What is a heat engine?

What is a heat engine? What is the best way to increase efficiency of a heat engine? Is it possible to design a thermal engine that has 100% efficiency?

A heat engine is a device (or a combination) which converts heat into work.Its efficiency,


From above expression, we can see that for 100% efficiency,
{ T }_{ 1 } = ∞
temperature of source is very large compared to that of sink.