Plenums.... why bother?

[Big rob]

OK
First off look at what you are trying to achieve. We want to make power, and the way to do it is to get as much air into the engine so we can add fuel. Now lets forget FI as they are a complete different kettle of fish.

So to acheive maximum power we must fill each cylinder with the most amount of air possible, thus eliminate any restrictions in the intake path. When a 4 stroke engine is opperating intake valves are closing and opening rapidly, when high velocity air traveling down the runner hits the back of valve as it closes it changes direction back into the intake. This air traveling into the intake is like an opposing force which in essence creates a restriction in the that particular runner, and once it bounces around in the plenum it effect other cylinders. So by accumulating a mass of air in the plenum we can try to damp out these occilating pressure waves.

So why not build a massive plenum? Well besides packaging limitations, as you increase the volume of air after the throttle body you reduce resposnse which is another desirable factor. If you move the throttle possition shut, all of the air after the throttle body must be combusted before a noticble effect will take place.

This is wear ITB become the beez neez. If you think about ITB , the whole atmosphere is the plenum which in comparrison is extreemly massive. This provides excelent dampning characteristics and little cylinder interaction thus excelent filling properties. But before we said that with a massive plenum you will have very poor response, well that only applies to the air after the throttle and with ITB this is very small incomparrison to a single TB giving excelent response.

When I think about it, the top reasons the majority of manufacturers use plenums is there simplicity in and cost. You only have one throttle body, no need for complicated engine managment systems and linkages. There is only need for one air filter ect.
In racing where plenums are used could be related to a limitation to number of throttles or the use of a restrictor in the air intake etc.

In FI applications power is made through turbo and superchargers which dont get affected so much by cylinder interaction and pressure pulses due to the nature of positive pressure of the system. Plenums are usefull in this case to provied positive pressurised air to each cylinder in which case ITB still can be used to recieve there benifits.

On another note, velocity stacks work on relationship of mass flow in = mass flow out. With out actually mentioning the formulas, as air enters the larger area on one side, the same amount of mass must leave through the smaller area. The relationship stipulates that for this to occure, the velocity of the air must increase. With an increase in velocity, combustion eficiency increases, which increases volumetric efficiency and therefore power.

If any of this wasnt clear, please let me know and i'll try and go into futher detail.

Cya,
Rob

[kingmick]

all very intresting internet knowledge.I like the FI cars with superchargers and turbo chargers.
very good read.

[Big rob]

all very intresting internet knowledge, i like the FI cars with superchargers and turbo chargers.
very good read.

??????????? I like forced induction motors too

[oldcorollas]

all very intresting internet knowledge, i like the FI cars with superchargers and turbo chargers.
very good read.

good to see you making another worthwhile contribution to toymods
kids been typing while you were busy again?

edit: previously, you have only referred to "internet knowledge" in a highly derogatory way, accusing people of only having learned information fromthe internet, as opposed to the real world. so it is natural to assume that you were taking the piss.
only a few days ago you were saying that most of the people on toymods were shit because they have only learnt information on the intahweb....

saying "i like FI with SC and turbo"... umm. ok...

[river]

Hi,

First off look at what you are trying to achieve. ... snip snip snip...
If any of this wasnt clear, please let me know and i'll try and go into futher detail.

Good stuff.

However, does this mean that having a carby-throat per cylinder, bolted straight onto the engine, side-draught style, offers the least resistance - which is what we're trying to acheive? In effect, isn't this providng an independant throttle per cylinder? And also isn't the whole atmosphere being used as the plenum in this case?

It was mentioned earlier in this thread that this arrangment is good for >75% throttle. Why is it not good for lower throttle settings?

seeyuzz
river

PS: In regards to super/turbo FI engines..... I guess I'm one of the rare few who likes their engine NA. Same as I prefer athletes to be all-natural, rather than pumped with drugs and other performance enhancing stuff. I'd take something like the superlative Honda F20 engine over a mediocre engine that gets its grunt for forced induction any day.

[kingmick]

??????????? I like forced induction motors too

No need for the question marks, i put a comma instead of a fullstop.
I like them.

[Ben Wilson]

Great thread - If we had more like this and less 'look what I found on the Internet', the world would be a better place

For everybody Another advantage of a plenum is secondary resonance tuning. On a NA car with individual runners there is a fairly simple harmonic tuning going on. Valve closes, positive pressure wave bounces up the runner, leaving a wake behind it, air rushes into the low pressure area hopefully coinciding with the next inlet event*

What happens to this positive pressure wave after it exits the runner? On a open throttle body setup or an extractor style inlet it's gone. With a plenum setup there will be a measurable increase in pressure inside the plenum, if you can time this pressure increase with the inlet pulse of another cylinder you've got a win/win situation.

Theoretically, you can either arrange this pulse to coincide with the times when primary inlet tuning is working against you to get a wider torque band, or time it to arrive at the same time as a positive pulse to get more peak power. Unfortunately, you either need a PHD in maths and physics to crunch the numbers on this sort of setup or a bunch of prototypes and heaps of dyno time - Neither of which is easily available to the backyard enthusiast..

* ridiculous oversimplification, but the concept is sound.

For Turbo guys - A plenum will usually mean lower inlet losses. As pressurised air is 'thicker' than atmospheric, losses due to pumping friction will be higher. The more pipe wall the air has to go past, the more losses there will be. A plenum allows you to evenly distribute air to all the cylinders without as many frictional losses (it's also a hell of a lot easier to ensure even air distribution).

[kingmick]

good to see you making another worthwhile contribution to toymods
kids been typing while you were busy again?

So i cant just say "intresting read" like most people i have to give information?
Things seem to be going fine,so i see no need to add any practical knowlege.
Yes i am very busy at the moment so dont have time to sit and write and essay on it.
I am sure you will never bring my kids into it again, I can tell you im showing restraint.

