Impact of Blending Gasoline with Isobutanol Compared to Ethanol on Efficiency, Performance and Emissions of a Recreational Marine 4-Stroke Engine

This study evaluates iso-butanol as a pathway to introduce higher levels of alternative fuels for recreational marine engine applications compared to ethanol. Butanol, a 4-carbon alcohol, has an energy density closer to gasoline than ethanol. Isobutanol at 16 vol% blend level in gasoline (iB16) exhibits energy content as well as oxygen content identical to E10. Tests with these two blends, as well as indolene as a reference fuel, were conducted on a Mercury 90 HP, 4-stroke outboard engine featuring computer controlled sequential multi-port Electronic Fuel Injection (EFI). The test matrix included full load curves as well as the 5-mode steady-state marine engine test cycle.

Analysis of the full load tests suggests that equal full load performance is achieved across the engine speed band regardless of fuel at a 15-20°C increase in exhaust gas temperatures for the alcohol blends compared to indolene. This increase as well as the observed 2.5-3% point improvement in brake thermal efficiency of both alcohol blends compared to the reference fuel are caused by changes in air/fuel ratio; an effect ultimately attributable to the open loop engine control strategy. This control strategy also explains the reduced CO as well as the increased HC+NO_x emissions of E10 and iB16 compared to indolene consistently observed across the 5 operating modes of the steady-state test cycle. The study also suggests that formaldehyde and acetaldehyde emissions increased with alcohol blend level.

With equivalent performance and emissions compared to E10 iB16 could be a viable option for increasing renewables utilization in recreational marine engines.