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. Key to the Spindt Method comparisons were the series of emissions tests providing total hydrocarbons, oxygen, carbon monoxide, and nitrogen oxides data from a series of small utility engines over a 0%vol to 50%vol range of oxygenate addition. The oxygenates ethanol and MTBE were used in this study. The seven popular (high volume sales) engines emissions tested were comprised of four 3.7 kW (5 hp) engines (two side valve and two overhead valve) and three 9.3 kW (12.5 hp) engines (two side valve and one overhead valve).
The emissions were measured by a 5-gas analyzer reporting total hydrocarbons, carbon monoxide, nitrogen oxides, oxygen, and carbon dioxide. EPA small engine test procedures, of which the Society of Automotive Engineers J1088 testing specification is a subset, were followed. The fuel flow method option was used, as is the practice of small engine manufacturers in the United States, requiring the use of the Spindt Method to determine equivalence ratios.