The use of lower or higher heating value, heat release rate and heat loss in internal combustion engines

Abstract

The heat release rate in internal combustion engines obtained from in-cylinder pressure data is a fundamental method to analyse the combustion characteristics of engines. As the measured in-cylinder pressure is lower than the pressure in the absence of heat loss to the walls, the methodology typically leads to the apparent rate of heat release as the heat loss to the cylinder walls cannot be segregated. Heat loss can then be inferred by reference to the chemical fuel energy expected to be released by the fuel. Typically, in engine thermodynamic analysis, the lower heating value is used to determine the energy released by the fuel. However, in this article, we argue that when detailed comparison with validated combustion modelling was done, it was concluded that the higher heating value is the more appropriate calorific value. In this research, the analysis of heat release rate and its determination using the first law of thermodynamics with constant ratio of specific heats γ and also varying γ is discussed. It was noted that the use of the “3rd term” (term due to the dγ/dϑ) in the heat release rate is advisable as it gives a more reasonable heat loss even in the compression stroke.