The wider flammability limit of lean natural gas-air mixtures offers potential for operating spark ignition engines on lean air-to-fuel ratios. However, at very lean equivalence ratios, the development of the initial flame and its subsequent propagation becomes highly sensitive to physical and chemical state of the mixture. This in turn, can adversely affect engine performance, particularly the cyclic variation in the combustion process.
This paper discusses the effects of lean-burn operation on the flame development durations and the cycle-by-cycle variations in a natural gas fuel injected engine. The study was conducted on a 8-cylinder, 4.6 liter, spark-ignited engine. A data acquisition system is used to acquire 300 consecutive in-cylinder pressure cycles. A heat release model was used to estimate the initial flame development time and the rapid burn duration. The engine was tested over a wide range of loads, speeds, ignition timings and fuel equivalence ratios ranging from stoichiometric to 0.6 (lean).
It was found that changing the mixture equivalence ratio from stoichiometric to 0.6 increased the initial flame development (kernel growth) time and the rapid burn duration by about 33% at fixed spark timing. However, at MBT timing the kernel growth time increased by almost 60% when the equivalence ratio was changed from stoichiometric to 0.6. The coefficient of variation in the IMEP, as a measure for the cyclic variation, was less than 5% at 0.6 fuel equivalence ratio which is an indication of the good stability of the combustion process at such lean conditions. The results of the study have been used to develop a relationship between the kernel growth time and the mixture conditions at spark discharge. The validity of the correlation was checked against limited data currently available on flame development period in natural gas engines.