Biodiesel remains an alternative fuel of interest for use in diesel engines. A common characteristic of biodiesel, relative to petroleum diesel, is a lowered heating value (or per mass energy content of the fuel). For same torque engine comparisons, the lower heating value translates into a higher brake specific fuel consumption (amount of fuel consumed per unit of power produced). The efficiency at which fuel energy converts into work energy, however, may remain unchanged. In this experimental study, evaluating nine unique engine operating conditions, the brake fuel conversion efficiency (an assessor of fuel energy to work energy efficiency) remains unchanged between 100% petroleum diesel fuel and 100% biodiesel fuel (palm olein) at all conditions, except for high load conditions. Several parameters may affect the brake fuel conversion efficiency, including heat loss, mixture properties, pumping work, friction, combustion efficiency, and combustion timing. This article describes a study that evaluates how the aforementioned parameters may change with the use of biodiesel and petroleum diesel, and how these parameters may result in differences in the brake fuel conversion efficiency.