To save energy..
Why don't cars have a built in waste heat energy recovery systems? i.e., a steam engine type booster that can convert heat into mechanical energy.. Is this really that difficult to implement? Seems like this could improve fuel efficiency by quite a bit.
How about energy recovery system from breaking, with hydrollics, flywheel, or magnetic/generator device.. I know this is in some hybrid vehicles today, but why wasn't it here 20 years ago? Sounds like the start of an energy crisis.