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. The RBOB+10% EtOH fuel blend is similar to one of the fuel blends currently used in Ozone Non-attainment Areas (a slightly higher oxygen content) while the higher ethanol fuel blends in this study provide insight into the behavior of air emissions as the ethanol concentration is increased beyond current levels of oxygenate addition mandates. The fuel blends ranged from RBOB+0% EtOH to RBOB+50% EtOH.The emissions were measured by a 5-gas analyzer which measures total hydrocarbons, carbon monoxide, nitrogen oxides, oxygen, and carbon dioxide. EPA small engine test procedures were followed. The fuel flow method option was chosen as is the practice of small engine manufactures. A method to deduce the air-fuel ratio from the fuel characteristics (carbon fraction and hydrogen fraction) and emissions was introduced by R. S. Spindt for conventional (nonoxygenated) fuels in 1965 . This study uses a modified Spindt Method to accommodate highly oxygenated fuels developed in a related study for use with this engine and fuel set . The Spindt method technique allows estimation of the actual (operating) air-fuel ratio from exhaust constituents. With the actual air-fuel ratio, the data may be assessed in terms of the equivalence ratio. By using the equivalence ratio, emissions from engines operating on varying oxygenate content fuels may be directly compared since the varying stoichiometric air-fuel ratios are stabilized.The primary results indicate that increasing the concentration of ethanol in gasoline is effective in reducing regulated carbon monoxide emissions from small engines designed to operate on nonoxygenated fuels but ineffective in reducing regulated combined hydrocarbon and nitrogen oxide emissions. This result has implications on one small engine emissions reduction strategy - blending a specialized “lawn and garden” highly oxygenated fuel for delivery through alternative distribution outlets.The secondary results indicate evidence that the overhead valve technology engine exhibits lower hydrocarbon and carbon monoxide emissions characteristics but has no discernible effect on nitrogen oxides and negligible effect on combined hydrocarbon and nitrogen oxides emissions. This result has implications on another small engine emissions reduction strategy - the shift from side-valve engines to overhead-valve engines in small engine applications.