Experimental Investigation of Instantaneous Cyclic Heat Transfer in the Combustion Chamber and Exhaust Manifold of a DI Diesel Engine under Transient Operating Conditions

In this paper, the results are presented from the analysis of the second stage of an experimental investigation with the aim to provide insight to the cyclic, instantaneous heat transfer phenomena occurring in both the cylinder head and exhaust manifold wall surfaces of a direct injection (DI), air-cooled diesel engine. Results from the first stage of the investigation concerning steady-state engine operation have already been presented by the authors in this series. In this second stage, the mechanism of cyclic heat transfer was investigated during engine transient events, viz. after a sudden change in engine speed and/or load, both for the combustion chamber and exhaust manifold surfaces. The modified experimental installation allowed both long- and short-term signal types to be recorded on a common time reference base during the transient event. An updated version of an existing code implementing one-dimensional heat conduction theory with Fourier analysis techniques was used for the calculation of local surface heat flux evolution during each transient event. From the analysis of experimental results it is confirmed that each thermal transient event consists of two distinguished phases the “thermodynamic” and the “structural” one which are appropriately configured and analyzed. In the case of a severe variation, in the first 20 cycles after the beginning of the transient event, the wall surface temperature amplitude was almost three times higher than the one observed at the “normal” temperature oscillations occurring during the steady-state operation. At the same time, peak pressure values in the same cycles are increased by almost 15% above their corresponding values at the final steady state. The same phenomena are valid for the exhaust manifold surfaces but on a moderated scale. The analysis of long term temperature and heat flux responses during the structural phase, revealed characteristic differences existing in the mechanism of heat transfer among the different positions of the engine combustion chamber, confirming theoretical model predictions reported by the present authors in the past.