Density Ratio

Hi,

I’ve spent a large amount of time understanding compressor maps and how to utilize them. There is a part I am stuck on and that is Density Ratio. I understand that Density Ratio is essentially the compressor inlet air density compared to the density of the air after turbocharging including all variables. What I don’t understand is how the Density ratio can be multiplied by the CFM to produce the CFM airflow of the engine after it is turbocharged. I know that this relates to the Ideal Gas Law but I’m not quite understanding how simply multiplying the Density Ratio by the CFM airflow will equate to the airflow under boost.

Thank You

I usually go about the calculation the other way where if you know which engine youre dealing with then you would know the CFM required at a certain engine speed and Pressure Ratio. Adding an assumed temperature you get mass flow (which can be used to calculate fueling). Density ratio is a unitless ratio that has a temperature/ efficiency built into it so in a sense you're still estimating the outlet temperature difference, and mainly you are interested in the air temp as it actually enters the engine.

So maybe the answer to your question is that when multiplying with a unitless ratio or efficiency number your units will not change and those types of numbers have certain assumptions built into them.

Ok if I'm understanding correctly, you're using the Pressure Ratio as a multiplicative and then afterwards factoring in the actual temperature to get you real mass flow rate as opposed to using a Density Ratio?

Basically yes. If you make a few educated assumptions namely air temp and pressure you can come up with the mass flow. There are a number of other compensations you can consider also. This is the basis of VE fueling calculations that ecu's with no air mass meter are performing. it should get you in a good ballpark of whether or not you are reaching the limits on a compressor map. A couple of other things to note is theres a big difference in compressor outlet temperature and air temp as it fills the cylinder (with an intercooler) . VE calculations rely heavily on the temp as air enters the cylinder. And the efficiency rings on a compressor map are related to the compressor outlet temperature.