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In the design of small off-highway and utility engines for compliance with increasingly stringent emissions standards, one component which can potentially reduce engine exhaust-gas emissions without necessitating changes in other, more costly parts is the spark plug. From studies carried out in automobile engines, benefits have been reported when using different spark-plug electrode shapes or when aligning the plugs in the cylinder head in preferred directions. However, these benefits, observed in automotive overhead valve engines with well-mixed charges, have generally been modest? and spark plugs of conventional shape remain the most widely used today. In the case of off-highway and utility engines, which operate at substantially higher air-fuel ratios, often with poorly-mixed charges, the potential for improving performance by changing spark-plug shape has not been explored.

The exhaust-gas emissions of a small four-stroke, carbureted, single-cylinder spark-ignition engine have been studied as functions of ambient conditions, using gasoline as the fuel. In steady-state dynamometer tests at fixed engine speeds/loads, carried out under different climatic conditions, the concentrations of exhaust-gas components have been measured. Their dependence on ambient conditions has been analyzed principally in terms of the influence of ambient temperature, pressure, and humidity on the air-fuel ratio metered by the carburetor. While the air-fuel ratio of carbureted utility engines at fixed loads varies by only a small percentage during modest changes in ambient air conditions, these changes can correspond to significant changes in the production of regulated pollutants. Using a correction for air mass flow and fuel density at wide open throttle, the scatter in observed air-fuel ratio and % CO data could be reduced by about one third.