Maths

[Memphetic]

Okay, let's see if I can figure this out here. Say for our example, the engine is operating at 3500 RPM, is a 3100 cubic centimeter engine, has an intake manifold pressure of 20 pounds per square inch, with an intake charge temperature of 70 degrees. We'll say we're at sea level, for an atmospheric pressure of 14.7 psi.

So our variables (constants) are as follows:

Displacement = 3100cc = .0031 cubic meters
Engine Speed = 3500 RPM
P = 20 PSIg = 34.7 PSIa = 239.2 kPa
T = 70*F = 21.1*C = 294.3*K

Each rotation displaces 3100 cubic centimeters of (atmospheric pressure) air divided by two, as there is only one intake stroke every other revolution. Using our constants, we can find the number of moles of air.

PV=NRT
(239.2 kPa)(3.1x10^-3 cubic meters)(1/2) = n (8.314 J*K^−1*mol^−1)(294.3 K)
Solving for n using a TI-89 because I don't wanna deal with that bullshit gives us:

n=0.171079881880499 moles of air per full revolution.

(0.171079881880499 moles/revolution)(3500 revolution/minute) = 598.779586582 moles/minute of airflow (instantaneously)

Our target AFR is 11.5 to 1. At STP, 1 mole of gasoline occupies 22.4L. This means we need (598.779586582 moles per minute)/11.5 = 52.07 moles per minute of fuel to obtain it. This translates to 1166.368 cc/min of gasoline at this instant.

Does anyone see any flaw with this math? There is a reason for wanting to know this :-P Discuss.

[IPD]

lol. reminds me of oxyrageous "discuss" threads on 3sdie.

[Memphetic]

The main factor that I'm not understanding... is CFM from the turbo. Obviously a larger turbo pushes more CFM at the same PSI. Or does it? Where is it reflected in the ideal gas law? The only place I can figure would be in the temperature of the intake charge.

The reason for this thread... I will be developing an air/fuel controller that will include standalone logging onto an SD card (or android/pc app via bluetooth or cable, both supported), as well as a 3D fuel map based on engine speed and load, that runs at a 100mHz clock speed. Essentially this would convert any Mitsubishi car into one with speed density, amazing logging capabilities, and more precise tuning than can be achieved with most of what is available from big name companies. In addition, my target price is around $300 ;-)

[Valhallaz]

Does that math take being a 4 stroke into account? We actually only combust every other revolution...

[Greg E]

So you're making a Speed Density setup?

[aaronatstate]

I may be wrong, but you're doing the whole engine. I'd say since there are 6 360cc/min injectors for a total 2160cc/min of fuel stock it's ok. And considering you're using 20psi gauge, that's only 6lbs of boost.

[Memphetic]

Does that math take being a 4 stroke into account? We actually only combust every other revolution...

Ah, no. It's rough at 3AM Let me revise.

I may be wrong, but you're doing the whole engine. I'd say since there are 6 360cc/min injectors for a total 2160cc/min of fuel stock it's ok. And considering you're using 20psi gauge, that's only 6lbs of boost.

You're thinking PSIa. If it was 20 PSIa, it would only be 6 pounds of boost (I BELIEVE pascals take atmospheric pressure into account, but didn't really find much supporting evidence when searching last night). The part that threw me is that the intake stroke is a vacuum.

And yes, it's for the whole engine. I believe these injectors are constant flow and pulsewidth controlled, so if (in our previous example) you require 595.5 total CC/min, it's simply divided by six giving us 99.25 cc/min per injector, and with a max flow of 360 cc/min on stock injectors, you obtain a duty cycle of 27.6%.


And that in itself seems weird... that's super low for stock injectors and 20PSI of boost.

[RealMcCoy]

If I'm reading your calcs correctly, you solved for an 11.5 AFR by volume...? shouldn't that be by mass?

[Memphetic]

It was solved by moles. There is a ratio of 11.5 air molecules to 1 fuel molecule.

Also interesting note, if you go back and solve for the amount of moles of air at ambient temp (70*F), you find that the airflow volume is 5425.05742089084 L/min. On a karman vortex, this would equate to about 800Hz. Does this sound right? I've never seen any datalogs to be able to compare.

[striker2]

seems about right
Stealth 316 - 3S MAS Liter Per Hz