Combustion Chamber

Figure 12. Thermal and mechanical loads on a diesel engine valve.

Figure 13. Portion of 2-dimensional axisymmetric finite element model of valve and coating geometry.

Figure 13. Portion of 2-dimensional axisymmetric finite element model of valve and coating geometry.

0.0 0.3 0.6 0.9 1.2 1.5 DISTANCE FROM COATING SURFACE, mm

Figure 14. Transient behavior of circumferential stresses within the PSZ coating of intake valve.

0.0 0.3 0.6 0.9 1.2 1.5 DISTANCE FROM COATING SURFACE, mm

Figure 14. Transient behavior of circumferential stresses within the PSZ coating of intake valve.

the spectral region where most of the thermal energy associated with the combustion process is concentrated. This causes concern about the effectiveness of such ceramic layers in reducing heat transfer in diesel engineswhere it is known that radiation can account for a significant portion of the total heat transfer.

The gas-to-wall heat transfer in diesel engines is produced by convection from cylinder gases and by radiation from the soot-laden burning zone. In the high output turbocharged engines typically used in highway truck applications, the heat radiation accounts for some 20 percent of the total heat transfer from gases to walls at the rated engine conditions. When the combustion chamber is insulated, the convective component of the heat transfer is selectively reduced. The proportion of heat transfer that is due to radiation then rises to over 50 percent, thereby becoming a much more significant fraction of the overall heat rejection and a more important design consideration.

The present work is concerned with the nature of the radiation heat transfer at the gas/ceramic interface. Specifically, it addresses the question of the importance of the translucence of ceramics, and investigates how this translucence influences the net heat transfer from the gases to the engine structure. If the translucence of the material is large enough, the radiant heat could penetrate in significant amounts to the underlying substrate and/or to bonding agent at the substrate/ceramic interface and raise the temperatures there, this could have significant detrimental effects with respect to heat rejection.

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