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Originally Posted by jamesqf
Not to me :-) And I still remember the (possibly apocryphal) story about the WWII-era engine designers searching for the optimum turbocharger size for piston engines - which turned out to be eliminating the piston part entirely.
But AFAIK those engines are all intended as replacements for gasoline engines, not turbines.
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Let's use WWII as the starting point.
The Junkers Jumo Aircraft engines were technically advanced even for current times in that they were 2 stroke, opposed piston engines with excellent scavenging and produced about 750 hp for just under 1700 pounds of weight. The license was sold to Napier who produced an after war engine that had a Delta configuration and became better known as the Deltic. Though it was not used in aircraft it was widely used in motor patrol boats and other utility marine craft as well as the Deltic line of rail cars and some municipal emergency vehicles. It produced around 2500 hp with a weight approaching 6000 pounds though it is unclear to me if that includes accessories. The engine was not a light weight by any means.
The famous Wright R-3350 which powered the B29 and other craft of the era eventually saw a development cycle that had multi turbos geared to the shaft to capture lost exhaust energy and add efficiency and power. It could produce over 2500 hp and weighed in at just under 2700 pounds.
Pratt & Whitney's line of PT turbo shaft engines which is now represented by the PW 127 and is widely used in commuter aircraft such as the Bombadier Dash 8 and the Fokker 50/60. It produces about 2500 shaft hp at just above 900 pounds!
The simplicity and weight advantage of the gas turbine is ideal for aircraft. The increase in fuel consumption is more than compensated for by the massive increase in lifting power and the ability to fly higher and faster than piston propeller aircraft.
Yes, there are continuing advancements in turbine technology but they are focused on the issues of reliability and costs as development is up against the limits of the laws of physics and material science. Compressor ratio and heat differential dictate your power production and efficiency much like in a piston engine. Compressor ratios are already up from the 3-5 of the WWII jets to 30 in engines found in the likes of the F15 and up above 40 for the high bypass turbines found on commercial transports. One just keeps stacking more vane/stator stages. But flight parameters also effect your engine design because effective compressor ratio increases with aircraft velocity.
None of this is germane to the discussion at hand except for the fact that turbine engines do not like to run at much less than full rated power without a severe drop off in thermal efficiency. The TE plateau starts to really drop off at 70% of max power and by the time you are idling at 30% max, your TE is halved. Aircraft designers will specify an engine so that it operates on this plateau with the knowledge they can call on a massive amount of take off power for at least a few seconds or minutes before all the inconel turbine blades melt.
The micro turbine to power a Class 8 tractor would be woefully inefficient as a direct drive. Using it as a hybrid charger in the Nikola truck makes sense even if it may be only at 35% TE as it will run for only a period of time to charge the batteries. It looks like Nikola has dealt with the fuel cost aspect by providing the "free fuel".