Engines that can operate using a mixture of two different fuels are called dual fuel engines. Frequently, diesel and natural gas fuels are used together within dual fuel engines. Often, dual fuel engines that mix diesel and natural gas can also operate using diesel fuel only if the natural gas is temporarily unavailable. 

Beyond natural gas and diesel, some dual fuel engines can also use varying mixtures of biodiesel, landfill gas, bio-gas and other fuels. 

Are all dual fuel engines the same? 

While the working principles of dual fuel engines are the same, those that operate dual fuel engines experience remarkable differences in total cost of ownership (TCO) and uptime. Things like natural gas substitution rate, quality of the natural gas, emissions produced, and equipment reliability can all effect operational efficiency.

Substitution rate is a key word associated with dual fuel engines. Substitution rate is the portion of fuel energy provided by natural gas. When comparing dual fuel engines, there are two important considerations regarding substitution rates:

1. Load factor: It is important to compare substitution rates of different engines at the same load factor, which is where your engines usually operate. Strictly comparing ‘maximum’ substitution rates of different engines could mislead you, and prevent you from maximizing the benefits of dual fuel engines.
2. Diesel fuel consumption: Consider evaluating the diesel fuel consumption rates of the engines while comparing substitution rates. If an engine delivers better diesel fuel economy, then that engine starts the substitution rate comparison with an important advantage.