Hydrogen effects on the combustion stability, performance and emissions of a turbo gasoline direct injection engine in various air/fuel ratios

Lean combustion has been identified as a way of improving the thermal efficiency of gasoline engines. However, it is difficult to operate gasoline engines in lean conditions, because the stability of the combustion deteriorates. Hydrogen has properties that can be exploited to improve the combustion stability, and hence solve the combustion instability problem. In this paper, we describe an experimental study into the effects of hydrogen on the combustion stability, performance, and emissions at various air/fuel ratios using a four-cylinder 2.0-L turbo gasoline direct-ignition engine. The engine speed was fixed at 2000 RPM with a brake mean effective pressure of 2-18 bar. The results of these experiments indicate that combustion is improved, thermal efficiency enhanced, and emissions reduced except for NOX. Especially, in terms of combustion stability, the cycle-to-cycle variation and cylinder-to-cylinder variation decreased when hydrogen was added. This was caused by two effects. First, when the brake mean effective pressure was 10 bar or below, the burn duration was reduced by the fast flame speed, high adiabatic temperature, and high diffusion coefficient of hydrogen. This led to an increase in the combustion stability and thermal efficiency. Second, when the brake mean effective pressure was above 10 bar, knock resistance was enhanced by hydrogen: spark timing could be advanced upon hydrogen addition. Therefore, the combustion stability and performance improved. Additionally, the improvements to the combustion, especially in terms of combustion stability, became more pronounced as the in-cylinder air-fuel mixture became leaner: the lean limit was thus extended.