Correlation between spark plug electrode gap and engine performance-emission characteristics in a single-cylinder petrol engine

In this study, we conducted an experimental investigation into how five different sparks plug ground electrode gap settings (0.5 mm, 0.75 mm, 1.0 mm, 1.25 mm, and 1.5 mm) affect the performance and exhaust emissions of a single-cylinder petrol engine. The experiments were conducted on a single-cylinder, four-stroke spark-ignition engine operated at constant speed and throttle, with a stoichiometric air–fuel mixture, and instrumented for in-cylinder pressure and full exhaust emission analysis. Key performance metrics including brake mean effective pressure (BMEP), brake specific fuel consumption (BSFC), and exhaust gas temperature (EGT) were assessed, along with exhaust emissions of carbon monoxide (CO), unburned hydrocarbons (HC), and nitrogen oxides (NOx). The engine was operated at a constant speed and throttled with stoichiometric mixture to isolate the influence of spark gap. Among the tested configurations, the 1.0 mm spark gap delivered the best performance, achieving a peak brake mean effective pressure (BMEP) of 7.2 bar and the lowest BSFC of ~300 g/kWh. Emissions of CO and HC followed a U-shaped trend, minimizing at the 1.0 mm gap (CO: 0.48%, HC: 300 ppm), while NOx peaked at this same setting (~2000 ppm) due to elevated flame temperatures. Wider gaps (1.5 mm) induced partial misfires, resulting in increased CO and HC emissions and a 17% drop in BMEP. The results confirm that spark gap size strongly influences combustion quality, and the optimal range of 0.9–1.0 mm offers a practical trade-off between efficiency and emissions. Smaller or larger gaps caused deteriorated performance: a narrow 0.5 mm gap produced weaker ignition leading to slower combustion, while an overly wide 1.5 mm gap caused partial misfires. Consequently, CO and HC emissions followed a U-shaped trend, minimizing at the ~1.0 mm gap and rising at the extreme small and large gaps because of incomplete combustion at those conditions. In contrast, NOx emissions were the lowest at the smallest and largest gaps and peaked at the mid-gap, inversely tracking the combustion efficiency and peak temperature trends. It was concluded that a larger spark gap improves the initial flame kernel and combustion stability up to a point, beyond which ignition becomes erratic. The optimal spark plug gap ~0.9–1.0 mm achieved the best trade-off between complete combustion (low CO/HC) and high thermal efficiency/BMEP, at the cost of increased NOx because of higher combustion temperatures.