Ok folks, this has been the bain of my existence for a long time.... so let's hash it out until we all get it right!

Pressure is a force applied to all surfaces based on the level of compaction of air molecules... flow relates to the mass or volume of air as it moves through an engine.

So pressure is important because it tells us about latent heat production due to compression of air - but flow will tell us how the mass of air moves through a motor.

Pressure of storage of air vs pressure of an engine: Pressure of a fixed volume of air relates to the environmental factors surrounding the storage of air. So an 80 cu/ft cylinder holding 3000 psi at 80F will increase in pressure by 10% for every 10% increase in temperature (see general gas law). NO MATTER WHAT - changes in temperature effect changes in pressure and vice versa; this is important as relates to timing.

Flow is king on a street car (factory internals) because it relates to the mass of air an engine can move. Power is derived from flow because and engine is not a static environment.... so using our 80 cu/ft cylinder as an example... it would be emptied in less than 5 seconds on a rotary at boost!!!!! So a 1 degree temperature change is not as important as a 1% change in flow (an 80 cu/ft cylinder vs an 8.1 cu/ft cylinder).

Pressure = heat = timing changes is totally different than (flow = power = fueling changes.)

Pressure and heat are tied while flow and power are tied... as long as you can account for heat you can run whatever pressure you desire.


Ok Ray, Jeff and company - how did I f-up the explanation?

Picture a positive displacement compressor... the pressure it produces efficiently is better than any centrifugal compressor because it gains pressure on each RPM - at the sacrifice of heat generation. This compared to a centrifugal compressor which may only increase the PR fractionally per RPM, however the efficiency over the flow characteristics of an engine are better..... so overall the engine moves a higher mass of air in the operating range. But in both these examples - the air charge gets heated and the latent heat has to be accounted for.

Compressors use pressure because that is what they do; make air pressure. This is why there are all these pressure signals for a turbo, it doesn't care about flow it needs pressure to tell it what to do. Engines use flow because that is what they do - move air in and out and turn a shaft to make power.

This is why when people talk about running XYZ psi of air it means nothing - that just refers to the compressor not the engine. With the exception of timing, and once you get REALLY high pressure - building the intake to contain it; measure flow and keep an eye on pressure.

 

So - if you make 300WHP at 10 PSI on a turbo what does that mean?



Jack ****....

What it means is that your compressor is heating the charge based on its efficiency; while you are flowing enough air to make 300 WHP.

They are not related since there is no temperature measurement in the mix.

Now if you are making 300WHP with the proper flow rate at 10 PSI and your latent heating is 20 degrees F.... you are still in good sorts; as long as you realize 20F hotter air is going to change your timing by a few degrees. If you are making 300WHP with a 50F change in intake temps.... well you may run into problems.

The key is to understand what these sensors mean. Boost means NOTHING!!! Other than an idea of the latent heating of the intake charge.

 

now you have gone and made me dizzy again.
OD

 

glad this thread is up for discussion now that i am starting to understand what you rotards are talking about

 

the RX-8 is NA, so why is this in the Tech area rather than in the Major HP area?

and you accomplish nothing posting a "lets hash it out", you either know what you're talking about and post it up, or you argue endlessly with a bunch of numb-skulls which results in the smart people walking away shaking their head and the rest coming up with a completely wrong answer that makes them warm and fuzzy because they're all on the same page of fail

 

...

 

 

That's the flaw in your discussion. Pressure directly affects mass flow, since the higher the pressure the more molecules per unit volume. Volume flow through an engine is governed mostly by the mechanical properties of the engine. Blowing is all about increasing the density of the incoming air. Compression by a blower does heat the air, since the compressor is doing some work, but that's incidental. In fact, heating by the blower undesireable, since it reduces density at a given pressure. That's why the intercooler was invented.

Ken

 

Depends how you look at it... while the compressor will make say 10 psi of boost, the intercooler will knock it down to say 8 PSI while cooling the air etc....boost pressure is important because that is how the wastegate controls itself.

At 10 or 8 psi it's irrelevant the same mass of air is going to the engine; so the flow is what the engine cares about, not the pressure (except for heating).

Which brings us to the last phase - static vs dynamic compression. At 8 PSI there is a lot less heating of the charge due to static compression than at 10 PSI if they are both at the same temperature.

Where pressure ratio can also be viewed as heating during the compression stroke.

 

Yes - you can look at it paying attention to the laws of physics and thermodynamics, or not.

Ken

 

You lost me on that statement...

 

Such negativity in this thread (from posters, not the op.) Instead of dead end statements attempt to prove him wrong, thats the point of the thread. If you state "youre wrong" you will continue to not have any credibility in the discussion. Anyone can post "youre wrong." If you don't wish to, then leave.

Kane, for what its worth I enjoy your tech postings. {internet high five}

 

/subscribed.

I think I see what you are saying Kane. You are saying that increased pressure from the turbine (boost), can be used to calculate the air temp increase, but that is not exactly the same thing as increased flow. For example, if you were to freeze time for a moment with the intake ports closed, you have alot of pressure, but no flow.

It is entirely possible to stick a massive turbine on the engine, and generate a ton of boost, i.e., pressure in the intake, but there is still a limit to how much air that boost can push into the engine.

Boost and temp are 100% linked all the time
Flow and Power are 100% linked all the time
Boost and Flow are only indirectly linked, and not all the time.

You need pressure to achieve the flow, but pressure in and of itself doesn't mean anything. Without pressure, you have no flow. But you can have pressure without flow. (or pressure 'beyond' flow)


I have that right?

