Study of Surface Insulation Structures to Reduce Cooling Loss in Heavy-Duty Diesel Engines 2023-01-0948

Cooling loss reduction is essential to enable further increases in thermal efficiency of reciprocating internal combustion engines. Many in-cylinder cooling loss reduction studies have been carried out by applying various thermal barrier coatings to the piston and/or other in-cylinder surfaces, taking advantage of the lower thermal effusivity of ceramic materials. However, the end result was mostly minimal or in some cases, negative. In our previous study, significant cooling loss reduction was experimentally confirmed by utilizing a mirror-like polished stainless-steel thermal sprayed surface (HVOF: high velocity oxy-fuel) on a forged steel piston. This study firstly investigated an alternative insulating layer material to stainless-steel, along with effects of its thickness on heat transfer by a one-dimensional unsteady numerical model. Results showed that lower thermal effusivity doesn’t always reduce heat transfer, but increases nonuniformity of surface temperature. Next, a modified insulation structure composed of a thin aluminum coating overlayed by physical vapor deposition (PVD: physical vapor deposition) on a stainless-steel layer of both the piston and cylinder head, was tested in a heavy-duty single cylinder engine. Aluminum was initially selected due to its high reflectance, to reduce absorption of flame radiation, but selectively absorb it at soot deposits, where the flame interacts with the wall, and reduce local convection. Experimental results suggested that higher surface temperatures of exposed areas are caused by better heat conduction outward from hot spots during and after the combustion period, through the aluminum coating. Such surface temperature equalization could also result in further reduction of cycle-integrated heat transfer.