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Off to a quick start: Eaton's Corp.'s Hydraulic launch assist system demonstrated in shuttle bus, delivery van applications.

It is generally accepted that hydraulic hybrids are well suited for the frequent start-stop duty cycle of medium-duty vehicles like shuttle buses and package delivery trucks. In both parallel and series hydraulic hybrid systems, benefits include fuel savings that occur when stored hydraulic energy is used to assist the engine during acceleration. Acceleration can also be significantly improved due to the high power density of hydraulics, and the energy transfer is virtually transparent to the driver. Also, the regenerative braking inherent in the systems reduces brake wear.

Prototypes for both parallel and series hydraulic hybrid vehicles have been developed using Eaton Corp.'s Hydraulic Launch Assist (HLA) technology. Eaton supplied its parallel HLA system for use in a hybrid shuttle bus demonstrator program managed by IMPACT Engineering Inc. As part of the U.S. Army's HAMMER (Hydraulic Hybrid, Advanced Materials and Multifuel Engine Research), the program is aimed at reducing fuel consumption in military ground vehicles.

Ford Motor Co. was responsible for the design and manufacture of the vehicle, MKP Engineering preformed structural analysis and the University of Toledo, Ohio, was responsible for hydraulics research.

In a parallel hydraulic hybrid configuration, the conventional powertrain is augmented by a hydraulic system consisting of a reversible hydraulic piston pump/motor coupled to the driveshaft through a clutch, a high-pressure accumulator, a low-pressure accumulator or reservoir and associated plumbing.

Fuel savings come from regenerative braking and by shutting off the engine rather than letting it idle when the bus is stopped. When a driver steps on the brake pedal, kinetic energy from deceleration drives the pump/motor as a pump to force hydraulic fluid out of a low-pressure fluid reservoir into the high-pressure accumulator. The fluid compresses nitrogen gas in the accumulator to pressurize the system. At start-up or in acceleration conditions, the HLA system switches from pump to motor modes. The nitrogen gas forces the hydraulic fluid back into the low-pressure accumulator passing through the pump/motor, which applies torque to the driveshaft through the clutch.

In the hydraulic hybrid shuttle bus, a 14,000 lb. GVW Ford E-450 Cutaway chassis is used with a bus body supplied by Glaval Bus. The bus is powered by a 6.0 L Ford Power Stroke V8 diesel engine rated 235 hp with 440 lb.ft. of peak torque. Eaton's Gen 2 HLA system boosts the overall output of the drivetrain to approximately 300 hp and 1000 lb.ft. of torque.

High-pressure fluid is stored at 5000 psi in the 10.5 L accumulator, which is part of the "filled for life" closed-loop system. When combined with a reduced axle ratio, the shuttle bus has demonstrated a 26% increase in fuel economy, and vehicle noise has been reduced during acceleration.

In the delivery vehicle application, a UPS package truck was developed through a partnership between the EPA, Eaton, UPS, International Truck and Engine Corp. and the U.S. Army. In this series hydraulic hybrid truck, a high-efficiency 6.0 L International VT365 V8 diesel was combined with a hydraulic propulsion system that replaced the conventional transmission and driveshaft. The series hydraulic hybrid gains its improved fuel efficiency through regenerative braking, by operating the engine at its optimum speed range and by shutting the engine off when not needed.

Again high- and low-pressure accumulators are used. A hydraulic pump driven by the engine pressurizes the system, while a combination, piston pump/motor connected to the differential drives the wheels and recovers kinetic energy during braking.

This hydraulic-diesel hybrid technology has achieved a 60 to 70% improvement in fuel economy and more than a 40% reduction in carbon dioxide emissions in initial laboratory testing. The EPA estimates that the technology has the potential to save more than 1000 gallons per year for each urban delivery vehicle. UPS will begin testing the vehicle this year on the road in the Detroit area and then will bring it to Cleveland for additional testing.

According to the EPA, when manufactured in high volume, the added cost of the hybrid components could be recouped in less than three years through lower fuel and brake maintenance costs. The series HLA has the advantage that a conventional transmission and driveshaft is not needed, while with the parallel HLA, the conventional drivetrain is still available for backup. In both systems, the hydraulic component is designed to last the life of the vehicle.

Bill Siuru, PhD, PE, is a Diesel Progress field editor based in Temecula, Calif.
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Author:Siuru, Bill
Publication:Diesel Progress North American Edition
Date:Apr 1, 2007
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