Turbos & intercooling

[Deere Freek]

The way i see it you can't have more cfm at 10psi with the GM-8 turbo as with the a team turbo. 10 psi in the intake is 10 psi and you will get the same cfm if the turbo can maintain that pressure.

Instead I think the turbo should be rated as 100 cfm @ 10 psi or 200 cfm @ 10 psi. The one at 100 will just lag and the pressure will drop if the engine requires more than 100 cfm whereas the att will be able to keep up with demand.

There is no way that one turbo can pump more into the engine than another if they are both at 10psi. but one can sustain the 10psi longer than the other. It just so happens that the att turbo can do so more efficiently as well.

Did i get this right?

 

[Turbine Doc]

Not exactly go back to the data I earlier in the replacement turbo thread, All theory aside, read the data; the ATT has physically larger wheels than GM-X turbos, it does with 3 psi boost, what GM-X does with 6 psi boost, more cfm from the ATT. I got ridiculed by Buddy with my compressor analogy maybe it would hve been better stated as 3 Hp compressor vs 5 Hp compressor one will move more cfm air than the other, sort of like engines 4 cyl @ 5000 rpm vs 8 cyl @ 3000 rpm both making 150 hp one more efficiently than the other.

IIRC what slim was saying turbo ratings were done at a standard condition of 15psi boost, and depending on size of the turbo you can different cfm from each though at 15psi.

 

[buddy]

I didnt really ridicule you, thought I was making fun of unrealistic ratings on little pumps. Deere Freek was pretty much correct. Its the demand that consitutes how much CFM is pushed through, and with our small demand it doesnt need to push 800cfm to make 15psi, just the 400cfm at 15psi, although put it in a 12litre engine and it would be pushing 800cfm at 15psi, but hte GM-X would only be able to make 7psi because the engine would consume its 400CFM too fast and the GMX couldnt compress the air any more to meet the demand.

Its physically impossible to flow more air at a lower psi in this case. a few people in this thread have understood that.

You might create the same power at a lower psi which is awesome and completely makes sense for the greater air density and the engines ability to make RPMs with less work (backpressure). A great accomplishment which everyone is very thankful for.

 

[JiFaire]

Before we get too far down the road of assuming our own little over-simplifications apply to this particular situation, take a minute and look up compressor efficiency mapping...

It might help to sort of explain the whole PV=nRT thing for fluid systems, of which this qualifies as one.

The ATT handles a greater volume of air at less pressure than the GMx series. Due to this, the ATT causes less dynamic thermo-loading, which eliminates the need for an IC. I would strongly suspect that a cross-sectional MAF would show more laminar flow and less thermal gradient in air of equivalent pressures, and since said air is cooler, there is indeed more of it at a given pressure.

Back-pressure minimization has little to do with this; the design of the turbo is entirely different, and therefore they should not be compared in this fashion (using back-pressure as a metric). The efficiency difference between the two turbos would best be measured using mass airflow; unless you have a nuclear densometer handy to perform those tests, then I suggest that in the absence of active measures, we stick with the proxy measures we now have... what it does to the engine.

It would appear that the ATT is capable of running with more power, less IAT, and lower manifold absolute pressure in similar circumstances as the GMx series.

No report has indicated otherwise. not one.

By induction, then, if the proxy measures are valid, we can infer the existance of the underlying efficiencies as also valid.

All those courses in Fluid Dynamics actually stuck with me all these years... amazing.

 

[buddy]

I completely agree, it flows more air mass, its a great turbo.

 

[Slim Shady]

Thanks JiFair,:thumbsup: the amount of technical experts we have here in varying fields has always been humbling. :rolleyes5: Only here (DTR) could we have a discussion like this and be civil to each other. ):hI can't contribute to the technical discussion from an expert's perspective, I just know that the turbo works well. Also, as you stated, the two turbos are vastly different physically, and in the way they accomplish their intended task, TD's pictures in the original post show the physical differences. http://dieseltowingresource.com/showthread.php?t=2015

Like you said everyone that is running an A-Team Turbo, ATT reports the same results, less boost at cruising but lower EGT and better throttle response and better fuel mileage. If it had not been for TD you and others this project would have went by by a long time ago.

