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Before turbocharger temperature vs after cylinder temperature

Posted: Mon May 09, 2016 6:50 am
by hugerich
Hey guys, got a question on the above.

I was in the engine room one day and listened in to the chief grilling the 2nd on engineering knowledge, but something he said caught my attention. He asked the 2nd why is it that before turbocharger temperatures are higher than exhaust outlets, to which the apparent right answer was to do with kinetic energy from the exhaust gas becoming heat energy before the turbine inlet.

To me this didn't make any sense, considering the distance between the closest cylinder to the inlet of the turbine, we are only talking 30cm at most. Luckily after some digging I found a page on ABB's site that explained everything, which I have attached in case someone is interested. Turns out it is simply because of temperature sensor placement and the effect of charge air flowing over the exhaust valve temperature sensor.

So one mystery solved I now have my own question which I can't find an answer to on google, perhaps because I'm using the wrong words but I'm sure someone here will know the answer. If exhaust gas entering the turbine has a certain amount of heat energy and kinetic energy, and that kinetic energy is driving the turbine, considering energy is converted and not "used up", why is it the heat energy also decreases on the way out of the turbine? My point being that the temperature before turbine may be 500 degrees then outlet will be around 350. My thermodynamics education was some time ago, but as far as I know a turbocharger is not capable of converting heat energy into kinetic. Is it simply a cooling effect from passing through the turbine which is in close contact with inlet gases from the compressor?

Any information would be appreciated.

Re: Before turbocharger temperature vs after cylinder temperature

Posted: Mon May 09, 2016 9:27 am
by JollyJack
P1TI = P2T2 Pressure pre-turbine is higher than pressure after the turbine. Energy, which can neither be created nor destroyed, changes from pressure to velocity across the stator and rotor blades, imparting kinetic energy to turbine blades. The gas residue leaves the exhaust with less energy (and therefore a lower temperature) and this can be used in an economiser exhaust boiler.

Re: Before turbocharger temperature vs after cylinder temperature

Posted: Mon May 09, 2016 11:43 am
by Merlyn
Don't think there is much to add to that explanation.

Re: Before turbocharger temperature vs after cylinder temperature

Posted: Wed May 11, 2016 2:20 am
by Big Pete
But is P1T1=P2T2 the right equation here?

Surely that equation only applies if we are dealing with a constant volume, in this case the gas is expanding through the Turbocharger and it's Pressure will drop, as its Volume increases. At the same time expansion will cause the Temperature to drop, but because the expansion through the Turbocharger is not Isentropic,(100% Efficient) internal friction will convert some of the Kinetic Energy into a Temperature rise.
Meanwhile the water cooled jacket around the Turbo is removing heat and lowering the temperature.
Help! Now I am confused...

Big Pete.

Re: Before turbocharger temperature vs after cylinder temperature

Posted: Wed May 11, 2016 9:40 am
by JollyJack
Maybe it should be P1V1 P2V2
----- = -----
T1 T2

Nah, V in this case is Velocity, not Volume. There's a Venturi effect at the nozzle ring, velocity is increased and that energy impinges on the turbine blade. That's where the transfer of energy, and therefore the temperature drop, occurs. I think the friction losses generated by the turbine gasses would be negligible.

Well that's a bummer! Can't get the "T1" under the "P1V1"

Re: Before turbocharger temperature vs after cylinder temperature

Posted: Thu May 12, 2016 3:55 am
by Merlyn
"Turbocharger temperatures are higher than exhaust outlet temps"

Turbine Efficiency.

The efficiency of the turbine may be obtained from a study of the heat given up by the exhaust gases on their passage through the turbine. The gases passing through the nozzles and blading are subject to friction, this causes a temperature rise in the gases above the temperature expected if expansion has been Isentropic ( Adiabatic )
This is known as reheating and causes an increase in the entropy of the gases.
Now I would love to say that this has been straight off the top of my head but alas it comes courtesy of "LAMBS" .
Don't know what manuals you use in Canada?
For further interest I enclose the necessary equations which of course we must recognise and apply to the necessary calculations in order to obtain Compressor Efficiency .
These Q&A 's are from my old LAMBS Chiefs reference books, long time no see but I do remember where to look for various references concerning different subjects from time to time ( as you do.)
You will no doubt observe that there is no mention of PV/T= C nor are there any references to Volume nor Velocity.
A T's and P's exercise here alright

Re: Before turbocharger temperature vs after cylinder temperature

Posted: Thu May 12, 2016 7:33 am
by hugerich
As always, useful answers all round. Thanks for the education guys

Re: Before turbocharger temperature vs after cylinder temperature

Posted: Thu May 12, 2016 10:53 am
by JollyJack
Got a Lambs here too, as well as a Macgibbon's Red Book, Pounder, Sothern, Osbourne (Vol 1 and 2). Grundy, Audel and Martens....oh, and Reeds too!

Re: Before turbocharger temperature vs after cylinder temperaturet

Posted: Thu May 12, 2016 11:20 pm
by Merlyn
Well I don't think the answer to my as yet unanswered question re the "difference between a clockwise machined cylinder liner V an anti clockwise one " lies in the Reeds N.A.
That should narrow the field somewhat

Re: Before turbocharger temperature vs after cylinder temperature

Posted: Fri May 13, 2016 2:14 am
by Big Pete
No sure here what JJ is saying about PV where V is velocity?

Multiplying Pressure by Velocity doesn't appear to be a very useful product to me. J.J. is usually pretty good on this stuff too.

The old equations are : P1. V1/T1 = P2.V2/T2 or, alternatively, P. V to the power n is a constant, where P is the absolute Pressure, V is the Volume (where the gas is enclosed in a cylinder), ( v with a dot over it would be volume flow rate i.e. cubic metres per seconds, where the Gas is flowing through a system, T is the absolute Temperature, and n is the Adiabatic Index.
These equations purely reflect the relationship between Temperature, pressure and volume.

My understanding was that the nozzle ring (fixed blading) guides the Gas / steam onto the rotating blades at the correct angle, according to whether they are impulse or reaction blades and at the same time, the nozzle ring acts as a series of divergent nozzles, expanding the gas and causing the pressure to drop as the velocity increases.

There are fundamentally 2 types of turbine, impulse and reaction.

Impulse is the defined as the change in momentum. Momentum is calculated as Mass ( or mass flow rate) multiplied by velocity.
so here we have (M1 V1 )- (M2. V2) for steam or exhaust gas hitting a turbine blade will be the momentum lost by the Gas.
One of the Fundamentally Laws of Physics is the conservation of Momentum, so this momentum is GAINED by the Turbine Blades.
Momentum is a Vector, it always has a direction as well as a quantity so you can use Vector Triangles to calculate the direction of the Momentum applied to the turbine blades from the original and final direction of the Gas.

In a reaction turbine, the gas flowing over the blades acts in the same way as air flowing over an aircraft's wing and generates "Lift" in accordance with Bernoulli's Theorem. I can't remember the equations for that off the top of my head! But I don't think Kinetic energy comes into that either.

Big Pete