Déjà une map de compresseur ne peut s'exprimer qu'avec rapport de pression et débit, on ne parle que de compresseur pas de turbine.
Une explication : (en anglais (cours), j'ai repris en Français le plus intéressant)
Compressor Selection
When using the formula's below, you will need to use compressor flow maps and work with the formulas until you size the compressor that will work for your application. Compressor flow maps are available from the manufacturer, or do a search on the web, you'll find that they are readily available. On the flow maps, the airflow requirements should fall somewhere between the surge line and the 60% efficiency line, the goal should be to get in the peak efficiency range at the point of your power peak. In this article I will walk through an example as I explain it, once you understand it, you can get the the formula's in the Sizing Formula's tech article for quicker reference.
Engine Airflow Requirements /
(Le souci c'est qu'on parle en unités US )
In order to select a turbocharger, you must know how much air it must flow to reach your goal. You first need to figure the cubic feet per minute of air flowing through the engine at maximum rpm. The the formula to to this for a 4 stroke engine is:
(CID × RPM) ÷3456 = CFM
For a 2 stroke you divide by 1728 rather than 3456. Lets assume that you are turbocharging a 350 cubic inch engine That will redline at 6000 rpm.
(350 × 6000) ÷ 3456 = 607.6 CFM
Autre souci : le rendement n'est pas parfait (en gros) :
The engine will flow 607.6 CFM of air assuming a 100% volumetric efficiency. Most street engines will have an 80-90% VE, so the CFM will need to be adjusted. Lets assume our 350 has an 85% VE.
607.6 × 0.85 = 516.5 CFM
Our 350 will actually flow 516.5 CFM with an 85% VE.
Presure Ratio
ça c'est important, puisque ensuite on s'en sert pour se repérer sur les map compresseur.
The pressure ratio is simply the pressure in compared to the pressure out of the turbocharger. The pressure in is usually atmospheric pressure, but may be slightly lower if the intake system before the turbo is restrictive, the inlet pressure could be higher than atmospheric if there is more than 1 turbocharger in series. In that case the inlet let pressure will be the outlet pressure of the turbo before it. If we want 10 psi of boost with atmospheric pressure as the inlet pressure, the formula would look like this:
(10 + 14.7) ÷ 14.7 = 1.68:1 pressure ratio
le reste, c'est de la thermo, en pratique on ne s'en sert pas trop pour déterminer un turbo, car il faut enir compte de l'offre du marché ...
Temperature Rise
A compressor will raise the temperature of air as it compresses it. As temperature increases, the volume of air also increases. There is an ideal temperature rise which is a temperature rise equivalent to the amount of work that it takes to compress the air. The formula to figure the ideal outlet temperature is:
T2 = T1 (P2 ÷ P1)0.283
Where:
T2 = Outlet Temperature °R
T1 = Inlet Temperature °R
°R = °F + 460
P1 = Inlet Pressure Absolute
P2 = Outlet Pressure Absolute
Lets assume that the inlet temperature is 75° F and we're going to want 10 psi of boost pressure. To figure T1 in °R, you will do this:
T1 = 75 + 460 = 535°R
The P1 inlet pressure will be atmospheric in our case and the P2 outlet pressure will be 10 psi above atmospheric. Atmospheric pressure is 14.7 psi, so the inlet pressure will be 14.7 psi, to figure the outlet pressure add the boost pressure to the inlet pressure.
P2 = 14.7 + 10 = 24.7 psi
For our example, we now have everything we need to figure out the ideal outlet temperature. We must plug this info into out formula to figure out T2:
T1 = 75
P1 = 14.7
P2 = 24.7
The formula will now look like this:
T2 = 535 (24.7 ÷ 14.7)0.283 = 620 °R
You then need to subtract 460 to get °F, so simply do this:
620 - 460 = 160 °F Ideal Outlet Temperature
This is a temperature rise of 85 °F.
Adiabatic Efficiency
The above formula assumes a 100% adiabatic efficiency (AE), no loss or gain of heat. The actual temperature rise will certainly be higher than that. How much higher will depend on the adiabatic efficiency of the compressor, usually 60-75%. To figure the actual outlet temperature, you need this formula:
Ideal Outlet Temperature Rise ÷ AE = Actual Outlet Temperature Rise
Lets assume the compressor we are looking at has a 70% adiabatic efficiency at the pressure ratio and flow range we're dealing with. The outlet temperature will then be 30% higher than ideal. So at 70% it using our example, we'd need to do this:
85 ÷ 0.7 = 121 °F Actual Outlet Temperature Rise
Now we must add the temperature rise to the inlet temperature:
75 + 121 = 196 °F Actual Outlet Temperature
Density Ratio
As air is heated it expands and becomes less dense. This makes an increase in volume and flow. To compare the inlet to outlet air flow, you must know the density ratio. To figure out this ratio, use this formula:
(Inlet °R ÷ Outlet °R) × (Outlet Pressure ÷ Inlet Pressure) = Density Ratio
We have everything we need to figure this out. For our 350 example the formula will look like this:
(535 ÷ 656) × (24.7 ÷ 14.7) = 1.37 Density Ratio
Compressor Inlet Airflow
Using all the above information, you can figure out what the actual inlet flow in in CFM. Do do this, use this formula:
Outlet CFM × Density Ratio = Actual Inlet CFM
Using the same 350 in our examples, it would look like this:
516.5 CFM × 1.37 = 707.6 CFM Inlet Air Flow
That is about a 37% increase in airflow and the potential for 37% more power. When comparing to a compressor flow map that is in Pounds per Minute (lbs/min), multiply CFM by 0.069 to convert CFM to lbs/min.
707.6 CFM × 0.069 = 48.8 lbs/min
Now you can use these formula's along with flow maps to select a compressor to match your engine. You should play with a few adiabatic efficiency numbers and pressure ratios to get good results. For twin turbo's, remember that each turbo will only flow 1/2 the total airflow
Les map ...
![[Image: compressormap.gif]](http://perso.wanadoo.fr/civic16/compressormap.gif)
Operating characteristics: The compressor operating behaviour is generally defined by maps showing the relationship between pressure ratio and volume or mass flow rate. The useable section of the map relating to centrifugal compressors is limited by the surge and choke lines and the maximum permissible compressor speed.
Surge line: The map width is limited on the left by the surge line. This is basically "stalling" of the air flow at the compressor inlet. With too small a volume flow and too high a pressure ratio, the flow can no longer adhere to the suction side of the blades, with the result that the discharge process is interrupted. The air flow through the compressor is reversed until a stable pressure ratio with positive volume flow rate is reached, the pressure builds up again and the cycle repeats. This flow instability continues at a fixed frequency and the resultant noise is known as "surging".
Choke line: The maximum centrifugal compressor volume flow rate is normally limited by the cross-section at the compressor inlet. When the flow at the wheel inlet reaches sonic velocity, no further flow rate increase is possible. The choke line can be recognised by the steeply descending speed lines at the right on the compressor map.
En rapide, il faut se placer dans la zone du milieu ( neta is V c'est le rendement isentropique du compresseur.
Cela suppose d'avoir déterminé abscisses et ordonnées, là je ne sais pas à quel turbo correspond cette map, il est surement pas très gros vu les vitesses de saturation.
Bon c'est de la bonne méca flux, mais c'est assez chiant et en plus ya pas mal d'approximations (surtout ici), donc ...
Bien evidemment il y a des tas d'autres choses à prendre en compte (efficacité volumétrique etc), voila pourquoi en pratique le choix se fait dabord par rapport à l'offre et aux objectifs.What turbo should i run
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