Based on Ford's 350-series Super Duty chassis, the Tonka concept is as muscular and weighty as a pickup gets. The vast majority of vehicles in its class are in commercial service, hauling dirt, cement, or refuse. But a fast-growing proportion of owners are employing these behemoths for personal use, ranging from hauling horses to towing RVs. Such work requires raw, fuel-consuming power. Until recently, the power demands made dramatic fuel savings seem unattainable. The Tonka's new approach to conserving energy in stop-and-go driving promises to change that.
The key is recovering the energy normally lost while braking. The raw torque required to push more than 3 tons up to highway speed is quickly dissipated as heat by the brakes in bringing the same load to a halt. Up to now, most systems that recapture lost braking energy have relied on some kind of electric drive -- using energy from a battery pack to assist while accelerating, then using the motor as a generator to recharge while slowing the vehicle. "The problem is that you change energy forms too many times, losing a little efficiency each time," says Cliff Carlson, senior specialist of advanced technologies at Ford. "Mechanical energy becomes electrical, then electrical becomes chemical (in the battery). And then you reverse it."
Ford has promised a 25 percent improvement in light-truck fuel efficiency in the next five years, partially as a reaction to pressure from federal regulators. Up to now, the company has directed its efforts toward gas-electric hybrids. In the heavier weight categories, both General Motors and Ford have been working on so-called mild hybrids, which use starter-generators that allow start-stop operation, killing the engine at idle and providing a limited amount of boost during acceleration. Some test results, however, have been casting doubt on the potential of such systems to achieve significant fuel economy.
Carlson points out the alternative Ford is pursuing: Mechanical energy can be stored directly, using a hydraulic accumulator. The accumulator is a high-pressure tank, partially filled with nitrogen. Hydraulic fluid is pumped into the tank, pressurizing the gas above the liquid as the tank fills. The hydraulic pressure can be quickly retrieved as shaft horsepower by running the pump as a hydraulic motor. "You can store lots of energy quickly, which is what you need while braking," says Carlson. "It's efficient: As much as 70 percent of the braking energy can be recovered. An electric hybrid is lucky to get 25 percent."