F2008-06-210
Cyclic Losses Evaluation of Heat for Engine Combustion Chamberwith Semitransparent Heat Insulating Coatings
Radiant heat exchange processes under intensive short wavelength infrared radiation (q0 ~ 1 MW/m2) of the red-hot soot particles in the combustion chamber (CC) of high speed diesel engines is considered. The ways of optimization of CC temperature condition are discussed. The mathematical and physical simulation of the heat losses in CC with the semitransparent heat-insulating coatings (SHIC) is studied. It is shown that these coatings can accumulate penetrating radiant heat of the total thermal flux. The accumulation as function of optical parameters scattering (σ~100-1000 m-1) and absorption (κ~1-20 m-1) indexes is due to volumetric heat absorption. It should be noted that penetrating radiation provides more rapid internal heating than conduction alone in semitransparent coatings especially during transient heat condition. In the case the SHIC surface temperature values are lower then for opaque coatings ones in ~ 1.5-2 times, temperature gradients are smaller and thermal stresses are lower, subsurface temperature of the SHIC being higher than surface one. Then subsurface positive temperature gradient stimulates returning accumulate heat as conductive flux back into CC volume (so called heat-regenerating effect). Evaluation is made of volumetric heating condition for model thin SHIC based on zirconium oxides. A temperature maximum forms at optimum retio between optical parameters, thermal physical ones and convective-radiant fluxes in CC during cyclic heat transfer. In any case the temperatures of thin semitransparent coatings provide more low temperature of metal substrate under intensive radiation. The radiant heat flux is generated during time of about 1 ms for modern CC of high speed diesel. The temperature of the gas atmosphere in the CC volume changes as a function of the piston movement in each cycle of engine during some millisecond. A selective absorption and scattering of the radiant heat flux in the SHIC allows the temperature to be controlled in the CC surface wall in the diesel engines. From the theoretical simulation the SHIC accumulates radiant energy in the beginning of “hot” phase (the compression stroke) and then acts as a heat source in the rest part of the cycle. Cyclic losses of heat for CC with and without SHIC are evaluated for the M-50 engine with turbo-compressor at 1600 rpm. There is decrease in heat losses for СС with SHIC by ~10-20 % unlike those for CC with traditional opaque coatings. The regulation of the CC walls temperature allows one to create the optimal conditions for the fuel combustion, many of which have different own critical values of vaporization and auto ignition. Thus SHIC is a sink and a source for thermal fluxes. Application of SHIC enables one to decrease heat losses improving the efficiency of engine by controlling surface temperature. This also provides decrease in the concentration of toxic gases.
Poster presentation: Future powertrain solutions