Bosch’s new, hydraulic full-hybrid powertrain, developed in collaboration with PSA Peugeot Citroën, is intended for passenger cars as well as light delivery trucks operating in urban environments.
The hydraulic hybrid combines a conventional internal-combustion engine 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, 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.
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% percent. 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.
What’s more, the hybrid system makes use of energy that would normally go to waste. Kinetic energy captured during braking is converted into hydraulic energy and stored in the pressure accumulator. When the vehicle is travelling at a constant speed, the engine can be run within an efficient range while also filling the hydraulic energy accumulator.
Detailed technical description of 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 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 pressure accumulator has a limited capacity and range compared with lithium-ion batteries but it is much quicker to recharge and can use the extra energy provided by the internal-combustion engine more efficiently.
Collaboration between Bosch and PSA
The collaboration between Bosch and PSA Peugeot Citroën dates back to an engineering alliance set up in 2008. In 2011, this strategic partnership saw Peugeot launch the 3008 HYbrid4, the world’s 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.
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