CNG Buses Explained: How They Work and Their Advantages

Combating climate change requires innovation from city authorities, and public transportation is no exception. One way to greener and more efficient mass transportation is to use eco-friendly means like compressed natural gas buses (CNG buses) that could revolutionize public transit’s carbon footprint.

As the US government introduces many climate-positive transportation measures, CNG buses could be the key to reaching new emissions reduction benchmarks and decreasing reliance on nonrenewable fossil fuels.

But what is a CNG bus? Are CNG buses any more environmentally friendly than their diesel counterparts? Let’s take a look under the hood, uncover the key benefits and answer some common questions.

What Is Compressed Natural Gas?

Compressed natural gas (CNG) is a fuel that can power vehicles and machines, including buses. Compressibility is a property of all gasses, including natural gas.

Fuel manufacturers create CNG by substantially compressing natural gas. As a result, CNG takes up less than 1% of the volume it does in its original form. But why compress natural gas in the first place?

Using natural gas in its original extracted form for vehicle fueling isn’t possible. To mirror fuel combustion performance, vehicles must have access to a significantly higher volume of natural gas than traditional fuel. But, since compressing the gas doesn’t change its chemical composition, pressurizing it can solve the volume problem for natural gas-powered vehicles and machinery.

How CNG Buses Work

It takes four steps to fuel buses with compressed natural gas.

1. Manufacturers build buses specially to run on CNG. While the two types of buses appear similar, their interior fueling systems and how fuel enters the vehicle are significantly different. The two types of buses perform similarly on the road.
2. Compression devices are installed on natural gas lines at fuel stations. Natural gas enters a compressor station through station yard piping. It is passed through scrubbers and filters to extract any liquids and remove solids or other particulate matter that could be in the gas stream. Once the natural gas stream has been cleaned, it is directed through additional piping to compressors.
3. Natural gas is compressed so it can be used as fuel. As natural gas is compressed, heat is generated and must be dissipated to cool the gas stream before it can fuel a vehicle.
4. Buses are connected to hoses and fueled up. The time it takes to fuel a bus depends on the number of vehicles, compressor size, and the amount of buffer storage. Filling a bus with CNG could take several minutes to a few hours.

4 Advantages of CNG Buses

Let’s explore four significant benefits of incorporating CNG buses into transportation infrastructure.

1. Cost Savings

CNG is markedly cheaper than diesel fuel. For example, in April of 2024, the average per gallon price of diesel was $3.50, while CNG was only $2.90 per gallon.

So, that makes CNG vehicles less expensive to operate than their diesel counterparts.

However, it’s important to note that CNG buses come at a higher upfront cost. According to the U.S. Department of Energy, CNG buses typically cost $25,000 to $30,000 more than conventional ones, which is often partially offset by government incentives. However, with $0.60 per gallon saved on fuel, CNG buses rapidly pay for themselves as the fuel savings grow to offset the increased purchase price.

2. Environmental Impacts

There are always two byproducts of combustion engines: Carbon dioxide and water.

Fuel + Oxygen (O2) → Carbon dioxide (CO2) + Water (H2O)

In diesel combustion, forty-eight parts of carbon dioxide are released for every four parts of fuel burned. However, only four parts of carbon dioxide are emitted for every four parts of CNG combusted — a significantly lower emissions profile than diesel combustion.

3. Noise Reduction

Many suburban areas regulate noise pollution. Buses can be distractingly loud, and noise ordinances can impede the expansion of public transit.

In addition, the United States Environmental Protection Agency (EPA) sets limits on noise sources, including construction equipment, transport vehicles, trains, and more.

The CNG engines are up to ten decibels quieter than comparable diesel engines.

Municipalities can bolster their compliance efforts by implementing CNG buses, improving the quality of life for riders and residents.

Increased Performance

All in all, CNG performs better than diesel fuel in three ways.

1. Particulate matter emissions: While diesel engines emit visible and detectable particulate matter (PM) as black soot, CNG engines emit nearly zero PM. According to the United States Department of Energy, a study on a Colorado bus system demonstrated that CNG buses emitted 97% less PM than diesel buses and 57%less in nitrous oxide emissions.
2. Collision safety: CNG fuel tanks are much more robust and safer than diesel tanks. Since CNG is stored in vehicles at high pressures, fuel tanks are robustly designed, preventing explosions in the event of bus crashes involving CNG-powered buses.
3. Vapor safety: CNG vapors are odorless, colorless, and non-toxic. By contrast, diesel and gasoline vapors are dangerous to ingest and breathe. While vapor buildup in enclosed spaces can lead to asphyxiation, fuel inhalation during fueling is a significantly lower concern for CNG vehicles.

