Effect of Port Gas Injection on the Combustion Instabilities in a Spark-Ignition Lean-Burn Natural Gas Engine

The combustion instabilities in a lean-burn natural gas engine have been studied. Using statistical analysis, phase-space reconstruction, and wavelet transforms, the effect of port gas injection on the dynamics of the indicated mean effective pressure (IMEP) fluctuations have been examined at a speed of 800 rpm and engine load rates of 25% and 50%. The excessive air coefficient is 1.6 for each engine load, and the port gas injection timing (PGIT) ranges from 1 to 120 degrees of crankshaft angle (degrees CA) after top dead center (ATDC) of the intake process. The results show that the PGIT has a significant effect on cyclic combustion fluctuations and the dynamics of the combustion system for all studied engine loads. An unreasonable PGIT leads to increased combustion fluctuations, and loosened and bifurcated structures of combustion system attractors. Furthermore, for both low and medium engine loads, the IMEP time series at earlier gas injections (PGIT = 1 degrees CA and 30 degrees CA ATDC) undergoes low-frequency fluctuation together with high-frequency fluctuations in an intermittent fashion. For other PGITs, high-frequency intermittent fluctuations become persistent combined with weak low-frequency oscillations. Our results can be used to understand the oscillation characteristics and the complex dynamics of combustion system in a lean-burn natural gas engine. In addition, they may also be beneficial to the development of more sophisticated engine control strategies.