The hydraulic full-hybrid powertrain technology, which Bosch is developing in collaboration with PSA Peugeot Citroën, is clear in its aims: to supply a hydraulic hybrid powertrain that will significantly reduce fuel consumption and CO2 emissions in compact cars.
The hydraulic hybrid is designed to enable a boost effect that would normally be offered only by complex electric drives. Here, a conventional internal-combustion engine combines with hydraulic units and an accompanying nitrogen pressure accumulator to provide a brief boost to acceleration. The hybrid system is able to support gasoline and diesel engines in ranges where they do not work at optimum efficiency.
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, acceleration 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.
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 45%. As a result, the range of a compact car can be greatly increased using this alternative powertrain.
What’s more, the hybrid system makes use of energy that would normally go to waste. Braking, for instance, quickly fills up the hydraulic accumulator: the kinetic energy captured during braking is converted into hydraulic energy and stored in the pressure accumulator. The advantages of a hybrid powertrain are equally evident when the vehicle is travelling at a constant speed. Here, the engine can be run within an efficient range while also filling the hydraulic energy accumulator.
This hydraulic-mechanical system makes for a low-cost, robust, and easy-to-service hybrid powertrain. Since it needs no specialized infrastructure, it can be used around the world.
Detailed technical description of hydraulic hybrid operation
In addition to a conventional internal-combustion engine, a hydraulic hybrid powertrain also includes a pressure accumulator and a reservoir. 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 over 300 bar. 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 compression 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.
It is true that the pressure accumulator has a more limited capacity and range than the lithium-ion batteries found in electric cars. Nonetheless, it is much quicker to recharge and can use the extra energy provided by the internal-combustion engine more efficiently.
I guess that complete system simulation could answer if such a technical solution is better than using electric hybrid in terms of fuel economy over a cycle. However, even if it has a better fuel consumption gain, I’m not sure that it is more packageable than an electric hybrid. Do you think that the pressure accumulator can be integrated in a car of the B or C segment?