject.
The piston crown with the ring shaped (fig. 4) was based on the idea that directing the pressure waves around
the chamber in a circular motion could reduce the momentum transfer from wave reflections and hence reduce
the transmitted sound energy. This was however not the case, possibly because the cylinder liner was fully
exposed to the pressure waves.