Combined Cycle Turbines are a gas turbine whose exhaust heat powers a second turbine running on steam. These can get thermal efficiencies greater than 50%, whereas a plain turbine would get 30 to 35%. Large power plants tend to use combined cycle for obvious reasons.

But I want to know if this can be small enough to power a train?

Example of a very small jet engine, which is very similar to a gas turbine.

Desired specs:

  • Fuel: either natural gas or kerosene

  • Horsepower: ~4500 (taken from a typical diesel train engine (GE Dash 9))

  • Max volume: 25 x 3 x 6 meters (about the volume of a train car)

  • Max weight: 100 metric tons

I'm assuming a fuel car can follow behind the engine car. The turbines would generate electric power sent to the car wheels, which is what a modern diesel-electric train does. I was imagining combined cycle turbo trains for freight and passenger services, but at "normal" speeds (100 - 150 kmh).

So can this be done with today's technology? The engine would still need to be insulated enough and efficient enough to retain 50+ % thermal efficiency.

BTW, I'm aware of this failed example from Amtrack starting in the 1970's. I don't think those were combined cycle turbines, and I could not find an explanation for why "The high cost of operating the trains led to their withdrawal from the Midwest in 1981." But I did notice those were tiny trains, 5 cars total (2 engine cars, 2 coaches, and a food car). That is probably far too small to get economy of scale.

  • $\begingroup$ This is not a very qualitative question and I doubt many on here will be able to give you much advice on it. It is like asking if we could put a super computer into a watch. Theoretically it is possible, but it is realistically not possible right now because we don't have the technology to do it. You need to be much more specific with your questions. $\endgroup$
    – user88720
    Feb 20, 2017 at 8:31
  • $\begingroup$ @user88720 I specified the limits on volume and mass. I specified the Horsepower and thermal efficiency desired. I specified two fuels that could be used. I don't know what else to specify. If it is unanswerable, then the correct "answer" would say so and explain why, ideally with cites. What is the technology required to do it? Is it something to do with lightweight insulation, in which case I would look into aerogels? Something to do with metallurgy? I can't imagine the steam part would be limiting, as we used to run huge steam trains. And gas turbines can be very small, see edit. $\endgroup$
    – DrZ214
    Feb 20, 2017 at 12:15
  • 2
    $\begingroup$ It's definitely possible to make them small enough to fit on a train, but will they be enough to power it? Primarily, steam condensers tend to be rather large facilities. OTOH it can work only on gas turbine at low speeds, activating the second cycle when it has all the coolant air it might desire from train movement. $\endgroup$
    – SF.
    Feb 20, 2017 at 12:46
  • $\begingroup$ For transportation, diesel engines are generally more efficient than gas turbines but also larger and heavier. My guess for the Amtrack turboliner is that they chose gas turbines to have a lot of power for high speed trains without having very heavy and expensive engines. When fuel prices started rising the lack of efficiency would have become a problem. $\endgroup$
    – JanKanis
    Jan 16, 2023 at 15:14

1 Answer 1


We can try to answer this question by looking at existing equipment in terms of size and weight and make an educated guess from there if all this fits on a train. Our CC will consist of:

  • Gas turbine
  • Exhaust HX/Boiler
  • Steam turbine with condensor and other aux. parts
  • Transmission

For the following, I will not match components exactly, but pick stuff that sits in the right power range, just to get a feel if the parts could fit into the design envelope given above.

Basic sizing
You give 4500hp, that's a bit more than 3MW. Looking at wikipedia, the gas turbine is typically twice the power of the steam turbine. Soe we look for a 2MW gas turbine and a 1MW steam plant.

Gas turbine
What are some turbine around 2MW?

  • Kawasaki has one at 1.7 MW, it seems the 'naked' turbine fits into a 6.5m container + there's another one stacked on top for air vents. No mass given, does not seem to include exhaust HX.
  • This 2MW Opra engine again fits into a 6.5 m container (without air supply), the naked engine and weighs maybe 13.5 t.

For the following, I'll take the stats of the Kawasaki engine and assume the weight of the Opra engine is not too far off.

Electrical Power 1.690 kW
Fuel input 6.360 kW
Efficiency 26,9 %
Exhaust gas mass flow 8,00 kg/s
Exhaust gas temperature 521 °C

Boiler/Exhaust HX
If we take the Kawasaki engine ebove, we have about 480kg/min exhaust gas. Looking at this table of exhaust HX, we would need 7 of the largest, each about 600kg and length x diameter 2.6 x 0.7 m. Roundabouts. At up to 700kW heat recovery each, we should get north of the 2MW worth of steam we need for our steam turbine.

Steam turbine
Siemens has one that covers the 800kW-6MW range. Without condensor it fits into a LxWxH evelope of 1.5 x 2.5 x 2.5 m. No weight is given, I guess a few tons.

We need to shed about 1MW of heat for our 1MW steam engine. Let's take some measurements from this product sheet from Güntner. Note that these a condensors for refridgerant. I guess the bottleneck is rather the airflow and the heat transfer towards air, then these should be ok.

The largest have a nominal capacity of 500kW, weigh 2t and are about LxWxH 10x1.2x1.7 m. Two of those side by side.

Transmission My guess is that an electrical trasnmission would be used, then you'd have to add a 3MW motor and a 1MW and 2MW alternator. Because by now I'm lazy I'd say that these add maybe 2-3 m to our 'length budget'.

We don't seem to really reach the mass budget of 100t (which seems way large for just the engine, OTOH).

Our train could look like, from from to back:

  • 6m length dedicted to air intake, filters etc.
  • 6.5 m for the gas turbine (13t)
  • 3m for a stack of HX (3.5t)
  • 3 m for the steam engine (3t?)
  • 10m for the condensors (4t)
  • 2-3 m for alternators and motor (10t?)

For a total of 31m. This is larger than the size envelope given by in the OP, but not by much. So my conclusion is: You can fit 3MW worth of combined cycle plant on a train car.

There's lots of room to improve the whole thing at the HX and condensors, OTOH we need to fit large electrical systems on the train. In summary I believe you could build a combined cycle powered train & you would not have to invent too many components to do it.

  • $\begingroup$ Thanks for the detailed analysis. I originally imaged one component stacked onto the other, so maybe the two turbines are stacked and the boiler at the front. That might get us to 25 meters or less as long as the turbines aren't too tall. And yea the mass budget is just the max, which I based on freight train cars. OTOH it's total mass, so engine + car frame and wheels. BTW I did not try to tackle the air intake scheme here. Trivially you could put a huge intake at the front, a lot like a jet engine, but then you risk foreign object ingestion. $\endgroup$
    – DrZ214
    Feb 21, 2017 at 0:47
  • $\begingroup$ P.S., when you say "We need to shed about 1MW of heat for our 1MW steam engine." The system is supposed to be 50% or more efficient, so I think it should be 2MW of heat shed for the 1MW engine. (However, this is only one part of the thermodynamic cycle. This part might be 75% efficient, and another part maybe 67%, so the product will be 50%.) $\endgroup$
    – DrZ214
    Feb 21, 2017 at 0:53
  • $\begingroup$ The gas turbine has maybe 25%, efficiency, if the steam engine has 33% efficiency it valorizes that much of the 75% energy 'wasted' by the gas turbine for a total efficiency of 50% ... You are right about the 2MW. $\endgroup$
    – mart
    Feb 21, 2017 at 7:25

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