Effects of Ceramics Translucence on Heat Barrier Effectiveness in Diesel Engines 890569

Ceramics are being used for heat barrier applications in diesel engines. Being partially translucent, there is a concern about the penetration of heat radiation through them. An analytical investigation was carried out into the effects of translucence on heat transfer. It was found that when the ceramic is translucent its capabilities as heat barrier material are severely reduced. Significant effects begin to be felt for absorption coefficients smaller than 10,000 m^-1 and scattering coefficients less than 20,000 m^-1. In the experimental part of the study, radiative properties of several zirconia plasma sprayed coatings were determined from transmittance and reflectance measurements. These ceramics were found to have high scattering coefficients (30,000-40,000 m^-1) and relatively low absorption coefficients (less than 2000 m^-1). Thus, these particular ceramic coatings may be considered as optically opaque on account of their high scattering coefficient. Other ceramics, e.g. monolithic partially stabilized zirconia, may be substantially more translucent.

CERAMIC MATERIALS are being developed for application as heat barrier materials in insulated diesel engines. Their purpose is to reduce the heat transfer rate from the gases to the walls, with the twin benefits of increased engine efficiency and of lower heat rejection to the coolant.

The idea of a heat barrier layer is to create a path of high resistance to heat transfer, and this is accomplished by using high temperature materials which have low thermal conductivity. Certain ceramics, e.g. zirconia, combine these properties with the potential for low material cost, which accounts for the interest in their development for this application. Computer simulations of in-cylinder heat transfer processes and of the heat conduction through the structural components of the engine indicate that ceramics can indeed provide very substantial reductions in engine heat transfer, translating into important thermal efficiency gains, reduced heat rejection to coolant and increased exhaust energy availability (Morel et al, 1984).

However, it is increasingly being realized that some of the ceramics proposed for diesel engine thermal barriers are partially transparent in the spectral region where most of the thermal energy associated with the combustion process is concentrated (Liebert, 1965 and 1978, and Makino et al, 1984). This causes concern about the effectiveness of such ceramic layers in reducing heat transfer in insulated diesel engines, where it is known that radiation can account for a significant portion of the total heat transfer (Morel and Keribar, 1986, Wahiduzzaman and Morel, 1987).

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 (Morel et al, 1986).

In all simulations of diesel engine heat transfer processes reported to date, the assumption has been made that the absorption coefficient of the ceramic is very large so that all of the incident radiation is deposited at the gas/ceramic interface -- i.e., the radiation has not been allowed to penetrate into the interior. This is a carryover from studies of metallic engines where this treatment provides a fairly accurate description of the actual process. However, since some ceramics of engineering interest are known to be translucent, especially at infrared radiation wavelengths, this approximation may be quite incorrect.

The work carried out on this contract is divided into two parts. The first part concerns an analytical study of the radiation process within engineering ceramics, and shows how heat transfer is affected by the radiation properties of ceramics (absorption coefficient, scattering coefficient, index of refraction and surface reflectivities). The second part of the work concentrates on an experimental determination of the radiative properties of some specific ceramics, and on predicting their effectiveness as heat barrier materials when used in a diesel engine environment. The details of the work are given in two reports to Oak Ridge National Laboratories (Wahiduzzaman and Morel, 1987, 1988).