 

[Goldsburg]

Before we get too far down the road of assuming our own little over-simplifications apply to this particular situation, take a minute and look up compressor efficiency mapping...

It might help to sort of explain the whole PV=nRT thing for fluid systems, of which this qualifies as one.

The ATT handles a greater volume of air at less pressure than the GMx series. Due to this, the ATT causes less dynamic thermo-loading, which eliminates the need for an IC. I would strongly suspect that a cross-sectional MAF would show more laminar flow and less thermal gradient in air of equivalent pressures, and since said air is cooler, there is indeed more of it at a given pressure.

Back-pressure minimization has little to do with this; the design of the turbo is entirely different, and therefore they should not be compared in this fashion (using back-pressure as a metric). The efficiency difference between the two turbos would best be measured using mass airflow; unless you have a nuclear densometer handy to perform those tests, then I suggest that in the absence of active measures, we stick with the proxy measures we now have... what it does to the engine.

It would appear that the ATT is capable of running with more power, less IAT, and lower manifold absolute pressure in similar circumstances as the GMx series.

No report has indicated otherwise. not one.

By induction, then, if the proxy measures are valid, we can infer the existance of the underlying efficiencies as also valid.

All those courses in Fluid Dynamics actually stuck with me all these years... amazing.

Jim -

There are a few points with which I take issue:

1. Again, the volume of air being provided by the turbo is relatively fixed because of the displacement and volumetric efficiency of the engine. Although SOME minute flow advantages are likely to be obtained through slightly improved volumetric efficiency as a result of lower exhaust backpressure. This is why it is best to speak in terms of mass flow with regards to engine induction.

2. Laminar flow is determined from the Reynolds number. The Reynolds number cannot be measured by a MAF nor can a MAF directly quantify "laminar" flow. There can be improved mass flow into the engine without that flow being laminar.

3. While I don't disagree with all of your thoughts on backpressure. It is perfectly valid to compare the backpressure of one turbo to another. That is because backpressure is detrimental to performance and longevity of the engine. The reduction in backpressure is a direct result of lowered required compressor drive energy (from improved aerodynamic efficiency of the compressor wheel, which is proven by the lower IAT's) and a greater turbine efficiency, which is demonstrated by the reduced exhaust backpressure even with reduced EGT. This turbine is just able to extract the energy needed to compress the required air with a lower P and T. Furthermore, it does not take a Nuclear Densometer to measure the efficiency of a turbo charger. If it did, then 3/4 of the turbocharger producers could not generate compressor maps or turbine maps. Efficiency is simply, the energy out divided by the energy in. You can directly measure compressor power (required), you can measure shaft losses (from bearing drag, imbalance, etc.), and finally you can measure turbine power (produced).

If you have a grasp of sound engineering principles, then you can often "simplify" the occurrences that are observed in machines. If you DO NOT have a sound grasp of engineering principles, then you SHOULD NOT try to simplify what is observed as you will likely come to the wrong or insufficient conclusion.

Although turbo chargers are a simple device, this whole discussion proves that turbo charger performance and sizing is still more of an art than a science...

Congratulations Slim and Turbine Doc!

Regards,

 

[buddy]

I think Jim was just saying backpressure has nothing to do with the amount of flow being put in, which is true for the most part, unless you take into account the very small(relatively) of exhaust during the momentary overlap period which we've been told doesnt work on IDI engine. I wonder if we can buy a crank that gets rid of overlap.

But yes the backpressure is obvious reason the turbo has lower IATs and it does reduce work the combustion stroke needs to do to drive the exhaust stroke of the other cylinders.

I'm going to look into those meters that will accept pitot tubes as inputs, that would be sweet to have. I played with them a lot in school where I had learned many things in engineering aircraft and spacecraft, although regrettably dont use them very much in the management role I have in the Air Force.

