A testing little question on flow rates

[Stan Weiss]

Originally Posted by FlowSpecialist

Now ignoring pulse tuning, normally aspirated engines operate at atmospheric pressure of 14.7 psi maximum pressure drop across the head and inlet manifold.

If the answer is not the same as for a flowbench then why not?

Dave

For this to be the case there would have to be an absolute vacuum in the cylinder.

So since we will have some pressure in the cylinder lets call ir "X"

Equation       (((2*14.7)-x)/(14.7-x))^.5
             Value of X       Evaluated Equation (Flow change)
           2.700000000000     1.49164338901763
           3.700000000000     1.528516809316678
           4.700000000000     1.571623364550171
           5.700000000000     1.622754859285078
           6.700000000000     1.684488052792302
           7.700000000000     1.760681686165901
           8.700000000000     1.857417562100671
           9.700000000000     1.984943324127921
          10.700000000000     2.162174831043965
          11.700000000000     2.428991560298224
          12.700000000000     2.889636655359978
          13.700000000000     3.96232255123179

[MAP]

Greetings,

Fun topic, this. Some factors that intrigue me:

1.) When the turbo compresses the air to 2 atmospheres, does it do so adiabatically or isothermally, or somewhere in between the two extremes? Gamma could make a big (1.4x) difference in airflow and power. And it is here, for example, that a flowbench and a turbo could create largely divergent results.

2.) The airflow inside the motor as a whole is probably not wholely turbultent, but probably has areas which are somewhat laminar too. Further, the balance between the two would vary as a function of time whithin each combustion cycle and, for turbulent flow, even from cycle to cycle. For this reason, I continue to wonder about the implied mathematical simplicity of thinking that airflow would go solely as pressure ^1/1 or pressure ^1/2. Maybe the best fit for a particular set of condtions would be pressure ^0.634, for instance! Or maybe the fit isn't best captured in this manner. But I speak in the most generall terms here...

At any rate, I'll be the first to admit that this is a very weak area of knowledge for me, so I'm looking forward to learning.

Best,
Mark

[FlowSpecialist]

Originally Posted by Lasse

Answer in not the same as for a flowbench, because flowbench measures volumetric flow, but engine likes mass flow. Charging air to twice atmospheric pressure doubles massflow.

Now that's an interesting point, of the type I was hoping would crop up. Actually a flowbench, or at least an airflow bench does indeed measure mass flow because you can't define a compressible fluid other than in terms of either mass, or volume at a given condition of pressure, temperature and density. Only a flowbench working with an incompressible fluid can work solely on volume.

Yes we talk about flow results in CFM but that is always CFM at a specified condition - usually standard atmospheric. In effect we are back to mass. The flow equations that relate flow to the square root of pressure drop are also stated in terms of mass flow. The actual conditions inside the flowbench will never be atmospheric because there's a pressure drop involved across the test piece so the equations have to correct this volume flow back to mass flow at standard conditions.

Nevertheless you are very much on the right lines. The full answer will come from an explanation of why.

Dave

[MAP]

Greetings,

Volume velocity and mass velocity are interrelated by mass density (rho.) Therefore, divergence between flowbench and global motor behavior for an n-bar boost turbo, ought to be reducible to a matter of rho. Since rho, in turn, depends on the manner of compression, I suspect we're headed back to the question of an adiabatic versus an isothermal process.

Dusting off decades worh of cobwebs on this topic speaking for myself, I believe our answer is that the compression is adiabatic, where PV^gamma = constant, and where gamma = 1.4 for a diatomic gas (O2, N2.)

And so the question comes full circle to the matter of the fundamental thermodynamics.

Change of topic - I'm enjoying the class, but I'd like to know more about its presumptive teacher.

Flow Specialist, who are you?

Thanks,
Mark

[rookie]

This is way over my head, but I'm wondering are ya'll factoring in the heat created by boosting an engine vs. the temps you would see on a flowbench

[rookie]

I think post #13 kinda answered my question, I guess I should read a little closer before opening my mouth.

[FlowSpecialist]

Originally Posted by Stan Weiss

For this to be the case there would have to be an absolute vacuum in the cylinder.

So since we will have some pressure in the cylinder lets call ir "X"

Whatever the pressure is inside the cylinder at any point in time you would at least expect it to be roughly twice as high inside the cylinder if the manifold pressure is 2 x atm so the differential pressure will always be double. You can't just call it X and apply that to both cases.

Dave

[FlowSpecialist]

Originally Posted by MAP

Change of topic - I'm enjoying the class, but I'd like to know more about its presumptive teacher.

Flow Specialist, who are you?

Thanks,
Mark

Now there's a question. If I'd wanted that to be known I'd have registered under my own name but in generalities a race engine designer and builder, a rightpondian, an engineer, a specialist in cylinder heads, flow and flowbenches if I had to pick one area but all aspects of engine theory, metallurgy, tribology are important to me. I've designed and made cams, cranks, conrods, pistons, valves and various other things in my time. These days I spend more time on engine and flow theory than actually building them and I intend to stop building them at all fairly soon. That side of it holds little interest anymore and I have other sources of income.

Passing the accumulated knowledge back on still gives me some pleasure and maybe if I ever get a roundtuit I'll write a book but given my propensity for putting things off it'll probably never happen.

Dave

[FlowSpecialist]

Originally Posted by rookie

I may be totally wrong.....

There, we've found something we can actually agree on. That wasn't so hard after all

but I feel this is a poke at the intelligent of the members of GFN and that you feel it needs time for more people like you.

I’m sorry, but I am not politically correct….

Anyhow most people hear do not debate David’s articles because they know his attention to detail (may be you should read his books they apply to more than Chevy’s) and it is somewhat senseless to argue with a dyno pull.

When David asked me to come and have a look on here at what he was getting up to these days I hoped to be able to answer a few questions, pose a few of my own and also learn something from the experience and knowledge of others on here.

I wasn't expecting it to turn into a pissing contest quite so soon or I perhaps wouldn't have bothered.

I have read plenty of David's work thank you, albeit not the Chevy books, and he doesn't need you to tell me what sort of regard I ought to hold his expertise in. He's known that well enough for 30 years.

BTW, the word 'debate' doesn't mean to contradict. It means to discuss, ask further questions.

They do say that England and America are two countries divided by a common language so I'll put this down to teething problems of that nature for now and say no more about it.

Dave

[MAP]

Dear Flow Specialist,

Thanks for the clarifications, but I vote we put this behind us once and for all. If you had explained yourself at the outset, we could have avoided this. Instead, you started with the tone of, "Now class, what do you think about this? And, if you get it wrong, I will correct you and lead you to higher planes of understanding...". This comes across as presumptuous and even a bit egotistical without a proper introduction. In retrospect, I'm sure you can appreciate this.

Your background sounds impressive and as I've written before, I look forward to learning from you.

So, I hope class is back in session.

Best,
Mark