Troels Dyhr Pedersen.indd - Solid Mechanics

12 Simulation of detonation phenomena
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This chapter deals with the physical phenomena called detonation, which can cause
strong pressure pulsations in HCCI combustion.
The first section contains a theoretical description of detonations, which is implemented
in a model that can predict stationary detonation wave properties.
The second section deals with a study that was setup to investigate if a CFD model is
capable of capturing the detonation event.
The last section describes an experiment that was setup to study detonation waves in
HCCI combustion. The presence of strong shock waves was shown, which indicate the
presence of detonation waves as well.
12.1 Background
The HCCI engine is operated with a nearly homogeneous charge. Therefore, an ideal
HCCI combustion should progress as a control volume explosion. This would result in a
relatively high pressure rise rate, but pressure gradients should not occur during
combustion. When HCCI combustion is intiated in an engine at lean conditions, it is
usually observed that combustion does indeed take place without introducing large
pressure gradients, which are indicated by pulsating pressure. The reason is that piston
and valves are cold, and hence the charge is not subjected to hot surfaces that may
introduce temperature gradients. When the piston and valves are heated up the
combustion changes and combustion knock increases. This is due to the temporal and
spatial variation in reaction rate, which causes high amplitude pressure waves to
propagate inside the cylinder. In the more extreme cases these pressure waves may
develop into detonations similar to those that cause SI engine knock.
It is important to note that flame propagation cannot be the cause of these pressure
oscillations, since normal flame propagation does not create pressure waves. Besides,
flame propagation is very slow at lean conditions and will not be able to consume the
charge in the short time span typical of HCCI combustion.
Explosions may be responsible for amplification of pressure waves, as will be argued in
the following. The nature of such waves is that they will be acoustic of nature, meaning a
limited pressure gradient and amplitude.
Detonations, although only in a partly developed stage, are characterized by supersonic
propagation of a steep pressure wave of large amplitude. These may be the explanation of
some cases of HCCI engine knock, in particular when the amplitude of the pressure
oscillations is large. The best indication of a detonation is the measurement of a pressure
increase so fast that it is not consistent with normal acoustic waves created by explosions.
An acoustic wave will appear as a slow harmonic increase in pressure, while a detonation
wave will appear as an instantaneous pressure increase, likely within to data samples.