Induction CFM calculation

[CharlieWaffles]

I am wondering, how can I determine he maximum amount of air that is used by an engine/fuel injector? Specifically, I'd like to know how many CFM the engine requires for operation. Would that be a component of the fuel injector and listed somewhere? In my case, it's a Silverhawk XP on a YIO-540.

[BPoletti]

I would think it would be something like displacement x RPM / 2 for WOT.

[jetdriven]

There is also a volumetric efficiency adjustment. An O-360 wont draw in 360 cubes of air every two revolutions due to intake tract ineffiencies and manifold vacuum.

[tjo]

Why do you want to know?

Tim

[rv6ejguy]

1.62 CFM/HP works out pretty close for most engines.

[krwalsh]

A first-order approximation would be:

(MAP/ATM)*DISP*RPM/(2*12^3)

Where:
MAP is manifold pressure at WOT. Best measured at the cylinder intake, not just after the servo to count for induction tubing losses.
ATM is atmospheric pressure (29.92)
DISP is displacement of the engine in cubic inches
RPM is engine speed in revolutions per minute

The divide by two is because the engine only intakes on half of the crank revolutions. The 12^3 converts from the engine displacement in cubic inches to the desired cubic feet. Note that this does NOT convert to SCFM, just to CFM. The S requires standard conditions, which, for SCFM I would argue that there really is no standard.

So, as an example, your 540 cubic inch engine, running at 2800 rpm, at 28 inches manifold pressure, is taking in 409.4 CFM. As a first-order approximation, this should work.

[rv6ejguy]

When matching turbos, especially for aircraft which operate at different altitudes, I find it is better work with mass flow instead of CFM. It is easier to quantify, especially relating to fuel flows and to apply changes in altitude.

HP/ 8.07 gives approx mass flow in lbs. per minute.

[CharlieWaffles]

I've just been contemplating the low tech/prone to failure alternate air door for the FAB on the 540. I was wondering if something like a check valve or other simpler, less likely to break and get invested option was possible. But at 400CFM I doubt I could find a lightweight enough valve to fit the bill.