Experimental Comparison of Heat Losses in Stepped-Bowl and Re-Entrant Combustion Chambers in a Light Duty Diesel Engine

Heat loss is one of the greatest energy losses in engines. More than half of the heat is lost to cooling media and exhaust losses, and they thus dominate the internal combustion engine energy balance. Complex processes affect heat loss to the cylinder walls, including gas motion, spray-wall interaction and turbulence levels. The aim of this work was to experimentally compare the heat transfer characteristics of a stepped-bowl piston geometry to a conventional re-entrant diesel bowl studied previously and here used as the baseline geometry. The stepped-bowl geometry features a low surface-to-volume ratio compared to the baseline bowl, which is considered beneficial for low heat losses. Speed, load, injection pressure, swirl level, EGR rate and air/fuel ratio (λ) were varied in a multi-cylinder light duty engine operated in conventional diesel combustion (CDC) mode. Temperature measurements in the engine cooling media were used to set up the engine energy balance and find out how much heat was lost to cooling media in different parts of the engine. Based on these calculations and heat release analysis, conclusions could be drawn regarding how heat losses in different parts of the engine were affected by changes in these parameters. Results were compared to previously published CFD simulations and it was concluded how the heat transfer characteristics differ between the two piston designs.