Heat Engine: A Cycle That Turns Heat into Work
A heat engine draws heat QH from a hot reservoir, turns part of it into work W, and dumps the rest QC into a cold one. Running in a cycle, it returns to its start every loop, so the net work is just what it takes in minus what it throws away.
Drag to set how much of the incoming heat becomes work. The rest is dumped to the cold reservoir. In a real car engine or power plant only about a third of the fuel's heat becomes work, and the rest leaves as hot exhaust and coolant.
The engine runs around a closed loop on the P-V plane. The area the loop encloses is the net work per cycle. The loop yields positive net work only when it runs clockwise; traced the other way it becomes a refrigerator that needs work put in.
Over one full cycle the gas returns to its starting state, so its internal energy change is zero and QH = W + QC. Internal energy is a state function, so whatever path the gas takes, returning to the same state forces its net change to zero. Drag to split the intake into work and waste.
Efficiency is η = WQH = 1 - QCQH. Efficiency hinges only on the ratio of the waste heat QC to the intake QH, so cutting the heat dumped to the cold side is the one way to raise it. Switch presets to compare a weak engine with a strong one.
Highlight each of the three flows in turn. Because some heat must always leave through QC, the efficiency stays below 1 -- the next chapter, on the second law, says why. That unavoidable QC is why power plants sit beside rivers or giant cooling towers, shedding waste heat without pause.