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Many hybrid cars use an interesting system called “regenerative braking” to recapture some of the energy which is wasted with conventional braking systems. On a typical car, each wheel has a rotor disk, and braking is accomplished by causing the brake pads to squeeze the rotor and create friction which slows the car, converting the forward momentum into waste heat. But a hybrid doesn’t use the brake pads at all unless you hit the brakes hard… Instead, the car’s momentum is used to crank its electric motors, which slows the car while recharging the onboard batteries.

This brilliantly simple system is part of why hybrid cars are so fuel efficient in stop-and-go traffic. But hybrids have their downsides… For one, a modern hybrid’s batteries only last 8-10 years on average, and they are extremely expensive to replace, on the order of $3000-7000. Battery disposal is also a sticky problem, since Nickel-Metal Hydride batteries contain hazardous chemicals. In addition, although hybrids save some weight by including a smaller gas engine, they add it all back by including the heavy electrical components: two electric motors and the batteries.

Could there be a way of usefully recapturing a car’s kinetic energy on deceleration without adding so much weight, and without the expense and environmental impact of batteries? What about a wind-up automobile?

Just about every kid has played with some variety of wind-up toy, whether it’s wound with a tiny crank or by placing a wheeled toy on the floor and pulling it backwards. Such toys are propelled by a special spring called a torsion spring which is twisted when the toy is wound, and it acts as an energy storage device due to its inherent tendency to return to its original, unwound state. Often, gears inside such toys prevent the energy from being released too rapidly, so the stored energy is released at a steady rate. Garage door openers use the principle on a larger scale, using torsion springs to do much of the heavy lifting while a small, low-power electric motor makes up the difference.

An electric hybrid’s batteries essentially act as kinetic energy storage, and a lightweight car could theoretically use heavy-duty torsion springs to accomplish the same thing, using the springs’ torque to propel the vehicle whenever enough torque is available. Like a hybrid, a small combustion engine would start up to add additional power when needed, and a continuously variable transmission (CVT) would ensure that the stored energy is always used most efficiently. Such a vehicle might be called a Torsion Hybrid.

A torsion hybrid car could automatically wind its springs by engaging a winding gear during braking, or when traveling downhill. Additionally, its small combustion engine could be designed to constantly run at its most efficient RPM level, putting any excess horsepower to the task of spring-winding. One could also keep a winding station in one’s garage which rapidly charges the spring from an access shaft on the bottom of the vehicle. Even more creative ways of incrementally capturing momentum could be utilized, such as ratcheting gears in the shock absorbers which capture a bit of energy every time the car goes over a bump.

A torsion hybrid could utilize one large torsion spring, or an array of smaller springs. Winding more springs would mean decreased efficiency due to friction, but it would allow the car to use less expensive springs, and it would minimize the risk involved in the event of a sudden, unplanned release of a spring, which could happen in a car accident.

As compared to a conventional electric hybrid, a torsion hybrid could potentially have a lighter weight and cost less to produce, though one cannot know with certainty without a good deal of research. But the problem of battery disposal would certainly be eliminated, as would the need to replace spent batteries after 8-10 years. However several key questions remain… for instance, is there a metallic alloy capable of storing the energy that would be demanded in a torsion hybrid?

Ultimately, the real question is whether torsion energy storage is any more efficient than it’s electric hybrid cousin. Data indicates most electric hybrid regenerative braking systems work at less than 50% efficiency by the time the kinetic energy is converted into electricity, then put back into propelling the vehicle… but it is difficult to say what a torsion hybrid’s efficiency might be once friction is accounted for.

It is likely that I am not the first person to conceive of a torsion hybrid, and no doubt there are some serious engineering problems which I have not yet considered. But is it feasible? Is it moronic? Opinions are welcome.