Originally Posted by woah
ex and engine can be 85% efficent in converting potental energy into usuable energy but there is alway loss due to heat. 85% of the potental was transfered into usuable energy and 15% was lost due to heat, but mpg is not effeciency.
Internal combustion engines convert maybe 40% of chemical potential energy into kinetic energy to move the car. Roughly 30% is lost as heat dispersed in the exhaust and 30% as heat dispersed through the engine block and coolant. Very inefficient.
When you apply the brakes to slow down, that 40% kinetic energy also becomes heat through friction. One car may get better MPG but not be technically more efficient in converting energy, it simply uses less fuel because it produces and requires less power. However with a Hybrid car like the Prius, some of that energy normally expended as heat while slowing down is instead used to spin a generator, which converts it to electrical energy stored for later use. So while it is not directly linked to MPG, the Prius is more efficient than the Jag.