Effects of transient diesel engine operation on its cyclic heat transfer: an experimental assessment

This paper presents results from the analysis of experimental data with the aim of investigating the cyclic, instantaneous heat transfer phenomena occurring in both the cylinder head and exhaust manifold wall surfaces of a direct injection, air-cooled diesel engine, operating under transient events (long-term), viz. during a sudden change in engine speed and/or load, rather than the usual steady state. The experimental installation allowed both for long- and short-term signal types to be recorded on a common time reference base during the transient event. Processing of experimental data was accomplished using a modified version of one-dimensional heat conduction theory with Fourier analysis, capable of catering for the special characteristics of transient engine operation. Based on this model, the evolution of local surface heat flux during a transient event was calculated. Two engine transient events are examined, which present a key difference in the way the load and speed changes are imposed on each one of them. It is revealed that in the case of a 'ramp' transient the amplitude of wall surface temperature and heat flux variation in the combustion chamber during the first 20 engine cycles was increased by almost three times more than the corresponding ones during steady state operation. At the same time, temporal gradients of cylinder pressure and wall surface temperature are also sharply increased in the combustion chamber and exhaust manifold surfaces. The variation of cylinder pressure, surface temperature, and heat flux is altered significantly during the transient event, with their peak values either advanced or retarded compared with those for the corresponding steady state operation. The previous observations are moderated or even vanished in the case of a 'slow pace' transient variation, revealing the significant influence of engine speed and load triggered by the external command, on the response of heat transfer variables. During a ramp transient, the amplitude of wall temperature swing is increased significantly even at a depth of a few millimetres below the surface of the combustion chamber, thereby influencing the dissipation of heat towards its external layers. Under these circumstances, the depth of penetration for temperature oscillations inside the metal volume is also significantly increased.