The Effect of Stroke-to-Bore Ratio on Combustion Performance of a Lean Burn Heavy-Duty Gaseous SI Engine

This paper presents a numerical investigation into the effect of stroke-to-bore (S/B) ratio on the combustion performance of a lean burn heavy-duty gaseous spark-ignited engine. The S/B ratio was varied from 0.94 to 1.32 by changing the stroke length for a fixed bore and connecting rod length. Identical cylinder head and valve events were used throughout the analysis. The compression ratio was kept unchanged by scaling the volume of the piston bowl while keeping the piston squish areas and head volume constant. FLUENT was used to conduct 3-dimensional transient flow analysis of the intake process. A Cummins modified version of KIVA-3V utilizing a G-Equation combustion model was employed to conduct simulation of the combustion process. The intake processes predicted by FLUENT were mapped to KIVA-3V at the time of intake valve closing. Both FLUENT and KIVA simulations were validated with experimental results at S/B ratio of 1.2. Simulations were also conducted at various engine speeds for different S/B ratios. When ignited at the same spark timing, the most important observations are: (1) The S/B ratio has a significant effect on turbulence intensity during the intake stroke and thus affects the in-cylinder peak pressure, heat release rates, power, and NOx emissions. (2) The gross indicated mean effective pressure and fuel specific NOx emission increase with increase of S/B ratio and decrease with increase of engine speed. (3) The gross indicated specific fuel consumption and burn duration (in crank angle) decrease with increasing S/B ratio and increase with increasing engine speed. (4) In general, a larger S/B ratio leads to higher thermal efficiency through faster combustion, and better fuel economy can be achieved for a long-stroke engine at low engine speeds. The primary influences with S/B ratio observed at constant spark timing are valid also when spark timing is modified to achieve the same centroid of heat release rates.