BOXBERG, Germany – Hydraulic pressure, already key to steering, braking and lifting hoods and tailgates in modern vehicles, also can be effective and efficient in driving the wheels, as proved by a new concept from Robert Bosch and PSA Peugeot Citroen.

The German supplier and French auto maker are working on what is considered to be the world’s first hydraulic hybrid powertrain for passenger cars.

A pressure accumulator filled with nitrogen – and placed underbody in the tunnel between the front seats in a concept on display here at Bosch’s test track – stores braking energy that normally would be wasted, and a hydraulic motor then converts it into propulsion.

The system is designed to provide a boost effect similar to that offered by complex electric drives. Compatible with gasoline and diesel engines and well-suited for compact cars, Bosch says the hydraulic hybrid can reduce fuel consumption in normal driving conditions 30% and in city driving 45%, based on the European test cycle.

No additional battery pack is required, and the system’s overall weight is under 220 lbs. (100 kg). The fuel-economy gain is based on computer modeling, and physical prototypes already are on the road, says Rolf Bulander, president-Bosch gasoline systems division, at last week’s international press conference here, where the concept was unveiled.

For short journeys, Bosch says the vehicle can run exclusively on hydraulically generated power, enabling zero-emissions driving while the internal-combustion engine remains inactive.

For longer trips or when driving at higher speeds, accelerative force comes from the engine, which occasionally gets additional boost from energy stored in the hydraulic accumulator.

The nitrogen-gas cushion within the accumulator compresses kinetic energy from braking, much like a coiled spring. Once the pressure within the accumulator is relieved, the system works in reverse, Bosch says.

The gas expands once more, providing a compressive force on the hydraulic fluid and driving a hydraulic motor. The motor delivers the stored energy back to the vehicle via the transmission. The boost effect occurs when the combustion engine is running and the hydraulic accumulator is discharging at the same time.

Bulander describes the hydraulic-mechanical system as a low-cost, robust and easy-to-service hybrid powertrain that requires no specialized infrastructure and gives the driver loads of torque.

He admits the pressure accumulator has less capacity and range than lithium-ion batteries found in electric cars, and the benefit is less dramatic on long-distance trips. But he says this new concept recharges more quickly and can use extra energy created by the combustion engine more efficiently.

“It can drive short distances completely emissions-free, and its boost effect adds to driving enjoyment,” Bulander says. “Moreover, there are no worries about range.”

City conditions, with a lot of stops and starts, are ideal for the system because decelerating for a mere 328 ft. (100 m) can provide full storage of kinetic energy in the pressure accumulator. “So the next time you accelerate, you can easily benefit from that,” he says.

The hydraulic accumulator allows engine downsizing. Bulander says the system could mate well with a turbocharged engine as small as 1.4L.

Bosch says the system also could work well for light delivery trucks in urban settings.

Bulander says the system remains under development and that PSA has set a 2016 target date for production.

A cutaway of a Peugeot compact cross/utility vehicle was on display with the hydraulic hybrid here at the Bosch event. Bulander says he expects a version will be available for driving at the supplier’s next international press conference two years from now.

The relationship between Bosch and PSA is not exclusive, and Bulander says the parties are free to shop the technology to other customers.

It’s too early to discuss system cost, but he says the goal is to make it less expensive than hybrid-electric powertrains.

Bosch and PSA set up an engineering alliance in 2008 and in 2011 launched the 3008 Hybrid4, the world’s first series-produced diesel hybrid passenger vehicle with axle-split drive.

Another Bosch powertrain development unveiled here is the eClutch, an electrically operated clutch that shifts into neutral when the driver is not accelerating.

In addition to improving fuel economy 5%, Bosch says the eClutch will make driving with manual transmissions more convenient. In traffic jams, drivers can start off in first and second gears without using the clutch.

tmurphy@wardsauto.com