[kingmick]

Great thread - If we had more like this and less 'look what I found on the Internet', the world would be a better place

For everybody Another advantage of a plenum is secondary resonance tuning. On a NA car with individual runners there is a fairly simple harmonic tuning going on. Valve closes, positive pressure wave bounces up the runner, leaving a wake behind it, air rushes into the low pressure area hopefully coinciding with the next inlet event*

What happens to this positive pressure wave after it exits the runner? On a open throttle body setup or an extractor style inlet it's gone. With a plenum setup there will be a measurable increase in pressure inside the plenum, if you can time this pressure increase with the inlet pulse of another cylinder you've got a win/win situation.

Theoretically, you can either arrange this pulse to coincide with the times when primary inlet tuning is working against you to get a wider torque band, or time it to arrive at the same time as a positive pulse to get more peak power. Unfortunately, you either need a PHD in maths and physics to crunch the numbers on this sort of setup or a bunch of prototypes and heaps of dyno time - Neither of which is easily available to the backyard enthusiast..

* ridiculous oversimplification, but the concept is sound.

For Turbo guys - A plenum will usually mean lower inlet losses. As pressurised air is 'thicker' than atmospheric, losses due to pumping friction will be higher. The more pipe wall the air has to go past, the more losses there will be. A plenum allows you to evenly distribute air to all the cylinders without as many frictional losses (it's also a hell of a lot easier to ensure even air distribution).

very good points and especially resonance tuning, Good way to see this sort of thing in the flesh is on SU carby car or car that has injector placment outside the bellmouths{ment to add when they dont have air filters on}, very intresting how much fuel travels around.
" ridiculous oversimplification, but the concept is sound." that is the best way to explain it to young people as they learn.
No point jumping to most avanced point without learning the steps on the way.

[river]

Hi,

So, let me see if I understand this......

The air, that's in the plenum, gets sucked into the cylinder (on the induction cycle), and when the inlet valve closes there is a back-pressure (for want of a better term) of the air (that was rushing into the cylinder). Air, having some viscousity, bounces back from the closed valve and back into the plenum. This influx of air creates a small amount of pressure within the plenum, and this pressure aids the intake of air for the next cylinder.... and the cycle continues. Is this it?

If so, then a I have a question or two.....

One end of the plenum is open to the atmosphere to allow incoming air from outside. When the back-pressure occurs and it pressurises the plenum, then what is stopping the air from going back out through the opening of the plenum and thereby not pressurise the plenum? I suppose you could have a 1-way valve to allow air into the plenum without escaping, but this (I assume) would provide more resistance to incoming air and be too restrictive.

So, I assume (I'm doing a lot of assuming here) the plenum is designed to allow for the velocity of the back pressure so it doesn't make it to the opening of the plenum before the next cylinder sucks in air?

If so, then as the engine rpm changes so does the back pressure change and so does it's velocity? Which implies the plenum is a "best fit" design as it can't cater for all engine rpm and therefore some design compromise must be employed to get it to work across the widest (or most effective) range of engine speed?

This is good stuff. This is pure Toymods stuff... so, illuminate me!

seeyuzz
river

PS: KM, settle down big fella. OC was just having a light-hearted joke - as evidenced by the smiley icons.

[Screamn_Sleeka]

Besides the word "viscosity", which is for liquid and not gas, that’s what i understood as well river.
And for your question, there is no one way valve. Since the flow is still very strong it is only the air at the valve and up the runners that become pressurised and the air in the plenum isnt pushed out, the pulsing is only in the runners.

[Screamn_Sleeka]

So if you had a super fast cycle pressure tester in the runners it would read 1 second of high vacuum then 3 seconds of low pressure (to a ratio, with a normal cam)?

Although they might not exist...
Isnt anyone here doing engineering, this would be probabaly be a first yr topic.

[takai]

The pressure "wall" or more accurately pulse which is reflected off the back of the valve will continue out the runner and into the plenum. Now while a portion of this will follow the edges of the runner as per lamina flow principles, another pertion of the pulse will reflect off the plenum. Given an effective RPM range (normally dictated by the cams you are using) the rate of these reflections can be calculated, and then when you are designing the plenum you can calculate the Heimholtz resonations in order to see where the air is going.

And then you try and apply all of that to an intake which needs to be built and find that you cant build your ideal intake due to packaging restrictions, and so you go back to the drawing board and see what is the best package which will fit.

In an ideal world we would all have spaceframe chassis with remote pedalboxes (eh kingmick) and therefore not have to worry as much about packaging.

For the IPRA car my plenum ended up looking like this:




The slant on it isnt as much for packaging as it provides a surface of reflection for the reflected pulses. While its clearly not 100% it fits in with design limitations, and most importantly stops my engine bay looking like this again:

[takai]

Besides the word "viscosity", which is for liquid and not gas, that’s what i understood as well river.
And for your question, there is no one way valve. Since the flow is still very strong it is only the air at the valve and up the runners that become pressurised and the air in the plenum isnt pushed out, the pulsing is only in the runners.

Air is a liquid. Well it behaves enough like one for you to be able to treat it as such. As evidenced by the fact that you can talk about lamina flow and apply flow modelling principles to it. It just has a lower density and friction coefficient.

[Ben Wilson]

river - good question, hopefully I can answer it

Another gross oversimplification follows:

Imagine a rock dropped into a pond, as the ripples spread you can clearly see high and low pressure systems dissipating. A pressure pulse coming out of a runner has a similar effect. You can clearly imagine how you could have either a high or a low wave over the top of an adjacent runner.

The real world has it working in 3d and bouncing off the walls of the plenum, but you should be able to get the idea.