Biodiesel continues to be an alternative fuel of interest for use in diesel engines. Although biodiesel's properties are similar to petroleum diesel's, certain differences create possibilities for parameters, such as efficiency, to be different. The objective of the study is to identify the parameters that influence gross indicated fuel conversion efficiency and how they are affected by the use of biodiesel relative to petroleum diesel, in a production-ready diesel engine. This study of nine operating conditions observes mostly insignificant differences in efficiency among the three fuels. This seems to result from a tradeoff between biodiesel's inherently shorter combustion durations, which phases combustion better for higher efficiency, and its inherently lower air-fuel ratios, which causes a relatively lower ratio of specific heats resulting in lower efficiency. Both features seem to be linked to the fuel-bound oxygen of biodiesel. Because of the changing fractions of premixed and diffusion combustion and turbocharger performance at different engine speeds and loads, the influences of the two competing factors vary with operating condition. Significant differences in efficiency are mostly manifested by system response issues, where the engine controller alters due to a response of an interpreted higher load with the use of the lower energy-density biodiesel. (c) 2012 Elsevier Ltd. All rights reserved.