Calculating engine plenum pressure?
Discussion
Hello everyone, does anyone know how to calculate plenum pressure for a 5 litre V8 BMW engine. What it is is that I'm doing some CFD modeling of an air intake system and I just need to know what sort of vacuum the engine will be generating. I've looked all over the web but I'm unable to find anything of help.
Regards
willwoll100
Regards
willwoll100
willwoll100 said:
Cheers for the reply Steve, do you know of any websites that I could read up on this or recommend some books perhaps.
willwoll100
Sorry no to both.
I know this is how it works from my experience with Chevy engines. The most common Holley carbs have problems with the power valves as they sense vacuum so when this is low it is difficult to select the right valve.
You may get help on an EFI forum
Steve
The conditions I'm interested in is just knowing how the pressure could be calculated, the plenum is a bespoke design and the engine is the old 400bhp M5 engine. So what I'd like to know is what conditions actually effect it ie what I need to know to be able to calculate it. Accuracy isn't really an issue as such as long as its 'ball park', and how it differs throughout the rev range.
Cheers again
willwoll100
>> Edited by willwoll100 on Sunday 13th November 23:28
Cheers again
willwoll100
>> Edited by willwoll100 on Sunday 13th November 23:28
I'm quite familiar with the S62 motor.
It has a massive plenum and 8 trumpets in it- two of them are facing away from the other runners to minimise charge robbing.
Because the engine has individual port throttles the plenum will be at positive pressure even at part load-heavily throttled conditions.
So is it the vacuum you ARE interested in or the positive pressure in the plenum?
The vacuum generated depends on the position of the VANOS units and how heavily thorttled the engine is. The amount of ignition being used will also effect the vacuum generated as when the ignition is retarded (say-for emissions during car warm up) there will be alot less vacuum generated across the throttles. So it's a bbit like asking how long is a piece of string. Also what load speed site are you intersted in?
It has a massive plenum and 8 trumpets in it- two of them are facing away from the other runners to minimise charge robbing.
Because the engine has individual port throttles the plenum will be at positive pressure even at part load-heavily throttled conditions.
So is it the vacuum you ARE interested in or the positive pressure in the plenum?
The vacuum generated depends on the position of the VANOS units and how heavily thorttled the engine is. The amount of ignition being used will also effect the vacuum generated as when the ignition is retarded (say-for emissions during car warm up) there will be alot less vacuum generated across the throttles. So it's a bbit like asking how long is a piece of string. Also what load speed site are you intersted in?
As said in my previous post I'd like to be able to find out what the vacuum pressure is throughout the rev range, even if its just half throttle and full throttle. But rather than just measuring it I'f like to find out what parameters need to be taken into account to actually calculate it.
Thanks
Thanks
I assume you're using a dynamic CFD simulation?
For that is what you'd need. STEADY STATE pressure loss across the throttles won't give you much of a picture unless it's only a directional comparator you're after. Steady state losses will NOT take into account the massive pressure pulse fluctuations you will get with a port throttled engine that close to the intake valve.
I wouldn't even trust 1 D cycle simulation code to predict vacuum across a throttle- unless the model was extensively correlated to measured data. You could try coupling 1 D cycle simulation code to CFD - i.e CFD the highley 3 D local areas, of, say, the throttles.
Where abouts will you be taking your vacuum measurements on your model?
Factors that will effect vacuum include
Ignition timing
Cam timing
Thorttle angle
Internal EGR in the cyliner
External EGR (i.e the combustion in the cylinder will have an effect)
Engine speed
Ambient temperature
For that is what you'd need. STEADY STATE pressure loss across the throttles won't give you much of a picture unless it's only a directional comparator you're after. Steady state losses will NOT take into account the massive pressure pulse fluctuations you will get with a port throttled engine that close to the intake valve.
I wouldn't even trust 1 D cycle simulation code to predict vacuum across a throttle- unless the model was extensively correlated to measured data. You could try coupling 1 D cycle simulation code to CFD - i.e CFD the highley 3 D local areas, of, say, the throttles.
Where abouts will you be taking your vacuum measurements on your model?
Factors that will effect vacuum include
Ignition timing
Cam timing
Thorttle angle
Internal EGR in the cyliner
External EGR (i.e the combustion in the cylinder will have an effect)
Engine speed
Ambient temperature
willwoll100 said:
The measurement I'll be after will be ideally taken at the aperture of the plenum entrance.
As ALREADY discussed, there is NO vacuum at the aperture of the plenum entrance on a port throttled engine.
There is a slight delta P across the twin air boxes going into the plenum (say may be a loss of 10-20 mbar tops at peak air flow/engine speed).
Marquis_Rex said:
The measurement I'll be after will be ideally taken at the aperture of the plenum entrance.
As ALREADY discussed, there is NO vacuum at the aperture of the plenum entrance on a port throttled engine.
There is a slight delta P across the twin air boxes going into the plenum (say may be a loss of 10-20 mbar tops at peak air flow/engine speed).
I really don't know how to reply to this
GreenV8S said:
Sounds like the problem has a very easy answer then: the plenum pressure will be one bar give or take a bit. Not a particularly helpful answer, but I suspect that's because you aren't asking the right question.
Well it'll never be give a bit as it will always be less than atmospheric.
>> Edited by willwoll100 on Monday 14th November 21:31
>> Edited by willwoll100 on Monday 14th November 21:31
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