The idea is to use the waste water & heat of a hydrogen engine to power a steam turbine for more power. The way it works is by taking the exhaust water/steam. Putting the water/steam through a small steam turbine. After that routing it to tanks very close to the cylinders where the combustion is happening. Using the waste heat of the engine to heat the water into steam. Then the steam is run through a turbine. After which it is routed back into the starting tank. The tank has one way valves to keep the steam & water from going the wrong direction. If there is too much steam pressure or water some of it is diverted out of the exhaust to keep it from exploding.

Would this work to get more power out of a hydrogen engine?


4 Answers 4


What you're talking about is called a Compound Steam Engine

The image below (courtesy Wikipedia) shows a 3-stage compound engine.

enter image description here

You can have as many stages as you want, but there is a mathematical balance. There comes a point where the weight of the next stage is too great to efficiently accept the remaining steam resource.

In your case, you're using a hydrogen to create the initial burst of speed. Use a stage or two to reuse the excess heat, and you're off.

Note that if you're comparing this to real life (which you should never do on this site), the answer is "no." The reason is that any hydrogen engine with that much excess heat isn't worth using. The ideal solution for any engine is to use as much of the "combustible" (the fuel source, no matter what it is) to generate work. Any heat lost (and therefore capturable for use in a compound steam engine like you're thinking) represents a loss of efficiency. No matter how good your secondary heat capture system, its very existence means you could have improved the efficiency of the primary motivator and had that much more power. The laws of thermodynamics are ruthless: you can never recapture for new work energy equal to the loss you're working from, so it's always better to use it in the first place.

  • $\begingroup$ "you can never recapture for new work energy equal to the loss you're working from". But can we capture some (as long as it's worth the extra mass and complexity). $\endgroup$
    – RonJohn
    Commented Aug 15, 2021 at 10:36
  • $\begingroup$ The third stage exhaust should not be to air as shown in the picture : it should be to a condenser, cooled by a large reservoir of cold; such as the sea. That way thermal energy in the steam is used right down to about 30C, rather than the 100C heat you would need to throw away to the atmosphere. This is crucial. An example ( * ) steamed past my window not long ago; the LP cylinder works at about 16 psi; the steam in your yellow pipe is 2 psi above atmosphere; the other 14psi is the vacuum created in the condenser. (* PS Waverley, world's last seagoing paddle steamer) $\endgroup$ Commented Aug 15, 2021 at 12:34
  • $\begingroup$ @RonJohn In reality, there comes a point where the energy available for recapture falls below a necessary threshold to even try. The odds of a hydrogen engine producing enough heat for this idea to work at all are vanishingly small because the heat lost must be great enough to generate steam. You could recapture more using a thermoelectric generator, but there's always a point where you simply can't recapture more without paying a higher price than it's worth. $\endgroup$
    – JBH
    Commented Aug 16, 2021 at 18:16
  • $\begingroup$ @user_1818839 You need to be submitting your complaint to Wikipedia, who hosts the picture. All I'm doing is presenting a suspension-of-disbelief solution. (Please don't get too caught up in the ugly details, this site is far, far too "based on reality" as it is. Unless the hard-science tag is used, the goal is always suspension-of-disbelief.) $\endgroup$
    – JBH
    Commented Aug 16, 2021 at 18:19
  • $\begingroup$ @JoinJBHonCodidact it wasn't a complaint, but an observation. $\endgroup$ Commented Aug 16, 2021 at 18:44

Certainly it would work. You don't even need a hydrogen-burning engine: it'll work just the same with a conventional engine. Indeed, that's what turbochargers do: use some of the energy in the exhaust stream to compress the intake air.

There have been experimental devices that do something similar: use the exhaust to generate electricity for hybrid cars. Here's one example: https://www.greencarcongress.com/2005/09/tigers_exhaust_.html AFAIK it's never been put into production, though. Perhaps the power generated doesn't justify the cost?

  • 2
    $\begingroup$ Your last sentence is spot on: The waste heat that's produced by an internal combustion engine is at a fairly low temperature, i.e. very rich in entropy. To make any significant use of the coolant heat, the motor block needs to work at insane temperatures, which would massively decrease the lifetime of the cylinders. Exhaust gas heat is somewhat usable, but it would be more efficient/effective to simply underload the cylinders so that the exhaust leaves the cylinder much cooler in the first place. $\endgroup$ Commented Aug 15, 2021 at 8:07

Perhaps you are using the wrong ICE

ICE engines beat out steam engines because of their higher efficiencies and power densities. But the standard Otto Cycle reciprocating piston design has a significant design limitation that limits its efficiency - it does not "over-expand" during the power stroke, resulting in exhausting the hot fuel-air mixture prematurely.

The Atkinson and Miller cycle engines attempt to address this limitation by incorporating over-expansion. If you take the time to read the articles that describe these alternatives, you will see that these engines also have downsides, but have been successful in certain markets because of the efficiency.

There is a newer engine design that I have read about that seems to be a very feasible improvement over these cycles by using a rotary engine (not a Wankel) that combines over-expansion with high-power density. Not a commercial design (at least not yet) seen in the HEHC Cycle being developed by Liquid Piston. This HEHC cycle has a very limited explanation on Wikipedia, but I've read the technical backgrounders on Liquid Piston website and it sounded very appealing to me (BSME and worked at Cummins in engine design, so I have odd hobbies). Diesel engines are more efficient than equivalent gasoline engines because they can run at higher compression ratios and temperatures. The HEHC cycle can also run at high-compression ratios.

I would suggest, that in situations where you would have considered using and secondary steam turbine (clearly you are not emphasizing overall power density), you would be better off with an Atkinson engine today, and perhaps an HEHC engine in the future.

So, it's not that a secondary steam turbine would not work to improve efficiency, it's just not the most cost-effective approach.


As Join JBH on Codidact answered, what you describe is a compound steam engine, and is indeed used in practice. However, I wanted to point out that there are practical concerns which affect "feasible."

One of the fundamental problems of using a compound steam engine is that, by necessity, the second stage acts as an insulator on the first, ensuring heat moves away from that stage slower than it would if the second wasn't in the way. Most engines use water cooling to get the heat away from the power generating centers as fast as possible. Wear and tear goes up at higher temperatures.

At some point the gains from the second stage are outweighed by the losses in efficiency from the first stage. The point where this occurs varies from engine to engine, technology to technology.


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