Citroen C1, Peugeot 107, 108 & Toyota Aygo Owners Club. (Discount code for CityBugStore: C1OC)

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PostPosted: Fri Jan 15, 2021 11:12 pm 
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This is the first lot of CFD simulations I've run; just to get to grips with it really.

Image


This illustrates the concept I'm trying to describe here about how airflow is distributed through the manifold plenum. It's a bit like turning your kitchen tap on full and soaking the entire room - If you can get air in faster it has more energy and kinda distributes itself everywhere evenly, so the cylinders furthest away from the inlet have just as high pressures at their intake as the no.1 cylinder closest to the inlet.


I realised when I started modelling the throttle body that I've made a bit of an oversight with how that original item is designed to flow. It's essentially and eccentric reducer and it's orientated so that airflow turns in from the front of the engine so the short radius of that turn isn't so extreme.
The way I've got things setup brings the air in from the side; the short radius is therefore a lot sharper and the air wants to blast past the throttle opening instead of turning into it when the intake is at at peak velocity (high revs).

You can see that here:

Image
(The dark blue traces pictured on the left side of the narrow throttle mouth is actually air circulating in the closed idle control valve port, overlayed)

From my road testing I've concluded the design is working well at lower revs though, like I'd hoped, but yeah the top end could really use some improvement.

Although, with the CFD analysis done so far I've discovered that by approaching the eccentric reducer from the side I'm actually creating a vortex (spinning) in the airflow as it travels through the throttle body, which can potentially be used to further increase flow rate into and distribution through the manifold plenum. I think if I can get the angles right to promote this vortex effect and do something about turning flow around that short radius then this oversight might prove to have its own benefits. I'm also approaching the throttle plate side-on so it poses less of a restriction in the WOT conditions we're interested in here - so that's another positive.


Keep me right guys. I'm not doing this for likes or for attention but I do still really like to know that you like this stuff - it picks me up.


I also have to give Gary a mention here; pages and pages ago he made a comment suggesting CFD simulation and I kinda shrugged it off thinking "how that hell am I supposed to afford something like that?" - Well props to your forward thinking Gary.
I'm using a service provided by http://www.simscale.com - These calculations are based on 3D models I've built in Fusion and uploaded to their site; they're run in the cloud as it requires a huge amount of computational power but it's all free to use with a limited number of core hours (unlimited private use via paid subscription). What an incredibly awesome thing. What an incredibly cool world we could build for ourselves with free access to educational resources like this. Money is a great system for trading luxury items/services but for our needs; education, wellfare, basic necessities - these things should be resource based and not priced out of people's reach. Your life isn't about working in order to survive; it's about the enjoyment and progression of this most incomprehensibly incredible sandbox we live in called real life - whetever the f-ck life is :lol: Is that not true respect and appreciation? (Yeah I remember your pre-covid stance on the NHS Boris)
I'm not doing this stuff because somebody is employing me to do it, not even because I intend to offer this stuff to purchase through the website as a means of supporting myself - I'm doing it because its cool and I enjoy it. Maybe one day I or someone like me will use the experience gained through projects like this one to create 100% efficient engines, self-sustaining farms with free to access global food output, technology allowing the propogation of other planets etc. This is progression, people. If that makes me crazy or a 'lost cause' then good luck to you; if it weren't for new ideas, diversity, positivity and hopefulness you'd still be a mindless single-cell organism :thumbs:

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Last edited by Bunkey on Sat Jan 16, 2021 2:51 am, edited 10 times in total.

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PostPosted: Sat Jan 16, 2021 12:04 am 
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Have you thought about putting a spiral on the inlet manifold to get some movement into the intake?


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PostPosted: Sat Jan 16, 2021 12:31 am 
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...Out of nowhere :wave:

That's a really interesting idea - I could mould it into the bellmouth of the induction system like that to spin the airflow down the pipe toward the throttle body. I'll run some tests to see if it makes much difference to the air distribution by the time it gets there

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PostPosted: Sat Jan 16, 2021 9:07 am 
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Two things that come to mind - on a non-turbo engine, the whole inlet system works on reduced air pressure as the air is being sucked into the engine - with the throttle body upstream of the plenum chamber, the more the plenum is "filled" the slower the throttle response.

For maximum response, you'd need individual throttles on each inlet, but with all opening/closing at the same time.

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PostPosted: Sat Jan 16, 2021 5:48 pm 
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Black Grouse wrote:
Two things that come to mind - on a non-turbo engine, the whole inlet system works on reduced air pressure as the air is being sucked into the engine - with the throttle body upstream of the plenum chamber, the more the plenum is "filled" the slower the throttle response.

For maximum response, you'd need individual throttles on each inlet, but with all opening/closing at the same time.


It's maybe easier to try and think of the pressure in the engine as what's relative to whatever else you're comparing it to, as opposed to just 'sucking and blowing'.

