Performance and emission testing of a bi-fuel outboard spark-ignition engine

With the increasing concern about the future availability of gasoline and continuously stringent engine emission regulatory standards, the need for an alternative to gasoline as a primary engine fuel is becoming increasingly important. One proposed alternative is to use natural gas due to its availability, low cost, and generally reduced engine emissions. However, due to volumetric efficiency losses, the conversion of a standard spark-ignition engine originally operating on gasoline to run on natural gas has been shown to produce considerable power reductions. Additionally, improperly tuned natural gas conversion kits could potentially increase certain emissions’ quantities compared to gasoline. This work reports the results of an experiment designed to characterize the performance and emissions of a commonly used gasoline outboard boat motor converted to run on natural gas. Reported are the comparisons between gasoline and natural gas for the widely used engine operating parameters for gauging engine performance related to power output and harmful emissions, including oxides of nitrogen, unburned hydrocarbons, and carbon monoxide, for comparison of the two test fuels. The results indicate that operation on natural gas greatly reduces harmful hydrocarbon and carbon monoxide emissions at the cost of reduced engine power output and potentially increased oxides of nitrogen emissions.