The class opens with a question that sounds like it should have an easy answer: is there a best possible engine? In 1824 Sadi Carnot proved there is a ceiling. Any engine running between a hot reservoir at T₀ and a cold one at T₋ can convert at most a fraction 1 − T₋/T₀ of the heat it draws into work.
The ideal cycle that reaches the ceiling is reversible: two isothermal strokes trading heat with the reservoirs, two adiabatic strokes gliding between them, and no wasted step. On the temperature–entropy diagram it is a perfect rectangle whose area is the net work.
Widening the gap — a hotter source or a colder sink — raises the ceiling itself, but you can never pass it, and never reach 100% without an impossible T₋ = 0. Every real engine is irreversible: friction and finite-rate heat flow generate entropy, and by the Gouy–Stodola result that entropy destroys a slab of work exactly T₋ × Sₛₜₙ. That is why no working engine ever quite touches Carnot.
The simulation stopped unexpectedly — the lesson continues without it. You can move on; nothing you did was wrong.