This year, according to the Hybrid Truck Users Forum, manufacturers will sell more than 5,000 hybrid trucks, compared with fewer than 200 just three years ago. These vehicles range from medium-duty package delivery vans to cherry-pickers, garbage trucks and even massive "18-wheelers" used for long-haul shipping.

Slide Show: Hybrid Trucks

Hybrid trucks use 20 to 50 percent less diesel than conventional vehicles do, depending on how they're used, and that adds up: FedEx calculated that its fleet of 170 hybrid–electric trucks has racked up 3.5 million miles (5.6 million kilometers) of service. The 200 hybrid trucks United Parcel Service (UPS) will add to its fleet later this year are expected to save 176,000 gallons (665,000 liters) of fuel per year, which is the equivalent of taking 100 conventional package delivery trucks off the road. Coca-Cola Enterprises is the most avid consumer of hybrid trucks in the U.S., deploying 327 hybrids out of a total fleet of 30,000.

Depending on how its driven, a typical medium-duty hybrid delivery truck from Peterbilt will average 12 miles per gallon (5.1 kilometers per liter), versus nine mpg (3.8 kpl) for the conventional version of the same truck, according to Peterbilt. The largest trucks on the road average six mpg (2.5 kpl), so the increase to eight mpg (3.4 kpl) achieved by Peterbilt's hybrid technology is significant.

As conventional hybrids go mainstream, researchers including Andrew Frank, a professor of mechanical engineering at the University of California, Davis, and lead engineer at Efficient Drivetrains, Inc., are already working on what comes next: plug-in hybrids that can be charged overnight or at intelligent charging stations. (Plug-ins carry more batteries than conventional hybrids do, and so can go some distance on electricity alone; intelligent charging stations allow for quick recharges.), With his team, he is building a medium-duty plug-in hybrid delivery truck for the U.S. military. With conventional hybrids, he notes, "We might improve efficiency 20 to 30 percent. [But with plug-ins] we could conceivably displace 90 percent of [the] fuel used by conventional vehicles."

The transition to hybrids and plug-in hybrids will only happen if these trucks come down in price, says Mark Duvall, director of electric transport at the Electric Power Research Institute (EPRI). Buyers typically pay a premium of between 30 and 60 percent above the price of a conventional truck for its hybrid version. For example, hybrid utility trucks used by Florida Power and Light Co. (FPL) cost $175,000, compared with around $115,000 for a conventional version.

Lee Slezak, a vehicle technology analysis and evaluation manager at the U.S. Department of Energy (DoE), says that one reason hybrids are expensive is that manufacturers simply aren't making enough of them. Eaton Corp., which is the nation's largest manufacturer of hybrid truck power trains, would have to produce between 5,000 and 10,000 trucks annually for these vehicles to be cheap enough for them to make sense economically for a majority of fleet operators, Kevin Beaty, hybrid business unit manager for Eaton, told Fleet Owner magazine.

Eaton expects to sell 2,000 power trains this year, says Dimitri Kazarinoff, general manager of emerging technologies at Eaton—quadruple what it sold last year. Government incentives are helping, Kazarinoff says, including $156 million in grants from the National Clean Diesel Funding Assistance Program (part of the American Recovery and Reinvestment Act of 2009), which will pay for 25 percent of the price of a new hybrid truck. (Because of the way the law works, however, almost all of the vehicles eligible for this credit are still going to be more expensive than conventional trucks.)

Even without economic incentives, some classes of trucks benefit more than others from hybridization and can pay for themselves with reduced fuel consumption in eight to 10 years, EPRI's Duvall says. (The payback was even quicker last year when diesel fuel peaked at $4 per gallon, before failing back to a little more than $2 per gallon today.) Utility vehicles, commonly known as cherry-pickers or bucket trucks, are a best-case scenario because they spend so much of the day idling in order to power their lifts, according to Dahlia Garas, program manager at the Plug-In Hybrid Electric Vehicle Research Center at U.C. Davis.

George Survant manages a fleet of 300 hybrid vehicles for FPL, 60 of which are medium-duty hybrid utility trucks. "A conventional truck will burn 1.9 gallons, [or 7.2 liters, of diesel fuel] an hour, and our hybrid trucks burn about a gallon [3.8 liters] an hour," Survant says.

Unlike passenger vehicles or long-haul trucks, a conventional utility truck spends on average half of every workday idling to power its bucket and other electronics. Survant's trucks, which were built by Navistar International Corp. using Eaton power trains, can run their bucket lifts for up to two hours on the electricity stored in their batteries.

Trucks that spend their days stopping and starting, such as garbage trucks,  also see a significant benefit from hybridization. "In refuse're accelerating from house to house and slamming on [the] brakes to cover a lot of houses," Kazarinoff says. Package delivery trucks have similar "duty cycles," which is why in 2006 UPS partnered with the U.S. Environmental Protection Agency (EPA) and Eaton to develop an innovative "hydraulic hybrid" that stores energy recovered from braking as a pressurized fluid rather than as electricity.

Peterbilt has deployed 14 hydraulic hybrid refuse trucks that use the energy recovered from braking as a "launch assist" to accelerate them faster than conventional vehicles. Peterbilt says this makes these vehicles both more fuel-efficient and productive because they can go from one stop to the next more quickly. In the next five years, hybrid vehicles of every kind will benefit from innovations in almost every component of their power trains—from batteries to motors, says Steven Boyd, a technology development manager at the DoE.

"In a future time when there [are] lots of [options for consumers] and batteries are less expensive, you could ultimately make the case for hybridizing almost every type of vehicle," industry veteran Duvall says.

As fleet operators buy more hybrid trucks, their prices will come down, he notes. Because hybrid trucks use less diesel, they make fleet operators less vulnerable to swings in the price of fuel.

"The model that we've used in the past as fleet operators—to wait and see what the market brings us, and wait and see what the government requires us to do—is frankly not a viable alternative in today's world," FPL's Survant says. "We can't afford to have the lassitude we've had historically.... I think we need an increased sense of urgency."