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According to the internet, an internal combustion engine is powered by the pressure differential when the fuel expands into gas and has a maximum thermal efficiency of 50%. And a steam turbine is powered by the temperature differential when the fuel releases its chemical energy as heat and has a maximum thermal efficiency of 90%.

What would happen if you sunk the heat from the internal combustion engine into the boiler of the steam turbine, instead of just venting it into the atmosphere as a waste product? I'm guessing it wouldn't have a thermal efficiency of 140% but WHY wouldn't it? Does harnessing the gas expansion reduce the amount of waste heat? According to which physical laws would THAT happen?

Why isn't a combined cycle power plant 140% efficient?

Lorry Laurence mcLarry
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This is known as a combined cycle implementation.

This is extremely common on gas turbine electric generator plants. The exhaust heat goes into a heat recovery steam generator, which makes steam to run a secondary generation system.

Other possibilities for the "waste" heat: Greenhouses, drying paper in a paper machine, building heat, etc.

This is considerably more difficult for a traditional piston engine, but the same rules apply. There is limited potential, however, for the intense capital required for this for use in a regular diesel engine, which will only be used in smaller cheaper applications (typically emergency power). Gas turbines have taken over the market, largely because of the advantages of the combined cycle process.

The second process in the combined cycle extracts work from losses of the first process.

So if Process 1 is 40% efficient, we lose 60%. If process 2 is 50% efficient, we can reduce the losses (60%) by 50%, so the total cycle is now 70% efficient.

Tiger Guy
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It appears you're already aware of the Carnot limit: the efficiency of a heat engine cannot exceed the fractional difference in absolute temperature between its hot and cold reservoirs. In all cases considered here, the cold reservoir is the ambient environment. Therefore, for the steam engine to reach that maximum efficiency of 90%, its boiler must be at at least ten times the absolute temperature of the ambient environment. The only way you could make the exhaust gases from an internal combustion engine that hot is to reduce the efficiency of the internal combustion engine well below 50% (because the enthalpy of the exhaust gases is "waste" from the ICE). Therefore, when the two are operated in series as you suggest, either the internal combustion engine is less than 50% efficient, or the steam engine is less than 90% efficient, or both.

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  1. Your numbers for efficiency aren't consistent. The steam turbine efficiency isn't a cycle efficiency, it's just the efficiency of the expansion machine. The efficiency of the power stroke in a four-stroke ICE is greater than 90 %. You need to treat the steam machine as a cycle and include all the parasitic losses and the thermal losses of other components. You also need to realize the steam condenser would need to be carried on a trailer and would double the aero drag of a car (It's pretty big.)

  2. The efficiency numbers are a design choice. We can build more efficient ICEs if we choose to. But they are pretty cheap. The cost of the iron components in a long block is about the same as the cost of a tire or a starter motor. If you wanted to spend an extra $2 per horsepower on the iron, you could do quite a bit better. But they end up larger and heavier and want a higher grade of fuel. Basically, as engines have gotten better, we just keep feeding them worse and worse fuel. If you want to buy 100 octane gas for your daily driver, we can bump the efficiency quite a bit.

  3. ICEs are really versatile. There isn't any other technology that has the throttle response and wide efficiency band of an ICE. You can beat ICE's efficiency at any single design point, but you just can't beat their efficiency over the range of power levels needed over the road.

  4. We can augment these features of the ICE with physically divers compounding, but that's what we do - augment the ICE. We don't use the compounding for shaft power except in a very few instances.

  5. A few gas and diesel engines were compounded or had multiple expansions. They were tried from about 1890 to 1910. Then we just gave up on it as not worth the trouble. Wikipedia has a list of them - https://en.wikipedia.org/wiki/Compound_engine

Phil Sweet
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