Stephen Max

iTrader: (59)

How do you estimate the volumetric efficiency of a boosted car?

I have seen one approach where you just use the pressure ratio as the volumetric efficiency, which seems a bit optimistic.

Maybe a more realistic estimate is NA volumetric efficiency times pressure ratio?

MardiGrasMax

iTrader: (14)

With boost I thought it was 100%

Stephen Max

iTrader: (59)

I guess I'm thinking of an effective volumetric efficiency that includes the effect of the supercharger.

For instance, if you wanted to calculate air flow into the engine. I have seen an equation for a supercharged engine where the effective volumetric efficiency was just the pressure ratio produced by the blower. So for instance, if you are running 9 psi at 6500 rpm:

PR = (14.7 + 9)/14.7 = 1.61

so: cfm = (183 x 6500 x 1.61 x .5)/ 1728 = 554 cfm

where:
183 = engine displacement in cubic engines
6500 = engine rpm
.5 = number of intake strokes per engine revolution
1728 = conversion from cubic inches to cubic feet

But using the pressure ratio for the effective volumetric efficiency seems a tad optimistic to me.

Stephen Max

iTrader: (59)

Ooops. Silly me. I forgot to factor in the thermal efficiency of the blower. So if the blower is operating at about 65% efficiency, you get an estimated airflow of around 360 cfm.

Confused

iTrader: (4)

since you are measuring the pressure ratio at the engine, wouldn't you ignore the blower efficiency?

Stephen Max

iTrader: (59)

Hmmm. Good point. I think you're right.