|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.)