Hello Doctor! Going back some two-to-three years, I seem to recall that you wrote about a seemingly clever mild hybrid system that had been developed by Peugeot, using compressed air as the energy reservoir, in place of a battery, and thus employing a lot less technology than something with a lithium-ion battery and all the associated and probably costly electronics. Do you know what happened to this sensible system, or whether it just died, and for what reasons?  

Len Asher

You are quite correct Len regarding this proposed Hybrid Air PSA system, and it was apparently prepared and ready to be incorporated in 2017 Peugeot 208s. But the EU regulatory and taxation authorities (bless them!) apparently collectively declined to offer similar new purchase subsidies for cars with such systems to those offered on more conventional battery supported hybrids, but please don’t ask me why! Considering that subsidies on newly registered hybrids are rapidly disappearing anyway, with the primary government support being focused on plug-in pure electric cars, Citroën and Peugeot may have been fortunate in killing Hybrid Air, or at least putting it “on the back burner” at an early stage. (Maybe the Government already had it in mind to cut the amount of hybrid subsidies?) 

The system used was a combination of hydraulics and compressed air, basically comprising of two hydraulic units and their associated air pressure accumulators. The hydraulic components (motor and pump) recover and store the energy generated by the internal combustion engine, and by braking and deceleration (kinetic energy that would otherwise be lost as heat) is converted into hydraulic energy, which is stored in a pressurised air accumulator. This energy can then be used to drive the car for short periods, primarily in city driving, just like a normal hybrid and, according to Peugeot, had very beneficial effects in lowering carbon dioxide emissions in the official urban test cycles. Yes, I know you’re thinking what this hydraulic accumulator is all about, if the storage medium is compressed air. Well, hydraulic fluid, pressurised by a hydraulic pump that draws energy from the deceleration of the car, forces the piston of the accumulator to compress the gas reservoir in the air chamber. The compressed gas stores the energy just like a spring, and when the stored energy is needed, it is released to drive the car, according to the needs and the available energy, as assessed by the management system. 

Almost as complicated as electronics? Maybe? That’s the story though, and I fear that we shall probably not hear any more of the Hybrid Air system. 

Best regards,

Doc