Troels Dyhr Pedersen.indd - Solid Mechanics

The frequency content of the acoustic noise generated in the cylinder is important since these frequencies are
transmitted directly to the surroundings but with different efficiency due to the frequency response of the
engine. The nature of in-cylinder oscillations in cylindrical chambers were investigated by [6], [7] and [8].
These studies showed that the resonance frequencies in a cylindrical combustion chamber follow acoustic
theory for closed cylindrical chambers.
The lowest resonance mode in a cylindrical chamber is a transverse wave with a single node through the center
of the cylinder as shown to the left in figure 1. This mode usually has the highest amplitude. In the figure, dark
represents high and low pressure zones and bright shows the node between them. The pressure zones shift from
high to low and back again at the given frequencies. The frequency of the modes in figure 1 are calculated at a
temperature of 1900 K in a chamber with a diameter of 85 mm.
Figure 1: Illustration of resonance modes in a cylindrical chamber. From the left, the modes and corresponding
frequencies are: (1, 0): 5.9 kHz; (2, 0): 9.7 kHz, (0, 1): 12.2 kHz and (3, 0) 15.5 kHz
The index (a, b) refers to circumferential and radial modes of vibration. There is also a third mode which is the
axial modes, but due to the limited distance between piston and cylinder head the axial modes have very high
frequencies and are rarely exited at magnitudes of importance. In experiments the first circumferential mode (1,
0) is most often dominating, while the first radial mode (0, 1) and the second circumferential mode (2, 0) are
present at lower amplitudes. Higher modes can be present, but their amplitudes are generally much lower than
the first circumferential mode.
When looking at the chamber geometries which are presented on the next pages, the theory for cylindrical
cavities does no longer support accurate calculations of the frequencies. In general however, similar patterns
will be established in the chamber while the differences in piston geometry will alter the frequencies and
possibly reduce the amplitudes.
EXPERIMENTAL SETUP
PISTON CROWNS
Being the focus of this study, these piston crowns are presented first. Seven different types were designed for
the study. All piston crowns were designed to a compression ratio of 10. In all cases, except for the flat piston
crown, the distance from piston top to cylinder head was 1 mm, which was the minimum requirement for
avoiding valve contact.
The purpose was first of all to test different geometries that could possibly reduce the interaction between
pressure and reaction rate, in order to avoid the larger pressure oscillations found in the typical cylindrical
chamber.
The cylindrical chamber is formed by a flat piston crown (fig. 2) which has a compression height (distance to
the cylinder head) of eight millimeters. The acoustic behavior of the flat piston is relevant in SI type combustion
Page 3 of 21