Knock criteria for aviation diesel engines

Abstract

The objective of this study was to develop knock criteria for aviation diesel engines that have experienced a number of malfunctions during flight and ground operation. Aviation diesel engines have been vulnerable to knock because they use cylinder wall coating on the aluminum engine block, instead of using steel liners. This has been a trade-off between reliability and lightweighting. An in-line four-cylinder four-stroke direct-injection high-speed turbocharged aviation diesel engine was tested to characterize its combustion at various ground and flight conditions for several specially formulated Jet A fuels. The main fuel property chosen for this study was cetane number, as it significantly impacts the combustion of the aviation diesel engines. The other fuel properties were maintained within the MIL-DTL-83133 specification. The results showed that lower cetane number fuels showed more knock tendency than higher cetane number fuels for the tested aviation diesel engine. In this study, maximum pressure rise rate, or Rmax, was used as a parameter to define knock criteria for aviation diesel engines. Rmax values larger than 1500 kPa/cad require correction to avoid potential mechanical and thermal stresses on the cylinder wall coating. The finite element analysis model using the experimental data showed similarly high mechanical and thermal stresses on the cylinder wall coating. The developed diesel knock criteria are recommended as one of the ways to prevent hard knock for engine developers to consider when they design or calibrate aviation diesel engines.