1985-10-01

Thermodynamic and Flow Analysis of an Indirect Injection Diesel Combustion Chamber by Modeling 851686

Two computer codes, zero and two-dimensional, were used to analyze thermodynamic efficiency and flow characteristics of a prechamber diesel engine. A thermodynamic model was developed and various experimental coefficients were calibrated by measurements performed on an actual automotive engine. Heat release in the prechamber and the main chamber was thus computed. This was found to improve as engine speed increases and to be highly sensitive to injection timing. The poor efficiency of an IDI diesel compared to that of a DI diesel can be explained especially by the high heat-transfer level in the prechamber. Passage loss at the transfer orifice is comparatively less and almost independent of load. Moreover, combustion timing, which is necessarily fairly late in the cycle, has only a small effect on overall losses. Under such conditions, assuming that the technological obstacle can be overcome and that the same combustion quality can be maintained, thermal insulation of the prechamber walls has the avantage of improving specific fuel consumption with almost no loss in filling efficiency.
However, since an increase in wall temperature would affect combustion, a two-dimensional code was used to analyse the air/fuel mixture formation process in the prechamber.
An unsteady swirling structure was revealed with a lifetime about five times greater during the compression phase than during expansion. These findings are supported by experimental published results. Injection simulation showed that the gaseous jet was entrained faster by the flow and that a fuel-rich zone was formed at the impingement point of the liquid spray. Thermal insulation of the prechamber walls was simulated by an appreciable increase in wall temperatures. Computing showed great heterogeneity of gas temperature characterized by the presence of high spatial temperature gradients in the swirl zone. However, despite the very great increase in mean gas temperature, there was almost no change in the aerodynamic structure and rotating velocity of the swirl.

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