Investigation on characteristics of ion current in a methanol direct-injection spark-ignition engine

• Combustion of methanol was detected with ion current method.
• Typical ion current has two peaks.
• Ion current can reflect the knock intensity.

Investigation on the characteristics of ion current in a direct-injection stratified-charge engine fueled with methanol was conducted. The test engine is a retrofitted 4-cylinder diesel engine with installation of a spark plug. Ion current was measured using the spark plug as ion sensor. Cylinder pressure was recorded and cylinder average temperature was calculated. The band-pass filtering and low-pass filtering method were chosen to extract the frequency and amplitude of ion current for the knock evaluation. Results shows that ion-current signals measured in the methanol engine under normal combustion conditions contain the front-flame stage and the post-flame stage. The characteristic parameters of ion current relate to the methanol engine operating conditions. The pattern of ion current shows high frequency oscillation under the knock conditions, and the amplitude of ion current frequency can reflect the knock intensity. The frequency of ion current signals under knock conditions is ∼7 kHz. The knock evaluation index determined by band-pass filtering and low-pass filtering methods is in good agreement with the experimental results. Ion current method can effectively diagnose the knock of direct-injection methanol engines.