Strategies for improving the idle performance of a spark-ignited gasoline engine

Because of the increased residual gas fraction and low combustion temperature, the traditional spark-ignited (SI) gasoline engines tend to encounter the dropped thermal efficiency and increased emissions at idle. This paper experimentally investigated the effects of hydrogen addition, cylinder cutoff, lean combustion and idle speed reduction on improving the combustion and emissions performance of gasoline engine at idle. The tests were conducted on a modified 1.6 L SI engine equipped with a hydrogen port-injection system and a self-developed hybrid electronic control unit (HECU). In the experiments, cylinder cutoff was realized by stopping the hydrogen and gasoline injections to the specified cylinders. The HECU was adopted to control the opening of idle bypass valve and spark timing at the reduced idle speed of 600 rpm. The test results showed that hydrogen addition, lean burn, cylinder cutoff and idle speed reduction were all effective on decreasing the engine fuel consumption at idle. Under the given test conditions, the combination of cylinder cutoff and hydrogen addition was the most effective, which reduced the fuel consumption by 33.25% at a hydrogen volume fraction in the intake of 2.87% and two-cylinder cutoff mode, compared with that of the original engine. Moreover, engine cyclic variation, HC and CO emissions were also decreased after hydrogen addition and cylinder cutoff. Because of the dropped combustion temperature, NOx emissions were decreased at the lean combustion and decreased idle speed. However, due to the increased residual gas fraction and weakened charge flow, HC emissions from the engine operating at an idle speed of 600 rpm and a hydrogen addition fraction of 3.20% were slightly higher than those from the original gasoline engine at idle.