Trucks at Work
Say hello to hydraulic hybrid school buses

Say hello to hydraulic hybrid school buses

There should be a quip in here about how “yellow” is fast becoming the new “green,” but despite trying, looks I’m coming up empty today, sad to say.

Yet that certainly isn’t the case when it comes to the ubiquitous yellow school bus – a staple of the American education system, yet also one that consumes a heck of a lot of fuel.

Chew on these numbers for a moment, courtesy of the American School Bus Council:


• Number of school buses operating in U.S.: 480,000

• Average annual miles per unit: 12,000

• Total annual mileage: 5.74 million

• Average fuel mileage for a diesel-powered bus: 7 mpg

• Average gallons consumed per year per unit: 1,714

• Total gallons consumed per year: 822 million

For that reason, many school bus operations – both public and private – have for years tested (and re-tested) a variety of different alternative propulsion technologies, using fuels such as natural gas, propane, and biodiesel, alongside a variety of hybrid systems.

Now a new path is being explored: the use of hydraulic hybrid vehicle (HHV) technology to power the “yellow fleet.”


The Ford Motor Company Fund and the Georgia Institute of Technology recently joined forces to help jump-start the very first conversion of a traditional school bus to an HHV that runs on recycled biofuel, using a school bus donated by the Atlanta Public School (APS) system.

The project – headed up by a team from Georgia Tech – is being financed by a $50,000 Ford College Community Challenge Grant, one of five given annually for a student-led project that matches university resources with an urgent community need related to sustainability.

Michael Leamy (at left), Georgia Tech’s assistant professor of mechanical engineering, and his students have designed and developed the hydraulic hybrid system for the 16-passenger school bus, and its installation is nearly complete, he said.

Now, HHV technology certainly isn’t new – in fact, United Parcel Service is using it to power about seven of its package delivery vehicles as part of a project conducted with Eaton Corp. (which designed the HHV system being used in UPS's HHV trucks) that began back in 2008.

[Below is a short video from the Environmental Protection Agency (EPA) explained how an HHV works.]

UPS tested its HHV delivery trucks along Detroit routes as part of a joint public-private partnership with the EPA, and its HHV prototypes achieved a 45% to 50% improvement in fuel economy compared to conventional diesel delivery trucks.

Big Brown also believes similar fuel economy improvements and a 30% reduction in carbon dioxide (CO2) are achievable in daily, real-world use, with the EPA noting HHV technology can perform equally well in other applications such as shuttle and transit buses, as well as refuse trucks.


Indeed, Peterbilt Motors Co. rolled out a type of HHV refuse truck back in 2008 – what it dubbed a “hydraulic launch assist” or “HLA” system – installed on its Model 320 vocational chassis. Peterbilt estimates the truck delivers 15% to 30% better fuel economy, reduces emissions and reduces annual brake realignment services by 50%.

Now, with a diesel “series” hydraulic hybrid of the type being purchased by UPS, a diesel engine is combined with a hydraulic propulsion system – replacing the conventional drivetrain and transmission. UPS said its HHV package trucks use hydraulic pumps and hydraulic storage tanks to capture and store energy, similar to what is done with electric motors and batteries in a hybrid electric vehicle.

However, in the HHV’s case, the diesel engine is used to periodically recharge pressure in the hydraulic propulsion system, with fuel economy increased in three ways: vehicle braking energy is recovered that normally is wasted; the engine is operated more efficiently, and the engine can be shut off when stopped or decelerating.

The EPA estimates that when manufactured in high volume, the added costs of the hybrid components can be recouped in less than three years via lower fuel and brake maintenance costs.

Now, the Ford/Georgia Tech HHV project is also going to conduct a cost-benefit analysis of a large-scale conversion of a school bus fleet to hydraulic hybrid powertrains, to see if this technology can pay for itself via the same type of fuel and maintenance savings, noted Georgia Tech’s Leamy.

“We expect our research will lead to cleaner, more efficient school buses that will help school districts like APS significantly reduce fuel costs and greenhouse gas emissions,” he noted.

Indeed, it may turn out that “yellow” can not only be “green” but save some greenbacks, too.