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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.

This paper describes how the image of a flame within an enclosure can be reconstructed from multiple measurements of capacitive impedance, between non-intrusive sensing electrodes distributed around the combustion zone. Measurements across the combustion zone respond to the ionisation inherent in the combustion process, which changes the permittivity and hence the capacitance measured between electrodes. For this feasibility study, experimental data is presented from a laboratory analogue of an engine combustion chamber, demonstrating flames being imaged in real time. The sensing system is modelled using 3-D finite element methods, enabling analysis of practical electrode geometries for engine combustion chambers.