turbo Q

[Mhannink]

ok so maybe this is an idea for you know you are a dieselhead when fourm but i was sitting in my government glass supposed to be doing homework but instead in trying to figure how much air is going in and out of 5. 9 liters my question for u is normal air pressure at sea level is 14. 7 and if a turbo is pushing 20 lbs of boost is the air 20 psi or is it 14. 7*20 or is it something else im just asking so that when i get bored i can run these numbers and figure it all out just for fun anyways thanks in advanced

 

[Alan Reagan]

It's not 14. 7 * 20, it is absolute pressure which is 14. 7 + 20. That is what is used in compressor calculations when you are trying to find the volume or lbs of air per minute.



So the answer is 34. 7 psia (psi absolute).

 

[Alan Reagan]

To get pounds of air:

n(lbs/min)= )P(psia) x V(cu. ft. /min))/(10. 73 x T(deg R))



T(deg R) it the ambient temperature plus 460 for Rankin.



To get the volume of air:

V(cu. ft. /min) = (n(lbs/min) x 10. 73 x T(deg R))/(29 x P(psia))



The engine takes 5. 9 liters per 2 revolutions. So it's the displacement times the RPM divided by 2. Then convert that to Cu/ft and you have the

V(cu. ft/min)



volume of air (cu ft/min)= (engine rpm x engine cid)/(1728 x 2)



Engine cid is 360 cu/in isn't it for a 5. 9 liter.

 

[TFucili]

How do you take volumetric efficiency into account on a turbocharged engine?

 

[Alan Reagan]

actual air flow = ideal air flow x volumetric efficiency



Above calculates ideal air flow. I'm not sure what the VE is on these engines but it's probably 80-85%.

 

[TFucili]

actual air flow = ideal air flow x volumetric efficiency



Above calculates ideal air flow. I'm not sure what the VE is on these engines but it's probably 80-85%.

I knew the equation, but was wondering if the VE varied with boost, and, if so, is the relationship linear??



You'd think I had better things to worry about :-laf

 

[Alan Reagan]

Volumetric efficiency is fairly linear. However, turbo efficiency is not. The turbo efficiency can vary 10-15% depending on what it is being asked to do. You have to have the compressor map for the turbo you are looking at to determine the efficiency at particular boost levels. The higher the boost, the less efficient the turbo usually is. As a matter of fact, getting outside the prescribed parameters of operation can cause surging in the turbo which can make it self destruct. Turbo efficiency then is another data point to use when calculating air flow.



Back to Volumetric efficiency... ... . that is a function of residual exhaust gasses left in the cylinders, heat, friction, etc that are fairly constant in the engine at specific operating temperatures and loads. That's why it is pretty much linear.

 

[TFucili]

OK, so pumping losses are inherent to the engine design, intake, and exhaust tracts, etc. Forced aspiration overcomes some or all of this inefficiency, right? Actual VE, then would be a curve which would rise and fall with the turbo map, no? With enough boost the engine should be able to, in theory at least, exceed 100% VE.

 

[Alan Reagan]

You can't exceed 100%. Theoretically with no losses you could reach 100%. That only means that you could put in the entire compressed volume of air every two revolutions. You are exceeding the displacement of the engine every two revolutions but you are not putting all you could put in every two revolutions. "All you could put in" is the ideal. The leftover exhuast gases, etc stop you from putting all of that into the cylinders. "All you could put in" would be 100%. What you do put in is less.



I think what you are thinking is that since you are compressing the air you are putting in more than 100% of the displacement. That is a little misleading. The volume of the displacement never changes. That's why you have to look at pounds of air or take into account the PSIA in the volume over time calculations. When you compress air you put more in the same space. When time is taken into account you get a better picture of how much air is moving. Same with converting that to pounds of air per minute.



To clarify:



n(lbs/min)= )P(psia) x V(cu. ft. /min))/(10. 73 x T(deg R))



The volume of air moving as a result of engine rotations remains constant at a constant RPM based on the displacement of the engine. What changes the amount of air being moved at the constant RPM is the pressure and the temperature.

 

[TFucili]

Right you are, Alan! Displacement versus "all you can"... great explanation.



Now, where's that Excedrin