Absolute zero
The Second Law of Thermodynamics says that all engines that are powered by heat (like car engines and steam train engines) must release waste heat and can not be 100% efficient. This is because the efficiency (percent of energy the engine uses up that is actually used to do the engine's job) is 100%×(1-Toutside/Tinside), which only is 100% if the outside temperature is absolute zero which it cannot be. So, an engine cannot be 100% efficient, but you can make its efficiency closer to 100% by making the inside temperature hotter and/or the outside temperature colder.