Knock Characteristics and Their Control with Hydrogen Injection Using a Rapid Compression/Expansion Machine

A rapid compression/expansion machine (RCEM) based on a single-cylinder engine was developed to understand the fundamental phenomenon of knock during spark-ignition (SI) combustion. In order to cause auto-ignition in the end-gas mixture during the flame-propagation process, and also to visualize the processes, the original head of the engine was replaced with a specially designed combustion chamber. The effects of spark timing, compression ratio and equivalence ratio on knock intensity were systematically investigated using the RCEM with n-butane fuel. In addition, the possibility of knock control by the injection of hydrogen into the end-gas region is also discussed. The experimental results indicate that a higher compression ratio, spark-ignition timing at -10 °ATDC and a stoichiometric equivalence ratio cause heavy knock. However, the knock intensity is drastically decreased with hydrogen injection. It was found that the direct injection of hydrogen into the end-gas region enables the reduction of knock intensity more effectively than the premixing of hydrogen with the hydrocarbon fuel. A higher degree of knock intensity reduction is achieved when the hydrogen injection timing is adjusted close to the spark timing. Also, the knock intensity may be reduced to a degree related to the amount of hydrogen quantity injected when the spark timing is near top dead center (TDC). However, the knock intensity may be increased with earlier spark-timing, in which case an excessive flame-propagation speed is observed in the unburned mixture including hydrogen.