The effects of spark ignition parameters on the lean burn limit of natural gas combustion in an internal combustion engine

Abstract

A full factorial experiment was conducted to determine the effects of internal combustion engine ignition parameters on the air-fuel ratio (A/F) lean limit of combustion with compressed natural gas (CNG). Spark electrical characteristics (voltage, current, power, energy and duration), electrode design, electrode gap and compression ratio were the control variables and A/F lean limit, fuel consumption and hydrocarbon and oxides of nitrogen emission concentrations were the response variables. Experiments were performed on a General Motors' 2.2 liter four cylinder engine. Spark electrical characteristics were varied by applying various primary voltages and secondary resistances to the production inductive ignition system, with the engine operating at two operating conditions, a light load and a road load, and with two compression ratios. Cylinder pressure data was acquired to quantify load and combustion stability. Spark electrical characteristics were acquired with a digital oscilloscope to quantify secondary spark electrical characteristics. The results indicated that the response variables were generally insensitive to all the control variables, except for compression ratio. However, contrary to the literature, the A/F lean limit and fuel efficiency degraded with a higher compression ratio. Single and multi-variant linear regressions were studied between the A/F lean limit and the spark electrical characteristics. The only statistically significant and notable finding was a multi-variant linear regression of the A/F lean limit to increasing spark duration and decreasing spark energy at the road load operating condition. Statistical significance of the effect of the ignition system control variables on the response variables was higher at the road load than the light load operating condition. Emissions responded as expected with the higher compression ratio.