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Some automakers (Mercedes-Benz, Toyota) have recently been making news about the improved thermal efficiency of their engines, stating that they reached the 50% mark. I was wondering if it was the same for airplanes.

I think I'm pretty sure the aviation industry is well beyond the 50% mark due to the high compression ratio, but what is the most efficient jet engine in existence (currently in use or tested prototype) per category (fan, turbojet, bypass, turboprop, ...)?

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Thermal efficiencies are very rarely quoted for aviation gas turbines. The metrics of interest are specific fuel consumption, and power to weight ratio. While a higher thermal efficiency will increase these, SFC and thrust/weight are performance terms that are easier to comprehend, and describe the performance in terms that can directly be used in performance calculations of the aircraft. However, this article from Pratt & Whitney indicates the best aviation gas turbines had just reached about 55% in 2010. But there is no indication of which engine has the highest figure.

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Land based gas turbines are more efficient than avaition gas turbines, for a number of reasons. According to this article, the most efficient land based gas turbines, where just under 60%, in 2010. The most efficient installation was a Japanese company, with 59.1% verified on an M701G2 gas turbine at the 1500 MW Tokyo Electric Kawasaki power station in Japan.

Since then GE claimed on the 28th of April 2016, a world record for thermal efficiency, with 62.22%, for a combined cycle plant, in France. GE state that it normally takes about a decade to gain 1% in efficiency. The high thermal efficiency is significantly due to operating at a very high temperature. GE state this plant runs at 2,800 deg. F, but the real number is probably proprietary. They say it's like running the engine at takeoff conditions for its entire flight. As you could imagine, this makes cooling very important. It also takes about 30 minutes to get the turbine from idle to maximum speed. Clearly, that won't work in an aircraft.

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This is the GE 9HA gas turbine. You can see the fuel nozzles (one circled) - and that weight does not matter. So, an aviation turbine will never match this level of performance.

In comparison, the first simple cycle model, developed in 1939 is said to have had a thermal efficiency of just 18%.

But maximum efficiency is not always the ultimate goal. A product director from Rolls Royce says:

Everyone would love 60% but the cost is often prohibitive. Not everyone needs a heavy-duty combined-cycle plant. Machines that can achieve baseload within ten minutes such as the Trent 60 are important for the peaking market and a lot cheaper.

Land based combined cycle gas turbines are more efficient than aviation turbines, because;

  • their design can be optimised with out having to worry about weight. Fuel nozzles in particular are optimised in land based gas turbines.
  • they can use steam or water inlet fogging (see above!)
  • they operate at a single rotational speed, so can be optimised for that.
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    $\begingroup$ I almost lost interest in this answer as nothing about aviation was to be found until the end - I'd consider moving the part actually answering the question to the top and add the land based part to the bottom (it is interesting, but not directly connected to the question, it gives a hint on the upper ceiling) $\endgroup$ – Arsenal Apr 23 '18 at 15:09
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    $\begingroup$ @Arsenal Because the question is about something that aviation hardly cares at all. For aircraft, the pure thermal efficiency is of very distant concern. What matters is the overall flight efficiency, which depends on many other parameters, like weight. So question about pure thermal efficiency has no useful answer in aviation context. It's like asking about what car has the lightest drivetrain. Nobody cares, we care about MPG, and heavy hybrids deliver more than weight-optimized transmission. Exactly opposite as in planes. It's reasonable that Penguin focused on area where efficiency matters. $\endgroup$ – Agent_L Apr 23 '18 at 15:40
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    $\begingroup$ In addition to the three reasons given for land-based combined cycle gas turbines being more efficient than aviation turbines, there is the whole 'combined cycle' thing - the use of a second heat engine (invariably a steam turbine, I believe) to extract power from the gas turbine's exhaust heat. $\endgroup$ – sdenham Apr 23 '18 at 16:35
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    $\begingroup$ @Agent_L - the question is quite clear, yeah it might not matter to the average jet engine aviation enthusiast. But inaliahgle is interested in this. If there would be no answer to be found, that's alright, compose an answer out of that and why it doesn't matter. But there is an answer, the latest figure to be found was 55% in 2010, so why not put that first? $\endgroup$ – Arsenal Apr 23 '18 at 20:53
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    $\begingroup$ @Arsenal. Your comment is a fair one, and I generally prefer and write answers with the bottom line up first, then the background. In this case, it happened the other way around, simply because that's the order of info I found as I researched the topic, and I didn't have time to re-organise. I have fixed it now. $\endgroup$ – Penguin Apr 23 '18 at 21:38

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