5 Things You Didn’t Know About Bi-Fuel and Dual-Fuel

1. What Bi-Fuel Is

A bi-fuel system uses two types of fuel, but the fuels are not mixed together during operation. The engine is capable of switching between the two so that it is always using the type of fuel most efficient for the conditions at hand. Some of these engines are automatically controlled by a governor, which switches between the two types of fuel when that specific fuel is the most efficient choice.

Other bi-fuel engines allow the operator of the machine to switch between the two fuels at will. These engines are capable of using either fuel exclusively in the absence of the other fuel source, but will operate less efficiently when the right fuel isn’t available.

Some bi-fuel generators use LP vapor (propane vapor) and natural gas as the two fuel sources. Others switch between operating on natural gas (compressed or liquid) and gasoline, or natural gas and diesel. Certain fuels are best for certain applications–bi-fuel cars may be designed to run on regular gas and natural gas, while a bi-fuel engine used in industry uses diesel and LP.


2. What Dual-Fuel Is

A dual-fuel system is capable of using two types of fuel at the same time in a mixture. It usually cranks up on one type of fuel, and a governor built into the system gradually adds the secondary fuel source until the optimal mixture of the two fuels is achieved for efficient running.

Like the bi-fuel system, a dual-fuel engine is usually capable of operating on just one of the fuel sources in the absence of the other. However, in many dual-fuel engines, a specific fuel is required to start the engine.

For example, a dual-fuel generator starts by using diesel fuel and gradually adding a mixture of natural gas. The diesel ignites at a mere 500-700 degrees Fahrenheit.

However, the natural gas will not ignite until temperatures reach 1,150-1,200 degrees Fahrenheit. So, once the engine starts it can run on natural gas only or diesel only. But natural gas can’t be used to start the engine because the diesel fuel is necessary to bring temperatures up to the point at which natural gas will ignite.


3. How the Terms Get Confused

The confusion between the terms “bi-fuel” and “dual-fuel” stems from how the federal government uses them. The Department of Energy uses them the opposite way of most industries. The definitions used in this article are those used by the Department of Energy and may not apply to all consumer or industrial products in the same way.

But once you understand the concepts of bi-fuel and dual-fuel systems, it’s quite easy to determine which is meant to refer to a specific system. Also, bi-fuel systems and dual-fuel systems are often used in different applications, which gives us another reference point.

Simply put, when you shop for an engine, find out whether its fuels are used in a mixture or separately. That’s the key. Then you’ll understand which type of engine you’re dealing with. Be sure to understand whether the engine can use either fuel to crank up, or if you need both fuels to start the engine. Also, shop for an engine that is specifically designed for your particular purposes.


4. What Bi-Fuel Is Useful For

Bi-fuel systems are useful in a variety of products including grills, residential heating systems, and ovens and ranges. But two applications are the most common:


Dual-mode Cars

Hybrid vehicles have been on the market for more than two decades, but it might surprise you to learn the typical gas-electric format is not the only type of hybrid car. Any car that uses at least two distinct types of power qualifies as a hybrid, which means bi-fuel engines in vehicles technically make them hybrids. Still, it may be easier to think of them as “dual-mode” or simply “bi-fuel vehicles.”

This is actually the most popular use of the bi-fuel system. These cars require two separate fuel tanks feeding one engine. For example, a hybrid vehicle that uses diesel + natural gas injects the accelerant-efficient diesel fuel into the engine until a steady speed is reached, then the engine transfers to the more efficient natural gas. Dual-mode cars are designed so that if one fuel source is exhausted, the engine can continue to operate on the fuel source that is still available.



Generators made with bi-fuel engines may be either consumer-grade or industrial-grade. Diesel has long been the favored fuel type for large commercial generators because it’s so reliable, yet many operators are interested in reducing their diesel usage due to today’s stricter emissions standards. To many, a diesel + natural gas generator offers the best of both worlds.


Benefits of Bi-Fuel Engines

  • Efficiency: The greatest advantage of a bi-fuel system is that the most efficient fuel for the task is always being used, so these systems are significantly more efficient to run overtime, compared to a single-fuel engine such as a gas-powered car. Fuel storage is also streamlined, allowing more time between diesel fill-ups or reducing the size of necessary storage tanks. 
  • Sustainability: Limiting the burning of diesel fuel, as a bi-fuel generator or other engine does, can dramatically lower the amount of particulate matter, CO2, and other emissions released into the atmosphere. They can also aid in flare gas recapture, making use of by-products that would otherwise be wasted.
  • Cost savings: The difference in fuel cost expenses can be upwards of 50% when compared to a single-fuel system. 
  • Longer runtime: During extended power outages, operators can keep critical applications running longer by not being so dependent on diesel fuel.


5. What Dual-Fuel Is Useful For

Most dual-fuel engines are used in industrial applications and are less common in consumer applications. As with bi-fuel engines, vehicles and generators are the most popular applications:


Flexible-fuel Vehicles

Dual-fuel vehicles or “flex-fuel” cars are especially popular in the trucking and busing industry, typically combining diesel and natural gas for a dependable, economical solution. Usually, the most efficient use of fuels within this type of engine is a mix of 75 percent natural gas to 25 percent diesel fuel. But either a computer system or fumigation system controls the ratio of natural gas to diesel depending on the load to ensure the engine is operating at peak efficiency.

Other dual-fuel cars, such as the E85 compatible vehicle, are equipped to use gasoline alone or a blend of gasoline and ethanol, methanol, or biobutanol. In the case of E85, the optimal fuel mixture is 85 percent ethanol to 15 percent gasoline. Real-world tests have found no differences in performance between cars running on 87 octane gasoline and E85 ethanol, while the latter is believed to keep fuel systems cleaner than gasoline, potentially making for lower long-term maintenance costs.



When it comes to non-transportation engines, besides drilling rigs, generators are the most common application. Although many portable generators are marketed to residential consumers as being “dual-fuel,” they’re in fact bi-fuel because the operator must choose whether to use propane or gasoline, for example. But industrial dual-fuel generators work with two fuels simultaneously for the most optimal performance, which can have a huge impact on the bottom line when scaled up in large operations.


Benefits of Dual-Fuel Engines

  • Cost savings: Dual-fuel systems cost significantly more than traditional engines to buy, usually carrying a price tag 15-30 percent higher. But since the engine is capable of running on a single fuel or a mix of fuels, it is less costly to run over time as it allows you to select whichever fuel is currently the cheapest.
  • Simple storage: Dual-fuel engines eliminate the need for large-scale on-site storage of expensive fuels, such as diesel. They also lower the maintenance costs of diesel storage.
  • Reliability: If the source of natural gas is temporarily cut, the engine can continue to operate on diesel alone, insulating it from any fluctuations in the power grid for as long as the diesel supply holds out.
  • Flexibility: A dual-fuel engine can allow you to make use of alternative fuels such as sewage gas or landfill gas yet pair it with a more traditional fuel. 
  • Sustainability: Like bi-fuel systems, dual-fuel models can lower harmful emissions by burning less diesel.

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To summarize: both systems use two fuels, but the bi-fuel system uses just one fuel at a time, whereas the dual-fuel system uses a mixture. So if there is ever a doubt about whether you’re getting a bi-fuel or dual-fuel product, as defined by the Department of Energy, simply find out whether the fuels are mixed during operation (dual-fuel) or if the system switches between one fuel and the other (bi-fuel).