Some Effects of Spark Plug Electrode Geometry and Orientation on Small-Engine Emissions

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. In this paper, we discuss the results of a series of experiments in which the emissions of a two-stroke chainsaw engine, a four-stroke side-valve engine, and a four-stroke overhead-valve engine are measured using conventional spark plugs and spark plugs with crown-shaped electrodes. In general, the use of a crown-shaped electrode resulted in improvements in brake-specific fuel consumption and reductions in hydrocarbon emission of the order of 5% to 10%, at the cost of emitting slightly more oxides of nitrogen. These trends are consistent with theories of ignition and combustion in-somuch as the reported effects can be explained as a consequence of crown-shaped electrodes generating flame kernels which lead to faster burn rates, reduced heat losses and higher burned-gas temperatures.