Vegetable oils, commonly used as a feedstock for the production of biodiesel, are also combusted directly as a diesel engine fuel in their neat, non-esterified form. A review of literature and experimental research conducted by this group and presented here shows that the combustion of vegetable oils, generally favorable at moderate and high loads, is problematic mostly at low engine loads and temperatures, characterized by a large increase in particulate matter emissions, penetration of the fuel into the engine lubricating oil, and deposition of liquid fuel within the engine and the exhaust system. Increase of the overall engine load, avoidance of low-load regions, and control of engine operating point, all typical for a hybrid-electric system, could resolve many of the drawbacks of neat, non-esterified vegetable oils, which can be a local, renewable fuel, with considerable benefits in terms of the very low "excess" CO₂ emissions.