Performance and emissions data for a four-cylinder diesel engine operating with diesel fuel, vegetable oil and emulsions of these with 10 percent water are compared with data for a laboratory single-cylinder engine. Neither the lower Cetane Number of the vegetable oil nor its emulsification with water presented obstacles to operation of the commercial engine at the higher operating speed. For this engine ignition delays were similar for all test fuels. and operation was leaner, thermal efficiency higher and NOx and smoke levels lower, for vegetable oil. These emission levels were further reduced for this fuel by emulsification.