Heat transfer investigations in a modern diesel engine

Abstract

An experimental investigation has been undertaken to study operating temperatures and heat fluxes in the cylinder walls and cylinder head of a modern diesel engine. Temperatures were measured under a wide range of speed and torque at more than one hundred locations in the block and cylinder head of the engine employing conventional thermocouples arranged to obtain one-dimensional metal thermal gradients and subsequently deduce the corresponding heat fluxes and surface temperatures. Results observed in the cylinder bores revealed that in addition to heat transferred by convection and radiation from combustion gases, the temperature and heat flux distributions are considerably affected by heat conduction from piston rings and skirt through the oil film, and by frictional heat generated at these components. The heat fluxes and surface temperatures obtained in the cylinder head combined with gas pressure measurements were used to evaluate existing formulae to predict heat transfer coefficients from combustion gases to the chamber walls. The evaluation confirmed the significant variation previously observed between the various methods. As a consequence, a modified correlation has been proposed to estimate the gas-side heat transfer coefficient. This new correlation is considered to be an improved tool for estimating the heat transfer coefficients from combustion gases in modern diesel engines. Additionally, the results observed in the cylinder bores were used to develop a simple model from first principles to estimate the heat transferred from piston rings and skirt to the cylinder wall.