How close to the limit is this engine?

Carnot, applied at the temperature the heat is actually delivered at — not the peak.

eta = [(Th−Ts)/(Th−T0)] × [1−T0/T_eff], with T_eff = (Th−Ts)/ln(Th/Ts). Carnot 1824. Three temperatures, not two.

Every temperature below is converted to kelvin first, by adding T_ABS = 273.15. Carnot is a ratio of absolute temperatures, and a ratio of Celsius readings is not wrong by a little — it is meaningless, and it still returns a number.

Your machine

turbine inlet, °C

stack, °C

ambient, °C

measured efficiency, %

The stack temperature is not a detail. A combined cycle exhausts near 100 °C; a simple cycle throws its gas away at 550–600 °C. Same turbine, same firing temperature, and two different ceilings. A tool that asks for two temperatures instead of three cannot tell them apart — try the last two rows below.

Five published machines

Tap a row to load it.

Machine	Inlet	Stack	Measured	Ceiling	Of it

What it refuses

Which heating value

ASME PTC 46 says plainly that it “does not mandate the use of either HHV or LHV” — so a reported efficiency arrives with no basis attached. Rather than ask, the law is applied: if one reading would put the machine above its own ceiling, that reading is impossible.