As emission limits become increasingly stringent and the price of gaseous fuels decreases, more emphasis is being placed on promoting gas engines. In the field of large engines for power generation, dual fuel combustion concepts that run on diesel/natural gas are particularly attractive. Knock in diesel/natural gas dual fuel engines is a well known yet not fully understood complex phenomenon that requires consideration in any attempt to increase load and efficiency. Thus combustion concept development requires a reliable yet robust methodology for detecting knock in order to ensure knock-free engine operation.
Operating parameters such as rail pressure, start of injection and amount of diesel injected are the factors that influence oscillations in the in-cylinder pressure trace after the start of combustion. Oscillations in the pre-mixed combustion phase, or ringing, are caused by the rapid conversion of large parts of the injected diesel. This effect may lead to misinterpreting non-knocking combustion cycles determined from the in-cylinder pressure trace data or knock sensor data.
This paper describes a new knock detection methodology based on the in-cylinder pressure trace and knock sensor data applied to a diesel/natural gas dual fuel large engine. The approach compares oscillations in the pre-mixed diesel combustion phase to oscillations in the main combustion phase. Parameters that provoke oscillations (rail pressure, start of injection and amount of diesel injected) as well as other parameters influencing knock (IMEP, excess air ratio, manifold air temperature, methane number and compression ratio) are investigated and discussed. This specific approach requires only two steady threshold limits to cover all these parameters influencing knock and to distinguish between knocking and non-knocking combustion cycles.
