Air cooled engines run very efficiently on propane as their fuel source. When properly installed, it is clean burning and safe fuel to use. Keep in mind that you need to figure about 5% less power output when using propane than gasoline as a fuel. Over the last thirty plus years, I have often been involved in propane fueled engines on generators and various other equipment. I'll try and pass along some of the knowledge I have gained in this article.
In addition to this, I have also written an article about Emission Compliant Engines. You may want to read this as the new EPA emission regulations are causing plenty of grief for those trying to modify new fuel systems. Propane conversions are easy to do, but specify guidelines must be followed to end up with an efficient safe system. Finding the proper LPG kit can be a challange with all of the carburetors available today.
- Propane Storage Tank - Where the propane is stored in liquid form under pressure.
- Primary Fuel Regulator - Reduces the pressure down to 6 ounces, before entering the demand regulator.
- Demand Regulator or Zero Governor - Where the fuel is held prior to entering the engine upon a vacuum demand from the carburetor intake.
- Load Block - Where the gas enters the carburetor through a load adjusting screw, many also have an idle adjusting screw.
- Electric Shutoff Solinoid - Used to positively shutoff the fuel to the demand regulator when the engine is not running, in case of a leak or rupture.
- Vacuum Switch - This switch is used to activate the electric solinoid in some applications. May also be designed to act as a stand alone disconnect.
Propane is usually stored in a tank in liquid form. Most small engines run on Propane vapor, which is formed above the liquid in the tank. That is why the fuel is drawn from the top of the tank. The fuel is sent from the tank as high pressure vapor, up to 250 pounds. It is then reduced down to 6 ounces pressure by a primary regulator, before entering the demand regulator. The demand regulator is operated by a vacuum signal from the engine as it runs. The vacuum to the diaphragm of the regulator opens a needle valve and lets fuel flow to the carburetor. It is important that the vent hole in the regulator is not obstructed or the diaphragm cannot operate properly. Some systems have the primary and demand regulator all in one unit.
To meet safety precautions a shutoff solinoid or vacuum switch is installed before the demand regulator. This is paricularly important if you do not turn the fuel supply off to the engine when the engine is stopped. In some applications a vacuum switch operates the electric solinoid. The important thing to remember here is that if the engine stops the fuel must be shut off to the demand regulator, by shutting off the power source to the solinoid.
When the engine cranks over, the solinoid circuit is activated and fuel is drawn from the demand regulator to the load block at the carburetor, where it is metered into the engine. Some models have a primer button on the demand regulator, a great help when starting with a recoil rope starter. Another important thing to do that is often overlooked is to set the spark plug gap at .018 when running propane on a small air cooled engine.
Precise Engine Repair does not offer conversion kits for sale. To find an LPG kit for your engine, do a search under propane conversion kits or see a local propane dealer. They may be able to help find a kit for your engine. I hope this article has answered a few questions and given you a better understanding of propane as a fuel - Bruce Perrault