Performance improvements in a longer-stroke,spark-ignition gas engine

The engine performance with several stroke/cylinder diameter (S/D) ratios from 1.0 to 2.0 was investigated in a small spark-ignition gas engine by experiments and with computational fluid dynamics (CFD) engine analysis. The experiments were conducted on a specially manufactured, single-cylinder, spark-ignition gas engine with the cylinder head of a commercial diesel engine with a spark plug instead of a fuel injector. The cylinder diameter was fixed at 70 mm, the same as the original engine, and the stroke length was varied at 70 mm (S/D = 1.0), 105 mm (S/D = 1.5), and 140 mm (S/D = 2.0) under the same compression ratio of 12:1 by changing the piston, the cylinder liner, the cylinder block, the crankshaft, and the connecting rod. The engine speed was set at 1200 r/min, and the test fuel was commercial compressed natural gas provided from a bottle. The indicated thermal efficiency improved significantly with S/D ratio, mainly due to the reductions in cooling loss, maintaining the degree of constant-volume heat release at high levels, and also due to improvements in the combustion efficiency. The optimum S/D ratio is about 1.5, as the improvement in indicated thermal efficiency with increase of S/D from 1.0 to 1.5 was more significant than that with S/D increase from 1.5 to 2.0. The indicated mean effective pressure (IMEP) with S/D = 1.5 was the largest among these S/D ratios, and the IMEP with S/D = 2.0 was much smaller due to lower volumetric efficiency. The indicated thermal efficiency improved with longer strokes in the stoichiometric and lean conditions of excess air ratios below 1.5, but the deterioration in thermal efficiency with leaner conditions, above 1.5, was more pronounced with longer strokes. The CFD analysis also showed the improvement in thermal efficiency with longer strokes, and the indicated thermal efficiency improved more in the case of the same cylinder diameter than in the case of the same displacement volume. This is mainly due to the larger reduction in cooling loss in the case with the same cylinder diameter. However, even with the same displacement volume, the reduction in cooling loss and the improvement in indicated thermal efficiency with longer strokes were reliably obtained. The experimental equations for the heat transfer coefficient with mean piston speed, including Woschni’s equation, do not express the change in heat transfer at the higher S/D ratios.