With a turbo system it's not that you're 'blowing' air into the engine vs. N/A 'sucking' air - What you're essentially doing with a turbo is raising the pressure differential between the intake and the combustion chamber to much higher levels (atmospheric pressure + boost pressure) than what you would be if referencing the combustion chamber against the intake at atmospheric pressure alone (N/A engines). This is why MAP sensors are manifold absolute pressure, referenced to total vacuum, as opposed to just a normal pressure guage where 0 is atmospheric; because the atmosphere is already a positive pressure that fluctuates, so the engine has to know exactly what that pressure is to compensate for things like altitude and weather conditions. With a turbo you're artificially increasing that atmospheric pressure way beyond what the Earth's atmospheric pressure already is. It's not really about blowing or sucking air at all; but the pressure differences between regions and how the air itself wants to distribute through those regions.

For me, seeing it in that way makes it easier to get a handle on what's happening to airflow.


In terms of fast and slow throttle response:

When the throttle is closed and you open it; the standard manifold plenum is at full vacuum regardless of how well the intake system distributes air through it - so it's actually empty at the point where throttle response is a concern. The faster and more 'full' you can fill that plenum when opening the throttle therefore actually increases the throttle response as the relative pressure between the plenum and combustion chamber (also at full vacuum) will now be greater more quickly - Which then leads into the description on the induction system product page.

The throttle response is of course only attributed to that initial 'throttle opening: plenum from vacuum to atmospheric pressure' stage. Once you're in WOT conditions and accelerating; the more consistently 'full' and closer to atmoshperic pressure the plenum is, the greater the airflow into the combustion chamber on account of the difference in pressure between these two regions. Therefore the greater the volumetric efficiency and power output of the engine. You can also disregard the location of the throttle plate in WOT conditions because it essentially disappears.

When you have ITBs, the engine bay and outside world is your plenum which is always at atmosheric pressure and consistent across all cylinders - so in that sense the 'plenum' is 'totally filled' all the time - Which is why ITBs generally make more power when fitted and are considered an upgrade. They do have the benefit of increased throttle response and this is largely due to the fact the throttle valve is much closer to the combustion chamber so the air has less distance to travel (less time) to fill that chamber and is always at the maximum possible pressure differential between the 'plenum' and combustion chamber.

Unfortunately the cost of an ITB setup is out of reach for most people, including me right now, as it's a much larger undertaking with completely different fuelling and sensor requirements. It's kind of a different path to what we're doing here but one I hope to explore at some point in the future :thumbs:


By better distributing the air through the standard inlet manifold plenum as I'm trying to do here to 'fill' it up more; I'm basically getting closer in function to what an ITB setup represents in its ideal scenario of full atmospheric pressure distributed evenly across all cylinders - but at a much more reasonable cost that doesn't require completely replacing the inlet manifold and fuel management system.


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PostPosted: Sun Jan 17, 2021 4:04 am 
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Black Grouse wrote:
with the throttle body upstream of the plenum chamber, the more the plenum is "filled" the slower the throttle response.


Black Grouse, it's occurred to me that you might be getting throttle response and intake velocity mixed up - In which case you'd be correct for a static example. In this case though you have to consider it's a dynamic system: The air which the engine displaces (sucks out of the plenum) has to be replaced somehow and that's what's coming in through the induction system...

Accounting for this is a slightly different kettle of fish. (I've never used that expression before in my life :lol: )

So in the case of these CFD simulations I calculated how much air the engine is displacing at a given RPM using an estimated value for VE and then what the velocity of the air coming in through the intake pipe diameter would have to be to meet those requirements.

This is basically a way of simplifying the analysis because, as you've probably just discovered, trying to wrap your head and your calculator around the actual pressure displacement in the dynamic plenum is just crazy at this level; its far too complicated. By calculating using displacement instead you get the net value for air velocity coming in which allows you to determine how it's going to flow through the induction system and enter the plenum.

I hope that makes sense. Sorry if I misunderstood what you were saying.

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PostPosted: Sun Jan 17, 2021 9:33 am 
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On this sort of arrangement, the plenum is almost always under negative pressure - because the cylinders are sucking and the throttle body is throttling even at Wide Open Throttle (WOT) - maximum air flow occurs at WOT and the plenum is at slight negative pressure and is there just to even out the suction pulses from each cylinder.

Is it possible to buy/make a larger diameter throttle body? It would make the idle and low throttle less driveable but potentially improve WOT flow, which is ok for a race engine.

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PostPosted: Sun Jan 17, 2021 12:14 pm 
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What's the real benefit/effect of these tuned length inlet manifolds that are appearing even on the 2019/2020 bugs?

I heard the idea was to get a pulse resonance going that created a slight positive pressure by the inlet valve at certain rpm ranges to improve the cylinder filling and hence improve mid rage torque?

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PostPosted: Sun Jan 17, 2021 12:41 pm 
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PetrolDave wrote:
What's the real benefit/effect of these tuned length inlet manifolds that are appearing even on the 2019/2020 bugs?

I heard the idea was to get a pulse resonance going that created a slight positive pressure by the inlet valve at certain rpm ranges to improve the cylinder filling and hence improve mid range torque?


That's exactly the idea.

