In order to meet recent stringent emission regulations, the exhaust catalyst should be heated as early as possible to activate the purifying reactions. In a direct injection spark-ignition engine, a combination of late fuel injection during the compression stroke and late ignition in the expansion stroke is a common strategy to quickly raise exhaust gas temperature for subsequent rapid activation of exhaust catalysts. However, this approach under cold start-up of an engine often results in incomplete and unstable combustion. In this study, to explore the conditions of stable ignition and combustion, the effect of injection timing on indicated mean effective pressure and early combustion duration (MBD0.5) are first investigated by an analysis of the pressure indicator diagram. As this analysis shows a strong correlation between indicated mean effective pressure and MBD0.5, the mechanism of initial flame propagation is investigated intensively using optical diagnostics. Namely, mean equivalence ratio of mixtures in the propagating flame front is measured by focusing on the ratio of C 2 * to CH* emission intensities. The flow velocity and turbulence intensity around the spark electrode are measured by the back-scattering laser Doppler anemometry. Two major conclusions are derived from this study: First, when the injection timing is retarded, the mean equivalence ratio increases as the time for the injected fuel to travel and diffuse is shortened. The most preferable mean equivalence ratio for fast initial combustion is found to lie in a range from 1.2 to 1.4. Second, when the second injection timing is retarded further, the mean equivalence ratio increases exceeding 1.4, and this results in slower and more fluctuated initial flame propagation. But, if the turbulent intensity is increased by means of the spray induced air motion, the slowed initial combustion can be recovered.

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.

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