NefMoto

Hi guys! :)
It's me again with another random question...

Let's say we know air mass that engine takes and boost at some point. How would we go about to convert it to approximate RPM (error could be ~+-500RPM). I know we would need some other constants, but let's start with someone pointing out simplified formula... :)

Is that even possible!

A freer better flowing engine will have a different air mass and boost to a crap engine!

It's all about the VE of the engine...

For a given maf grams airflow, you will get a different boost depending on the ve of the engine, better ve lower boost , poor ve higher boost.. So I'd say its very hard to give a rpm , I maybe wrong but I think you could only do it for your engine and not a generic formula without like you say knowing all the variables

So I've logged g/s and found out that boost doesn't affect much air mass taken...

Here's from my log:
RPM vs air mass
2000 30
2250 37
2500 43
2750 53
3000 57
3250 63
3500 67
3750 71
4000 75
4250 80
4500 85
4750 87
5000 93
5250 97
5500 100
5750 103

Next thing I did was finding the difference between each measurement:
RPM vs air mass vs diff
2000 30 0
2250 37 7
2500 43 6
2750 53 10
3000 57 4
3250 63 6
3500 67 4
3750 71 4
4000 75 4
4250 80 5
4500 85 5
4750 87 2
5000 93 6
5250 97 4
5500 100 3
5750 103 3

As you see boost doesn't affect that much air mass taken. Average was ~5g/s increase per 250RPM.

So now I could go and predict air mass:
RPM vs PR air mass
750 5 <- correct!
1000 10
1500 20
2000 30
2500 40
3000 50
3500 60
4000 70
4500 80
5000 90
5500 100
6000 110
6500 120
7000 130 <- correct!


So all in all I think there has to be a way to atleast approx rpm from air mass and boost.... We just need some kind of formula..

Pv = nrT

so you need IAT, pressure, VE, and displacement to get a ballpark number .. which ends up being the reverse of  of the speed/density calc.

Honestly, I dont see a good reason to do this.

I see one reason... I'll try to reverse tune to kompressor map. Don't know if this will be faster than doing usual way and then working with k.m. though.

BTW: Could you define variables in that formula? Like Pv = RPM, n = ?, r = ?, T = ?

http://en.wikipedia.org/wiki/Ideal_gas_law

So you're trying to determine your compressor map?

http://www.intengineering.com/garrett-turbo-speed-sensor-kit-w-gauge

I'm interested in a way to determine kompressor map air mass axis relation to RPM so I could initialy get to wanted k.m. "island" without logging and reworking whole damn torque model...

I have drawn few things on the k.m. I would like to initially land as near as possible to the yellow line without much guessing..

Volume is basically RPM/2 * VE * displacement.

The rest is arithmetic :)

finally something I can work with :) Thanks nyet! Few small questions:
Does this VE for 1.8T looks ok?:
VW 1.8T VE%
0000-3000 85%
3000-4000 95%
4000-6000 100%
6500-7200 95%

Is this displacement in one Cylinder or in all? Cubic centimeters? :)

Displacement == volume of air a cylinder can take in one cycle so whatever units you're using for displacement will be what you get for volume.

You tell me. How many turbos do you have per cylinder? :P

And I am assuming you know how dimensional analysis works, or you will never get the right units.

Keep in mind google calculator can do it all for you.

https://www.google.com/search?q=2.7+liters+*+1000+(1%2Fminute)+in+m^3+per+minute+%3D (https://www.google.com/search?q=2.7+liters+*+1000+(1%2Fminute)+in+m^3+per+minute+%3D)

Oh, Ok now I understand everything :) Thank you both

P.S> Would be nice to have turbo for each cylinder, huh? :D No intake mani FTW :D

I'm pretty sure you'd not want them directly connected to the cold side w/o an IC :)

Ok, I feel dumb now..

VOL = RPM / 2 * VE * DISP
RPM = VOL * 2 / VE / DISP

Is this correct?

If so then I'm trying this:
DISP = 1780 / 4 = 445 cm^3 = 0.000445 m^3
VE = 95

RPM = VOL * 2 / 95 / 0.000445

Am I correct? :) Because something does not add up... If I try to use 0.02 from k.m. then I get some nonsense <1 RPM :D

if an engine has a displacement of 1.8 liters, that is for all the cylinders combined. if you do the math using bore and stroke you'll see the whole number of displacement can be divided by the number of cylinders. So in a 1.8L 4 cylinder engine, the displacement for each cylinder would roughly be a quarter of the total engine displacement. in a 6cyl, 1/6 of the total displacement equals individual cylinder displacement. same with a 8cyl and so on.

I've already divided that into 4...
DISP = 1780 / 4 = 445 cm^3 = 0.000445 m^3

VE 95 is 95%, not 95...

Oh, stupid me! So I have to use 0.95 :) Now it makes sense!

EDIT:
Still get weird numbers...

0.02	0.04	0.06	0.08	0.1	0.12	0.14	0.16	0.18
95	189	284	378	473	568	662	757	852

Using: VOL * 2 / 0.95 / 0.000445

Please include units and use dimensional analysis to make sure you got the unit conversion right.

I can't make head or tail of what you are trying to do.

Ok, so let's try this again...

VOL = RPM / 2 * VE * DISP
vvvvvvvvvvvvvvvvvvvvvvvvvv
RPM = VOL * 2 / VE / DISP

Where:
VOL = volume per time (from k.m.)
RPM
VE = volumetric efficiency (I'll be using 95% == 0.95)
DISP = displacement (1780 cm^3 / 4 = 445 cm^3 = 0.000445 m^3)

After this formula becomes:
RPM = VOL * 2 / 0.95 / 0.000445

Now, I try to use VOL from k.m. => 0.02; 0.04; 0.06; 0.08; 0.1... and result is weird

rpm is per min

what are your target units for volume?

what is "VOL from k.m"

Your air flow is m3/s and rpm is revolutions per minute, you need a factor of 60 to convert between the two.

This is exactly what I mean by dimensional analysis.

Try all of your calculations with only conversion constants

rpm * minutes * grams = grams

rpm * seconds * (60 seconds/minute) = 60 * grams