Hydraulic hybrid cars and trucks

Written by Camilo H. Rueda

The new HYDRAULIC HYBRID technology is designed mainly to improve both fuel efficiency and overall operating economy of vehicles with Stop and Go duty cycles by capturing braking energy and then using it to help accelerate the vehicle.

Depending on the type of vehicle, the Hybrid system and the stop an go cycle used, the fuel savings can range from 20 to 70%.

While all hydraulic hybrids use reservoirs, accumulators and pumps, there are two ways those components can be coupled with a vehicle. The first, a parallel hydraulic hybrid, simply connects the hybrid components to a conventional transmission and driveshaft. While this allows the system to assist the gasoline engine in acceleration (when the gasoline engine is working its hardest), it doesn't allow the gasoline engine to shut off when the vehicle isn't in motion. That means the vehicle is always burning gas (unlike gas/electric hybrids, which have engines that shut off at slow speeds or during a stop).

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The Hydraulic Axial Piston Unit (Pump/Motor) is coupled to the mechanical drive train through a gear box. When the brakes are applied, the axial Piston Unit acts as a Pump converting kinetic into hydraulic energy and pumps Hydraulic fluid into the accumulator increasing the pressure inside of it.

At start up, The Hydraulic Axial Piston Unit (Pump/Motor) acts as a motor converting the hydraulic energy stored in the accumulator into kinetic energy helping the main transmission to accelerate the vehicle. The control valve helps the system from overpressure and the filling and discharging process. The Hydraulic Axial Piston Unit remains coupled to the mechanical drive train until the pressure accumulator is discharged.

Courtesy of BoschRexroth)

HYDRAULIC AXIAL PISTON UNIT (PUMP/MOTOR)

Series Hydraulic Hybrid Systems use the same process as Parallel, but do not use a conventional mechanical transmission and transmit power directly to the wheels.

As we discussed before, reducing the number of components energy has to pass through before hitting its target makes it more efficient. Because the hydraulic system itself is turning the wheels, not a regular transmission or driveshaft, the vehicle's gasoline engine can be shut off, resulting in even more fuel savings. With the added efficiency and the ability to shut off the gasoline engine, series hydraulic hybrids are estimated to improve fuel economy by 60 to 70 percent, and lesser emissions.

Courtesy of BoschRexroth)

SERIES HYDRAULIC HYBRID SYSTEM

MOTOR

PUMP

ACCUMULATORS

HYBRID CONTROL

In contrast to the Parallel Hydraulic Hybrid System, the SHHS uses components in the hydrostatic transmission. Actual vehicles can therefore be retrofitted with the series system easily with low cost and effort. This kind of system can be implemented in fork lifts for example.

Almost all the big player in the Hydraulic Market are already developing this technology. Eaton, Parker, Rexroth, Sauer Danfoss and others are providing and testing Hydraulic Hybrid Systems to make them something for the present not for the future.