How Peugeot-Citroen's Hybrid Air System Works: The Car That Runs on Air

Air: it allows humans to breathe, airplanes to fly, fills balloons, propagates sound waves and is basically a crucial part of our everyday lives. But can it fuel a car? Most people can only dream about a car that runs on air, but engineers may have found a way to turn it into reality.

Hybrid: the offspring of two animals or plants that becomes a breed all of its own, with different characteristic. When it comes to cars, we know hybrids as being the mating of internal combustion engine and electric motors. However, the French are fond of fusion cooking and decided there's a better way to make a mule other than combining the hard working donkey genes, the electric motor, and the horse genes, good ol' HP!

You see, Citroen has always been the sort of car company that has a strong fascination with fluids and gasses, long before Toyota made its first Prius. And so, when they decided to invent the new hybrid engine, they replaced the hardy donkey, the electric motor, with a puffer fish, an air compressor. We'd call it the "Puffahorse" (like a Pokemon), but Citroen call it simply "Hybrid Air".

Hybrid Air technology made its official debut in March 2013 at the Geneva Motor Show. “We are not talking about weird and wacky machines. These are going to be in everyday cars,” a PSA Peugeot Citroen spokesman said. The system will be in use on everyday cars by 2016 and will be cheaper than the equivalent Prius. With claims like this, we simply had to know how it works!

But first, let’s talk physics. In simplified terms, combustion engines transform fuel into power, which is then used to propel the cars up to speed. But all the energy used to get you up to speed is then wasted when you have to brake again, as calipers bite into your disk rotors and slow you down using friction force. The fact that your brakes are really hot to the touch is the most obvious indication that, from a kinetic point of view, you are wasting energy.

Conventional hybrids get around this problem using electric motors and generators. Instead of slamming on the brakes, they convert the braking energy into electricity which is then stored into a battery. The distance a car travels while braking looks very small because modern disk brakes are very strong, so it might seem like an insignificant problem, but in theory hybrids recapture the energy used to get you up to speed in the first place.

The Hybrid Air system uses much the same principle to run. It still uses a conventional but very efficient 3-cylinder combustion engine for highway cruising, but also has a motor that runs on air around town. This is stored in a pressurized tank situated in a tunnel under the middle of the car is compressed using a regenerative braking pump.

Just like a hybrid can run on petrol or electricity, or a combination of the two, so too will Citroen’s system run on gasoline or air, or both. Air power would be used only during city driving, automatically activated below 43mph (70 km/h) and available for ‘“60 to 80 percent of the time in city driving,” according to the French. The air tank has enough pressure for a daily commute of up to 50 minutes under ideal conditions, a lot better than the 15 or so km range some hybrids of today offer.

On paper at least, there are three major advantage. The carmaker says we can expect a reduction in our monthly fuel bill by up to 45%. Secondly, the Hybrid Air technology could allow a supermini like Citroen C3 or Peugeot 208 to emit as little as 69g/km of CO2. Lastly, air compressors and tanks are cheaper than advanced battery technology, meaning the cars will be much cheaper to buy in the first place.

Unfortunately, full details are not yet available, and as such we’re concerned about a few technical aspects. For example, compressed air isn’t exactly the ideal storage medium. Compressing air usually creates a great deal of heat which will probably have to be vented though an extra radiator. And while the system previewed in Geneva conserves most of the interior space but leaves no room for a spare wheel. What’s more, there’s no such thing as complete energy recovery, otherwise we’d have a perpetuum mobile.

For now, Hybrid Air is only the stuff of prototypes and dreams. But the automotive world will definitely welcome a cheaper, simpler to service hybrid with few compromises.