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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.