by Jim Kerr

When Ford first showed off the Hydraulic Power Assist in their Mighty F350 Tonka concept truck at last January’s North American Auto Show in Detroit, it looked like an interesting concept. Most concepts never go beyond that stage of development. Fortunately however, Ford saw several advantages in this concept. This summer they announced that a demonstration fleet of E550 delivery vans will be equipped with this new innovation and be on the road by 2004.

Hydraulic Power Assist (HPA) is an addition to a vehicle, complementing the regular engine and drivetrain. The concept is simple. Store energy during deceleration, and release it for acceleration. The fuel savings could be dramatic. Ford research has shown that in stop and go driving situations, the delivery vans equipped with HPA could increase fuel economy by 30 to 35 percent and reduce exhaust emissions by at least 20 percent. An added benefit is up to 70 percent reduced brake wear because the HPA provides driveline braking.

How HPA Works
Click image to enlarge

So how does it work? Add a shaft driven reversible hydraulic pump/motor to a strengthened four-wheel-drive transfer case, and use two accumulator tanks to hold the hydraulic fluid. One tank is for low pressure fluid and the second tank stores high pressure fluid. The tanks
use a composite construction and are similar to those used for compressed natural gas on-vehicle storage.

During deceleration, a computer controlled coaxial clutch is used to connect the pump motor to the transfer case. Hydraulic fluid is pumped from the low pressure tank into the high pressure tank and stored at up to 6,000 psi. When the vehicle starts out again, the computer operates solenoid valves, directing the fluid out of the high pressure tank,
through the reversible pump/motor, and back into the low pressure tank. As the pump is driven, it transfers its torque through the transfer case to the vehicle’s driveline, assisting the vehicle to accelerate. The HPA system is over 70% efficient at converting the energy lost in deceleration to power used for acceleration.

Testing done at Ford’s Dearborn Proving Ground has shown the HPA system provides a significant boost in power. An F350 pickup research vehicle could make it up a 30 percent grade, 120 foot high hill in 6.6 seconds, compared to 12.6 seconds in a similar vehicle without HPA. That’s a 52 percent improvement. An E550 van loaded to 19,000 pounds showed a 37 percent improvement in 0 to 50 kph acceleration – 4.4 seconds vs. 7.0
seconds for a similar vehicle without HPA. The current model HPA system in Ford’s Scientific Research laboratory was producing up to 600 horsepower hydraulic assist to the conventional driveline!

Will we see HPA and all the power it recycles on regular passenger vehicles? Not likely. The HPA system is quite large. Although it could be reconfigured to many different shapes, the size of the storage tanks is the limiting factor. Large truck frames offer the room and strength to mount the HPA assembly. The unit is quite heavy. Strong storage tanks, and heavy duty pump add weight, which would have a significant impact on light passenger vehicles, but not much on heavily loaded work units.

The biggest advantages of HPA are gained in stop and go driving. City delivery trucks are the logical choice for HPA installation. Highway trucks that drive mainly at constant speeds would gain very little from HPA, and still have to carry the extra weight of the unit with them all the time.

Finally, with up to 600 horsepower on tap, Hydraulic Power Assist might make an interesting addition to someone’s drag racing car. With all that torque available from zero rpm all the way up, stronger drivelines and bigger tires would be suggested. Acceleration times could be very interesting. Hmmm.

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