Mr. Green Car: Flywheels in your future?
By Allen Penticoff
This week, I’ll veer into a technical report, thanks to regular reader, Henry Tideman of Oregon, Ill. Henry sent me a link to an article in The Economist about the advancing state of flywheels in transportation that I found to be full of good information. I’ll share the basics with you here.
Flywheels are attached to every gasoline and diesel engine crankshaft. Usually, they serve to smooth out the pulses of the cylinders firing and vibrations by providing mass that keeps all those crankshafts, rods and pistons in motion. Heavy flywheels are slow to accelerate but can transmit a lot of power in the kinetic energy they carry — thus, heavier flywheels are found on large diesel engines, where lighter flywheels are found on small engines that need to rev up quickly (think sports cars or motorcycles). Most of the time, the flywheels also serve as gears for the starter system. But the future lies in freewheeling flywheels with a far different purpose.
Flywheels store kinetic energy. They will give back the energy that was put into making their mass spin. The energy can be stored in one of two ways or a combination. You can have a very heavy flywheel that spins relatively slowly — the large mass generates a lot of torque when the flywheel is no longer accelerating. You can also spin a smaller flywheel at a higher speed and derive the same amount of energy as the large, slow-turning flywheel. Think of it this way: a speeding little bullet can do more damage than a slow moving football that weighs many times more. Mass and velocity compound to equal force.
In the past, flywheels have been used on large trucks, such as garbage trucks and buses that start and stop frequently. They could carry the heavy weight of the flywheel and the mechanism to engage and disengage the flywheel. As the truck coasts/brakes, the flywheel spins up. Then, at the next stop, the flywheel has all its energy stored up ready to help the truck get moving again, thus saving on fuel by delivering free power. Large flywheels are also used on electric locomotives and trams to carry them past areas, such as intersections and switches where there is interrupted power.
But these big, heavy flywheels don’t work well in automobile and light truck applications. Standard steel or iron construction is too heavy and works against acceleration and fuel economy. With these materials, the fast-spinning flywheels can be a hazard, as they are prone to catastrophic failure. Those that are used in big trucks and locomotives have heavy protective cases around them.
Enter modern materials — carbon fiber composites. Stronger and lighter than steel, engineers are experimenting with making very small, lightweight flywheels that spin to incredible speeds. In use already are carbon fiber flywheels with the mass in their rims (where it has the most effect) spinning at 60,000 revolutions per minute. Computer studies have shown that flywheels with speeds of 145,000 rpm are feasible.
What this means is that simple, lightweight flywheel systems will be added to future automotive engines/transmissions. While coasting or braking, the flywheel will store energy; when you stop, it will sit there spinning with the energy waiting for you to press the accelerator — indicating it is time for it to engage and help the car move — quite likely without the engine even starting for quite a distance. This is exactly the same thing that is happening with hybrid systems, but with expensive electric motors and batteries doing the work. According to the The Economist article, a flywheel the size of a hockey puck could reduce fuel consumption by as much as 20 percent. This is because an electric hybrid with its complex chemical conversions can only recapture 35 percent of the kinetic energy, while a flywheel can store in excess of 70 percent of the energy in regenerative braking.
Flywheel systems could be so inexpensive that they may be standard equipment or low-cost options on most vehicles. Flywheel technology could spell the demise of the electric hybrids like the Prius, but probably not the plug-in hybrids like the Volt that are essentially electric cars with engines. Jaguar and Volvo have working prototypes, as well as some Formula 1 race cars. Because it holds a lot of promise, I look forward to seeing this technology reach down to the lower-priced consumer auto market. For more in-depth reading, visit http://www.economist.com/node/21540386.
From the Dec. 28, 2011-Jan. 3, 2012, issue
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