air pressure vs air volume, theory vs reality - (10lbs here is not 10lbs there)
12-18-02, 12:36 PM
#51
Rotary Enthusiast
Originally posted by cover8
Alright let me take a crack...
Basis:
Air can be assumed to behave as an ideal gas at pressure below 2atm (1atm=14.7psia) and temps below 500F.
Therefore from the ideal gas law:
PV = nRT
Isothermal (constant temp) or Boyle's Law:
p1V1 = p2V2
Isobaric (constant pressure) or Charle's Law:
T1/V1 = T2/V2
Clearly, the temps change (intercooler), pressure change (lower deltalP)
What is important is to not confuse gravimetric with volumetric flow. So once the air hits the turbos and then a temp drop exists after you do not suddenly get more air by mass because of the temperature drop across the intercooler. After the turbos the air in essence, is in a closed system so dropping temps does not suddenly produce more air by mass it simply occupies less space (lower partial pressures P*)
For example, commonly turbines are sized by displacement or cc/rev or ft^3/rev so a number results from this being volumetric but what really is important is the mass number so the volume must be multiplied by density to get rate.
(ft^3/rev) * (lbmol/ft^3) = lbmol(air)/revolution
Well, we are not done yet...to complicate matters even further, the density of any fluid is a direct funtion of temperature. So as temp increases, the same amount of gas (isochoric = constant volume) occupies more space per unit volume which is measured as partial pressure.
So here is what does not make sense to me:
I do not know the operational basis for the FD ecu fuel maps but...
The manifold pressure pressure cannot be a good operational basis for fuel addition if total volume and temp of the system is not known. Really it is the mass flow rate of air that is important. So yes, you can run the same pressure (boost) and get different amount of air flow by mass if the system volume changes (lower deltaP, bigger inlet pipes, lower frictional losses blah blah) and the inlet air temp changes (more air by mass is displaced per turbine revolution)
But what continues to fail to make any sense are the singles running the 15psi boost with the same intake system claiming all this exceptional horsepower.
If the system volume does not change ie cai, pipes, flow factors and you run the same inlet temp and manifold pressure YOU ARE NOT GETTING MORE AIR OR HORSE POWER by running a bigger turbo...your turbine (assume larger) is simply running more efficiently (or not) depending on the turbine design.
Lastly, for those that may have forgetten, the air/fuel ratios are STOICHIOMETRIC or MASS numbers...specifically molar ratios.
Alright let me take a crack...
Basis:
Air can be assumed to behave as an ideal gas at pressure below 2atm (1atm=14.7psia) and temps below 500F.
Therefore from the ideal gas law:
PV = nRT
Isothermal (constant temp) or Boyle's Law:
p1V1 = p2V2
Isobaric (constant pressure) or Charle's Law:
T1/V1 = T2/V2
Clearly, the temps change (intercooler), pressure change (lower deltalP)
What is important is to not confuse gravimetric with volumetric flow. So once the air hits the turbos and then a temp drop exists after you do not suddenly get more air by mass because of the temperature drop across the intercooler. After the turbos the air in essence, is in a closed system so dropping temps does not suddenly produce more air by mass it simply occupies less space (lower partial pressures P*)
For example, commonly turbines are sized by displacement or cc/rev or ft^3/rev so a number results from this being volumetric but what really is important is the mass number so the volume must be multiplied by density to get rate.
(ft^3/rev) * (lbmol/ft^3) = lbmol(air)/revolution
Well, we are not done yet...to complicate matters even further, the density of any fluid is a direct funtion of temperature. So as temp increases, the same amount of gas (isochoric = constant volume) occupies more space per unit volume which is measured as partial pressure.
So here is what does not make sense to me:
I do not know the operational basis for the FD ecu fuel maps but...
The manifold pressure pressure cannot be a good operational basis for fuel addition if total volume and temp of the system is not known. Really it is the mass flow rate of air that is important. So yes, you can run the same pressure (boost) and get different amount of air flow by mass if the system volume changes (lower deltaP, bigger inlet pipes, lower frictional losses blah blah) and the inlet air temp changes (more air by mass is displaced per turbine revolution)
But what continues to fail to make any sense are the singles running the 15psi boost with the same intake system claiming all this exceptional horsepower.
If the system volume does not change ie cai, pipes, flow factors and you run the same inlet temp and manifold pressure YOU ARE NOT GETTING MORE AIR OR HORSE POWER by running a bigger turbo...your turbine (assume larger) is simply running more efficiently (or not) depending on the turbine design.
Lastly, for those that may have forgetten, the air/fuel ratios are STOICHIOMETRIC or MASS numbers...specifically molar ratios.
This one statement is relavant and key:
"Well, we are not done yet...to complicate matters even further, the density of any fluid is a direct funtion of temperature."
The ecu reads temp, meas's pressure, and knows the exact density in the manifold. It doesn't matter what was done to the control volume upstream, big/small pipe, ... the manifold density is known.
stock ecu has the stock system VE tables included in the maps, and mass flow is properly prediced, (and overly fueled).
If upstream upgrades allow same pressure and temp to occur more efficiently, this means less work is done by turbine, and less backpressure for more complete chamber filling, ie higher system VE.
Either your thoery is misguided, or all thoses dynos with more hp at same boost, with same upstream mods are wrong. I believe the dynos. Anyone that has viewed the stock twin turbine exh path, including poor wg vent path, vs a single T04, will know which has higher port backpressure at 15 psi.
FYI, it is not unusual to find backpressure to boost ratios of 1.5 - 2 for stock turbos that are cranked beyond design range.
12-18-02, 05:35 PM
#52
Senior Member
Join Date: Jul 2002
Location: Los Alamos, NM
Posts: 592
Likes: 0
Received 0 Likes
on
0 Posts
Originally posted by SilverRX7
LARacer, its not impossible to understand... just take your masters in powertrain modelling and control and it all falls into place
LARacer, its not impossible to understand... just take your masters in powertrain modelling and control and it all falls into place
Seriously, I've said it before and I'll say it again; this is complicated stuff. There are alot of interdependent variables and many of the relationships between them are not well understood. The best you can do (without some serious CFD work) is to learn from other people's experiences and find out what your A/F ratio is.
It doesn't hurt to try and understand this system, it's just that the more you learn the more you realize you have no idea what's going on. If someone claims they understand it completely they are either a genius or an idiot.
Honestly, I'm pretty impressed with the knowlege and understanding of most people here since this is an RX-7 forum and not a physics forum. I like talking to smart people
.
12-19-02, 12:41 PM
#53
KevinK2 Thanks for your response. I agree with your statements which I may have failed to communicate in my writing.
LAracer:
Exactly, very complicated, dynamic events...I was trying to provide some basic understanding to improve fundemental cause and effect relationships.
LAracer:
Exactly, very complicated, dynamic events...I was trying to provide some basic understanding to improve fundemental cause and effect relationships.
Thread
Thread Starter
Forum
Replies
Last Post
troym55
3rd Generation Specific (1993-2002)
23
05-25-16 12:42 PM
