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PSA Peugeot Citroën and Bosch developing hydraulic hybrid powertrain for passenger cars; 30% reduction in fuel consumption in NEDC, up to 45% urban; B-segment application in 2016

Elements of the “Hybrid Air” hydraulic hybrid powertrain under development for B, C, and D vehicles. Click to enlarge.

Following the introduction of the Bosch electric axle-split hybrid in PSA Peugeot Citroën diesel vehicles (earlier post), the two companies are now planning to develop a hydraulic hybrid powertrain for passenger cars. The hydraulic system, which uses compressed air for energy storage, basically comprises two hydraulic units and their pressure accumulators. PSA says the technology—called “Hybrid Air”—will be fitted on B-segment models starting in 2016.

The hydraulic components (motor and pump) recover and store the energy generated by the internal combustion engine and by braking and deceleration (kinetic energy); kinetic energy from braking that would otherwise be lost as heat is converted into hydraulic energy and stored in a pressure accumulator. This energy can then be used to drive the car.

A specific continuous transmission enables optimal use of the different energy sources depending on the type of driving in three operating modes:

  • Gasoline power, with the gasoline engine as the sole source of propulsion.

  • Air power, with the hydraulic motor alone transmitting energy to the wheels via the accumulators.

  • Combined power, with the internal combustion engine and hydraulic motor working together. At low loads, the internal-combustion engine can be run at a more economical operating point.

The result is a potential average CO2 reduction of 30% in the new European driving cycle, and as much as 45% in a purely urban driving cycle. Certified fuel consumption stands at 2.9 l/100km (81.1 mpg US) in combined-cycle driving, for CO2 emissions of around 69 g/km for standard body style models such as the Citroën C3 or Peugeot 208.

In comparison, PSA Peugeot Citroën’s 3-cylinder gasoline engines with manual gearshift produce 104 g of CO2 per km in combined-cycle driving. Vehicles equipped with Hybrid Air technology can also run on Air Power alone (with no CO2 emissions) 60 to 80% of the time in city driving, depending on traffic density, thanks to the optimal efficiency of energy recovery during braking.

PSA suggests that the technology—based on materials that are both plentiful and easily recyclable,—offers a CO2/price trade-off unmatched by any of the current hybrid technologies, representing a breakthrough. The company has filed 80 patents on the work.

Price vs. emissions positioning of the Hybrid Air technology. Click to enlarge.

In principle, this technology can be combined with any conventional engine. In an initial phase, it is to be used in the compact car segment, but it is also suitable for other passenger-car segments and light delivery trucks in urban traffic. This hydraulic-mechanical powertrain system results in a hybrid powertrain that is more cost-effective, robust, and service-friendly. In addition, it does not require any special infrastructure, and can be deployed anywhere in the world, Bosch noted.

The technology works on the same hydraulic powertrain principles as those used worldwide on a wide scale by the Bosch Rexroth. Bosch and PSA see great potential for adapting this technology to passenger cars.

The close collaboration between PSA Peugeot Citroën and Bosch dates back to an engineering alliance set up in 2008. In the context of this strategic partnership, Peugeot launched the 3008 HYbrid4 in 2011, the first series-produced diesel hybrid passenger car with an axle-split powertrain.

In close collaboration with Bosch, PSA Peugeot Citroën developed the electrical components (electric motor, power electronics, and high-voltage generator) as well as the special technical design of the ESP electronic stability program required for hybrid vehicles. This powertrain concept now also features in PSA’s Peugeot 508 (both the RXH station wagon and the HYbrid4 sedan) and Citroen DS5 HYbrid4 models. For these models as well, Bosch supplies the components for the electrical powertrain.

Automotive technology is the largest Bosch Group business sector; its sales came to €30.4 billion, or 59% of total group sales, in fiscal 2011.





Very low according to their diagram.

It looks very good - it isn't flashy like HEV or PHEV, but it might be cheap enough to roll out across most cars and so would save a lot of fuel and CO2 (and Peugeot).
It is a pity it will take till 2016 - they should go as fast as they can.

Lets hope it is more than a load of hot, compressed air.


cost for the Prius hybridization is about $1800.. so they have a target there


I don't know much about hydraulics, thus I find it difficult to evaluate this concept.

Looks like we all have some homework to do.


Hydraulic hybrids have been around for a while, I'm sure there's nothing all that tricky about this.  The high specific power means that the energy storage can be sized for the kinetic energy of the vehicle, not several times as much (which wastes capacity).


If the performance numbers hold up this is a nice way to get around the high cost of Li ion hybrids as well as their inherent safety (fire) issues.


I think that their graph is quite optimistic. Maybe not for cost but certainly for implied efficiency. Hydraulics are probably cheaper and maybe easier to maintain especially with a less sophisticated workerforce but they are not more efficient nor are they more reliable. Also compressed air (actually dry nitrogen) is quite limited in the amount of energy that be store for a given mass compared to batteries.

Where hydraulics would probably be best is for trash trucks and mail delivery where you continuously start and stop but never drive very far or fast between stops. Maybe European traffic is closer to this, but this system would not work well in most American cities were the distances are further and the speeds are greater.


The neat thing is that this can still work as a plug-in hybrid, and the battery doesn't have to have any high-power capacity; it just has to be able to run a hydraulic pump to top up the accumulators during low-speed operation.

A battery which cannot discharge at a high rate would appear to be resistant to failure by self-heating and fire.


Initial cost + on-going repair cost may be a challenge.

Toyota Canada is currently pricing (list) for the 2013 Camry XLE HEV at $30,800** and the regular 2013 V-6 ICE Camry XLE at $32,035. Both of the above include the $1,595 transportation and set up charges but not the sales taxes.

**A Peugeot 508 Hydraulic Hybrid would cost about twice as much?

** The 2013 Camry HEV seems to be a much better buy (at $1,235 less) than the regular 2013 Camry ICE.


I'd like to know the round trip energy efficiency of a hydraulic hybrid system. I recall having read somewhere that for NiMH batteries it's only about 50%, due mainly to poor charge acceptance; and it's somewhere around 80-90% for LiIon batteries. If anyone has better numbers please share your info and sources. Thx.


Interesting question.  Ecogeek's page claims 75-85%.


Air hybrid are much more efficient than batteries at returning brake energy, on the Prius I think barely 30% of the brake energy is recycled. It is really good thing that PSA develops that approach a diesel with an air hybrid would leave a gasoline HEV behind both in fuel economy and price. Also such a system would be much more suited to light truck than HEV that don't perform well on heavy cars. Yes 2016 is late but EV won't be around anytime soon so there is still room on the market for this type of approach

Nick Lyons

I like this for a city car--simple tech, low cost materials. Mechanical systems will wear more than electric hybrid components, but no expensive battery to replace down the road, fewer recycling issues. I like it.


a flat, thin pancake hydraulic motor placed in-between the engine and transmission would be interesting


'Air hybrid are much more efficient than batteries at returning brake energy'

Only kind of.
It is true for most lithium chemistries, the issue being the ability to rapidly charge/discharge.
Lithium titanate batteries as used in the Honda EV can do this a lot faster, as can electric systems using capacitors to assist by accepting and supplying energy fast.

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