The next time you fill up your car, instead of watching the numbers spin and musing over how much of it is helping to finance extravagant luxury in far-away places like Dubai, you might ponder instead the dark brake dust coating your wheels, because that is where a fair amount of the gasoline you’re pumping is going to end up.
An automobile engine is a very specialized machine for converting chemical energy into mechanical, and mechanical energy is what is needed to make an automobile do something besides depreciate in the driveway. Because to go anywhere at the speeds we’re accustomed to, the engine has to supply a steady flow of energy to do things like flex rubber treads and push large quantities of air out of the way very fast. That’s why soft tires and a large boxy exterior mean lower gas mileage, because the rubber flexes more, it’s harder to push air out of the way with something shaped approximately like a barn door, and because that flow of energy is fed by a corresponding flow of gasoline from the tank.
But first you have to accelerate.
Every driver is familiar with pressing the gas pedal harder to get the car moving, whether just a little bit harder for a longish time to slowly get up to speed, or quite a bit harder for that gratifying jack-rabbit start. Either way, extra gasoline is burned to create extra energy, and this is converted into kinetic energy, which is the energy represented by something massive moving fast.
So where does all that kinetic energy go? Well, as long as the car is moving, nowhere. It’s only gotten rid of when you stop.
What the brakes do is to use friction to drain the kinetic energy by converting it to heat and dust – the particles that wear off the brake pads. This is where the gasoline you used to accelerate ends up.
The more stopping you do to get from point A to point B, the more gasoline you use wearing out your brakes and heating up your wheels. Thus, EPA mileage estimates for city driving (stop and go) are always less than for highway driving (stop once).
Or, almost always.
What if, when decelerating the car, its kinetic energy could be recovered and reused?
This is called regenerative braking, and it’s built in to most hybrids. The principle is that the electric motors that are part of a hybrid gas-electric car can be switched to work like generators instead, using the power of the moving car to help recharge the big lithium battery pack. It works when you’re decelerating from cruising speed, and then conventional brakes take over to finally bring the car to a halt.
This isn’t the only way to do regenerative braking. Engineers at Peugeot decided there might be an even better approach.
What they came up with was a way to recover the kinetic energy of a moving vehicle using hydraulics. In their Hybrid Air car, the car is slowed by a hydraulic motor that compresses gas in a long cylinder. The action is then reversed to allow the highly-compressed gas to assist the conventional engine in accelerating the car.
This is not a hybrid in the sense most of us are familiar with. The compressed gas can only power the car forward a few hundred yards, at best, but that may be enough to nearly double fuel economy in heavy stop and go driving. (One reason it is so effective is that the hydraulics and compressed gas cylinders are quite a bit lighter than the massive battery packs employed in more familiar hybrids.)
Another advantage cited by Peugeot is that, lacking the powerful electric motors of a Prius, say, there are no ‘rare earth’ magnets needed (except, perhaps, for the small engine starter). This frees them from the uncertainties of minerals that are mined almost exclusively in China.
If it still sounds a little farfetched, consider other examples of compressed gas supplementing or replacing internal combustion in a vehicle – functional, if not truly commercialized. For example, a company called Motor Development International has been testing a small runabout (or large golf cart, depending on your perspective) that runs entirely on compressed air. The driver recharges it at home with an air compressor. The technology is not very efficient yet, and it may never be, due to some irritating laws of thermodynamics, but it demonstrates that compressed-air motors work well enough to power a vehicle at speeds appropriate to city driving.
Then there is the liquid nitrogen car, which runs on the cryogenic gas that is a cheap byproduct of manufacturing liquid oxygen, re-using energy that might otherwise go to waste.
Peugeot’s technology is quite a bit less ambitious, combining as it does more conventional components to supplement a run-of-the-mill gasoline engine and make it drastically more efficient. Whether it will ever grace the polished floors a dealer’s showroom, of course, remains to be seen. But it seems like it ought to.