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Hi,

My question is regarding compressor maps and density ratio. I understand that Pressure Ratio cannot be multiplied by the NA flow of the engine because the pressure ration does not take into account the real world temperatures. Everywhere I read says to multiply the density ratio by the NA volume flow of the engine to get the turbocharged volume flow.

This confuses me because the volume flow is less when multiplying by density ratio as opposed to multiplying by pressure ratio when in my mind the volume after turbocharging should stay the same but the density would be different after accounting for the density ratio. Can someone explain this.

I understand that the volume can stay constant while achieving a higher mass flow, but what I dont understand this:

Example:

The process to properly match a turbo for a specific application is straight forward.

1. Calculate engine CFM airflow NA for a specific rpm(s) at a certain VE %

2. Calculate the Pressure Ratio required to achieve a certain manifold pressure

3. Calculate compressor outlet temp at certain adiabatic efficiency

4. Calculate Intercooler outlet temperature at a certain efficiency

5. Calculate Density ratio after computing 3 and 4

6. Multiply Density Ratio by NA cfm to produce turbocharged cfm and then multiply turbo cfm by standard density to get turbocharged mass flow.

I understand everything with that. Except number 6 which is what I dont understand.

if you multiply the 2.0 Pressure Ratio x NA cfm it will give you a certain cfm value after turbocharging. Then to get mass flow you would do turbocharged cfm x .069(standard density) however the engine will not flow that mass of air due to the fact that the compressor efficiency and intercooler outlet temperature isnt taken into account by simply multiply the pressure ratio by the engines NA cfm airflow. Thats where the Density Ratio comes in.

So now to get a more realistic mass flow value you take into account the compressor efficiency and intercooler efficiency and you compute a Density Ratio (or how much more dense the air is as opposed to ambient conditions) of 1.7.

So then you’d multiply 1.7 x NA cfm to get turbocharged cfm and then multiply turbo cfm by standard density to get mass flow. and obviously you will get a lower value than multiplying by 2.0 (PR). This is how you plot lines and match turbochargers and this is how it is explained from many different reliable sources.

I can understand how for the same boost pressure the mass flow can change due to temperature and compressor and intercooler efficiency.

However when you multiply the Density Ratio by the NA cfm it produces a lower number and this then to me implies that the volume flow is related to the density of the air and that goes against everything that I have learned.

What am I missing here?

The density changes between using Pressure Ratio and Density Ratio, but the Volume flow shouldn't. When you multiply Density Ratio by cfm the volume flow is less when it should not change. I don't understand that part

This is an example from a reliable source:

"Density Ratio = 1.481 Multiply the actual NA air flow by the density ratio to get the air flow under boost conditions Compressor Air Flow Turboed 158 CFM 10.9 Lbs/Min"

If you substitute Pressure Ratio, since it is higher the volume flow in cfm will also increase when in my mind only the mass flow should increase

So since the volume should stay constant because the boost pressure is constant multiplying by either PR or DR should yield the same cfm airflow but it doesn't

ex:

14.7 psi

2.0(PR) x 108 cfm = 216 cfm MASS flow is 0.069 x 216 = 14.9 lb/min

1.8 (DR) x 108 cfm = 194 cfm MASS flow is 0.069 x 194 = 13.3 lb/min.

The volume changes when it should stay constant and that is what I dont understand

You use wrong sequence of calculations that confuse you.

1) find out the displacement

2) find out RPM

3) find out VE at RPM choosen

4) find out volume moved at chosen RPM ( volume times RPM times VE)

5) find out PR

6) find our DR corresponding to PR ( two pictures i posted for aftercooled and not aftercooled applications).

7) Find out your mass flow per minute by multiplying volume moved by DR.

End of story.