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The ignition delay in a diesel engine is generally seen as consisting of two different consecutive although overlapping phases: the physical and the chemical ignition delay. As is commonly accepted, the physical ignition delay corresponds to the mixture formation, and the chemical delay to the time necessary to get an exponential increase in the chemical reaction rate. In this paper it is shown that if the assumption is made that the ignition of the spray is started by the ignition of a single droplet, the physical ignition delay is determined by the chemical ignition delay. If the results of the ignition delay measurements reported in the literature are interpreted with respect to this ignition model, better understanding of diesel ignition is obtained.

The use of hydrogen in a spark ignited engine is accompanied by a significant risk for backfire and knock, especially at full load, where the richest mixture is used. In fact, when attempting to maximize engine power, knock can (and usually will) lead to runaway surface ignition and backfire without much delay. Since backfire (and knock) has to be avoided at all cost, an attempt was made to detect and quantify knock from the measured pressure traces. For future use knock detection, combined with multipoint timed hydrogen injection, offers the possibility to avoid backfire by temporarily cylinder deactivation. In a first attempt the standard method using the third derivative of the pressure was tried, but proved to be too insensitive to be of any practical use, even though knock was very audible and the pressure oscillations are easily visible on the measurements. This insensitivity is caused by the very fast combustion achieved with hydrogen, compared to other fuels.