I remember something like that, probably in the 70s. Not sure it would have added greatly to the overall efficiency (though a stationary vehicle with the engine running is being 100% inefficient). Ironically, I don't think it would work on the city streets today - so much driving is stop/go that the engine would constantly be restarting. This, in itself is probably inefficient use of fuel, and I suspect the starter motor would be knackered in a very short time.ina wrote:a long time ago I read about a really efficient car made by VW - it just didn't take off (no pun intended!) because folk thought it too spooky that the engine turned itself off whenever it stopped, at lights etc!
However, the idea lives on in some forms; modern fuel injection systems cut off the fuel supply completely when the car is slowing down off the accellerator, but then switching it back on again before it drops below tickover.
And I suppose a modern extension of the idea is the hybrid car (eg Toyota Prius, some Hondas, and I believe even Ford are getting into it) - a petrol engine and an electric motor combined together. When conditions are right it runs solely on the electric motor, with the petrol engine there to provide extra input when needed, and to charge the batteries. This would have maximum benefit in city stop/go traffic.
Alas, even the most efficient petrol engine today struggles to get much better than 30% efficiency - 70% of that precious, poisonous stuff that we run our cars on is simply wasted directly to the atmosphere as heat before it can do any useful work at all.