Knocking Cylinder Pressure Data Characteristics in a Spark-Ignition Engine

An experimental study of the knock initiation and development period was conducted in a single-cylinder research engine. Cylinder pressure was recorded at two locations in the combustion chamber using a high sampling frequency, thus allowing the identification of spatial and temporal pressure characteristics introduced by knock. It was found that substantial local pressure differences exist inside the chamber during the initiation of autoignition. In most cases, knock-induced pressure waves are of acoustic nature, but they can become weak shock waves in heavily knocking cycles. Waves appear first in the pressure trace of the transducer closer to the site of autoignition, and are subsequently detected by the other transducer. The results show evidence of multiple sites of autoignition, and indicate changes in the nature and the direction of propagation of pressure waves. Cylinder pressure signals sampled at locations closer to the center of the chamber show significantly smaller amplitude of pressure fluctuation, making them more suitable for further mathematical processing.