What Is CNG Fuel? Understanding Natural Gas Vehicles

Compressed natural gas (CNG) is 70% to 90% methane, the second most significant greenhouse gas contributing to climate change, after carbon dioxide. In the United States, over two-thirds of natural gas comes from hydraulic fracturing, also known as fracking. The remaining percentage is a byproduct of crude oil drilling. Compressing natural gas reduces it to 1% of its original volume, making it usable as a transportation fuel.

Here, we explore the economic and environmental pros and cons of substituting gasoline and diesel with CNG fuel, and whether there are better alternatives.

What Are CNG Vehicles?

Natural gas vehicles are primarily medium- and heavy-duty vehicles. They are manufactured to either run on natural gas only or as “bi-fuel” vehicles that are able to run on gasoline or diesel when natural gas is not available. Bi-fuel vehicles require a second fuel tank, which increases the vehicle's weight and decreases its storage space.

Today, CNG fuels less than 2% of vehicles worldwide, but the number is growing. While few countries have more than 10% of their vehicles running on CNG, the total number of CNG vehicles has increased from less than a million in 2000 to an estimated 28.5 million in 2019, dominated mostly by China, Iran, and India.

Until recently, low natural gas prices and high gasoline and diesel prices have spurred investment in medium- and heavy-duty CNG vehicles. In India, for example, cities like Delhi converted their entire public transportation system, including taxis, three-wheelers, and buses, to compressed natural gas, leading to reductions in lower emissions of particulate matter—an important concern in cities with high levels of air pollution.

In the United States, the U.S. Department of Energy's Alternative Fuels Data Center lists roughly 100 CNG vehicles on the market, all of them medium- and heavy-duty vehicles such as street sweepers, garbage trucks, semi cabs, and buses. NGV America estimates that there are more than 175,000 natural gas vehicles on U.S. roads today.

Benefits of CNG Vehicles

Cleaner

“Runs on clean natural gas” is a common sight on the sides of city buses or other heavy-duty vehicles. CNG vehicles burn cleaner than gasoline and diesel vehicles, reducing greenhouse gas emissions by anywhere from 15% to 27%, depending on the study and the type of vehicle. Emissions of particulate matter (“soot”), nitrogen oxides, and other air pollutants are also significantly reduced.

Cheaper

While there is increasing concern about methane leaks at natural gas wells, compared to crude oil extraction, the energy required to extract natural gas is lower. And because CNG comes in gaseous form, no distillation is required, so its production costs are approximately 50% lower than gasoline or diesel. CNG vehicles also have lower operating costs than diesel and gasoline vehicles. A life cycle comparison of diesel and CNG refuse trucks in Canada showed that switching to CNG saved $100,000 Canadian ($77,230 U.S.) over a five-year time period.

Drawbacks of CNG Vehicles

Less Fuel Efficient

Compressed natural gas is 3.5 times less energy-dense than gasoline, meaning it requires that many times more volume to produce the same amount of energy. CNG fuel tanks thus need to be larger than gasoline or diesel fuel tanks, cutting into available storage or cabin space and increasing the vehicle weight. In “bi-fuel” vehicles, two fuel tanks are required, limiting storage or cabin space even further. Lower energy density and increased weight make CNG vehicles less fuel-efficient. In one study, heavy-duty CNG vehicles required 17% more energy to operate than diesel vehicles.

Not Always Cleaner

The promotion of CNG vehicles as “cleaner” depends on their use. In one study, CNG refuse trucks emitted “considerably higher” levels of greenhouse gases than diesel vehicles. This is due in part to the added vehicle weight, but also due to the high level of idling time for refuse vehicles (up to 50% of their operations), which leads to to higher concentrations of unburned methane being released.

Unstable Prices

Like gasoline, the global price of natural gas is subject to wild fluctuations. In India, for example, low natural gas prices spurred a boom in CNG vehicles between 2014 and 2022, as the number of CNG refueling stations grew from 938 to 3,500. Demand for CNG vehicles grew further with the surge in gasoline and diesel prices in late 2021, but rising natural gas prices in 2022 threatened to dampen the enthusiasm for CNG vehicles. Heavy-duty commercial vehicles are significant investments, where dramatic price fluctuations in fuel can determine the success or failure of the investment.

Few Refueling Stations

Public CNG refueling stations are rare, with only 865 CNG refueling stations existing in North America, according to the U.S. Department of Energy, including 821 in the U.S. (By comparison, there were 47,120 electric vehicle charging stations in the U.S. as of 2022.) This limits CNG vehicles to commercial fleets with standard routes and short-range vehicles which return to the same fueling station at the end of their workday.

In a 2017 study, nearly a third of CNG fleet drivers were solely reliant on public refueling stations, rather than stations owned by the fleet, which reduced the range of their operations and increased their refueling times. The low number of public refueling stations also adds to the cost of developing their own charging infrastructure for fleets considering the adoption of CNG vehicles.

Alternatives to CNG Vehicles

In the United States, CNG passenger vehicles are an experiment that came and went.

In the mid-2010s, as the fracking revolution made natural gas price-competitive with gasoline, several car manufacturers offered CNG or bi-fuel (gasoline/natural gas) versions of popular vehicles such as the Chevrolet Impala and the Honda Civic, among others. Like many of the other CNG passenger vehicles, the Chevy Impala lasted two model years, doomed by a drop in the price of gasoline.

But the threat to CNG vehicles comes more from cleaner electric vehicles than it does from the ever-changing price of gasoline. As the price of natural gas rises, the attraction of electric vehicles increases, potentially reducing the demand for CNG vehicles substantially. This is especially the case for commercial fleets which, unlike individual consumers, are more likely to calculate the total cost of ownership of a vehicle in making their fleet purchases. And while the adoption of electric vehicles is in part inhibited by the presumed limited availability of public charging stations, the same limitation applies to CNG refueling stations.

The triumph of electric vehicles is by no means inevitable, however. In the early 2010s, CNG vehicles began entering the market at the same time as the all-electric Nissan Leaf and Tesla Model S, but hopes were still high for CNG vehicles. In parts of the world where electricity is generated primarily through the burning of coal, electric vehicles are not only more expensive to own and operate than CNG vehicles, they are more polluting. In short, where electricity generation is dirty, CNG vehicles are cleaner.

But as the world commits to cleaner sources of electricity, the advantages of electric vehicles become clearer. Even considering the higher GHG emissions incurred during the production of electric vehicles, throughout their life cycle even light-duty electric vehicles emit lower levels of greenhouse gases and other pollutants. The benefits only increase with the vehicle's weight.

According to one California study, replacing gasoline and diesel-burning engines with battery electric and hydrogen fuel cell trucks could achieve 25% to 31% higher reductions in emissions than could CNG-burning replacements.

In 2011, the city of Los Angeles, with the second-largest bus system in the U.S., retired its last diesel buses and replaced them with CNG buses. A decade later, the city's Metro G (Orange) line became the city's first all-electric, zero-emissions bus line, with the goal of replacing all CNG buses with fully electric ones. In 2022, Clark County, Nevada pledged to convert 80% of its fleet from primarily CNG vehicles to electric ones by 2030, and to 100% by 2050. The same process has occurred in other major cities, as public agencies seek to both reduce costs and emissions.

Takeaway

The burning of natural gas instead of dirtier fossil fuels is often credited with helping reduce greenhouse gas emissions, leading many people to consider it as a “bridge fuel” that can accelerate the transition to clean energy. Yet with the increasing availability of electric vehicles, including heavy-duty ones, and with more clean, renewable sources of energy being used to produce electricity, the use of compressed natural gas for transportation may be a bridge we have already crossed.