Hybrid drive systems usually have to be part of a new car or truck, and usually they're expensive. But XL Hybrids of Boston is coming up with one that can be installed on new or existing vehicles at a moderate cost.

And while most currently available systems are for medium- and heavy duty trucks, the XL electric hybrid is for light trucks, says Justin Ashton, vice president of business development and a company founder. There are a lot of Class 1, 2 and 3 trucks out there, so the sales potential is great and so are the possible fuel savings.

It's designed for a specific light-truck model, General Motors' 2500 series G van, sold as the Chevrolet Express and the GMC Savana, with the 4.8-liter Vortec 4800 gasoline V-8.

"It's made for urban use with stops and starts," Ashton says. "We're targeting fleets with these vans operating high mileages, more than 25,000 miles per year. It will approach the performance of a diesel but at a gasoline engine cost."

Why this particular 3/4-ton van?

"We chose the GM van because quite a few customers in our surveys have them," he explains. "In the future, it will be designed for GM pickups and Ford vans and pickups. It can be adapted to heavier and lighter vehicles, and with larger engines, including diesels."

2-Wheel Drive Only

These would be 2-wheel-drive trucks. A version for 4x4's might come later, but would require much bigger changes to the vehicle.

The 2WD system requires no cutting into the floor, and won't affect cargo space because it's placed closer to the vehicle's front. A glance at the accompanying drawing shows a layout:

A motor-generator hangs next to the transmission and is mated to a gearbox; this connects to the driveshaft via a Kevlar drive belt, like those used on some large motorcycles. A few inches has to be cut out of the driveshaft to make room for a pulley.

The gearing is 4 to 1, with the motor-generator spinning that much faster than the driveline. This allows the 43-horsepower motor to make as much as 240 pound-feet of torque during launching. During deceleration and braking, the belt transfers kinetic energy from the driveshaft to the motor, which now acts as an electric generator. Energy is stored in a nearby 2-kilowatt lithium-ion battery pack. An XL system weighs less than 400 pounds.

"Speed is electronically limited to 70 mph, which customers want," Ashton says. "The XL controls read the GM engine control module, but don't put anything on it. The controls make all the complex decisions about when to use the electric motor, when to release it, and when to regenerate."

Parallel Hybrid System

The XL is a parallel system, so the Vortec engine loafs as the motor makes torque, works with the motor, or works alone at higher speeds. As with other parallel hybrid-drive systems, it all depends on the driving circumstance at hand. The engine runs all the time. Engineers are looking at an engine-off function during pauses and stops, but it would be more complex and expensive, he says.

The company hasn't yet set a price, but Ashton thinks it'll be "under $8,000" installed. Discussions with well-known upfitters who are most likely to do the installations will help determine a final price. Installation will probably take about a half a day.

XL Hybrids thinks its system will save $1,500 to $2,500 a year in gasoline, depending on fuel cost and miles run. The math for return on investment is therefore pretty simple: If it saves $2,000 a year and if the installed price remains $8,000, it would pay for itself in four years. If government incentives were factored in, the ROI would be faster.

"It's actually cost-effective for fleets with a scalable ROI," Ashton says. "It's simple, with the right kind of fuel savings at the right kind of cost."

XL is about to start pilot tests with fleets.

More info: www.xlfleet.com.