Troels Dyhr Pedersen.indd - Solid Mechanics

Coupling of pressure waves and reaction kinetics is a common cause for amplification of pressure waves in the
combustion chamber. One approach to reducing the noise is therefore to design pistons that prevent this
phenomenon.
In this study, DME was used as a fuel. It is used due to its excellent auto ignition properties which makes the
fuel usable in engines with compression ratios around 10. DME is a gas with a vapor pressure of approx. 6 bars
at 25 °C. It evaporates instantly when injected as a liquid in the inlet manifold, thereby creating a premixed
charge. The charge is furthermore homogenized during the intake process and thus the homogeneous charge
condition is reached. Since the charge burns homogeneously it does not produce soot. As the charge is
furthermore lean, formation of nitric oxides is very low. HCCI combustion of DME is therefore a clean
alternative to DI CI combustion as well as an inexpensive option due to a low pressure fuel system. The main
challenge is to avoid knocking combustion, which is the subject of this study.
THEORY
There are at least three possible mechanisms which are responsible for creating pressure waves in the chamber.
These are local explosions, pressure wave amplification and detonations.
Local explosions may occur when part of the gas in the chamber has a higher fuel concentration and/or
temperature. The explosion will cause a pressure wave to propagate through the remaining charge, thereby
compressing it. Pressure wave amplification may then occur as the temperature is increased by the adiabatic
compression. The phenomenon is described by [1] and [2]. When a pressure wave approaches a solid wall, e.g.
the cylinder liner, the pressure difference increases to twice the magnitude of the traveling wave close to the
wall. The corresponding rise in temperature will lead to an increase the reaction rate in the charge locally and
hence cause the pressure to increase further. The pressure wave is therefore amplified upon reflection on the
wall. This phenomenon can result in engine knock with moderate amplitudes.
Detonation is known to cause severe cases of engine knock in SI engines [3, 4, and 12]. Heat transfer is
enhanced when knock occurs due to disturbance of the thermal boundary layer [5]. This may result in melting
of the piston due to excessive thermal load. The destructive nature of the detonation on SI engines is due to the
excessive pressure and temperature in the shock front. Detonations can however not reach a fully developed
state in engines due to the small distance available for the detonation to grow in amplitude. The term
“developing detonation” is therefore more appropriate when referring to detonation in engines. Pressure waves
from developing detonations are however still clearly recognizable since they create a rapid and discontinuous
pressure wave, while explosions cause a slower rise in pressure and hence a more harmonic pressure wave.
The distance in which a detonation can develop is larger when the engine is operated with HCCI combustion. In
the SI engine the detonation can only develop within the end gas pocket, while in an HCCI engine the whole
combustion chamber is filled with partly reacted gas in which the detonation can increase its magnitude.
Therefore a logic approach to reducing pressure waves from developing detonations would be to reduce the
dimensions of the combustion chamber. This can be done by splitting the compression volume into smaller
volumes.
In this study, splitting the combustion chamber into smaller volumes proved successful in reducing the noise to
very low levels compared to the flat piston geometry. It was observed that the cylinder pressure fluctuations
were reduced. The engine vibration level and the sound pressure level of the impulse emitted from the engine
were reduced as well.
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