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Bosch offers more details on the light-duty hydraulic-hybrid drive developed with PSA

The concept and layout of the hydraulic hybrid system. Click to enlarge.

Bosch, in collaboration with PSA Peugeot Citroën, is developing a hydraulic full-hybrid powertrain to reduce fuel consumption and CO2 emissions in light-duty vehicles. (Earlier post.) PSA is applying the technology in “Hybrid Air” vehicles from both Citroën and Peugeot (earlier post, earlier post), showcased at the Geneva Motor Show.

The hydraulic hybrid is system is designed to enable a boost effect that would normally be offered only by more complex electric drives. A conventional internal-combustion engine combines with hydraulic units and an accompanying gas (e.g., nitrogen) pressure accumulator and a reservoir to provide a brief boost to acceleration. Like electric hybrids, the hydraulic hybrid system is able to support gasoline and diesel engines in ranges where they do not work at optimum efficiency.

Hydraulic hybrid powertrain. Click to enlarge.

In operation, hydraulic units compress a gas cushion using hydraulic fluid. Fluid and gas are kept separate from one another. The gas cushion stores energy by the gas being compressed rather like a coiled spring. At this point, the pressure in the system is above 300 bar (4,351 psi). The amount of energy that can be stored in the pressure accumulator depends on the size of the system.

As soon as the pressure within the accumulator is relieved, the system works in reverse. The gas expands once more, providing a compressive force on the hydraulic fluid and driving a hydraulic motor. This motor takes the stored energy and delivers it back to the vehicle via the transmission.

The kinetic energy captured during braking is converted into hydraulic energy and stored in the pressure accumulator. Normally, this energy would go to waste, turning into heat in the friction linings of the brakes.

The pressure accumulator has a more limited capacity and range than the lithium-ion batteries found in electric cars. However, it is much quicker to recharge and can use the extra energy provided by the internal-combustion engine more efficiently, Bosch says.

The power-split concept permits various drive options. For short journeys, stored energy can be used to run exclusively on hydraulically generated power, with the internal-combustion engine remaining inactive and the vehicle producing zero emissions.

For longer journeys, or when driving at higher speeds, accelerative force is provided by the internal-combustion engine. Alternatively, the two types of powertrain can also be combined. In this case, the energy stored in the hydraulic system and the fuel burned in the internal-combustion engine work together to drive the vehicle, which also provides a brief boost effect.

Bosch and PSA Peugeot Citroën see great potential in this technology. In the new European driving cycle, it has the capacity to reduce fuel consumption by up to 30% when compared to a conventional internal-combustion engine. For purely urban driving, this rises to as much as 45%. As a result, the range of a compact car can be greatly increased using this alternative powertrain. The improved efficiency is due to the careful configuration of the two powertrain components.

The close collaboration between Bosch and PSA Peugeot Citroën dates back to an engineering alliance set up in 2008. In 2011, this partnership saw Peugeot launch the 3008 HYbrid4, the first series-produced diesel hybrid passenger car with axle-split drive. PSA Peugeot Citroën developed the electrical components (electric motor, power electronics, and high-voltage generator) in close collaboration with Bosch, a collaboration which extended to developing the special technical setup needed to use the ESP electronic stability program in hybrid vehicles.

The hybrid powertrain concept now also features in PSA’s Peugeot 508 (both the RXH station wagon and the HYbrid4 sedan) and Citroën DS5 HYbrid4 models, for which Bosch supplies the electrical powertrain components.

Bosch has also been working on hydraulic hybrid drivetrains for heavy-duty vehicles, and showcased its parallel hydraulic hybrid drivetrain for that market at the IAA Commercial Vehicle Show in 2008. (Earlier post.)


Andy Kennedy

"In the new European driving cycle, it has the capacity to reduce fuel consumption by up to 30%"

There's your problem right there.

The New European Drive Cycle, aka the NEDC. Which is the biggest load of crap any government bureaucrat could ever have devised.

It vastly overestimates fuel economy figures by allowing grossly excessive acceleration times, thus rewarding car makers who produce boosted downsized engines that on the NEDC, never actually go on boost.

I mean, does anyone take anyone 26 seconds to accelerate to 30mph???
In 26 seconds of a morning, I'm halfway down the street having negotiated 2 corners and a stop junction, never mind at 30mph. And I'm not a particularly quick driver. Sit and count out 26 seconds to yourself, its quite a while.

The NEDC is ludicrously short as well, so a low energy content system like this could well save a load of fuel, but again, its not representative. Most normal drivers will probably use all the stored energy the first time they accelerate to 40mph.

Put it this way, a Ford Focus Ecoboost 1.0 litre turbo, is supposed to get 56 mpg (imperial) (46mpgUS) on the NEDC.
I own one of these and struggle to get 40mpg average even taking it easy on the power. And I'm not the only one, most owners struggle to post decent fuel economy. in contrast many diesel owners can often match their NEDC numbers, supporting the issue of car makers using cycle beating technologies, that offer little benefit to consumers in actual use.

On the NEDC, the urban cycle spends a disproportionate amount of time stationary, which with stop start technology fitted, means no fuel burn at rest.
Again, in reality, most cars spend most of their time moving (albeit slowly), only being stopped briefly at traffic lights.

I'd be surprised if the hydraulic hybrid was as good as the electrical versions, simply because the basic physics preclude them from storing much energy. Its ok on commercial vehicles, indeed on buses it would be ideal to avoid heavy engine load pollution when pulling away from a stop. In British cities, diesel is now the major source of inner city pollution, and it ain't the cars that are to blame....



Hydraulic hybrids are most beneficial for service vehicles with frequent stops (e.g., delivery vehicles, trash trucks, buses). The hoped-for advantage is that accumulators are much cheaper than batteries, and can absorb a higher precentage of the regenerative braking energy. The downside is that a reasonable accumulator only stores seconds worth of impulse power, they're noisy, and they usually require changes to the transmission that could affect durability. Lightning Hybrids of Colorado sells retrofits for delivery trucks and buses, and just got another US$4 million in funding a few months ago.


@Healthy, agree with your analysis. But imagine the billions of gallons of fuel we would save every year in the US if FedEx, UPS, USPS, every city bus and school bus, and every garbage truck were equipped with this. It would be incredible.

And it could be done in 5 years if we really cared to do it.


The saved gas will be sold to Europe by speculators and we will be left with the same high gas prices and the need for Middle East oil



Maybe so, but it's negawatts of energy we won't import, purchase, burn and suffer the soot from, so and the owners of the vehicles will save, win, win, win.

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