Natural Gas Vehicle Emissions

When used as a vehicle fuel, natural gas can offer life cycle greenhouse gas (GHG) emissions benefits over conventional fuels, depending on vehicle type, drive cycle, and engine calibration. In addition, using natural gas may reduce some types of tailpipe emissions.

Tailpipe emissions result from fuel combustion in a vehicle's engine. The emissions of primary concern include the regulated emissions of hydrocarbons, oxides of nitrogen (NOx), carbon monoxide (CO), as well as carbon dioxide (CO2). Due to increasingly stringent emissions regulations, the gap has narrowed between tailpipe emissions benefits from natural gas vehicles (NGVs) and conventional vehicles with modern emissions controls. That's because the U.S. Environmental Protection Agency (EPA) is requiring all fuels and vehicle types to meet the same thresholds for tailpipe emissions of air pollutants. Still, NGVs continue to provide emissions benefits—especially when replacing older conventional vehicles or when considering life cycle emissions.

Natural gas is increasingly used to replace gasoline in smaller applications, such as in forklifts and commercial lawn equipment. Because natural gas is a low-carbon, clean-burning fuel, a switch to natural gas in these applications can result in substantial reductions of hydrocarbon, carbon monoxide, oxides of nitrogen, and greenhouse gas emissions.

Life Cycle Emissions and Petroleum Use

Argonne National Laboratory's GREET model estimates the life cycle petroleum use and greenhouse gas (GHG) emissions of light-duty vehicles running on compressed natural gas (CNG) and liquefied natural gas (LNG). Based on this model, natural gas emits approximately 6%-11% lower levels of GHGs than gasoline throughout the fuel life cycle. The GHG emissions impacting the CNG and LNG life cycle are predominately the result of production-phase fuel leakage. When comparing the life cycle emissions of the two types of natural gas, CNG and LNG are nearly identical. CNG production uses less petroleum and emits fewer GHGs than LNG, because compressing natural gas requires less energy than liquefying it.

In 2007, a study for the California Energy Commission (CEC) found that CNG and LNG reduce life cycle GHG emissions in both light- and heavy-duty vehicles compared to their gasoline and diesel counterparts. Again, this is primarily due to the low petroleum usage in the production phase and the low-carbon intensity of the fuel during use.

An Argonne National Laboratory analysis and the CEC study In 2007, a study for the California Energy Commission (CEC) found that CNG and LNG reduce life cycle GHG emissions in both light- and heavy-duty vehicles compared to their gasoline and diesel counterparts. Again, this is primarily due to the low petroleum usage in the production phase and the low-carbon intensity of the fuel during use.

Because renewable natural gas (RNG), also known as biomethane, is chemically identical to fossil natural gas, yet yields far fewer GHG emissions during the production process, the blending of relatively small quantities of RNG with fossil natural gas can provide significant life cycle GHG emission benefits. In a 2011 study of RNG production methods, Argonne National Laboratory concluded that all RNG methods show significantly less GHG emissions and fossil fuel consumption than conventional fossil natural gas and gasoline.

Overall, CNG and LNG are both clean-burning fuels and perform well against current vehicle emissions standards.

Converted Vehicles

Converting conventional vehicles to run on natural gas is a good option for incorporating alternative fuels into light- and heavy-duty fleet operations. EPA's emissions requirements and regulations apply to vehicles converted to run on CNG or LNG.

EPA requires conversion system manufacturers to demonstrate that converted vehicles or engines meet or exceed the same emissions standards as the original vehicle or engine. For this and many other reasons, it's important that conversions be performed by careful and reputable qualified system retrofitters.