Light-duty natural gas cars work much like gasoline-powered vehicles with spark-ignited engines. This schematic shows basic CNG fuel system components.

2008 Honda Civic GX Natural Gas Car

CNG enters the car through the natural gas fill valve (A) and flows into high-pressure cylinders (B). When the engine requires natural gas, the gas leaves the cylinders and passes through the master manual shut-off valve (C). The gas travels through the high-pressure fuel line (D) and enters the engine compartment. Gas enters the regulator (E), which reduces the gas pressure used for storage (up to 3,600 psi) to the required vehicle fuel injection system pressure. The natural gas solenoid valve (F) allows natural gas to pass from the regulator into the gas mixer or fuel injectors. The solenoid valve shuts off the natural gas when the engine is not running. Natural gas mixed with air flows down through the carburetor or fuel-injection system (G) and enters the engine combustion chambers where it is burned to produce power, just like gasoline.

Some heavy-duty vehicles use spark-ignited natural gas systems, but other systems exist as well. High-pressure direct injection engines burn natural gas in a compression-ignition (diesel) cycle.

Compressed natural gas (CNG) and liquefied natural gas (LNG) are considered alternative fuels under the Energy Policy Act of 1992. Natural gas cars are either fueled exclusively with CNG or LNG  or are capable of natural gas and gasoline fueling.

What is a natural gas car?

Dedicated natural gas cars are designed to run only on natural gas; bi-fuel natural gas cars have two separate fueling systems that enable the vehicle to use either natural gas or a conventional fuel (gasoline or diesel). In general, dedicated natural gas cars demonstrate better performance and have lower emissions than bi-fuel vehicles because their engines are optimized to run on natural gas. In addition, the vehicle does not have to carry two types of fuel, thereby increasing cargo capacity and reducing weight.

2008 Honda Civic GX Natural Gas Car

Natural gas vehicles are fueled with compressed natural gas (CNG) or liquefied natural gas (LNG). These fuels are considered alternative fuels under the Energy Policy Act of 1992 and qualify for alternative fuel vehicle tax credits. As a new twist, tests are being conducted using natural gas vehicles fueled with HCNG, a blend of CNG and hydrogen.

Compared with cars fueled with conventional diesel and gasoline, natural gas cars can produce significantly lower amounts of harmful emissions. In addition, some natural gas vehicle owners report service lives two to three years longer than gasoline or diesel vehicles and extended time between required maintenance.

The driving range of natural gas cars generally is less than that of comparable gasoline- and diesel-fueled vehicles because of the lower energy content of natural gas. Extra storage tanks can increase range, but the additional weight may displace payload capacity. Natural gas car horsepower, acceleration, and cruise speed are comparable with those of an equivalent conventionally fueled vehicle.

Other benefits of the cars include increasing U.S. energy security and paving the way for fuel cell cars.

Propane, also known as liquefied petroleum gas or LPG, is considered an alternative fuel under the Energy Policy Act of 1992. There are more than 270,000 on-road propane cars in the United States and more than 10 million worldwide. Many are used in fleets, including light- and heavy-duty trucks, buses, taxicabs, police cars, and rental and delivery cars.

The availability of new light-duty original equipment manufacturer propane cars has declined in recent years. However, certified installers can economically and reliably retrofit many light-duty cars for propane operation. Propane engines and fueling systems are also available for heavy-duty cars such as school buses and street sweepers.

What is a propane car?

Propane, also known as liquefied petroleum gas (LPG), has been used in cars since the 1920s. It is considered an alternative fuel under the Energy Policy Act of 1992 and qualifies for alternative fuel car tax incentives.

Today, most propane cars are conversions from gasoline cars. Dedicated propane cars are designed to run only on propane; bi-fuel propane cars have two separate fueling systems that enable the car to use either propane or gasoline.

Propane car power, acceleration, and cruising speed are similar to those of gasoline-powered cars. The driving range for bi-fuel cars is comparable to that of gasoline cars. The range of dedicated gas-injection propane cars is generally less than gasoline cars because of the 25% lower energy content of propane and lower efficiency of gas-injection propane fuel systems. Extra storage tanks can increase range, but the additional weight displaces payload capacity. Liquid Propane Injection engines, introduced in 2006, promise to deliver fuel economy more comparable to gasoline systems.

Lower maintenance costs are a prime reason behind propane’s popularity for use in delivery trucks, taxis, and buses. Propane’s high octane rating (104 to 112 compared with 87 to 92 for gasoline) and low carbon and oil contamination characteristics have resulted in documented engine life of up to two times that of gasoline engines. Because the fuel mixture (propane and air) is completely gaseous, cold start problems associated with liquid fuel are eliminated.

