Calculating exhaust gas CFM............. 10

[JadedSilver]

I'm working on a little project here. Nothing serious or new like that, but I do need to figure out how to calculate the exhaust flow volume of a particular sized engine. the project is based around extracting power through efficiency, and as such we are gathering every last bit of information from the motor now before any other work commences.

I'd imagine there has to be some sort of formula or series of formulas that can solve this, similar to that of finding the charge temp of a force inducted motor, or the CFM intake of an engine(which I have already completed).

I'm also assuming there are multiple variables including but not limited to a/f, fuel octane, displacement, RPM, exhaust temp, OAT,ATM, humidity, etc. Even if I can locate some formulas or charts that would get me a rough idea of the exhaust CFM then that would probably suffice for what I need to do.

These would be based around a standard DOHC gasoline engine that is turbocharged. I have located a chart about this topic for turbo desiels, but they are based around different operating parameters, and fuels of course.

If you have any information I'd greatly appreciate the effort on your part in lending me a hand.

Sincerely,

-Adam
R247 Motorsports

 

[kmb1949]

I am developing a new diesel engine design and we are begining to machine and assemble the prototypes. During our dyno test research we found that we could make 340hp with 575cfm of ambient air. This was accomplished in a 305ci four cylinder at 2800rpm running 37psi boost. The new designs should be capable of much greater air flows. I do not have an engineering degree but am very interested in fuel system applications and cylinder combustion. The last post seemed a little short of our dyno results and I was wondering if you guys might help me understand more fully the relationship between air and fuel for optimum combustion. Our indicated BSFC was .32.We do not yet have the equipment to test emmisions but I was also wondering what a good BSFC might be for diesel engines.

 

[franzh]

kmb1949
This might be better addressed as a new thread.

Franz

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[sacem1]

I have just read this thread and I find the discussion quite interesting and the analisys of the variables not known are really the difference between values obtained by the different methods but I don't get to understand this reasoning from SBBLUE (Oct 4):

"That will approximately be the intake volume flow for an engine with the throttle wide open. If we assume that the throttle is only open 33%, the intake volume flow will still be 4500 liters, but the pressure will be one-third of an atmosphere. The equivalent mass of air will be the same as 1500 liters at one atm of pressure"

To my understanding if we are at 1/3 throttle the engine will be at 1/3 rpm's and still be filled at each stroke by the same amount of air at the same atmospheric pressure than at full throtle because the slight additional resistance in air flow at the throttle will automatically be compensated by opening the butterfly a little bit more, we are really setting the 1/3 throttle position by the 1/3 rpm's shown on a tach because the position of the carburetor butterfly is not proportional to rpm's so I do not see where is the 1/3 of an atmosphere pressure coming from.

Can anyone clear up this?

SACEM1

 

[ivymike]

I think that when he said 1/3 throttle he meant butterfly opening = 1/3 of max (on an effective flow area basis). That's really the only answer that would seem to make the statement approximately correct, and is fairly consistent with my understanding of how the terms "wide open throttle" and "part throttle" are used (independent of RPM).

 

[sacem1]

I agree with you on that, what I don't get is the drop in inlet presure, to me the atmospheric pressure is maintained what cuts the gases output is that the engine is going at one third the rpm's working at the atmospheric pressure.

SACEM1

 

[GregLocock]

I understand your confusion. At low rpms you are at full throttle (zero MAP) even though the throttle angle is only 30 or 45 degrees.

That's why we plot and calibrate everything against MAP, not throttle position. The throttle position sensor is mostly used for some predictive transient stuff, and fault detection.

Cheers

Greg Locock

 

[ivymike]

I agree with you on that, what I don't get is the drop in inlet presure, to me the atmospheric pressure is maintained what cuts the gases output is that the engine is going at one third the rpm's working at the atmospheric pressure.

While the actual throttle angle to achieve the desired pressure drop may be rpm-sensitive, I think that's beside the point that SB was trying to make. At any given rpm, it's still intake flow restriction via the throttle valve that sets the power output (up to the WOT power @ that rpm). If you wanted to estimate the CFM exhaust gas, you could use power/(WOT power) at the given rpm to estimate it.

 

[sacem1]

I'm not worried about the throttle angle, the point I don't get is if there should be a drop in inlet pressure to 1/3 atmospheric when you are at 1/3 lets call equivalent throttle opening, maybe just in the moment the driver "floors it" (pushes the pedal all the way down opening the butterfly completly) and the rev's haven't gone up yet but once stabilized at lets say 1000rpm's the inlet pressure should be atmospheric, am I wrong or not?

Cheers

SACEM1

 

[ivymike]

I'm not sure I understand what you're asking.
* If the throttle is wide open and the rpms are low, then intake pressure should be close to atmospheric. How quickly the throttle was opened and how long it has been open should have insignificant effects (other intake restrictions may be more important).
* If the engine is being "throttled" to reduce power output to 1/3 of max power at a given rpm, then the intake pressure should be about 1/3 of the WOT pressure at that rpm.
* If the engine is throttled to reduce power output to 1/3 of max power at 1000rpm, and you subsequently snap the throttle wide open, intake pressure will increase very rapidly and engine power output will increase almost immediately to the WOT power output @1000rpm. If you simultaneously increase load on the engine to keep rpm constant, then the intake manifold pressure will remain at atmospheric and the power output will remain constant at (wot@1000) from then on out.