Reduction of Heat Loss and Improvement of Thermal Efficiency by Application of “Temperature Swing” Insulation to Direct-Injection Diesel Engines

The reduction of the heat loss from the in-cylinder gas to the combustion chamber wall is one of the key technologies for improving the thermal efficiency of internal combustion engines. This paper describes an experimental verification of the “temperature swing” insulation concept, whereby the surface temperature of the combustion chamber wall follows that of the transient gas. First, we focus on the development of “temperature swing” insulation materials and structures with the thermo-physical properties of low thermal conductivity and low volumetric heat capacity. Heat flux measurements for the developed insulation coating show that a new insulation material formed from silica-reinforced porous anodized aluminum (SiRPA) offers both heat-rejecting properties and reliability in an internal combustion engine. Furthermore, a laser-induced phosphorescence technique was used to verify the temporal changes in the surface temperature of the developed insulation coating. This was found to quickly rise during the combustion stroke and then drop during the expansion stroke. Second, a SiRPA coating was formed over the entire surface of the diesel piston cavity, and the thermal efficiency and the heat-loss reduction were investigated for a single-cylinder direct-injection (DI) diesel engine with a bore of 86 mm and a stroke of 96 mm. An energy balance analysis showed that the SiRPA coating achieves heat loss reduction by means of heat rejection and an increase in not only the exhaust energy but also the piston work, which increases the thermal efficiency.