Hydraulic hybrids boost fuel economy

Oct. 12, 2006
Innovative propulsion system enhances efficiency, cuts emissions

UPS is currently road testing a delivery vehicle in which a hydraulicpropulsion system replaces the conventional driveshaft, differential, and transmission.

In the series hydraulic hybrid, hydraulic pumps and motors drive the rear wheels, and high-pressure accumulators recover and store normally wasted braking energy.

High fuel prices have most every driver feeling pain at the pump. But imagine you're UPS with a fleet of 91,000 delivery trucks. "Fuel conservation is critical to our business," says company Vice President Frank Whalley and, as a result, the company is actively investigating alternative vehicle-propulsion systems such as hydrogen fuel cells, liquified natural gas, propane, and electric power.

One groundbreaking project to boost fuel economy involves hydraulic-hybrid technology. The company teamed with Eaton Corp., International Truck and Engine, the EPA, and the U.S. Army, and recently unveiled a new fuel-saving delivery vehicle.

The first-ever hydraulic-hybrid diesel urban-delivery vehicle reportedly improves fuel economy by 60 to 70% and reduces carbon-dioxide emissions more than 40% in initial laboratory testing. EPA estimates that the technology has the potential to save more than 1,000 gallons/yr for each urban-delivery vehicle. It is now being road-tested in Cleveland and Detroit.

The design is called a series hydraulic hybrid truck. In it, a high-efficiency diesel engine combines with a hydraulicpropulsion system to replace the conventional drivetrain and transmission. The vehicle uses hydraulic pumps and motors to drive the wheels, and hydraulic accumulators to recover and store energy — similar in concept to what is done with electric motors and batteries in hybrid-electric vehicles.

The system layout is relatively straightforward: An enginedriven pump delivers high-pressure fluid to a rear-drive hydraulic motor. Fluid pressure turns the motor to power the vehicle wheels. When the vehicle brakes, the hydraulic motor acts as a pump, slowing the truck and recovering braking energy by generating high-pressure fluid that is stored in the accumulators. Low-pressure reservoirs store fluid after it has been used by the pump-motor. An electronic controller monitors the driver's acceleration and braking and commands the system components.

The system increases vehicle fuel efficiency three ways:

Regenerative braking. A hydraulic hybrid recovers and reuses braking energy that is normally wasted. As the vehicle stops, energy from the wheels pumps fluid from a low-pressure reservoir into the highpressure accumulator. When the vehicle subsequently accelerates, the stored energy propels the vehicle. According to EPA officials, this process recovers and reuses more than 70% of the energy normally wasted during braking. And it also reduces brake wear by about 75%, substantially increasing net savings.

Optimum engine efficiency. Because there is no conventional transmission and driveshaft connecting the engine to the wheels, this frees the engine to run at maximum efficiency and optimize fuel economy. It runs at a near-constant speed while the controller varies pump output to generate flow according to vehicle demands.

Engine shut-off. The engine shuts off when not needed, such as when stopped or decelerating. And energy stored in the accumulators can also propel the vehicle for short hops with the engine off. As a result, engine use is cut in half in stopandgo urban driving.

The added cost for a hydraulic-hybrid system is estimated at under $7,000, less than 15% of the base vehicle price. Thus, at current fuel prices, the technology will pay for itself in less than three years. And net lifetime savings over a vehicle's typical 20-yr life span would be over $50,000, according to EPA estimates. Hydraulic-hybrid technology also has significant commercial potential for other medium-size vehicles, such as shuttle and transit buses, and waste-disposal trucks.

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