Compared with cars fueled with conventional diesel and gasoline, propane cars can produce significantly lower amounts of harmful emissions. Another benefit of propane cars is increasing U.S. energy security.

How Propane Cars Work

Propane cars work much like gasoline-powered cars with spark-ignited engines. Propane is stored as a liquid in a relatively low-pressure tank (about 300 pounds per square inch). Liquid propane travels along a fuel line into the engine compartment. The supply of propane to the engine is controlled by a regulator or vaporizer, which converts the liquid propane to a vapor. The vapor is fed to a mixer located near the intake manifold, where it is metered and mixed with filtered air before being drawn into the combustion chamber where it is burned to produce power, just like gasoline.

Liquid Propane Injection engines, developed over the past 15 years, do not vaporize the propane. Instead, it is injected into the combustion chamber in liquid form. Liquid injection systems have proven reliable in terms of power, engine durability, and cold starting.

Propane Car Emissions

Compared with cars fueled with conventional diesel and gasoline, propane (also known as liquefied petroleum gas or LPG) cars can produce significantly lower amounts of some harmful emissions and the greenhouse gas carbon dioxide.

The EPA calculated the potential benefits of propane versus gasoline based on the inherently cleaner-burning characteristics of propane, summarized in Clean Alternative Fuels:

• Potentially lower toxic, carbon dioxide (CO₂), carbon monoxide (CO), and nonmethane hydrocarbon (NMHC) emissions.
• Rich calibration shows high NMHC and CO emissions, but lower nitrogen oxide (NO_x) emissions
• Lean calibration shows slightly higher NO_x emissions, but lower CO and NHMC emission

Sold by all U.S. vehicle manufacturers, flex fuel cars are capable of operating on gasoline, E85 (85% ethanol, 15% gasoline), or a mixture of both. There are more than 6 million flex fuel cars on U.S. roads today, but many flex fuel cars owners don’t know their vehicle is one.

What is a Flex Fuel Vehicle?

flex fuel cars are capable of operating on gasoline, E85 (85% ethanol, 15% gasoline), or a mixture of both. flex fuel cars qualify as alternative fuel cars under the Energy Policy Act of 1992 (EPAct). They also qualify for alternative fuel cars  tax credits and can provide emissions benefits.

Unlike natural gas and propane bi-fuel vehicles, flex fuel cars contain one fueling system, which is made up of ethanol compatible components and is set to accommodate the higher oxygen content of E85. E85 should only be used in ethanol-capable FFCs.

Other than fueling capability and ethanol compatible components, FFCs are similar to their conventional gasoline counterparts. Their power, acceleration, payload, and cruise speed are comparable whether running on ethanol or gasoline. The only noticeable difference: fuel economy is lower when FFCs run on ethanol.

Many drivers aren’t even aware their vehicle is an FFC. To find out if your car is a flex fuel car, visit the National Ethanol Vehicle Coalition Web site.

Flex Fuel Vehicle Emissions 

Using ethanol as a car fuel provides local and global benefits—reducing emissions of harmful pollutants and greenhouse gases.

Ethanol has been blended in low levels (10% or less) with gasoline for many years. This use of ethanol as an "oxygenate" promotes more complete combustion of the fuel, which can reduce exhaust emissions of carbon monoxide—a regulated pollutant harmful to human health—by 20% to 30% compared with pure gasoline.

Ford Expedition E-85

flex fuel cars fueled with E85 (85% ethanol, 15% gasoline) also emit less carbon monoxide than gasoline-powered vehicles. Emissions from E85-fueled FFCs of other regulated pollutants, such as hydrocarbons and nitrogen oxides (NO_x), are similar to those from gasoline-powered vehicles.

Tests indicate that vehicles fueled with ethanol blends produce lower emissions of some toxic compounds—such as benzene and 1,3 butadiene—than vehicles fueled with pure gasoline. However, evidence suggests that ethanol might increase toxic aldehyde emissions.

In addition to exhaust emissions, all vehicles emit hydrocarbons due to evaporation of fuel from their tanks and fueling systems, especially in warm weather. The common oxygenate blend E10 (10% ethanol, 90% gasoline) has a higher vapor pressure than pure gasoline and thus produces higher evaporative emissions. E85′s vapor pressure is lower than gasoline’s, so it produces lower evaporative emissions.

Mid-level ethanol blends, such as E15 and E20, can be used in flex fuel cars but currently not in standard gasoline-powered vehicles. See the AFDC’s mid-level ethanol blends page to learn about emissions and other issues associated with these blends.