Experimental Heat Release Rate Analysis in Both Chambers of an Indirect Injection Turbocharged Diesel Engine at Various Load and Speed Conditions

Abstract

A heat release analysis of experimental pressure diagrams, appropriate for indirect injection (divided chamber) diesel engines, is developed and used to obtain heat release rate profiles during the combustion process in each combustion chamber. Attention is paid to the correct processing of the data, due to the inherent complexity of the mass interchange between the two combustion chambers. The analysis concerns a turbocharged, indirect injection diesel engine, having a very small pre-chamber and a very narrow connecting passageway, operated at various load and speed conditions, located at the authors' laboratory. An extended experimental work, at steady-state conditions, is conducted on a specially developed test bed configuration of this engine, which is connected to a high-speed data acquisition and processing system. The experimental indicator diagrams from both chambers are processed in connection with the pertinent application of the energy and state equations, using relevant relations for the gas heat loss to the walls and mass exchange between the chambers. The analysis results for the heat release rates in the two separate chambers reveal some very interesting features, which aid the correct interpretation of the combustion mechanisms associated with this type of engine, over a wide spectrum of load and speed conditions. Thus, they can provide a sound basis for the construction of a combustion model that would be very useful, especially during the difficult case of transient engine operation. The analysis reveals the following features. The heat release in the pre-chamber begins suddenly and lasts relatively little, while just before 'hot' TDC (when the gas flow reverses direction) the heat release continues in the main chamber beginning as suddenly and extending relatively deep into the expansion phase. The magnitude of the heat release rate in the pre-chamber is nearly independent of load, provided this is above a certain value corresponding, roughly, to overall stoichiometric conditions there. At lower loads only one peak of premixed combustion type appears, whereas at the higher ones a second smaller peak also follows of rather diffusion type. On the contrary, the heat release in the main chamber increases with load, extending progressively deeper into the expansion phase with basically one peak, with the exception of the higher speeds where a small preceding peak makes its appearance. The heat release rate diagrams, at various speeds, show a significant improvement of the heat release rate pattern ('faster') with speed. Specifically, the heat release rate in the pre-chamber increases slightly, while at the same time the heat release rate in the main chamber is 'compressed' in duration, with its area center of gravity moving 'forwards' with speed.