 

[schiker]

But you can have more SCFM or "CFM" or throughput of air with the same intake pressure.

There is a conservation of mass law. Basically what comes out must go in or what goes in must come out.

There is no conservation of volume or conservation of pressure law.

You can have a cylinder at 100 psi shut no flow ie no velocity. You can have an air hose at 100 psi with many different flow rates depending on the exit differential pressure. The exit differential pressure will determine the velocity of the air.

If you have a set cross sectional area and you know CFM you can calculate the velocity or vice versa. If you know pressure you can calculate mass flowrate. Its the mass flowrate of the ATT that is better.

You just have to keep the context of the discussion and the same "CFM" reference or frame of mind.

 

[buddy]

Schiker, we havent changed the exit differential pressure, because its closed, there is not exit pressure when the air is flowing in, and the cylider pushes out the exit, not the turbo on the exhaust stroke.

So there is no volume of air increasing as CFM or SCFM relates to, but even as you say the mass flow is better, not volume cubic feet.

 

[Goldsburg]

I think Jim was just saying backpressure has nothing to do with the amount of flow being put in, which is true for the most part, unless you take into account the very small(relatively) of exhaust during the momentary overlap period which we've been told doesnt work on IDI engine. I wonder if we can buy a crank that gets rid of overlap.

But yes the backpressure is obvious reason the turbo has lower IATs and it does reduce work the combustion stroke needs to do to drive the exhaust stroke of the other cylinders.

I'm going to look into those meters that will accept pitot tubes as inputs, that would be sweet to have. I played with them a lot in school where I had learned many things in engineering aircraft and spacecraft, although regrettably dont use them very much in the management role I have in the Air Force.

Buddy -

Just to clean up a few loose ends:

1. The combustion stroke driving the exhaust stroke of the other cylinders is not my point with regard to back pressure. I am looking at it more from a "pumping loss" standpoint. In other words, having 20PSI of head pressure against the exhaust stream reduces the scavenging efficiency of the cylinder (imagine a one cylinder engine). This leaves unburnt gases in the chamber when the intake opens to fill. This will "effectively" reduce volumetric efficiency of the pump (engine) as less intake charge is needed to fill the cylinder.

2. Lower backpressure is not necessarily the reason for lower IAT's. As mentioned before, aerodynamic improvements to a compressor wheel, lower operating pressure (ratio), or improved operating conditions, these are reasons for lower IAT's.

Regards,

 

[ak diesel driver]

Just so you guys know I am still folloing this and I really enjoy ready all the different viewpoints. Kind of makes you rethink all the ideas and theory:thumbsup:

 

[ercaduceus]

Wow, what a great thread. I feel like a moron reading it tho. Should have paid more attention in school. Well, I think I'm starting to get it. Thanks

 

[schiker]

But we have changed the "exit" differential pressure. Not of the tailpipe but at the valves both intake and exhaust valves. ie the backpressure on the turbine wheel due to the more effecient turbo. Once a couple of feet down the the exhaust down pipe the exhaust gas cools some and a bigger pipe say 4" diameter pales the valve cross sectional area so the differential pressure in the exhaust pipe is not a big deal vs the exit and atmosphere for a SCFM measurement. But across the valve area the differential pressure is a big deal.

Some rough math....
At 6.5 liters displacement that's 0.295 left after 22:1 compression. or 0.295 liters per cycle or about 18 something oz's. Now look at the high temperature of exhaust gas on exit stroke. When air rushes into cylinder on intake stroke its warmed up some due to residual exhaust gas left and expands some and is working on equalizing but due to time of event gets cut off at the end before full equilization. Because there is friction of passing the valves and turbulence. And appreciate the intake or exhaust full stroke is what about 650 strokes per minute or 10 strokes per second (thats the piston time BDC to TDC or vice versa at 2600 RPM's so I imagine increasing the differential pressure across the valves does improve velocity and the flow is increased due to decreased backpressure between the turbine and ehxaust valve.

