Thermodynamics Myths: Why 100% Efficiency Is Impossible and What to Do Instead

You've probably typed this into a search bar: “Can a heat engine be 100% efficient?” I get it. It's the dream—turn all the heat into work, no waste. But thermodynamics says no, and not because engineers aren't clever enough. It's a hard physical limit. As an editor who's spent years covering mechanical engineering, I'm here to bust the myths and give you the real story. And I'll tell you what to do instead: stop chasing the impossible and start optimizing within the laws we've got.

Is 100% efficiency possible for a heat engine?

No. The second law of thermodynamics forbids it. The Kelvin–Planck statement is blunt: no process can have as its sole result the absorption of heat from a reservoir and its complete conversion into work (MIT Second Law). Every real engine must reject some heat to a cold reservoir. The best you can do is the Carnot efficiency, which is 1 − TC/TH, with temperatures in Kelvin (MIT Unified Engineering). That's the ceiling, and it's always less than 100% unless TC is absolute zero—which you can't reach. So, no, 100% is a myth.

Does a higher compression ratio always mean better engine efficiency?

In theory, yes—for the ideal Otto cycle, efficiency increases with compression ratio (MIT Otto Cycle). But in practice, there's a catch: if you push the ratio too high in a spark-ignition engine, the air-fuel mixture ignites before the spark plug fires—that's knock—and it wrecks your engine. So you can't just keep cranking up compression to get more efficiency. There's a sweet spot, and it's lower than you'd think.

Is entropy just a measure of disorder?

That's the oversimplified version, and it leads to confusion. Entropy is better understood as the directionality of energy transformations—it tells you which processes can happen spontaneously and which can't (MIT Unified Engineering). For an isolated system, entropy never decreases: ΔS ≥ 0. That's the second law in math. It's not just about messiness; it's about the arrow of time for energy.

Is the Carnot cycle the only way to get maximum efficiency?

No, but it sets the limit. The Carnot cycle—two isothermal and two adiabatic steps—is a theoretical benchmark (MIT Carnot Cycle). No real engine can beat it between the same two temperatures, but you don't have to build a Carnot engine to be efficient. You just need to get as close as possible. Real engines like gas turbines and combined cycles are engineered to approach Carnot limits by raising turbine inlet temperatures and using heat recovery.

Do higher temperatures always mean better efficiency?

For a gas turbine, raising the turbine inlet temperature T3 increases work output per unit mass flow, but it doesn't change the ideal Brayton efficiency—that depends only on the temperature ratio across the compressor, T2/T1 (MIT OCW Unified Engineering thermo mud T7). So you can't just crank up the heat and expect efficiency to climb. You have to manage the whole cycle. But higher temperatures do help in combined-cycle plants: DOE-supported systems pushed firing temperatures to 2,600°F, enabling combined-cycle efficiencies past 60% while cutting NOx to less than 10 ppm (DOE Advanced Turbine Systems success story). That's real-world progress, but it's not free—it takes metallurgy and cooling.

Is enthalpy the same as internal energy?

No. Enthalpy (h = u + pv) is internal energy plus the flow work needed to make room for the system (MIT Enthalpy). It's a state function, but it includes that pv term, which matters in open systems like turbines and compressors. Confusing the two leads to miscalculations in energy balances. I've seen it in student reports—don't be that person.

Quick tip:

When comparing heat engines, always look at thermal efficiency e = W/Q_H = 1 − |Q_L|/Q_H, and remember that the Carnot limit is your yardstick (MIT Unified Engineering). If someone claims 100% efficiency, they're selling something.

Comparison: Real-World Efficiency Limits

Bottom line

The single best move you can make as an engineer is to stop idolizing 100% efficiency and instead design for the Carnot limit—maximize the temperature difference between your hot and cold reservoirs, minimize irreversibilities, and recover waste heat where you can. That's how real progress happens, like the combined-cycle plants that pushed past 60% efficiency (DOE Advanced Turbine Systems success story).

Sources

• MIT Unified Engineering - https://web.mit.edu/course/16/16.unified/www/FALL/thermodynamics/
• MIT Second Law - https://web.mit.edu/course/16/16.unified/www/FALL/thermodynamics/notes/node37.html
• MIT Enthalpy - https://web.mit.edu/course/16/16.unified/www/FALL/thermodynamics/notes/node17.html
• MIT OCW Unified Engineering thermo mud T7 - https://www.ocw.mit.edu/ans7870/16/16.unified/thermoF03/mud/T7mud.html
• DOE Advanced Turbine Systems success story - https://www.energy.gov/hgeo/doe-technology-successes-breakthrough-gas-turbines