Alternative Fuel Systems - LPG 101

Liquefied Petroleum Gas (LPG), often called propane, is one of the leading alternative fuels in the U.S., and on a worldwide basis. It is the third most common vehicular fuel, after gasoline and diesel. There are over 270,000 on-road vehicles in the United States, most of which are spark-ignited gasoline engines adapted to use LPG (Source: US Department of Energy – Clean Cities Program). A large number of these are used in fleets that include light- and heavy-duty trucks, buses, taxicabs, police cars, and rental and delivery vehicles. Propane is widely used because propane is clean burning. Tests conducted by the EPA show that propane engines produce 60 percent less ozone-forming emissions than reformulated gasoline.

LPG is a petroleum derived, colorless gas consisting mainly of propane, propylene, butane, and butylene in various mixtures. Propane is safe. It is nontoxic and nonpoisonous and has a very small flammability range. It is produced as a by-product from two sources: natural gas processing and crude oil refining. LPG is a mixture of various hydrocarbons that are normally in a gaseous state at atmospheric pressure, but liquefy at higher pressures, approximately 200 psi or less. Most of the LPG used in the United States is produced domestically. More than 90% of all propane used in the U.S. is produced domestically.

FUEL GRADES
In the U.S., the Gas Producers Association (GPA) has established a LPG fuel grade for propane when used as a motor fuel called HD5. The HD5 standard requires a minimum propane content of 90%, and a maximum propylene content of 5%, on a volume basis. Typical LPG has an octane rating of 105-110 as compared to gasoline at 85-93.

LONGER VEHICLE LIFE
Many of the fleets using LPG have reported 2-3 years longer service life and extended intervals between required maintenance. Spark plugs from a propane vehicle last from 80,000-100,000 miles and propane engines can last 2-3 times longer than gasoline or diesel engines (Source: National Propane Gas Association). Propane is generally considered to reduce engine maintenance and wear in spark-ignited engines. The most commonly cited benefits are extended oil change intervals, increased spark plug life, and extended engine life. Gasoline fueled engines particularly carbureted engines require very rich fuel mixtures during cold starting and warm up. Some of the excess fuel collects on the cylinder walls, effectively washing lubricating oil off the cylinder wall and contributing to accelerated wear during engine warm up. Gaseous fuels do not affect cylinder lubrication.

Engines powered by gaseous fuels are generally considered easier to start than gasoline engines in cold weather because gaseous fuels are already vaporized before inducted into engine. However, under very cold temperatures, cold-start difficulty occurs for propane. In extreme cold weather environments a supplemental electrically powered heater will likely be necessary.

LPG EQUIPMENT

LPG FUEL TANK
LPG tanks are constructed of heavy gauge steel, in compliance with the Boiler and Pressure Vessel Code of the American Society of Mechanical Engineers (ASME) to withstand a pressure of 1000 psi. Normal working pressures within the tank vary depending upon ambient temperatures and the quantity of fuel in the tank. Common operating pressures are in the range of 130-170 psi. Propane tanks limit the liquid level to 80% of the total tank volume by using an auto-stop fill valve. Tanks are equipped with a pressure relief valve that can release propane vapors to the atmosphere to prevent tank rupture under abnormally high-pressure conditions. Under normal operating conditions the LPG system is essentially a closed fuel system without the typical vapor emissions associated with gasoline. Each tank also includes a manual shut-off valve. The propane fuel tank is installed along with a fueling port, fuel lines, and pressure safety valves. A filter removes particles and contaminants that may be present in the propane.

VAPORIZER
The LPG system draws fuel from the bottom of the tank and sends liquid propane to the vaporizer. The vaporizer converts the liquid to a gas. The primary heat source for this vaporization is engine coolant flowing through specially designed water jackets cast into the vaporizer body. Many vaporizers include an internal pressure regulator to control the pressure of the fuel sent to the engine. Some early propane systems with mechanical mixers used a separate pressure regulator.

FUEL METERING
Early propane systems used a mixer which operated as a conventional venturi device in a manner quite similar to a gasoline carburetor. Vaporized propane is drawn through a fixed orifice in response to engine air flow. As intake air enters the engine, a venturi effect is created through the mixer. This slight pressure drop acted on a spring-loaded diaphragm in proportion with air flow. The result was a simple yet fairly accurate flow meter which controlled the volume of fuel to the engine as a function of air flow.

Like gasoline carburetors, the mixer was limited in accuracy. Changes in altitude, ambient weather conditions, and even temperature cause significant variations in the fuel mixture that cannot be compensated for using a mixer . Electronically controlled, closed-loop fuel injection provides a much more precise method of metering fuel. Based on sensor inputs the electronic control module (ECM) determines the engine operating conditions, and then modifies the injector pulse width to maintain a stochiometric mixture.