Propane is the alternative fuel that’s added to a diesel or gasoline engine in a product from LP Energy Systems, a small firm in La Conner, Wash.

The device, called simply the Dual Fuel System, injects up to 20% propane vapor into the fuel stream to displace the more costly diesel or gasoline fuel and reduce exhaust emissions, according to William Farrell, the company’s president.

The introduction of propane to the regular fuel is called fumigation, and is used on many other dual-fuel systems.

Installation of the Dual Fuel System requires no permanent modifications to the vehicle or engine, and no connection to the engine computer is required.

“We don’t go near the engine computer,” he said. “There is no controller with our system. We monitor the boost pressure and the Jake Brake. Because we’re creating more energy, boost pressure is increased. When the Jake Brake is on, propane is off.” And there’s an optional exhaust temperature sensor. 

“In a typical fleet application the system can result in fuel savings of several thousand dollars per year in fuel costs, and as a result the fleet can achieve an return on investment on the system is as little as six months,” he said. “In addition, the idle circuit designed into the system provides a quieter idle and lower emissions at idle or while running a power take-off.”

He added the idle circuit to a little-known product that had been on the market for 14 years to get the Dual Fuel System.

The idle circuit injects a small amount of propane into cylinders while the engine’s idling, which smooth’s the engine’s combustion to reduce vibration. It also cuts nitrogen oxide emissions by 59% because propane vapor cools combustion.

Clean burning reduces diesel particulate matter sent into the exhaust and to the diesel particulate filter, reducing the number of regenerations needed, he claimed.

The Environmental Protection Agency has no category in which to place the propane Dual Fuel System, so does not certify it, Farrell said. But it’s allowed because it does not alter an engine’s emissions results except to improve them. And it will not harm any engine it's installed on.

The fast payback is due to the Dual Fuel System’s price of $1,500 for the diesel version, plus the cost of a propane tank that, along with installation, takes the total cost to about $2,700, Farrell said. A system for a gasoline engine is $1,200 plus tank and installation.

A propane tank is a stout but simple and inexpensive steel vessel, not unlike the LP Gas bottle used for propane grills. It’s stored as a liquid but injected into the engine’s inlet air stream as a vapor.

Farrell calls the type of tank he specifies a “propane vapor” tank because it contains a regulator that adjusts pressure and a valve that changes it from a pressurized liquid to the vapor used by the engine.

Propane “autogas” is widely available in the United States at a price similar to that for natural gas. But propane costs less to handle and store, and has more energy than gas so less is needed to maintain performance of the diesel or gasoline engine it works in, Farrell said.

He invites inquiries at  More product information is at