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Clean Automotive Technology

Hydraulic Hybrid Vehicle Layouts

HHV technology provides the possibility for multiple drivetrains or hardware configurations. Each drive train has a variety of advantages and disadvantages. Each drive train has a variety of advantages and disadvantages. EPA and its industry partners are currently researching different types of drive trains to determine which will be the most durable, efficient, clean and cost-effective option. There are three drive trains that have emerged among the different vehicle applications.

HHV Layout 1

The first layout shown at right includes an integrated hydraulic hybrid rear drive assembly. The rear drive assembly consists of two hydraulic pump/motors integrated into the rear axle. The pump/motors of this assembly are capable of performing regenerative braking. There is one engine hydraulic pump/motor that produces fluid pressure when needed. The energy storage system consists of two hydraulic fluid storage vessels. The first vessel is for high pressure hydraulic fluid or hydraulic fluid pressurized up to 5000 psi (pounds per square inch). It is called an accumulator. The second vessel is for low pressure hydraulic fluid or hydraulic fluid pressurized up to 180 >psi. It is called a reservoir. The entire closed loop fluid system holds about 22 gallons of hydraulic fluid, with the possibility of even smaller storage systems. Hydraulic fluid is fundamentally no different from transmission fluid currently being used in most vehicles.

Initial laboratory tests results for this HHV layout in a pilot vehicle showed greater than 70% improvement in fuel efficiency (mpg) with a corresponding 40 percent reduction in CO2 emissions. At $2.75 per gallon fuel costs and with the projected high volume manufacturing costs of $7000, this hybrid would pay for itself in less than 3 years and deliver a lifetime (20 years) fuel and brake maintenance savings of over $50,000. This vehicle was tested on-road by UPS during 2006 & 2007.

UDV 1 layout graphic
UDV 1 Layout
HHV Layout 2

The hardware configuration for the second layout is shown at right. This vehicle has a single pump/motor coupled with a four-speed computer shifter clutch-less transmission. This vehicle has a similar engine pump/motor and energy storage accumulator system to that of the first layout.

Originally this design was applied with a standard diesel engine in a UPS delivery vehicle with encouraging results. Work on this vehicle has continued into phase 2 efforts to include an alchol engine. Alcohol engines use E85 (Ethanol) or M85 (Methanol) and burner cleaner than diesel or gasoline.This removes the need for NOx aftertreatment. An optimized alcohol engine is capable of 40 percent peak efficiency.

UDV 2 layout graphic
UDV 2 Layout
HHV Layout 3  

This design uses a drive system that is similar to the first layout. In this design the drive pump/motor is rotated 90 degrees and mounted to the chassis frame and uses a driveshaft to provide power to the rear differential. By not having the pump/motors directly integrated into the rear differential they can be integrated into a vehicle in a way that maximizes space.

This layout demonstrates the versatility of HHV technology because it has been or is being applied to urban delivery vehicles, yard holsters and shuttle buses. Additionally, it has the same potential for being used with advanced engines. It is currently being evaluated with a diesel engine in a yard hostler and is set to be evaluated with an HCCI (Homogeneous Combustion Charge Ignition) engine in a shuttle bus.

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UDV 3 layout graphic
UDV 3 Layout

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