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.
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.