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Links between the RON, MON and Octane Index (OI) of a gasoline are explored and factors influencing knock severity are discussed. The OI was calculated by considering how the autoignition delay time changes with temperature and pressure. Three fuels were examined: a 65/35% toluene/heptane test fuel, and two primary reference fuels (PRF), one with the RON value of the test fuel and the other with the MON value. PRF autoignition times were taken from Adomeit et al and test fuel autoignition times were generated from mathematical models of RON/MON tests plus two experimental sets of engine autoignition data. The toluene/heptane OI depended strongly on engine conditions and could easily exceed the RON. With a lean mixture at high pressure it was 100.2 whereas the RON was only 83.9. Knock severity is governed by the nature of localized “hot spots”. Severe knock is associated with developing detonations towards the end of the delay time.

Controlled autoignition (CAI) engines ideally operate at very lean stoichiometries to achieve low NOx emissions. But at high loads, when combustion approaches stoichiometric, they become noisy and severe engine knock develops. A possible cause is the development of amplifying pressure waves near the hot spots that inevitably occur in the autoigniting gas. This paper presents the results from numerical solutions at realistic engine conditions of the detailed chemical kinetic equations with acoustic wave propagation. Those calculations that involve hot spots must include a spatial dimension. Because of this, they are much more time-consuming than for the homogeneous case. A model system of mixtures of 0.5 H2-0.5 CO with air for equivalence ratios, ϕ, between 0.45 and 1.0 has been used at engine-like temperatures and pressures. These calculations investigate the behaviour for various values of ϕ, hot spot size and temperature elevation.

In general, the rate of heat release during combustion in a spark ignition engine, can have two components: one due to normal burning in a propagating flame, and another due to autoignition in the end gas. It has been possible to separate these two components by analysing the pressure trace of a single cylinder engine. From this, the volumetric autoignition heat release rate can be inferred and studied in some detail. To approximate this rate in an Arrhenius form presents difficulties, in so far as it is not possible to measure the temperature at the instant of maximum heat release rate, at the onset of knock. However, it was possible to measure end gas temperatures by the CARS technique prior to autoignition and then to estimate the temperature at the onset of autoignition by extrapolation. Estimation of the temperature at the instant of maximum heat release rate has enabled kinetic parameters to be assigned in an Arrhenius expression for this rate over a range of temperatures.

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.