For decades, diesel has been the dominant fuel of choice for school bus fleets. But as emissions requirements change and interest in alternative fuels, electrification, and carbon reduction grows, many are turning to diesel modifications and/or new fueling sources. 

It can feel like a maze navigating all the options. So, let us help! Here’s a high-level guide to how each works for school bus use, and related terms and technologies. Plus, some recommended articles on each so you can dig deeper.

For decades, diesel has been the dominant fuel of choice for school bus fleets. But as emissions requirements change and interest in alternative fuels, electrification, and carbon reduction grows, many are turning to diesel modifications and/or new fueling sources. 

It can feel like a maze navigating all the options. So, let us help! Here’s a high-level guide to how each works for school bus use, and related terms and technologies. Plus, some recommended articles on each so you can dig deeper.

Propane & Natural Gas Options

CNG (Compressed Natural Gas)

Natural gas that’s been compressed to less than 1% of its volume to fit into high-pressure tanks. CNG, while still a fossil fuel, burns cleaner than diesel, though fueling infrastructure is limited. CNG school buses lost popularity in the mid-2000s, with manufacturers ending sales on those models in the early 2020s. CNG school buses are no longer available to buy new.

Natural Gas

An odorless, gaseous mixture of hydrocarbons predominantly made up of methane (think gas stoves). The vast majority of natural gas in the United States is considered a fossil fuel.

• Study Measures Methane Impact of Natural Gas Vehicles

Propane

Sometimes called propane autogas or LPG, it’s a clean-burning liquid fuel stored under pressure. Compared to diesel, propane engines run quieter and produce fewer emissions, often at lower fuel and maintenance costs. Diesel-powered school buses can be converted to propane using a conversion kit. 

Currently, only Blue Bird offers a propane bus due to an exclusivity agreement with Roush CleanTech through 2030.

• Alt-Fuel for Non-Electrifiers: 3 Reasons to Consider Propane

• 4 Reasons I Chose Propane Autogas Buses as a District Transportation Director

• Township 211: A Propane vs. Diesel Maintenance Experience

RNG (Renewable Natural Gas)

Also known as biomethane, it is produced from organic waste (such as landfills or farms) and then processed to clean any impurities. RNG, a renewable source, reduces greenhouse gas (GHG) emissions even further than CNG. While it is growing in popularity, its availability and infrastructure can be limited. 

You can’t buy an RNG bus, but you can convert a diesel bus to use it using an aftermarket retrofit system, such as Ingevity’s “CowFartBus” solution.

• Five Questions About the CowFartBus and Renewable Natural Gas with Nate Thacker of DEMI

• South Carolina Charter School Converts Fleet To Run on Renewable Natural Gas

Renewable Propane

Also called biopropane, it’s made from renewable feedstocks (like vegetable oils or animal fats) instead of fossil fuels. It is chemically the same as traditional propane and works the same, but its carbon intensity is four times lower than conventional propane, and five times lower than diesel. Diesel-powered school buses can be converted using a conversion kit.

• Bonny Eagle School District Puts Renewable Propane to the Test

• Renewable Propane Reduces Emissions, Provides Cost Savings

Liquid Fuels

Biodiesel 

Diesel alternative made from renewable sources like soybean oil or recycled cooking grease through a process that introduces oxygen. Generally blended with petroleum diesel (you’ll see the blend ratios listed as such, e.g., B5 has up to 5% biodiesel; B20 is 6% to 20% biodiesel; and B100 is pure biodiesel). 

Biodiesel can be used in existing diesel engines with no modification. It reduces GHG emissions by up to 86% compared to regular diesel and offers an easy point of entry.

• Enhancing School Transportation with Biodiesel: One Company’s Green Journey

• 5 Biodiesel Benefits for School Buses

Gasoline

Just like what you (probably) put in your car. Used in Type A school buses and now gaining ground in larger-capacity buses. Less expensive upfront than diesel, but generally less fuel-efficient and higher in emissions than other alternative fuels.

• Cobb County Shop Turns to Gasoline Buses to Cut Costs, Recruit Techs

Renewable Diesel

A hydrocarbon chemically equivalent to petroleum diesel (a fossil fuel made from crude oil) but made from renewable sources like soybean oil or canola oil. Renewable diesel buses do not require a conversion kit.

• What You Need to Know About Renewable Diesel

Diesel Emissions Systems

DEF (Diesel Exhaust Fluid)

Not a fuel, but an additive required in modern diesel engines. DEF helps reduce nitrogen oxide (NOx) emissions through the exhaust system.

• Wheels of Wisdom: Understanding Diesel Exhaust Fluid (DEF)

• How Old Is Your Diesel Exhaust Fluid?

• Getting the most out of DEF

DPF (Diesel Particulate Filter)

An emissions reduction device in the exhaust system of diesel buses that traps soot and fine particles. It requires regular regeneration (burning off the buildup) and maintenance to keep running smoothly.

• How Not to Hate Your Diesel Particulate Filter

Electric Vehicles & Zero-Emission Tech

Battery-Electric Vehicle (BEV or EV)

Runs entirely on electricity stored in onboard batteries that power the vehicle, charged by plugging into a charging station. These vehicles boast zero emissions.

• Planning & Considerations for School Bus Electrification 

• How Thermal Tech Drives Electric Innovation in School Buses

• Electric Buses in The Shop: How to Be Prepared for EV Maintenance

Hybrid

A combination of a conventional engine (like diesel or gasoline) and an electric motor. Hybrids improve fuel efficiency and reduce emissions but aren’t currently used in school bus applications.

Hydrogen Fuel Cell

Generates electricity through an onboard tank when hydrogen combines with oxygen captured from cells on the roof of the bus, producing only water vapor as exhaust. It offers long-range and fast refueling, but the technology is early in its adoption.

• Allison Transmission Introduces Hydrogen Fuel Cell Vehicle Testing

Repower

A repowered bus is an existing diesel or gas school bus that’s converted to run on electricity by replacing the engine and drivetrain. It offers a lower-cost path to zero emissions than buying new.

• Contemplating Repower? ChatGPT Has Some Suggestions

• EV Charging Solutions Providers Partner on Repowering Buses, Trucks for V2G

Solar

Used as a supplemental technology and support infrastructure, such as solar panels on bus charging depots. 

ZEV (Zero-Emission Vehicle)

A broad category of a vehicle that produces no tailpipe emissions, most commonly battery-electric or hydrogen fuel cell. Many state mandates and incentives focus on ZEV adoption.

Grid & Infrastructure

V2G/V2X (Vehicle-to-Grid/Vehicle-to-Everything)

Technology that lets electric buses send stored power back to the grid or a building. This can create new revenue streams or emergency backup power.

Infrastructure

Often overlooked, but key: The fueling or charging setup needed for a bus to run — like on-site fueling stations or EV chargers. 

• Creating a Successful Electric School Bus Ecosystem

• How to Choose the Right Chargers for Your Electric School Bus Fleet

• Electrifying Buses: How to Speak Utility

• Breaking Down Alternative Fuel Infrastructure Costs

Microgrid

A small, self-contained energy system — often combining solar, batteries, and even V2G buses — that can power a depot independently.

This list was based on industry research and compiled with the assistance of OpenAI.

For more, visit the U.S. Department of Energy’s alternative fuel resources page.