Knock Detection in a Hydrogen Engine 970039

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.
A different method, using the frequency spectrum of the apparent heat release rate, was capable of detecting knock with excellent selectivity. Although the time resolution used to sample the pressure trace (1° crankangle) is not sufficient to fulfil the Nyquist criterion for the knocking frequencies, it is shown that this condition can be relaxed without unduly influencing the accuracy of the method.


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