| Recently, "dual fuel" vehicle engines have been developed which retain the high thermal efficiency and desirable torque characteristics of diesel-fuelled engines while replacing most of the required diesel with cleaner-burning natural gas. One of the biggest problems facing dual fuel engine technology is the tendency for end-gas regions to violently autoignite, or knock, at full torque. In mass-production spark-ignition engines, knock is detected non-intrusively by analyzing vibrations transmitted throughout the engine structure. Since dual fuel conversions are based on diesel powerplants, the added stiffness and strength of their compression-ignition blocks tends to make vibration analysis more problematic. In this investigation, a test engine was outfitted with a block-mounted wide-band accelerometer, and a pressure transducer was installed inside one of the combustion chambers. The output of the pressure transducer during knock testing was correlated to operator-observed knock intensity to determine the most accurate pressure-based measure of knock. Accelerometer output was then compared to the pressure transducer's indication of knock severity to determine the best block vibration statistic. The maximum value of bandpass filtered acceleration amplitude was found to be the optimum vibrational statistic when analyzed within a data window extending 40 crankshaft degrees beyond the initiation of knock. This research concludes that there is an excellent possibility that conventional non-intrusive knock detection systems will also be useful for dual fuel engine knock control. |
