This study investigates the effects of engine bore size on diesel engine performance and combustion characteristics, including in-cylinder pressure, ignition delay, burn duration, and fuel conversion efficiency, using experiments between two diesel engines of different bore sizes. This study is part of a larger effort to discover how fuel property effects on combustion, engine efficiency, and emissions may change for differently sized engines. For this specific study, which is centered only on diagnosing the role of engine bore size on engine efficiency for a typical fuel, the engine and combustion characteristics are investigated at various injection timings between two differently sized engines. The two engines are nearly identical, except bore size, stroke length, and consequently displacement. Although most of this diagnosis is done with experimental results, a one-dimensional model is also used to calculate turbulence intensities with respect to geometric factors; these results help to explain observed differences in heat transfer characteristics of the two engines. The results are compared at the same brake mean effective pressure (BMEP) and show that engine bore size has a significant impact on the indicated efficiency. It is found that the larger bore engine has a higher indicated efficiency than the smaller displaced engine. Although the larger engine has higher turbulence intensities, longer burn durations, and higher exhaust temperature, the lower surface area to volume ratio and lower reaction temperature leads to lower heat losses to the cylinder walls. The difference in the heat loss to the cylinder walls between the two engines is found to increase with increasing engine load. In addition, due to the smaller volume-normalized friction loss, the larger sized engine also has higher mechanical efficiency. In the net, since the brake efficiency is a function of indicated efficiency and mechanical efficiency, the larger sized engine has higher brake efficiency with the difference in brake efficiency between the two engines increasing with increasing engine load. In the interest of efficiency, larger bore designs for a given displacement (i.e., shorter strokes or few number of cylinders) could be a means for future efficiency gains.

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