Final answer:
A heat engine cannot be 100% efficient due to entropy, which dictates that some energy is always lost as heat, making complete conversion of input heat to work impossible.
Step-by-step explanation:
The efficiency of a heat engine is never 100 percent primarily because of the law of entropy increase from the second law of thermodynamics. This natural law dictates that in any energy transfer or transformation, some energy is irreversibly lost in the form of heat loss to the surroundings. In addition to friction and dissipative processes, which further reduce efficiency by converting some engine work back into heat, the concept of entropy ensures there is always a percentage of the input heat that cannot be converted into useful work, meaning the thermal efficiency of an engine cannot be perfect.