A Carnot engine operating between 300 K and 600 K has a maximum efficiency of 50%.
The Carnot-efficiency formula is η = 1 − T₍C₎/T₍H₎, where T₍C₎ is the cold-reservoir temperature and T₍H₎ is the hot-reservoir temperature. Substituting the values gives η = 1 − 300/600 = 0.50, or 50%.
This is a theoretical upper limit for a reversible engine operating between those reservoirs. It does not mean that half of the fuel or heat in an actual engine becomes useful work. Friction, turbulence, heat leakage, finite-rate operation, and other irreversibilities make real engines less efficient.
The temperatures must be measured on an absolute scale such as kelvins. Using Celsius directly in the ratio would produce a meaningless result. Another common mistake is to calculate the temperature difference, 600 − 300 = 300, and stop there; the difference must be divided by the hot temperature, giving 300/600 = 50%.