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Old 05-13-2016, 01:10 PM   #21 (permalink)
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Quote:
Originally Posted by RustyLugNut View Post
The advantage of gas turbines in aircraft is self explanatory.
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.

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Nothing beats the power to weight advantage. But, small aircraft engines that are diesel are making a comeback for their fuel efficiencies and reasonable costs.
But AFAIK those engines are all intended as replacements for gasoline engines, not turbines.

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Old 05-13-2016, 01:20 PM   #22 (permalink)
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The Piper M350 and M500 are the same airplane on the outside.

The M350 has a 350 hp turbocharged six cylinder gasoline engine. It holds 120 gallons of fuel, has a maximum cruise speed of 245 MPH, and a maximum range of 1544 miles.

The M500 has a 500 hp PT6 turbine engine. It holds 170 gallons of fuel, has a maximum cruise speed of 299 MPH, and a maximum range of 1150 miles.

Those high efficiency gas turbine power plants have huge heat recovery systems that could not possibly fit into an airplane.
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Old 05-13-2016, 11:03 PM   #23 (permalink)
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Quote:
Originally Posted by JRMichler View Post
The Piper M350 and M500 are the same airplane on the outside.

The M350 has a 350 hp turbocharged six cylinder gasoline engine. It holds 120 gallons of fuel, has a maximum cruise speed of 245 MPH, and a maximum range of 1544 miles.

The M500 has a 500 hp PT6 turbine engine. It holds 170 gallons of fuel, has a maximum cruise speed of 299 MPH, and a maximum range of 1150 miles.

Those high efficiency gas turbine power plants have huge heat recovery systems that could not possibly fit into an airplane.
So a 500 hp engine uses more fuel then a 350hp engine. I don't think people 20 years ago thought you would be holding a battery powered supercomputer in you hand either, and it makes sense Orville and Wilbur's design might be more refined then Adolf's, especially when the technology is used widespread from lawn movers to supertankers. I still say give turbines a little more chance before writing them off.
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Old 05-14-2016, 01:34 AM   #24 (permalink)
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So a 500 hp engine uses more fuel then a 350hp engine.
Right. And it comes as a surprise that cruising at 299 mph burns more fuel than cruising at 245 mph?

As for heat recovery systems, a truck can carry quite a bit more weight than an airplane.
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Old 05-14-2016, 04:09 AM   #25 (permalink)
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What leads to a relatively high efficiency of turbine engines in aircraft is either the cruising altitude or the bypass ratio. For higher altitude a turbojet (or pure-jet as it's still often known) makes sense because the air is thinner, so there wouldn't be so much advantage with a high bypass ratio and the fuel consumption would be more related to the lower drag with the air and the lower oxygen concentration limiting the amount of fuel that could be burnt at those conditions, while for intermediate and lower altitudes a turbofan is more advantageous due to the bypass ratio. Turbine engines in general, even turboprops to some extent, are also more prone to damage from foreign object ingestion than piston engines, no wonder some operators still rely on piston-powered aircraft for harsh environmental conditions such as faced by agricultural aviation.
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Old 05-14-2016, 03:34 PM   #26 (permalink)
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Originally Posted by jamesqf View Post
Right. And it comes as a surprise that cruising at 299 mph burns more fuel than cruising at 245 mph?
.
I am afraid it is not entirely correct. 299mph is max cruising speed. If you need max range, you fly at economy speed and it is determined by mass, shape and propeller, so it should be pretty similar for both planes (EDIT: I must admit, I cannot prove it - I cannot find the corrrect data).
Lets do simple math:
1544 miles div 120 gals is 12.86 mpg
1150 miles div 170 gals is 6.76 mpg
500 vs 350 hp means 42% more power
6.76 vs 12.86 mpg means 52 % more consumption
Here you go. In similar aplications, turbine loses roughly ten percent in efficiency.

Last edited by seifrob; 05-19-2016 at 12:35 PM..
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Old 05-16-2016, 03:58 PM   #27 (permalink)
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Pardon me for assuming.

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Originally Posted by jamesqf View Post
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.
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".
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Old 05-16-2016, 04:12 PM   #28 (permalink)
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Let me address this question.

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Originally Posted by jamesqf View Post
As for heat recovery systems, a truck can carry quite a bit more weight than an airplane.
The idea of the micro turbine engine is to provide high power in a very small size and weight. To gain a few more percentage points of thermal efficiency would require not just an increase in mass but an increase in volume. Intake air can be pre-heated and compressed using the exhaust stream of the turbine, but at the gas flow volumes needed to be effective, the size of your power pack increases tremendously.

A 500 hp micro turbine with intake heater and pre-compressor would be larger and weigh almost as much as a 500 hp diesel engine. At that point, why go with the turbine? Though of course you would have to add back the weight and volume of a diesel's cooling and transmission needs.
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Old 05-16-2016, 11:13 PM   #29 (permalink)
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500hp might be somewhat overkill anyway, as the ICE is going to be used only to drive a genset. But still, even though a turbine engine has a higher power-to-weight ratio and fewer moving parts, it still sounds too hard to justify their use in a road-going vehicle even if it's a serial hybrid. Even a Wankel engine could make more sense.
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Old 05-17-2016, 03:52 AM   #30 (permalink)
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I always thought hybrid tech was perfect for heavy hauling. Reducing the need for a higher output engine just to accelerate or climb a grade should improve fuel economy and recapture on downgrades.

Why all the talk of a diesel engine operating under a wide load range? It would operate similarly to a turbine in that it would only run when needed to charge the battery, and during that time would operate at peak efficiency.

Another advantage of a hybrid design is to maintain cabin comfort without having to idle the engine.

The huge drawback to a hybrid design is that it weighs a lot more, and that cuts into the max payload the truck is allowed to haul. Since the primary purpose is to haul a load, this is not an insignificant drawback.

My question is, with all variables concerning payload and route being the same, what is the fuel cost per mile of a hybrid design compared to modern diesel-only tractors?

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