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Effects of changing ambient humidity and temperature have been studied on the performance and emissions of a hand-held two-stroke and a hand-held four-stroke engine. The main effect of changes in ambient conditions is to change the intake air density and therefore the air-fuel ratio metered by the carburetor. Trends in the effects of humidity and temperature on emissions are predicted reasonably well by theoretical thermodynamic models. They suggest an improved correction for the dependence of NOx on ambient conditions, as a function of both humidity and operational air-fuel ratio, which appears to collapse NOx production data better than the existing KH correction factor. They also suggest a simple procedure for tuning engines to design air-fuel ratios using the measured exhaust-gas %CO, which takes into account the prevailing ambient conditions.

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.