The Chevy Impala came our way for Wards 10 Best Engines testing recently, but it wasn’t a conventional gasoline-driven engine.

Instead, the bi-fuel 3.6L DOHC V-6 runs both on gasoline and compressed natural gas, an energy source in the U.S. that is expected to last about 100 years.

The CNG tank is 7.8 gallons (30 L) and is located behind the rear seat and takes up some of the trunk. The Impala runs primarily on CNG, with a range of about 150 miles (241 km) with a full tank. A button on the dashboard also lets the driver pick between the two modes.

Once depleted of CNG, the vehicle switches automatically to gasoline, which will get you another 350 miles (563 km) in range. The transition is indiscernible. To accommodate both fuels, this V-6 has hardened valves and valve seats for better wear resistance and durability.

How is it to drive? It’s very smooth, quiet and capable. The 3.6L makes 260 hp and 247 lb.-ft. (335 Nm) of torque in gasoline mode, but switching to CNG drops output to 230 hp and 218 lb.-ft. (296 Nm).

During our evaluations, the gasoline engine was achieving a respectable 22.6 mpg (10.4 L/100 km), while the mileage for CNG went as high as 16 mpg (14.7 L/100 km) of gas equivalent, according to the trip computer.

The Impala Bi-Fuel is not really intended for most consumers, but instead for companies with fleets that run reasonably predictable routes from day to day.

Still, if you have a ready supply of CNG, your local Chevy dealer might be able to track you down an Impala Bi-Fuel. Cost is a hurdle, however. A non-CNG V-6 Impala LT bases at $30,895. The bi-fuel version of that same car will cost an extra $9,500.

Be sure to visit on Dec. 10 to see our picks for the 2016 Wards 10 Best Engines.