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Old 05-12-2016, 12:05 AM   #11 (permalink)
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Simple cycle gas turbines do not have the efficiency needed.

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Originally Posted by Hersbird View Post
A gas turbine can be more efficient then a diesel when used as a powered for a generator. That is how I read this. The electic motors at the wheels are powered by a battery. The CNG turbine just trys to keep the battery charged. In 1200 miles both the batteries and the CNG is empty and it needs charged and refuled. The driver would be at his limit anyway before that point. Many heavy trucks are used less then 500 miles and may be able to go pure electic.
As pointed out, this is how a locomotive works but also how a submarine is powered.
Modern gas turbines are rated just above 39% thermal efficiency. Many class 8 diesels are at 48% thermal efficiency and some cutting edge applications are knocking on 52%. The only way a gas turbine exceeds this efficiency is via co-generation with a steam cycle using some of the high value exhaust heat from the gas turbine. I doubt they are using a steam cycle as a co-generator in the Nikola Truck.

However, the fuel is supposedly free for a period of time after purchase, so the efficiency is really a moot point when viewed by Nikola Motor Company.

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Old 05-12-2016, 01:37 AM   #12 (permalink)
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Gas turbines may have a lower weight and fewer moving parts than a reciprocating engine with the same power output, but their maintenance is more specialized and the time between overhaul is shorter, not to mention their air intake filters have a shorter life.
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Old 05-12-2016, 07:51 AM   #13 (permalink)
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I'll take always being used at peak load over a higher theoretical thermal efficiency any day- an idling engine is inefficient no matter what graph you can point to. That'll help with overhaul interval as well.
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Transmission type Efficiency
Manual neutral engine off.100% @MPG <----- Fun Fact.
Manual 1:1 gear ratio .......98%
CVT belt ............................88%
Automatic .........................86%

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Old 05-12-2016, 10:07 AM   #14 (permalink)
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Right, the gas turbine is always at max efficiency, the diesel sometimes. Plus you are comparing old gas turbines to the most modern diesels. There are modern turbines that do as well or better then the most modern diesels and they are getting better all of the time as they are more widely used in ships.
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Old 05-12-2016, 12:45 PM   #15 (permalink)
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If diesels are more efficient than gas turbines, then why are there many gas turbine-powered generating plants, while diesels are only a niche application, mostly for backup & emergency power? And why are there so few diesel-engined airplanes?
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Old 05-12-2016, 01:05 PM   #16 (permalink)
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The Nikola One design has 6 motors - and no need for a multigear transmission. Electric motors have almost flat torque output at all RPM's.

Another route would be to do something like this, instead of the turbine:



Since the trucks have batteries, they can still pass:

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Old 05-12-2016, 03:35 PM   #17 (permalink)
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Quote:
Originally Posted by jamesqf View Post
If diesels are more efficient than gas turbines, then why are there many gas turbine-powered generating plants, while diesels are only a niche application, mostly for backup & emergency power? And why are there so few diesel-engined airplanes?
Gas turbine power is relatively cheap and the turbines can be run at their max efficiency. Gas can be main lined into a facility much easier than diesel. Diesels are favored for backup generation because of their much quicker startup time vs a turbine generator. Diesel, as I understand, is also not favored in airplanes because of weight.
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Old 05-12-2016, 08:55 PM   #18 (permalink)
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Really?

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Originally Posted by Hersbird View Post
Right, the gas turbine is always at max efficiency, the diesel sometimes. Plus you are comparing old gas turbines to the most modern diesels. There are modern turbines that do as well or better then the most modern diesels and they are getting better all of the time as they are more widely used in ships.
Please show me the "modern turbine" that can improve on current thermal efficiencies of diesels. I have already defined this as the simple cycle gas turbine and not the combined cycle turbines you are thinking of. The Nikola truck does not seem to have a steam turbine hanging off the back of the gas turbine exhaust. The steam turbine would be exceedingly large and it's start up would be excessively long.

Turbines are ideal where power to weight is at a premium. Modern turbines have the ability to run at higher temperatures and compressor pressures but are still limited to around 40% TE. Yes, you can hang pre-compressors and thermal recuperators and such but now your small compact turbine is big and clunky for just a few percent gain. Capstone only claims about 35% TE for it's simple cycle micro turbine. Such a turbine would be ideal for a class 8 tractor with hybrid electric drive but not because it is more fuel efficient than current diesels but because it's small size and weight allows you to transfer more payload to your battery and fuel.

The advantage of gas turbines in aircraft is self explanatory. 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.

And you are making the assumption that diesels lose great efficiency when at part "throttle". They do not. They do not have throttles and can be optimized for part load efficiencies. The BFSC map of a diesel is far larger than that of a gas turbine or a spark ignition reciprocating engine.

Yes, I am familiar with super-critical CO2 turbines but these are more suited to larger power generators found on ships or land. An experimental system leverages the high quality heat of turbines (little heat is lost to cooling and friction) by running a gas turbine exhaust into a steam turbine whose exhaust powers a super-critical turbine. The thermal efficiencies are pushing 70%. Exciting stuff but not applicable to a class 8 tractor drive.
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Old 05-13-2016, 02:02 AM   #19 (permalink)
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How about US Army plans to replace Abrams power pack (originally gas turbine) with diesels? I assume that originally gas turbine was chosen because of its multi-fuel capacity and high power to mass ratio, but certainly not fuel effectiveness.

https://en.wikipedia.org/wiki/M1_Abrams#Tactical

Last edited by seifrob; 05-13-2016 at 03:37 PM.. Reason: grammar corrected (hopefully)
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Old 05-13-2016, 12:40 PM   #20 (permalink)
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The 30-40% turbines are the old design that date back to when diesel motors achieved similar numbers or worse. So diesels have evolved and improved and you yourself have pointed out so have the turbines even though much less attention is spent on them. So even the old simple turbine design is not bad thermal efficiency especially comparing it to a truck diesel operated at all kinds of rpms and loads. Now start using this concept's power plant in millions of trucks for 25 years and watch how much better turbine designs get.

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