thermodynamic efficiency
You pour a tankful of fuel into an engine and only some of its energy comes back out as motion; the rest escapes as heat through the exhaust and radiator. Thermodynamic efficiency is the simple score that captures this: the fraction of the heat you put in that the engine actually turns into useful work. Put in 100 units of heat, get 30 units of work, and the efficiency is 30%.
Written as a ratio, efficiency equals work output divided by heat input. The second law caps this ratio harshly: even a flawless, friction-free engine cannot reach 100%, because it must always shed some heat to a cold reservoir. The ceiling — the Carnot efficiency — depends only on the two temperatures: 1 minus the cold temperature divided by the hot temperature, both in kelvin.
Why it matters: efficiency tells you how much of your fuel becomes work versus waste, which drives the economics and emissions of every engine and power plant. It is also why engineers chase the hottest possible combustion and the coldest possible exhaust — widening that temperature gap is the only way to lift the ceiling. The honest caveats: real engines always fall short of the Carnot limit because of friction and finite speed, and 100% is forbidden outright, not merely hard.
A car's gasoline engine runs around 25–35% efficient: most of the fuel's energy leaves as hot exhaust and radiator heat, not motion. A modern combined-cycle gas power plant, by burning hotter and capturing the waste heat a second time, climbs past 60% — but never to 100%, because the cold reservoir always claims its share.
Efficiency = work out ÷ heat in; the Carnot value 1 − T_cold/T_hot is the ceiling.
Heat pumps and refrigerators are rated by a 'coefficient of performance' instead, which can exceed 1 (or 100%) — but they are not violating anything, because they move heat using work rather than converting heat into work.