Experimental Assessment of Instantaneous Heat Transfer in the Combustion Chamber and Exhaust Manifold Walls of Air-Cooled Direct Injection Diesel Engine

An experimental analysis is carried out to investigate several heat transfer characteristics during the engine cycle, in the combustion chamber and exhaust manifold walls of a direct injection (DI), air-cooled, diesel engine. For this purpose, a novel experimental installation has been developed, which separates the engine transient temperature signals into two groups, namely the longand the short-term response ones, processing the respective signals in two independent data acquisition systems. Furthermore, a new pre-amplification unit for fast response thermocouples, appropriate heat flux sensors and an innovative, object-oriented, control code for fast data acquisition have been designed and applied. Experimentally obtained cylinder pressure diagrams together with semi-empirical equations for instantaneous heat transfer were used as basis for the calculation of overall heat transfer coefficient. On the other hand, one-dimensional heat conduction theory with Fourier analysis techniques combined with an iterative procedure between calculated and measured temperature data are implemented, in order to calculate the instantaneous local engine cylinder heat transfer coefficients in the cylinder head surfaces. For the exhaust manifold, the local gas temperature was calculated based on a thermal-zone model approach developed. Analysis of experimental results revealed significant differences between the overall and local peak heat transfer coefficient values in the cylinder head surface. The effect of engine speed and load as well as the effect of air-swirling motion on cylinder head heat transfer coefficient variation are presented and quantified, revealing the significant influence of turbulence on local heat transfer conditions. The effect of valve motion on instantaneous heat transfer is also depicted and quantified. Analysis of the results for the instantaneous wall temperatures and heat transfer coefficient on the exhaust manifold reveals interesting details regarding the flow of exhaust gases, both in the blowdown and displacement phases of the engine exhaust stroke. From the results it is concluded that the instantaneous heat transfer coefficient variation for engine combustion chamber is highly non-uniform, unlike its values calculated from standard correlations that assume spatial uniformity. This is very important, since especially for air-cooled diesel engines limited relevant information seems to exist in the literature.