So yes the backpressure effeciency improvement of the ATT is pretty big. It allows more air to exit the cylinder and in turn allows more air to enter with a good compressor to turbine map.

Along with temperature improvements this combines to increase the mass flow rate and thus the SCFM throughput in effect increasing CFM through the intake with the same pressure due to velocity increase and density improvements across the intake valve.

The neat thing about turbo's is the mass improvement compounds itself. So a little more mass on intake makes more drive volume once heated for the turbine so more mass in is a win win situation.

 

[schiker]

reworded a bit before I get cut off

With improved differential pressure across valves more flow happens so in effect more CFM with the same intake pressure.

 

[Mudbath]

Before we get too far down the road of assuming our own little over-simplifications apply to this particular situation, take a minute and look up compressor efficiency mapping...

It might help to sort of explain the whole PV=nRT thing for fluid systems, of which this qualifies as one.

The ATT handles a greater volume of air at less pressure than the GMx series. Due to this, the ATT causes less dynamic thermo-loading, which eliminates the need for an IC. I would strongly suspect that a cross-sectional MAF would show more laminar flow and less thermal gradient in air of equivalent pressures, and since said air is cooler, there is indeed more of it at a given pressure.

Back-pressure minimization has little to do with this; the design of the turbo is entirely different, and therefore they should not be compared in this fashion (using back-pressure as a metric). The efficiency difference between the two turbos would best be measured using mass airflow; unless you have a nuclear densometer handy to perform those tests, then I suggest that in the absence of active measures, we stick with the proxy measures we now have... what it does to the engine.

It would appear that the ATT is capable of running with more power, less IAT, and lower manifold absolute pressure in similar circumstances as the GMx series.

No report has indicated otherwise. not one.

By induction, then, if the proxy measures are valid, we can infer the existance of the underlying efficiencies as also valid.

All those courses in Fluid Dynamics actually stuck with me all these years... amazing.

When I read this I keep picturing the bear in your avatar talking. ):h Must be 70's or 80's flash backs. :sifone:

 

[buddy]

Thank you Schiker for bringing that up. So I did the math, which took me a while with all the unit conversions. And it seems like the volume of exhaust left, if scavenging doesnt work like mentioned, does decrease with the lower pressure.

At 20psi backpressure at the valve, if the 22:1 ratio left .034L of exhaust at 20psi and maybe 500*F and then on the intake stroke it expanded to lets say 5psi the turbo is pushing in, and its at say 250*F then there would be 0.1L of exhaust and 0.713L of air. Reduce the backpressure to 10psi (when pushing 5psi boost) and the exhaust only expands to .05L and you have 0.7625L of air.

So you have gained about 6.5% more air flow into the cylinder. So there, you convinced me there is more air flow at the same psi, although Im sure my calculations are flawed one way or the other and completely messed up if that overlap part of the valves actually does anything.

However, I was willing to agree that there might be 5% increase before, and now with some actual math and physics thats pretty close.

I just used that ideal gas law and calculated the total number of moles=n at a given Temp, pressure and volume, and converted everything to work with the constant R=8.314, temp: K, pressure: KPa, volume: litres already.

 

[ak diesel driver]

So if we were to use the belt driven turbo and have 0 or close to 0 backpressure we wouldn't have to run as much boost to see a comparable gain .

 

[buddy]

So if we were to use the belt driven turbo and have 0 or close to 0 backpressure we wouldn't have to run as much boost to see a comparable gain .

I dont want to think that hard on my day off and I have to get back to the putting up a microhood where I had no cabinet to hang it from, so all from scratch and I hope 7 lag bolts into the wall studs will hold the weight with no vertical support coming down from the roof. since youre going to have to convert work into turning the supercharger to create the boost, so there is loss, I just dont know how much and I'm willing to bet its less efficient than a well designed turbo like the ATT.

 

[ak diesel driver]

I be interested in some hard data before I'd give up totally on this idea. This is after all a mental excerise that were doing here.