I caused a little confusion on my post on flywheels by including their use in vehicles under the category of energy reuse, although that was a correct category. A little further explanation is due however.
Flywheels have been used on engines since their inception and the technology for this application is quite mature. A possible new application for energy storage has developed with the introduction of hybrid vehicles. This is because batteries cannot absorb nearly all the power that is available from regenerative braking, the chemical reaction in a battery is much too slow for this. More than 50% of this energy is wasted. Flywheels and ultracapacitors can do this job much better. Flywheels have been used on hybrid subway and hybrid light rail vehicles with success. Their use is being developed for use on hybrid delivery vehicles. The ultracapacitor has been used on hybrid buses with very good success. Both of these technologies are expensive and add to the cost of already expensive hybrid vehicles. Where life-cycle costs are important, as in high mileage commercial vehicles, they can be justified. One flywheel company is developing a flywheel system aimed at the personal vehicle market, and they are working with on low volume producer. It is my opinion that flywheels are too heavy and too costly to be incorporated into automobiles with current or near term technology.
On the other hand ultracapacitors are lightweight, in fact it is claimed that the combined weight and size of an ultracapacitor and battery are reduced compared to the current weight and size of a battery. This is because the ultracapacitor can take over the starting duty of the battery, reducing the size of the battery considerably. (An ultracapacitor with enough capacity for a few starts is state of the art and increases in functionality with decreasing temperature, the capacitor can be recharged in about 15 minutes of driving--ultracapacitors are starting to replace batteries for starting of diesel engines because of these properties) The life of the battery is also increased considerably due to less cycling and no deep discharges. The cost of ultracapacitors can be reduced significantly with high volume production, so this looks like a much more suitable means of increasing the energy reuse in a car.
It seems to me that a very cost effective "mild hybrid" could be made by putting in an ultracapacitor combined with shutting off the engine when the vehicle is stopped. If this would increase the mileage by 10 to 15% with little increase in cost it should be a no brainer. So what is wrong with my reasoning or are the car companies just not motivated.