On Vauxhall twinport and some VW Group turbo-diesels, the two inlet valves for each cylinder have separate ports which are connected to different halves of the inlet manifold so that one inlet tract is long to optimise low rpm torque and the other is short to optimise the high rpm power - the short tract having an extra throttle which is closed at low rpm. This also achieves the same effect as Toyota/Daihatsui Variable Valve Timing.

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PostPosted: Sun Jan 17, 2021 6:36 pm 
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I had to take a break from this and come back to it cos overthinking has started to fry my brain a bit :lol: sorry if its changed since you read it.

Black Grouse wrote:
On this sort of arrangement, the plenum is almost always under negative pressure - because the cylinders are sucking and the throttle body is throttling even at Wide Open Throttle (WOT) - maximum air flow occurs at WOT and the plenum is at slight negative pressure and is there just to even out the suction pulses from each cylinder.

Is it possible to buy/make a larger diameter throttle body? It would make the idle and low throttle less driveable but potentially improve WOT flow, which is ok for a race engine.


So the diamter of the TB isn't really a limiting factor on this engine.
For reference the 1KR-F20 race engine makes 120hp (claimed) with a restricted 20mm intake on its plenum - The standard TB on the 1KR-FE is 40mm.

The best thing to do would be to optimise the topology of the TB instead which is in part what I'm trying to do here. Its shape can be improved on to further increase airflow by means of improving air velocity (or more accurately: by reducing drag) - That was kinda where the initial experimentation with porting the TB and shaping the bellmouth in the induction pipe etc started, as documented in the cylinder head rebuild thread; and what brought me to this point here. Of course that improved velocity ties into the whole 'distributing air through the plenum' thing.

If you did put a bigger TB on I think all it would do is make the throttle feel on/off in how the engine responds. It would also have a negative effect on this pressure distribution through the plenum due to the reduced velocity of air entering the plenum, so cyl.1 would have a large pressure differential whereas cyl.3 would have a comparatively smaller differential as that area of the plenum is subject to lower pressure most of the time.. as you can see here:

Image

This is a rough approximation of airflow based on induction velocity with cyl.2 as a pressure outlet but it illustrates the point.

PetrolDave wrote:
What's the real benefit/effect of these tuned length inlet manifolds that are appearing even on the 2019/2020 bugs?

I heard the idea was to get a pulse resonance going that created a slight positive pressure by the inlet valve at certain rpm ranges to improve the cylinder filling and hence improve mid rage torque?


The pulse timing of the intake is relative to the length of the intake runners on the inlet manifold that run from the plenum to the intake ports and is a different topic altogether.
It's exactly the same premise as getting the right primaries diameter/length on the exhaust manifold, so that when the positive pulse wave created from air slamming against the closing inlet valve is reflected back up the runner on one cycle, as that positive pulse escapes the end of the runner into the plenum it pulls air behind with it - the resulting negative pressure wave going back down the runner, and the positive in-rush that chases it, coincides with valve opening event of the next cycle to force more air into the chamber at that instant the valve opens when the piston is at the top of the exhaust stroke/beginning of intake stroke but not actually moving (to suck air in).
You can see therefore how the length of the runner with the fixed speed of the pulse wave -speed of sound - determines at what rpm this happens. After this, the pull from the piston moving down again takes over once it's further into the intake stroke. It is a different topic to what I'm covering here though they are interlinked..

Plenum pressure I'm interested in with the induction system design is more to do with whats happening at valve closing, the momentum the air has, to force more air in at the bottom/end of the intake stroke when the piston is barely moving just before the valve closes - and this momentum in turn would create a larger positive pressure pulse wave when it hits the closed valve, influencing this pulse wave timing at valve opening and thus everything plays nicely together.

The timing of the event is optimised - the effect drawn out over a larger rpm band for a given intake runner length - with VVT, so its not so much that one thing replaces the other but that they're all tuned together for the best VE. The reason this is prominent at low/mid revs is because the piston is moving slowly; so that period at the top (or bottom in my case) of the piston stroke when the piston isn't moving up or down at all is exaggerated and that's when these tricks come into play.

I think one of the coolest examples is the LaFerrari that has infinitely variable intake length that 'breathes', like a trombone slider, with engine RPM.
The technology has actually been around for years and was banned from F1 in 2006 for being so effective.

The actual pulse wave timing effect is present on every engine because its just a physical property of fluid dynamics; it contributes to how any engine behaves the way it does but engine designers obviously integrate that into their design - VVT is an economical way of exploiting it further - and sometimes if they put a lot of resources into designing around the pulse-wave timing aspect (like dual length runners), the marketing then reflects that expenditure :lol:
Tuned intake length and pulse wave timing is a pretty basic (albeit complex) fundamental of engine design that's been around for decades though.

There's a good article/test covering intake length that I linked in the OP of this thread:

"I think, in hindsight, the smaller internal diameter had a similar effect to increasing intake runner length -
If you're interested here's a great write up on the effect intake length has on an engine's power curve: https://www.emeraldm3d.com/articles/cat ... th-intake/"

Obviously I didn't have as much undertsanding of the process then because I alikened the pre-TB intake diameter to valve opening runner length as opposed to valve closing plenum pressure distribution via induction velocity; but the results are similar nonetheless.

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