Without doing the calc's to prove or disprove, I'd bet the the Kinetic energy of hot gas is negligible sine the mass flow rate is very low in relation to the internal energy U.

Mike

 

where does that velocity come from???
Mass is there.. but more importantly is volume.. try blowing a fan with your breath... its the same thing...

I incline more to the W=PdV equation more than anything.

 

Well, I'm certainly not as smart as many of the folks here so I can only go on what I do know. I'm not saying that thermal energy isn't responsible, I am saying it's *part* of the energy exchange. I don't have the data or time to do any calculations either so it's all conjecture

This is what I'm thinking. Pressure is a fuction of the temperature in a closed system. Mass is being moved through the exhaust/turbine so it's most definitely not a closed system and you can't distinguish a boundary. During the power stroke (with the exception of blow-by and the addition of fuel in a diesel) you are dealing with a closed system. The work performed on the crankshaft is a direct result of the thermal energy released from the fuel/air. When the exhaust valve is open work is also performed on the mass of gasses that reside in the cylinder (which under these conditions behave almost like a piston), as well as further down stream, that in turn performs work on the turbine. This isn't a closed system as mass enters and leaves (and hence carries with it KE) If this is all you had to worry about, it would indeed be "free" power.

The problem starts after the piston passes BDC. Because of this pressure we have working for us, the cylinder has not been evacuated. At this point the tables turn and now the piston must perform work on the gasses, which in turn perform work on the turbine. Now you are realizing very real pumping losses, which "steals" work that could be used to motivate the vehicle. This is the part we should all be concerned with and why a turbo isn't *totally* free. If anything you would actually be heating the gasses now (relatively).

It'd be difficult to determine real numbers to represent the exhange of energy with all the factors and being so dynamic. This is just how I can picture it with my simple little brain

 

Depends where you start. The velocity comes from the sun, it comes from the gas, it comes from the temperature, it comes from the temperature which in turn creates pressure which imparts KE into the airstream which imparts energy onto the turbine.

Mass is what matters. Volume is fixed, which is what creates pressure. The pressure starts because of temp but the mass moves in and out of the system. Technically speaking, the higher the velocity, the lower the pressure (all relative again!)

Are you saying the temp of my breath is what moves the fan??? Or is it the movement of mass across the blades?

I wouldn't completely discount the energy of moving air. Sure the the mass isn't all the great, relatively speaking, but the velocity is extremely high. K=1/2 mv^2 Velocity has a much larger affect on KE then you give it credit.

I'm no authority but I figured this could fuel an interesting discussion

 

I agree and was what I wanted to clarify... most of the info on the net put it as free power... and it has similar power losses as a supercharger belt.. the advantage of turbo is that you get more power down low as opposed to a centrifugal.. but still most of the time with the sizes for real hp cars... I cant find a real difference between them.. right now znines went 9.006@159 with a YS-trim.. i have yet to see that on a camaro lt1 with a turbo or twin turbo.. maybe the sleeperz28.

 

No.. the temp has some effect but its your lungs which was my point.. I thought your post was that mostly temp was what moved the turbine

 

My point was, like most things in the real world, that it isn't as simple as one thing. It's highly dynamic. It's proven simply by the fact that a turbo system will impose pumping losses, however much energy that "steals" is roughly how much work is being done by the kinetic energy, right? Because as soon as the thermal energy leaves the cylinder it must perform work or be lost to its surroundings.

That probably brings us to the most important point. From what I understand, it's so highly dynamic and complicated that the best results are usually seen by just throwing the dynamic crap out the window and simply balance pressures

I can't even afford a turbo, much less have much experience with them so I probably don't carry much weight in this forum

 

I think you guys are making this more complicated than it already is.

I have posted the accepted, 150 year old method of solving this sort of problem. It is still used today throughout the power-generating, refining, automotive, aviation, etc. industries.

Kinetic energy of the exhuast gas is negligible. It takes ~35 hp to spin a turbocharger. Put your car in neutral and rev the engine. If the exhaust had more than 2 or 3 hp KE available, the 1/2*m*v^2 would push the vehicle along, but it doesn't.

