A Study of Knock and Power Loss in the Automotive Spark Ignition Engine

Measurements were made with a piezoelectric pressure transducer in one cylinder of an automotive V-B engine having a compression ratio of 10 to determine the level of knock intensity at which knock becomes detrimental to power output (IMEP), and/or damage occurs at wide open throttle. A computer program was devised to permit the measurement of (IMEP), knock intensity, and peak pressure for 100 cycles. It was found that at an average knock intensity of approximately 6 to 12 psi (40 to 50% knocking cycles) the engine would begin to suffer a power loss of 1 to 2% due to knock. At trace knock (audible) it was found that about 10% of the cycles were knocking. The authors have found the definition of knock limits (trace knock) to be rather vague. The number of knocking cycles (10%) found here agrees with the senior author's experience with large aircooled aircraft engines in WWII in which 60 to 100 flashes per minute at 2400 engine rpm on the scope were considered knock limits using a Sperry vibration pickup mounted on the cylinder. A comparison was made between a piston designed with a step to reduce knock and a standard flat top piston. It was found in this case that the stepped piston was not effective, probably due to the fact that the detonation wave was traveling in a direction opposite to the normal flame direction.