Read more: CNG vs. LNG vs. Propane

Frequently Asked Questions About CNG Buses

Check out the answers to questions many people have about buses fueled by compressed natural gas. Much of the information in this section comes from the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy.

Will advanced admissions control systems make CNG unnecessary?

Diesel emissions from buses have improved significantly over the last decade. Most of the improvements have resulted from enhanced engine design. In the future, diesel engines will likely need to depend on emission control devices to reduce emissions further.

The issue: The durability of emission control devices on diesel and diesel-hybrid buses depends on fuel quality. If the devices degrade over time because of lower-grade fuel, emissions could be much higher, on par with those of the aging gasoline buses on the road today.

In addition, diesel buses emit toxins, including benzene and 1, 3-butadiene, that buses fueled with CNG do not. Based on the information available today, CNG buses will likely always have emissions benefits over diesel buses because natural gas combustion produces lower emissions.

Will ultra-low sulfur diesel fuel and diesel-hybrid buses negate the benefits of CNG buses?

Testing hybrid buses that use advanced emission control technologies coupled with ultra-low sulfur diesel fuel has resulted in particulate matter emission levels similar to those of natural gas engines, requiring minimal exhaust emission control.

However, nitrogen oxide emissions from these experimental buses were higher than those from condensed natural gas buses. In addition, the special diesel fuel needed for the emission control systems to work correctly on these engines will not be commercially available for at least five years, and it is expected to cost more than conventional diesel fuel.

On top of this, natural gas emission control devices will most likely be less expensive and more durable than those for diesel engines. That’s because there are far fewer nitrous oxide and particulate matter emissions with natural gas to degrade the devices.

Are fuel tanks in compressed natural gas buses more likely to explode?

The technology behind CNG bus fuel tanks has been around for a long time. CNG fuel tanks are much stronger and safer than traditional diesel or gasoline tanks, performing better when collisions happen.

Are CNG buses safer than other options?

Compressed natural gas buses come with different safety concerns than diesel fuel buses. However, no data shows buses fueled by compressed natural gas have a greater risk of fire or explosion than diesel buses. CNG buses are equipped with gas detectors and other equipment to ensure safety.

Are natural gas buses more difficult to maintain?

Mechanics must have the skills necessary to maintain different types of buses. They have to understand the properties of the fuels used in them.

This situation is similar to how diesel mechanics get different training than those working on gasoline buses. Advanced technology diesel propulsion systems, including hybrid-electric-diesel ones and the exhaust emission control systems needed to reduce emissions from diesel engines significantly, are far more complex than standard diesel power trains.

In short, maintaining CNG buses requires special training, tools, and test equipment, but nothing outside of what’s needed for other bus types.

Are CNG bus facilities and fueling stations hazardous?

No evidence supports the claim that CNG bus maintenance facilities are less safe than traditional ones. Natural gas and diesel fuels are both flammable. Each requires people working around them to use safety and fire protection equipment developed exclusively for the fuel type.

The reality is that diesel bus facilities typically store much larger quantities of fuel on-site than CNG facilities, putting them at greater risk. Ground soil contamination from diesel tank leaks is another issue that CNG facilities do not face.

Are CNG buses better than diesel?

As covered earlier in this article, CNG buses provide four key benefits that traditional ones do not. Buses powered by compressed natural gas are more cost-effective, environmentally sound, less noisy, and perform better, meaning they emit fewer particles, are safer, and release fewer vapors that are less harmful.

Choose FASTECH for Your Renewable Energy Fueling Projects

CNG buses are an environmentally friendly option, outperforming diesel and gasoline engines in emissions profiles, price, and safety.

Is your business prepared to embrace a fossil-fuel-free world as alternative fuel becomes more mainstream? If you’re ready to champion a clean energy future, turn to the experts at FASTECH. We engineer and build end-to-end renewable energy solutions, helping green companies crush their emissions goals.

You can rest assured knowing FASTECH is innovative, professional, an industry leader, visionary, and passionate about what we do.

We can’t wait to usher you into the next generation of energy — let’s explore your CNG bus options together today.