Work is irrelevant. A snail has the same ability to do work as a top fuel dragster, it just may take a little longer to do it. The idea here is that we need to do a certain amount of work in a small amount of time. Work/Time = Power.

The simple FACTS:

- Turbo's add backpressure.

- [After about a dozen assumptions] a turbo will add a MINIMUM of .4 X Boost Pressure as backpressure.

- A typical turbocharger creates backpressure equal to 2 X Boost Pressure.

- It is very difficult to get the backpressure to be less than boost pressure, but it is possible.

- Increased backpressure decreases engine hp.

- Pumping losses are in the neighborhood of 2 X Exhaust Backpressure.

- Intake dilution losses (due to backpressure pushing exhaust into the intake port during overlap) depend heavily on camshaft overlap and is nearly impossible to calculate.

Mike

 

Okay, Okay, my buddy that we are having a twinturbo LS1 built for sent me this link. After reading and watching, I just couldn't stand not to respond.

EngineerMike put it in the best terms. I have always used this analogy when talking about turbos. If it was jsut exhaust velocity alone that moved the turbine on the turbo, then a Diesel would be producing boost all of the time. Diesel's, with no throttle plate are just like being at full throttle all of the time. All that they varry is the amount and the timing of the fuel being injected into the combustion chamber. Which, when added in greater amounts, creates more heat, which, along with the exhaust velocity, spins the turbo.

Yes, a turbo "takes HP to run" just not in the same amount that a supercharger takes. Turbos are more efficient than a belt driven supercharger. Also, at light throttle the turbine wheel is hardly even a restriction.

One thing is for sure, turbos are just BAAAAAD

Pat

 

Well, step on the gas and a diesel gets more air and more fuel.

No, they don't have a throttle plate, but since throttle is controlled by fueling it's not like WOT all the time.


I will agree that turbos are BAAAAAD

 

Actually, the only reason that it gets more airflow when you step on the gas is from a turbo. If the Diesel doesn't have a turbo, it gets 100% of the airflow 100% of the time. There is no throttle plate controlling airflow into the engine is what I was explaining. Sorry you missunderstood.

 

Therein lies the flaw in your statement.

"Diesel's, with no throttle plate are just like being at full throttle all of the time"


Diesels are not "THROTTLED" through the control of airflow. They are "THROTTLED" through the control of fueling.

When more fuel is introduced, then rpms will increase, and airflow through the engine will increase, creating more exhaust which spools the turbo.

So no, a Diesel is not "like being at full throttle all of the time" just because it is not throttled by airflow.

 

Ok, I'll try to make it simple for evertone to understand. Compared to a gasoline engine, which uses a throttle plate, the diesel doesn't have a "throttle plate" and is getting ~100% of the CID in airflow every 2 RPM's

A gasoling engine has only a small percentage of it's CID in airflow filled at <100% throttle opening.

you said "Well, step on the gas and a diesel gets more air and more fuel. " which is incorrect. the only way that a diesel gets more air is either by increasing RPM or by the means of forced induction.

Also, one other point that you were confused on. Let me try to help you out. Just because more fuel is introduced into a diesel, as you stated, the RPM's won't always increase. It depends on load etc...


Hope this helps

 

Obviously... If my car is towing 20kpounds... no matter if the throttle plate is open 100% it wont gain rpm.. jaja

 

First off, a 350ci engine will use 350ci of air every two revolutions of the crankshaft no matter if the throttle is opened or closed. The only thing that changes with throttle position is air density. (Regardless of density it is still 350ci of air)

And I didn't say " step on the gas and a diesel gets more air and more fuel"

I said "When more fuel is introduced, then rpms will increase, and airflow through the engine will increase, creating more exhaust which spools the turbo."

Your argument is exhaust flow does not spin the turbine because diesels "are like at WOT all the time". Well, that just doesn't make any sense if the 'Throttle' is 'Closed', and there is no load on the engine to produce enough exhaust to spool the turbo.

So, the bottom line is, a Diesel is not "like at WOT all the time". If it was, than it would be, and it's not.

So I guess we will have to agree to disagree.