Analysis and evaluation of the thermal shock phenomena in the in-cylinder surfaces of a DI diesel engine during its transient operation

Abstract

This paper presents the results from the analysis of an experimental investigation with the aim of providing an insight into the cyclic thermal shock phenomena occurring in the internal cylinder wall surfaces of a direct injection (DI), air-cooled diesel engine during the initial stage of a transient operation. The mechanism of cyclic heat transfer is investigated during engine transient events, viz. after a sudden change in engine speed and/or load. The experimental installation allowed both 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. During the analysis of experimental results the most important parameters characterizing thermal shock, such as the heat wave velocity and length of penetration, are quantified for each event, providing a comprehensive insight into the causes and consequences of this dangerous phenomenon. The results, in addition, confirm the theoretical predictions for the development of the thermal field during an engine transient event, as presented by the authors in previous work. Each thermal transient event is characterized by two distinct phases, that is the 'thermodynamic' and the 'structural' one, which are appropriately configured and analysed. From the results it is revealed that in the case of a severe variation, in the first 20 cycles after the beginning of the transient event, the wall surface temperature and heat flux amplitude on the cylinder head was almost three times higher than that observed in the 'normal' temperature oscillations occurring during steady-state operation.