New hybrid powertrain works well both in theory and in practice, producing large economy gains and a smooth driving experience

What is it?

Underneath the skin of this Peugeot 2008 is Peugeot-Citroën’s new Hybrid Air transmission in prototype form. The French company says the new transmission is a big part of the company’s attempts to put a production car capable of an average 141mpg on the road by 2020.

Until now, a ‘hybrid’ car has been one that combines an internal combustion engine with an additional electric drive motor and a battery pack. PSA’s solution is simpler and claimed to be far more cost-effective than the petrol-electric drivetrain pioneered by Toyota and others.

While the system is dubbed ‘Hybrid Air’, it might be better described as 'Hybrid Hydraulic'. This 2008 is primarily powered by the company’s 1.2-litre, three-cylinder, naturally aspirated 81bhp petrol engine. A new gearbox has been designed for the car using a planetary gear set (similar in principle to the classic Strumey-Archer three-speed bicycle hub gear). 

This type of gearbox allows power to be split between the front wheels and a pair of hydraulic pumps, which are mounted between the transmission casing and the nearside front wheel. Mounted along the centre tunnel of the car’s floorpan is a high-pressure cylindrical tank. This tank contains a bladder filled with a combination of air and nitrogen. 

Under the rear seats is a conventional fuel tank while under the boot floor is another cylindrical tank, designed for low-pressure storage. The two cylindrical tanks and the hydraulic pumps in the transmission are all connected by a series of pipes to allow hydraulic fluid to flow around this closed-loop system.

While the engine and transmission can power the front wheels in the conventional way, it can simultaneously use the hydraulic pump to force oil into the centre tank. The oil compresses the air and nitrogen bladder as it is forced into the tank, creating an oil pressure as high as 220bar. 

When the car is coasting downhill, with the engine idle, the planetary transmission can also turn on the hydraulic pump and pressurise the centre tank with oil. 

When the centre tank is fully primed with hydraulic fluid under high pressure, it can be released back through the hydraulic pump, which then acts as a drive unit to turn the front wheels without assistance from the engine.

The depressurised oil is returned to the low-pressure tank under the boot floor before being drawn back into the hydraulic pump then the driver is coasting or heading downhill.

What's it like?

This all may sound a little improbable, but we tried the 2008 prototype in the heavy traffic of central Paris and found it to be pretty remarkable. 

The main visible changes to the 2008 include the adoption of a three-mode automatic transmission selector in the centre console, a hybrid performance readout on the instrument cluster and a ‘charge’ status for the central tank on the touchscreen.

The effectiveness of this deceptively simple drivetrain was demonstrated in the first few hundred yards of the test drive. According to the prototype's dashboard indicator, a few seconds coasting down into a Parisian underpass was enough to fully charge the central tank. That, according to PSA engineers, should store enough energy in the oil tank to power the 2008 for up to 200m without needing the engine.

Back to top

This doesn’t sound much, but in the stop-start conditions of central Paris, there was enough energy gathered from coasting (this hybrid is best charged by coasting rather than braking as in the case of conventional electric hybrid transmissions) to drive the car quite some distance. 

Indeed, on my 15min test route the engine was idle for a remarkable 9min. PSA engineers say that a standard car with the same petrol engine and a manual gearbox would return about 47mpg in city conditions. This prototype manages between 80mpg and 90mpg.

Even in this early form, the car was smooth and easy to drive, and it was very hard to tell when the full 29bhp was being added by the hydraulic drive.

Should I buy one?

On this brief showing, you might want to, but a production car with the Hybrid Air transmission is at least three and half years away. PSA engineers say the company wants at least one big partner to buy into this technology for production cars before it can proceed. 

Although it is estimated to be around half the factory cost of a conventional battery-electric hybrid transmission (as well as being around 15kg lighter), sources say the company will not commit to the technology unless it is rolled out on millions of B and C-segment cars each year. 

Aside from the big investments needed to bring the system to market, it is easy to detect PSA’s fear that, if the company went it alone, the technology would fall to the same fate as Betamax video.

But, as they point out, Hybrid Air does without batteries and electric motors, both are which are dependent on the extraction of rare-earth metals. Hybrid Air is simple, seemingly robust and based on well established technology. It just needs at least one other big carmaker to buy into the idea.

Peugeot 2008 Hybrid Air prototype

Price £15,000 (est) 0-62mph na Top speed na Economy 106mpg (combined, est) CO2 50g/km (est) Kerb weight 1165kg Engine 3 cyls, 1199cc, petrol plus pressurised hyrdaulic assist Power 112bhp Torque na Gearbox Planetary with power split.

Back to top

Join the debate

Add a comment…
dgate 11 July 2014

Not yet there

Any attempt toward efficiency is to be applauded but with this one there is no comparison to an electric hybrid.
A pitiful 15kg weight difference, 3 cyl 1.2 litre ICE and 200 metre travel on hydraulic power are hardly competitive with a Prius just check the figures. Its old technology, tanks, valves, pumps, hoses, pipes, that make it cheap but will customers want second best in efficiency.
One comment from above mentioned lack of fire risk compared to an electric hybrid which I find absurd since unaware of any. All conventional fuelled cars including the Peugeot are potential fire risks which don't seem to be a problem for buyers or solicit comments.
d79m 14 July 2014

If you read the article it

If you read the article it may only get 200m of travel but it also recharges in a few seconds of coasting! So how exactly is that no comparison to an electric petrol hybrid apart from not haing to worry about the thousands of pounds its going to cost when your batteries go one day.
Fazil 9 July 2014

Fiat Chrysler

Fiat Chrysler should definitely partner up with PSA to take down the Germans.
jonbon088 5 July 2014

Mixed up

Citroen is part of PSA Peugeot Citroen, they're looking for a partner other than themselves like Ford or GM for example.