Effect of Flight Altitude on the Knock Tendency of SI Reciprocating Turbocharged Engines 2016-32-0006
This paper provides an analysis of the effect of a flight altitude on knock occurrence in reciprocating SI turbocharged engines. It presents results of the computational study aimed at investigating reasons leading to knock occurrence and methods of alleviating the knock tendency of small aircraft engines. Turbochargers are frequently used to improve the performance of aviation platforms at high altitudes. Although a turbocharger provides the benefits of increased power, improved BSFC and a downsized engine, it can result in engine knock because of increasing the intake air temperature, due to a rise in the compression ratios as the air density drops. Aerial platforms experience environmental conditions that can change drastically in a matter of a few minutes. Therefore, it is important to be aware of the combined effects of altitude, initial ground temperature, humidity, flight velocity and fuel octane numbers on the emergence of knock following takeoff. A novel approach was suggested for assessing the joint influence of various ambient and operating parameters on knock appearance in a turbocharged aircraft engine that can be used for knock risk and severity evaluation prior to takeoff, during a flight and for taking measures to prevent knock at real-time operation. Conditions that may provoke knock during certain flight circumstances were identified and analyzed. Possible methods of in-flight knock prevention, such as water injection, retarded ignition, EGR, intercooling etc. were analyzed.