To improve fuel efficiency of engines, cooling heat loss is one of the most dominant losses among the various engine losses to reduce. The present work proposes a new heat insulation concept in combustion chamber, "TSWIN (Thermo-Swing Wall Insulation Technology)" that can reduce heat loss to the coolant without any sacrifice in other engine performances. Surface temperature of insulation coat on combustion chamber wall changes rapidly, according with the fluctuating temperature of in-cylinder gas. Reduced temperature differences between them lead to lower heat transfer. During the intake stroke, surface temperature of the insulation coat goes down rapidly, and prevents intake air heating. To realize the scheme mentioned above, a new insulation material with both low thermal conductivity and low volumetric heat capacity, "SiRPA (Silica Reinforced Porous Anodized Aluminum)" was developed and applied on the top surface of the piston. It was confirmed that the temporal changes in temperature of coated surface in the engine operating condition showed nearly identical behavior as predicted by 1-dimensional heat transfer simulation using laser-induced phosphorescence thermometry. It was applied to turbocharged direct injection diesel engines, and showed improvement of fuel efficiency, and increased exhaust gas temperature which contributes to earlier light off of the aftertreatment catalyst. As the result of energy balance analysis, cooling heat loss was reduced, on the other hand, brake power and exhaust loss was increased. Furthermore, on low temperature engine start condition, TSWIN showed remarkable improvements both in fuel efficiency and exhaust emissions.