Engine Heat Transfer Correlations
The heat transfer coefficient depends on the engine geometry, such as the exposed cylinder area and bore, and the piston speed. Due to the complex gas flow in the cylinder, it varies with location in the cylinder and in time with changing piston position. The value of the heat transfer coefficient is found from a Nusselt number - Reynolds number type correlation.
Nusselt # = a (Reynolds #)m
The characteristic gas velocity U is typically the mean piston speed:
Table III. Types of Heat Transfer Coefficients
The peak values of the instantaneous and local coefficients can be many times higher than the averaged values.
A frequently used correlation for a space and time averaged heat transfer coefficient is that of C. F. Taylor ("The Internal Combustion Engine in Theory and Practice", MIT Press, 1985), which uses the mean piston speed for the characteristic velocity.
the average heat transfer coefficient is