A bi-fuel vehicle has two separate fuel systems and can run on either one, but only one at a time. The most common setup pairs gasoline with compressed natural gas (CNG) or liquefied petroleum gas (LPG). A switch on the dashboard lets the driver choose which fuel to burn, and the vehicle can alternate between them freely. This gives you the range and convenience of gasoline plus the lower emissions and often lower cost of a gaseous fuel.
How a Bi-Fuel System Works
Both fuels are stored on board in separate tanks. Gasoline sits in a standard fuel tank, while CNG or LPG is stored in a pressurized cylinder, typically mounted in the trunk or under the vehicle bed. The engine itself is a conventional internal combustion engine, but it’s fitted with additional fuel lines, injectors, and a control module for the alternative fuel.
When you flip the fuel selector switch to the alternative fuel position, the system signals the engine’s computer to shut off the gasoline injectors and open the gas fuel pathway instead. If no signal is received, the system defaults to gasoline. In cold conditions, some bi-fuel systems start on gasoline automatically. LPG systems, for example, may adjust fuel delivery pressure based on engine coolant temperature, and a cold-start heater activates when temperatures drop below freezing to help the gaseous fuel vaporize properly. Once the engine warms up, you can switch over.
Bi-Fuel vs. Dual-Fuel
These two terms sound interchangeable, but they describe different engineering approaches. A bi-fuel vehicle runs on one fuel or the other, never both at the same time. The two fuel supply systems operate independently and sequentially. A dual-fuel vehicle, by contrast, can burn two fuels simultaneously, mixing them in a preset ratio. A common dual-fuel example is a diesel engine that blends diesel with natural gas during combustion. The distinction matters because the engine tuning, emissions profile, and driving experience differ between the two designs.
Common Fuel Pairings
Gasoline paired with CNG is the most widely available bi-fuel configuration. CNG is the same natural gas used in home heating, compressed to about 3,600 psi so enough fits in an onboard tank. Gasoline with LPG (propane) is the other major pairing, particularly popular in Europe and parts of Asia where LPG refueling infrastructure is well established. Both options use a spark-ignited gasoline engine as the base, which is why diesel engines rarely appear in bi-fuel setups. Diesel ignites through compression rather than a spark plug, making it harder to swap cleanly with a gaseous fuel.
Fuel Efficiency and Range
Running on CNG typically costs you a small amount of fuel economy compared to gasoline. Testing on a Chevrolet bi-fuel sedan found gasoline delivered about 21.5 MPG in city driving and 31.5 MPG on the highway. On CNG, the same vehicle averaged 17.3 MPG in the city and 29.8 MPG on the highway. The gap is larger in stop-and-go driving and nearly closes at highway speeds.
The bigger trade-off is range. CNG tanks hold less energy per volume than a gasoline tank, so the CNG-only range in that same test was 75 to 81 miles in the city and 113 to 143 miles on the highway. But that’s the point of bi-fuel: when the CNG runs out, you switch to gasoline and keep driving. Combined, the two tanks give you more total range than either fuel alone, which is especially useful in areas where CNG stations are sparse.
Emissions Benefits
Lower emissions are one of the main reasons people choose bi-fuel. When running on CNG instead of gasoline, nitrogen oxide emissions drop by roughly 60%, and particulate matter (the tiny soot particles linked to respiratory problems) drops by about 97%. LPG offers similar advantages: approximately 57% less nitrogen oxide and 88% less particulate matter compared to gasoline. Both fuels also produce less carbon monoxide. These reductions apply only when the vehicle is actually running on the alternative fuel, so your real-world benefit depends on how often you use CNG or LPG versus gasoline.
Extra Maintenance Requirements
A bi-fuel vehicle needs all the standard maintenance of a gasoline car, plus additional upkeep for the gas fuel system. The high-pressure fuel filter needs to be drained every 24 months and the filter element replaced on a schedule that varies by vehicle, typically every 24 months or at a set mileage interval. CNG tanks require a visual inspection by a certified inspector every 36 months or 36,000 miles, whichever comes first. Inspectors check for physical damage, corrosion, and deterioration of the tank material.
CNG tanks also have a hard expiration date. They must be replaced 15 years after their manufacture date, regardless of condition. This is a significant cost to factor in if you’re buying a used bi-fuel vehicle. Check the manufacture date stamped on the tank before purchasing.
Who Bi-Fuel Makes Sense For
Bi-fuel vehicles are most practical for fleets and commuters with predictable routes near CNG or LPG refueling stations. Municipal vehicles, delivery vans, and government fleets have been the largest adopters because they refuel at a central depot where a CNG compressor can be installed. For individual drivers, the math works best if you have access to affordable CNG or LPG and drive enough miles to offset the higher upfront cost of the bi-fuel system. The gasoline backup eliminates range anxiety entirely, which is the key advantage over vehicles that run exclusively on alternative fuels.