A heat engine runs between a 600 K flame and a 300 K environment. Estimate the maximum possible efficiency, in percent.

50 (answers within ±5 count)

$\eta_{max} = 1 - 300/600 = 50\%$, and no engineering cleverness can beat it, because it follows from the second law itself. Real engines between these temperatures manage perhaps 30%. Waste heat is not a design flaw; it is physics rent.