Experimental Analysis of Heat-Loss with Different Piston Wall Surface Conditions in a Heavy-Duty Diesel Engine

抄録

Reducing heat loss to cylinder walls should not be ignored when addressing currently sought improvements in thermal efficiency of reciprocating engines. In case of diesel engines, which have complex heterogeneous in-cylinder temperature distributions due to their combustion phenomena, it may be desirable to spatially investigate heat loss where spray flames directly interfere with external surfaces, in order to obtain further thermal efficiency improvement. Especially, spray flame interference zones with large temperature gradients could have significant influence on total heat loss, even with temporally short interaction times. One approach for reducing heat transfer from flames (or hot gases) to the wall could be changing the wall material, which was investigated with monolithic ceramics in 1980s and has been reconsidered recently. A 0.5mm Zirconia thermal spray coating has been applied on the piston cavity surface as a thermal barrier, aiming to increase both average and swing wall temperature beyond that of a baseline forged steel piston, to reduce the temperature difference between in-cylinder gas and the wall surface. Nevertheless, no measurable difference in heat loss was observed. Since the flame and wall surface temperature distributions and their interactions were influential, direct observation of the flame was carried out in this study, with different piston surface structures. A modified cylinder head with one exhaust valve replaced by a sapphire window enabled combustion visualization. To discriminate differences in heat transfer by flame observation, high-speed high magnification macro-photography was utilized. Results indicated that the thermal boundary layer thickness between the flame and zirconia-coated wall was thinner than with the baseline steel wall. This could result in a higher heat transfer coefficient. Numerical simulations showed that not only wall material but also the microstructure of the wall surface (e.g. surface roughness and/or open porous structure characteristic of the manufacturing method) could impact heat transfer to the wall. This study also experimentally compared those differences.