Engine Heat Transfer

Engine heat transfer is the propagation of the combustion heat from the chamber to the external wall of the cylinder. In regions of the chamber of high heat flux, thermal stresses must be kept below deleterious levels, which could cause fatigue cracking. Therefore, temperatures must be kept below 400° C for cast iron, and below 300° C for aluminum alloys engines. Meanwhile, the gas side surface of the cylinder wall must be kept below 180° C to prevent deterioration of the lubricating oil film. Heat is transferred by molecular motion, through solid or fluid materials, due to a temperature gradient. This gradient is the difference in temperature, which cause heat to dissipate.

Heat transfer affects engine performance, efficiency, and emissions. For a given mass of fuel within the cylinder, higher heat transfer to the combustion chamber walls will lower the average combustion gas temperature and pressure, and reduce the work per cycle transferred to the piston. Thus, power and efficiency are influenced by the magnitude of engine heat transfer. Friction is most affected by the heat transfer, contributing to the coolant load. The cylinder liner temperature governs the piston and ring lubricating oil film temperature, and hence its viscosity.

Within spark-ignition engine cylinders, the temperature of the charge relative to the wall temperature and the flow field vary significantly throughout the cycle. During the intake process, the incoming charge is usually cooler than the cylinder walls and the flow velocity are high. During compression, the charge temperature rises above the wall temperature as gas velocities decrease. Heat transfer now takes place from the cylinder gases to chamber walls. During combustion, gas temperatures increase sharply and the gas expansion, which occurs on combustion, causes increased gas motion.

Substantial heat transfer from the exhausting gases to the valve, port, and manifold takes place during the exhaust process.