What term describes an engine that can run on both diesel and natural gas?

An engine designed to run on both diesel and natural gas is called a dual-fuel engine. In the usual diesel–natural-gas arrangement, the two fuels are used together during operation.

Natural gas does not readily self-ignite under the conditions used by a conventional diesel engine, so a small quantity of diesel is injected as a pilot fuel. That diesel ignites in the hot, compressed air and provides the ignition source for the natural-gas–air mixture. The engine therefore retains compression ignition while substituting much of the diesel’s energy with gas.

The exact diesel-to-gas ratio depends on engine speed, load, injection equipment, and the conversion or factory system. High-speed engines may replace a substantial portion of diesel with natural gas, while low-load operation often requires more diesel to maintain stable combustion.

“Bi-fuel” is often confused with “dual-fuel.” A bi-fuel vehicle generally carries two fuels but uses one or the other, such as gasoline and CNG. A dual-fuel engine burns two fuels in combination. “Flex-fuel” usually refers to an engine that can use varying blends of a fuel, while a hybrid engine combines power sources rather than fuel types.