# Engine Idle air requirement calculation

1. Nov 26, 2007

### signorvivek

I want to know how to calculate engine idle air requirement.

Engine rpm = 1200 rpm = RPM
Rev/cyle = 2
Engine capacity = 395 cc = 0.395 Lit = L
Volumetric Efficiency = 90% = VE

Then VAF (Volume of Air Flow) is,

VAF = (L/28.317) x (RPM/2) x VE.

I want o know what is the meaning for 28.317
Can anyone tell be the derivation for it?

2. Nov 26, 2007

### FredGarvin

28.317 is the conversion from liters to cubic feet, i.e.

$$1 ft^3 = 28.317 L$$

3. Nov 26, 2007

### stewartcs

4. Nov 26, 2007

### signorvivek

Yes it is the unit conversion data only.
But what is the purpose of the 28.317 there.
See i am giving the Engine capacity in Litre only,

And my end unit of Volume of air is Lit/min only.

And also VAF = (Engine Displacement/1728) * (RPM/2) * VE

I am still confused on the 28.317 reason. Why??????

5. Nov 27, 2007

### FredGarvin

What is the source for your calculation? Did you get it from a US or other source that usually rates flow rates in terms of CFM? That would be the only reason to use that factor. The rest of the equation is simple geometry.

6. Nov 27, 2007

### stewartcs

You've answered your own question. It is for unit conversion only and serves no other purpose.

7. Nov 27, 2007

### stewartcs

8. Nov 27, 2007

### signorvivek

VAF = (Engine Displacement/1728) * (RPM/2) * VE
Yes the end unit for this equation is CFM only. But what is the purpose of 1728 or 28.317 in the equation. Without these constant we can achieve the units correctly.

9. Nov 27, 2007

### signorvivek

@Stewartcs,

I could see the link which u attached. Kindly dont mistake me.

CFM = (CID*RPM*VE)/3456

What is the purpose of 3456? Kindly expain me in detail.

10. Nov 28, 2007

### FredGarvin

Again, the 1728 is the conversion from ft^3 to in^3. The 3456 is 2 times that same conversion (you'll see there is an extra 2 in the RPM term).

11. Nov 28, 2007

### stewartcs

The conversion factor of 3456 is derived from the fact that there are 12 inches in 1 foot. So to get the volumetric flow in CFM (cubic feet per minute) you must convert from inches to feet (since cylinder size is measure in inches or whatever smaller unit of measurement you use in your country).

So, you have 1 in^3 = 1 / 12^3 = 1 ft^3, which equals 1/1728. The 3456 comes from the fact that the intake valve on a cylinder opens once every 2 revolutions of the engine. So, for every 2 revs the engine takes in some x in^3 of air (in this example I used inches for the cylinder size).

Hence you have another factor of 2 in the denominator of the conversion which gives you 2*12^3 = 2*1728 = 3456.

You just have to use the simple geometry of the problem like Fred told you to begin with. Just think about what it is you're trying to find...i.e. the volumetric flow rate.

So, like we've said all along, it is only a conversion factor.

12. Nov 28, 2007

### stewartcs

Not unless your cylinders are measure in meters or feet!

13. Nov 30, 2007

### signorvivek

Thanks guys

14. Dec 24, 2007

### divenpuke

Volumetric Efficiency!

IF YOU ARE IDLING YOU ARE NOT AT 90% VE!!!!
You are more likely around 10-50%! The whole point of that CLOSED throttle blade is to RESTRICT airfow! Also remember at idle that inside the intake manifold there is about 20"Hg vacuum.
JM

15. Jan 20, 2008

### pranay sharma

how to calulate ressure drop for air flow

how to calulate ressure drop for air flow

16. Jan 20, 2008

### pranay sharma

intake system

I have 220 hp engine 6 cylinder with 7.2 liter capacity can any one tell how to design intake system for this?