Two DD8V92s bolted together
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I’m really troubled by this brayton cycle as it applies to exhaust turbines. Does it apply? its not a closed loop. Going from 4 to 1, rejecting heat at constant pressure represents the gas leaving the system to the atmosphere, carrying its heat with it. But 1 to 2 is isentropic compression of the fresh charge… so at 1 we are talking about two different things. That whole 4 to 1 leg is so bogus. We’re not compressing exhaust. We’re not compressing anything at constant pressure. They just drew a crazy line there so that they can use the area to represent work. Bogus as those meaningless arrows in a circle graphics that seem to please the pants off of corporate HSE guys.
Okay, up to speed here! DD = Detroit Diesel, 16 V = V 16 cylinders(ergo two V8’s bolted together!), 95 = 95 cubic inches per cylinder!
Gotcha! I am from the DD 6 71 era. Seems that was the only diesel engine for a boat in the Navy. My short foray into big trucks taught me the DD 6V71 was the most efficient engine for turning diesel fuel into noise!
The Brayton cycle describes a complete gas turbine heat engine, not just the turbine itself. 4-1 represents the exchange of hot exhaust for cold air. The cycle appears closed because it uses ambient atmosphere as a common reference point. The same applies to the Otto and Diesel cycles, once you think about it.
But they are pinwheels that just so happen to be in a duct. If we get stuck on the temperature drop, I must point out that there is also a temperature drop across a windmill blade.
Also, this idea that the temperature of the working fluid must drop for energy exchange to take place is inaccurate. If you run a turbine on an incompressible fluid (such as water), the temperature of the fluid rises as it passes the turbine (because of friction etc). I’m talking the exact same hardware, different working fluid. Then there’s low compression Stirlings, where the temperature of the gas actually rises during expansion. It’s all about where and how you add the heat.
Sure, you can always go hunt down the delta-t somewhere no matter what type of engine we’re looking at, demonstrating that everything is a heat engine (or part of one) somehow. I love playing that game, and it makes you appear very sage and all, but it doesn’t contribute much to understanding exhaust gas turbines, at least not on this level.
I was advised that this post appears particularly condescending. Rest assured that it was not my intention; my position (not an actual engineer but just some dude who feels strongly about the deeper questions of energy transfer) is not one whence to condescend.
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I don’t think anyone said it had to, there are many examples such as the starting air turbine and those used in large scale compressed air energy storage systems (CAES) where heat transfer is not a major consideration. Adding heat to the air in the case of the CAES increases output and efficiency but is not critical to the function. Even in those cases though, heat was involved but the temperature drop occurred at another place and time.
In the case of exhaust gas turbines, heat is the major source of energy and that is why some say the turbine is not just a pinwheel. If you want a striking example of what impact heat has on a turbine, look at the thrust of a jet engine or the power output of a turboshaft engine when fuel flow is reduced. Rotational speed and gas flow taper off but power drops instantly. The reason rpm drops is because the power required to turn the compressor is substantial. Next time you fly pay attention to how quickly the engine spools down when fuel is cutoff, the only source of power to keep that engine spinning is heat from burning fuel.
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I understand your point here but I think the people who have posted here do understand that. There are concepts like adiabatic lapse rate and so forth in meteorology.
In my case it is true that I lack an intuitive understanding of thermodynamics beyond the absolute basics.
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Wonder how long it will be before a bearingless jet engine will be developed? When I saw my first bearingless motor I thought about that. Just need a magnetic field. Imagine the fuel savings if you could get a cheap magnetic field.
That will be a real challenge! The weight of shafts and rotors is considerable and both radial and axial loads have to be managed. Thrust loads alone are very high and sealing is critical as well. That is not to mention the fact that on aircraft the rotors turn from wind blowing through the engine while the plane is parked
In practice though, the ball and roller bearings used today are very low friction. More friction is presented by the lube oil. On one engine I used to operate, a snifter valve was opened during start that admitted air to the lube oil pump suction in order to reduce friction and spool up the engine for faster, cooler, starts.
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I was just dreaming because as you say the jet engine guys have their stuff together as fuel efficiency and reliability is what makes or breaks an engine manufacturer. The future of energy savings in machines is with smaller applications. AC compressors and the like. There are a lot more of them than jet engines,
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Did you ever clean a turbocharger exhaust side in service with rice. Hmm fried rice.
Crushed walnut shells were preferred but I believe crushed macadamia nut shells work as well.