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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.

Combustion chamber deposit (CCD) formation in SI engines is a complex phenomenon which is dependent on a number of fuel and engine parameters. A mathematical model has been developed, based upon a previously proposed mechanism of CCD formation, which describes the physical and chemical processes controlling the growth of deposits in SI combustion chambers. The model allows deposit thickness to be predicted as a function of time, taking into account gasoline composition and factors influenced by engine operating conditions. Piston top deposit thicknesses predicted by the model for 38 unadditivated fuels show a strong correlation with data from three different bench engine tests. The model offers the possibility of predicting the amount of CCD produced by unadditivated gasolines for a range of engine designs, operating conditions and test durations.

The octane requirement increase (ORI) observed in spark ignition engines essentially occurs due to the effect of deposits in the combustion chamber changing the heat transfer characteristics between the end-gas and the combustion chamber walls. In addition, the volume occupied by deposits produces a change in compression ratio inside each cylinder which also contributes to ORI. However, all the ORI observed in spark ignition engines cannot be explained by these physical effects alone and for some time the existence of a chemical mechanism of ORI has been postulated. Evidence is presented from a laboratory experiment which demonstrates that deposits are indeed able to influence the ignition delay times of fuel-air mixtures by providing a source of active species which help initiate autoignition. Such effects have also been observed in some engine experiments, thus confirming the existence of chemically based ORI.

The heavier components of a gasoline appear to contribute disproportionately to hydrocarbon emissions. To quantify this effect, a series of well defined bi-component fuels consisting of a mid-range volatility base (isooctane) with a variety of heavy hydrocarbons have been tested in a single cylinder test engine under steady conditions. The fraction of the heavy component in the fuel that was emitted in the exhaust was greater than that predicted from the correlation that has been developed with its OH reactivity. The extent of the discrepancy increased with the concentration of heavy component in the fuel or with the boiling point of the heavier hydrocarbon. The results are consistent with a mechanism based on an increased probability of wall impaction of heavy components during the intake stroke because of their slowness to evaporate.

The formation of combustion chamber deposits in modern SI engines is predominantly derived from hydrocarbon fuels and occurs as a consequence of the quenching action of the combustion chamber walls on the flame. A laboratory experiment has been designed which enables rapid generation of deposit material in the form of viscous brown liquids. Heating these deposits produces material that is consistent in composition and physical appearance with mature engine deposits. The deposit-forming tendency of a number of individual hydrocarbon species has been determined. The amount of deposit increases with i) the amount of unsaturation present in the molecular structure and ii) the boiling point of the hydrocarbon fuel being burned. A structurally derived parameter for each hydrocarbon molecule is found to correlate well with deposition rate, allowing a unified treatment of the different generic forms of hydrocarbons in which deposit-forming tendency is linked to molecular structure.

In order to estimate the influence of the fuel composition on speciated hydrocarbon emissions from gasoline engines a model has been developed for the processes undergone by the fuel which escapes the main combustion event. One of the most important ways that this occurs is by trapping in crevices followed by mixing and partial oxidation with the hot burnt gas during the power and exhaust strokes. This complex process has been modelled by recognising some important characteristics. It is observed that the fraction of a fuel species emitted is well correlated with its rate constant for reaction with OH radicals and that this is independent of the rest of the fuel composition. This means that (a) the chemistry is significant (not just mixing) and (b) the radicals carrying out the oxidation originate from the burnt gases. The decoupling of radical concentrations from the fuel composition considerably simplifies the modelling.