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Emissions of total hydrocarbons, carbon monoxide, nitrogen oxides, and combined hydrocarbons and nitrogen oxides from small utility engines were used to judge the effect of ethanol addition (zero to 50%vol) to a hydrocarbon fuel with factory air-fuel ratio carburetor settings. In this study, emissions from two 4-stroke 9.3 kW (12.5 hp) side-valve engines and one 4-stroke 9.3 kW (12.5 hp) overhead valve engine were assessed to support conclusions. Emissions differences due to the differing valve orientations were addressed as a secondary objective. A series of RBOB plus ethanol (EtOH) fuel blends were used in this parametric study. RBOB is the base fuel in which oxygenates are blended to produce reformulated gasoline (RFG) for use in Clean Air Act Amendments-designated Ozone Non-attainment Areas. RBOB+EtOH fuel blend (an oxygenated fuel) emissions were compared to RBOB (a nonoxygenated fuel) emissions to assess the effect of ethanol addition on emissions.

A method to deduce the operating air-fuel ratio from the fuel flow rate, fuel characteristics, and emissions was introduced by Spindt in 1965 for conventional (nonoxy-genated) fuels. This study expands the original method to encompass oxygenated fuels. The use of the expanded Spindt Method allows the equivalence ratio to be estimated more accurately at high oxygenated fuel blends. Two generalizations are developed and proposed. One of the methods is shown to provide a 8-10% improvement in equivalence ratio estimation at a 17%wt oxygen typically under the maximum SAE J1088 load condition. To evaluate the two proposed generalized Spindt Methods, a series of small engines and fuels were emissions tested to determine the utility of the generalized Spindt Methods for analysis of oxygenated fuels. Air-fuel ratio estimates from the proposed Spindt Methods were compared to the original Spindt Method to assess equivalence ratio estimation improvements.