Natural Gas Vehicles

Natural gas powers about 150,000 vehicles in the United States and roughly 15.2 million vehicles worldwide. Natural gas vehicles (NGVs), which can run on compressed natural gas (CNG), are good choices for high-mileage, centrally fueled fleets that operate within a limited area. For vehicles needing to travel long distances, liquefied natural gas (LNG) is a good choice. The advantages of natural gas as a transportation fuel include its domestic availability, widespread distribution infrastructure, low cost, and inherently clean-burning qualities.

CNG and LNG are considered alternative fuels under the Energy Policy Act of 1992. The horsepower, acceleration, and cruise speed of NGVs are comparable with those of equivalent conventional vehicles. Also, compared with conventional diesel and gasoline vehicles, NGVs can produce some emissions benefits.

There are many heavy-duty natural gas vehicles—as well as a growing number of light-duty NGVs—available from original equipment manufacturers. Qualified system retrofitters can also economically, safely, and reliably convert many vehicles for natural gas operation.

Types of Natural Gas Vehicles

There are three types of NGVs:

  • Dedicated: These vehicles are designed to run only on natural gas.
  • Bi-fuel: These vehicles have two separate fueling systems that enable them to run on either natural gas or gasoline.
  • Dual-fuel: These vehicles are traditionally limited to heavy-duty applications, have fuel systems that run on natural gas, and use diesel fuel for ignition assistance.

Light-duty vehicles typically operate in dedicated or bi-fuel modes, while heavy-duty vehicles operate in dedicated or dual-fuel modes. On the vehicle, natural gas is stored in tanks as CNG. LNG, a more expensive option, is used in some heavy-duty vehicles. The form of natural gas used is typically chosen based on the range an application needs. Because it is a liquid, the energy density of LNG is greater than CNG, so more fuel can be stored onboard the vehicle. This makes LNG well-suited for Class 7 and 8 trucks requiring a greater range.

In general, dedicated NGVs demonstrate better performance and have lower emissions than bi-fuel vehicles. Additionally, dedicated NGVs only have one fuel tank, so they aren't as heavy as bi-fuel NGVs and offer more cargo capacity. The driving range of NGVs is generally less than that of comparable conventional vehicles because of the lower energy density of natural gas. Extra storage tanks can increase range, but the additional weight may displace payload capacity.

Source: Adapted from Compressed Natural Gas: A Suite of Tutorials. Courtesy of Thomason & Assoc. Inc.

How Natural Gas Vehicles Work

Light-duty natural gas vehicles work much like gasoline-powered vehicles with spark-ignited engines. The schematic at the right shows the basic CNG fuel system components.

A CNG fuel system transfers high-pressure natural gas from the storage tank to the engine while reducing the pressure of the gas to the operating pressure of the engine's fuel-management system. The natural gas is injected into the engine intake air stream the same way gasoline is injected into a gasoline-fueled engine. The engine functions the same way as a gasoline engine: The fuel-air mixture is compressed and ignited by a spark plug and the expanding gases produce rotational forces that propel the vehicle.

Many 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, where a small amount of diesel fuel is injected in addition to the natural gas to facilitate ignition. Heavy-duty engines can also burn diesel and natural gas in a dual-fuel system.