US EPA

8537743 

Journal Article 

The influence of air-fuel ratio, spark timing and combustion chamber deposits on exhaust hydrocarbon emissions 

Jackson, MW; Wiese, WM; Wentworth, JT 

1962 

1 

SAE Technical Papers
ISSN: 0148-7191 

SAE International 

Detroit, MI 

English 

10.4271/620153 

1. Air-fuel ratio was found to have a significant effect on exhaust hydrocarbon emission. It was determined that operation at air-fuel ratios between 16 and 18 produced minimum exhaust hydrocarbon emission as well as best fuel economy. Leaning the air-fuel ratio from 14 to the air-fuel ratio for minimum hydrocarbon emission in a single-cylinder engine resulted in decreases in emission of 25 to 50%. Leaning air-fuel ratios by only 1 to 9% in several current production passenger cars reduced the California composite concentrations by approximately 10%. Since these cars were not operating at the air-fuel ratio for minimum hydrocarbon content, further reduction would be expected if still leaner air-fuel ratios were utilized. However, the problem of vehicle "surge," associated with leaner air-fuel ratios, may present a practical limitation to reductions that otherwise might be obtainable. 2. Retarding the spark timing was generally found to reduce exhaust hydrocarbon emission in both a single-cylinder engine and in passenger cars. In the single-cylinder engine tests, retarding the spark 10° from the optimum economy value resulted in reductions in hydrocarbon emission of 7 to 31%. Similar reductions were observed in three passenger cars. Reductions achieved by this method, however, will be accompanied by losses in fuel economy. 3. The effects of leaning the air-fuel ratio and retarding the spark timing were additive when used in combination and consequently produced large reductions in exhaust hydrocarbon content. Potential reductions in any given car are, obviously, limited by the initial settings of air-fuel ratio and spark timing. Hydrocarbon reductions obtained by operation at leaner airfuel ratios are accompanied by improved fuel economy, whereas reductions obtained by retarded spark timing are accompanied by reduced fuel economy. These fuel economy effects tend to offset each other when both methods of hydrocarbon reduction are used. 4. The accumulation of combustion chamber deposits in both a single-cylinder engine and in a passenger car caused a significant increase in exhaust hydrocarbon content. Therefore, it appears that engine deposit condition is an important factor which cannot be neglected in making long-term comparative tests or in establishing absolute levels of exhaust hydrocarbon emission from engines or vehicles. 5. The hydrocarbon emission measured by means of a nondispersive infrared analyzer sensitized to n-hexane was always less than the total emission measured with a flame-ionization analyzer. The ratios of the values obtained with the flame-ionization analyzer to those obtained with the infrared analyzer ranged from 1.5 to 5.5. Variations in air-fuel ratio had little effect on this ratio but retarding the spark timing caused it to increase markedly. This suggests that spark timing has a significant influence upon the type of hydrocarbons in exhaust.