Engine that runs off of heat

Modern internal combustion engines have an efficiency of about 25-30%. Every year car manufacturers come out with small new innovations and technology the increase the efficiency by very little. The problem is a large percentage of  the 70% of unused energy is wasted in heat which most car companies have yet to harness. A solution to this lack in efficiency could be solved by the Stirling engine which runs of temperature difference. In other words a stirling engine could be used to power electrical components or attach to the driving train to reduce the work the engine does. It would be doing this with energy the engine would be otherwise throwing away, it would not take any power away from the engine or increase fuel use and would also help keep the engine cool. In the picture and the video attached you can see a small Stirling engine a made myself and learn a bit about how they work. Keep in mind it was made with scrap materials and a machined Stirling engine has more power, speed and efficiency. if you look into Stirling engines and watch more videos you will see they have a large potential in increasing efficiency of cars, and are very amazing with some so refined they can run off the heat of your hand

My stirling engine: http://www.youtube.com/watch?v=wW6-T9NQqOs
One that runs off hand: http://www.youtube.com/watch?v=Ebu6Vcds_40
A fast one: http://www.youtube.com/watch?v=xF15NA4vR2w