 

Pretty close yep! The extra power comes from the extra molecules of Oxygen moving through the engine - not the pressure. The extra pressure is how we cram in more O2. So a perfect system would have 50 PSI at 500g sec of air (from the compressor) at 0 psi (intake) because all the flow would be eaten by the engine with none of it being wasted by pushing against the intake pipe. Not the that is possible per se, but just to visualize.

Compressors make pressure - engines move air; they are only related because denser air has more O2.

 

So ideally, when the port is closed, no pressure is being generated. The port opens, the turbine pushes alot of air into the intake, into the engine, the port closes, the turbine stops generating pressure, etc...

Obviously, this isn't realistic due to the frequency of open/closed, the time it takes to move the air needed through the intake, etc... But that would be ideal.

Throw in the mechanicals and reality, and that is where efficiency comes into play. Any period of time that the turbine is generating pressure without flow (ports closed) is wasted energy and generated heat without a benefit.

Hm... Wouldn't that also be increasing intake pressure during port-closed, decreasing pressure during port open, back to increasing during port closed, etc...? It would make sense that it would be.

 

Thus the BOV and Wastegate....

Pressure is the signal in these systems to actuate these parts to relieve and prevent excess "work" by the compressor that isn't needed.

At several thousand RPM's port opening and closing happens so fast that it generally doesn't cause pressure fluctuations... well maybe REALLY small ones.

 

Same here Dude and I dont think Kane pays much attention to the things that are not important to him.
Kane---as I mentioned in the pettit thread it helps me to think of the engines air in different viscositys. 10psi of boost yeils a different "viscosity" of charge air than 5 lbs for example.
Thicker gas/fluid has different flow characteristics so this helps me to not forget that.
As Richard Paul said "Air is funny stuff".
Pressure does heat and that is why intercoolers and lets not forget WATER/METH INJECTION were invented.
So lets stay away from the physics of heat production in the compression of air as we do have some control over the intake charge temps?
By the way what is the best intake charge temp you can get?

It is interesting that intercoolers and water meth are after the compressor but before the intake port. By that I mean the compressor spits out a certain volumne of air at a certain temperture, then the intercooler cools that volumne of air which makes that same volumne more dense (increases viscosity lol)and therefore at the same time reducing the volumne.
Thats why i like to think of air in terms of its viscosity. Pre cooler viscosity is different than post cooler. Pre water meth viscosity is totally different than post in terms of viscosity and mixture!
The higher the viscosity of air/liquid the more difficult it is to make that 90% turn in our intake ports. And the intake charge mixture pattern changes. That influences the flame front and any quenching going on.
Basically saying anything that helps to increase flow in a na engine will also help a boosted engine.
Staic versus dynamic compression is one hell of a topic. Its why I strongly advocate to never boost this 10/1 engine without a water meth system. Period.
And now do we start speaking of lowing the compression ratio by using reworked rotors?
Oh boy
OD

 

Actually, I found his reply to my post ("Depends on how you look at it") to be pretty negative. He clearly did not try to understand what I wrote.

The key to making power is how much air/fuel mixture you burn. The fundamental principle of forced induction is to cram more air into the engine. If you don't pay attention to that, don't pay attention to the difference between mass flow and volume flow, think that the blower is there to heat the air, etc., then a deep discussion like Kane is attempting will go nowhere.

So my comment about paying attention to the laws of physics was not at all negative.

Ken

 

Huh? It seems like we are saying the same things.....I am not sure what you want to clarify.

A blower is there to compress air - heat is a byproduct. We only care about heat because it affects tuning to prevent detonation.... that is it.

 

I dont get what this whole topic is about anymore...

flow is a byproduct of pressure differentials, charge temp is a byproduct of intake temp/compression/intercooling. How confusing is that?

opening or closing ports simply alters what area of the efficiency chart the compressor operates in, which produces changes in temps.

and what is this nonsense about volume flow vs mass flow, they are directly interrelated. volume flow = mass flow at a given temp: take the same mass flow rate and alter the temp, you get altered volume flow rate. Volume is not what matters, mass is. you can look at volume, but its the harder way to derive the same math then it is to look at mass and temp.

 

We care about heat for several reasons. One is tuning, like you say, but that can be dealt with in various ways. Like running alcohol instead of gas. There's also the fact that we're dealing with heat engines, so the higher the inlet temperature the lower the efficiency. But a really big reason is that it reduces density, so for a given boost pressure you're not packing in as many molecules.

Ken

 

Ohhhh! I think I understa.......er..no...oh wait there it is! Nope, lost it again....not a clue.

 

Exactly - which is why boost only matters to the compressor not the engine.

Density can be ignored in the sense that the engine doesn't care about volume - it cares about flow (PPO2) aka mass.

If you are caring for, designing, tuning, etc on the compression system then deal with pressure, temperature, efficiency and all that.

If you are after power then measure and worry about flow. Keeping in mind temperature for tuning...

 

hold on now --i am getting confused, density and volume are different things
"Density can be ignored in the sense that the engine doesn't care about volume " It cares about mass.
Mass is equal to density plus volume---right ?
So my word that I use "viscosity" could be substituted with "mass"

Power has and always will be about mass and flow. Where it gets tricky is getting these through the engine. Hince the discussion?
Somer thoughts of mine:
1- Pressure can increase flow and affect mass,
2-lowering restrictions can increase flow, no affect on mass by itself
3- temps can affect mass, and pressure
4- engine compression can affect flow and temperture
5- fuel choices can affect mass
6- mass itself can affect flow
All of this affects my cash FLOW!
OD

 

density is a measure of how much mass per volume
mathmatically: density = mass divided by volume

1- exactly, increasing the pressure differential will increase flow
3- temps do not affect mass, but will affect density