Role of Exothermic Centres on Knock Initiation and Knock Damage 902136

The nature of autoignition and knocking is investigated experimentally and theoretically in an optical engine by high speed direct light photography and laser schlieren filming. Special emphasis is devoted experimentally and theoretically to the role of exothermic centres in the end-gas in initiating knocking combustion and subsequent knock damage to the combustion chamber walls. The optical engine is a modified single cylinder ported two stroke engine equipped with a large head window for unlimited access to both the entire combustion chamber and the ring crevice region.
In some experiments the formation of exothermic centres was stimulated by microscopic aluminium particles that deposited on the mirrored piston surface.
The data are analysed by numerically modelling the transition from normal combustion to autoignition with a simplified 2D-code. This code models the interactions between exothermic centres, normal combustion and the resulting time dependent fields of pressure, temperature and gas velocities.
The distinction between the three modes of propagation of reaction from an exothermic centre by Zel'dovich and co-workers becomes blurred under engine conditions but they are, nevertheless, useful:
deflagration: associated with steep temperature gradients; knock may be non-existent to moderate.
thermal explosion: associated with small temperature gradients; knock may be moderate.
developing detonation: associated with intermediate temperature gradients; knock can be violent.
In practice a pure thermal explosion, with homogeneous reaction is improbable in engines.
The chemico-hydrodynamic coupling is especially strong near walls where compression heating by reflected pressure waves is most effective. Thus knocking combustion is preferentially stimulated near cylinder walls.
The predominant occurrence of knock damage is in the ring crevice region where knocking combustion is associated with soot formation both in the crevice and the endgas region.


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