After PSA Peugeot-Citroën announced its plans to introduce a new "hybrid air" solution as part of its powertrain strategy, IHS Automotive's Global Powertrain Forecasting team has taken the opportunity to explore this technology.

As part of its strategy to remain a leader in low carbon dioxide (CO2) emitting vehicles, PSA Peugeot-Citroën revealed its "hybrid air" powertrain at a recent Innovation Day. The technology, which is being developed in partnership with German component supplier Bosch, is essentially a hydraulic-based system and combines an internal combustion engine (ICE) with a system of storing energy as compressed nitrogen. The energy is generated from the ICE and the kinetic energy from braking and deceleration. The compressed nitrogen is then forced at high pressure through a hydraulic motor and onward to drive wheels, either alone or in combination with the ICE through an automatic transmission with an epicyclic gear train. PSA claims that as a result of the technology, the potential average CO2 reduction compared to a standard vehicle featuring its three-cylinder gasoline (petrol) engines with manual gearshift will be 30% in the new European driving cycle (NEDC). This would equate to a certified fuel consumption for a B-segment model of 2.9 l/100 km in combined-cycle driving, with CO2 emissions of around 69 g/km. It added that this could be as much as 45% in a purely urban driving cycle, as vehicles featuring the technology can run on air power alone for 60-80% of the time in city driving, depending on traffic density, thanks to the optimal efficiency of energy recovery during braking. Although it can be used in a wide range of applications, PSA is intending to begin fitting the technology to its B-segment models from 2016.

Outlook and implications

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Faced with many alternative propulsion concepts, IHS Automotive's Global Powertrain Forecasting team believes that hybrid air, while not technically a breakthrough technology, is likely to be an interesting and valuable addition to its powertrain strategy as it attempts to diversify its alternative powertrain portfolio from those currently on offer. Immediate thoughts turn to classifying the system as a parallel hybrid. There is a direct connection between ICE and driven wheels and so it cannot be considered as a series-based range-extender format. However, we do see potential for this architecture to offer a "parallel-series" mode whereby the ICE can act as a generator to "re-gas'" the high pressure storage tank.

With regards PSA's expectations for CO2 emissions and the improvement in fuel efficiency these are always a function of the test cycle performance, in this case NEDC. Given the ability for the vehicle to complete 80% of the urban speed trace in "air only" mode with non-running ICE, it is perhaps of no surprise that such headline figures have been achieved. What is interesting however is that we assume air-hybrid vehicles will be subject to the regular ICE laboratory procedures for type approval, and not the separate procedures being developed for electrically-based powertrain vehicles in the Worldwide harmonised Light vehicles Test Procedures (WLTP). Therefore, although we expect these figures to worsen once run on the Worldwide harmonized Light duty driving Test Cycle (WLTC) and WLTP ICE test procedure (which have a higher average speed and less emphasis on low speed phase). The apparent abilities of the regeneration and boost function of hybrid air could provide an impressive countermeasure to the transient nature of the WLTC compared to NEDC.

There are other advantages from the use of this system too. One of the most obvious is the lack of batteries, electric motors, power electronics and other components, and the benefits that this brings in terms of weight, cost and safety. Nitrogen is non-flammable and it seems that PSA has accounted for the risk of high-pressure gas leakage in its tank design and location.

However, while the system appears to have a great deal of benefits, packaging issues regarding the high pressure gas cylinder compared to batteries make it unlikely that PSA would be able to offer all-wheel-drive variants. Therefore, the technology should be seen as complementary to the electric rear axle drive (E-RAD) system that it currently uses in the Peugeot 3008 and Citroën DS5.

Other questions also remain about certain aspects of the technology.

For now, IHS Automotive's Global Powertrain Forecasting team continues to discuss these open questions and gather further intelligence. Once a point of comfort is reached regarding market entry point, we shall reflect the system in our engine and alternative propulsion forecasting suites. For now, we consider that it looks set to be unique in the marketplace, with OEMs that already have plans for compact hybrid vehicles unlikely to change course, although we will watch with interest. Obviously General Motors (GM) is a prime candidate through its joint-venture (JV) with PSA, and Ford has attempted in recent years to use similar technology, although it decided on battery energy storage instead.

For further information, please contact Andrew Fulbrook, Senior Manager Global Powertrain Forecasting on andrew.fulbrook@ihs.com or +44 (0) 20 3159 3455.

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