Port Barrel Throttles 1

[NeilRoshier]

In a brief conversation with another engineer it was reported that the principle advantage of port barrel throttles at part throttle was the biasing of the flow to one port wall and thus promoting tumble and good mixture distribution within the combustion chamber. In testing there was an advantage in throttled economy/emissions/power (in some circumstances) but no advantage in WOT performance.
However in looking at the barrel throttles available through the aftermarket/performance suppliers the barrel section would seem to be prone to promoting turbulence even within the barrel itself (at part throttle) as the edges of the machined 'port' section through the barrel were quite sharp and the 'port' had an even section. These do not sem ideal features to me but I can accept that the manufacturer needs to proved a product that can be applied to a wide range of engines and thus cannot be specifically tailored to one application (unless its an elise!)
It would seem (to me) that an improvement could be had by continuing the included angle (venturi) of the port into the barrel port and also by radiusing the edges of port to the incoming air, but would there be further advantages in attempting to retain attached flow within the barrel port at part throttle? Specifically in the transition from part to WOT with a quicker re-establisment of full flow and thus throttle response (or am I just thinking myself into more problems?)
Incidentally the PBT I have seen on a MX-5 produced an increase of 10kW@6000rpm in admittedly a modified engine, though the only changes pre/post was the change in manifold from stock to PBT and recalibration of the Motec M4 to suit. Unfortunately this was not my work!

 

[Marquis]

Hi Neil, port throttles is an interesting one!
We had port barrel throttles that were teflon sealed which we tested on a V8,we also did extensive tests on the ones used on the BMW M cars.

In terms of the comments about biasing the flow toward one side to give good in cylinder motion at part load, I couldn't possibly comment- it's possible- I guess you could look at part load combustion data with cylinder pressure tappings in the cylinder head- if what was said was true - it would manifest itself in these burn times- also looks at the ignition values required for MBT ignition- did it need less ignition? Usually a more stable burn manifests itself by being able to take more cylinder EGR without becoming unstable, and/or leaning off without becoming unstable and /or being able to take alot of igntion retard .
Combustion sability is measured in Coefficient of variance of IMEP- or Cov of IMEP.

Now all I will say is that the explanation given ISN't the usual accepted reason why port throttled/barrel throtles engines give better part load economy. The reason outlined in SAE paprers ( there's an excellent one by Roger Duckworth)- is that you're cutting your pumping loop when you consider a P-V diagram- you're changing load conditions much quicker-not having to evacuate the whole section of the inlet manifold/or indeed fill when going to a different engine load site.

In terms of power or gain in performance, you're TOTALLY right that when you bolt one of these systems on- to a like for like engine configuration you get nothing. And before all the small block Chevy /MGB afficionados start to pull their beards out with exception to that comment-remembering trick engines on multiple carburetteurs- WAIT!! When you DO gain is when you design a whole engine system around the port throttles. Let me elaborate ort throttles allow you to have more overlap and under idle conditions there is less tendency for exhaust gas residual to flow back up the intake tract and cause combustion instability. This is because your compressible throttled volume is so much more less now - that there’s less likelihood of this happening. So there- you already have a constraint on cam timing and duration is removed.
The next big benefit is that you're no longer constrained on plenum volume- or throttle volume, and Neil you hit this one on the head too. Basically your throttle response can be a lot better as the distance from back of valve to throttles is much smaller and so is this volume so the engine will recognise changes in load much much quicker! Because of this down stream of these port throttles you can size the plenums and general system mamothley for peak power and torque requirements rather then compromising for transient response requirements (the BMW M3 s have had plenum volumes of about 14-18 litres!!).

I’ve often thought along the lines you said too. SC, or swirl control valves have often been tested on various cars to increase part load cylinder motion and if this is combined with a port throttling barrel-and placed close enough to the cylinder- you could potentially combine these devices.
Unfortunately most run of the mill manufacturers won’t look at port throttles for production, because typicall they don’t under stand the benefits ad when they do try them – undertake the test totally incorrectly. There’s also the issues of trying to set them up in volume production.
To my mind the benefits far outweigh the disadvantages but most typical manufacturers are more into short term thinking- and not looking into the big technical picture.

 

[NeilRoshier]

Marquis thank you for replying, as usual I have had to sit back and have a good hard think on what you have said and thank you for your kind thoughts on my own thinking! The Mazda did need slightly less ignition advance from report, though exactly how much I do not know. As you note, the sate of tune of this engine may have been such that any port throttling system could have had a positive effect as described and not just the PBT
I am aware of Roger Duckworth's paper but as I am not a member of SAE I was not able to get a copy from SAE. Roger did give me some of his valuable time (when I queried him about the paper)and we had a brief discussion and offered to provide more information should I need it (a very generous offer) but I have been distracted by other things and have not yet bothered him with another query.
In looking at the action of the barrel movement in the throttle assembly it seemed (again perhaps only to me!) to be a crude mechanism. I thought that an advantage of the system had been lost in the purely rotational movement of the barrel as a throttle, rather the throttling action of the barrel could also be used to bias the flow to one section of the port and thus one of the two intake valves and thus keep port speed sufficiently high to ensure good in cylinder motion (and possibly allow a different motion at such low throttle levels). I thought that this could be engendered by a 'screw' type motion to the barrel such that at say 20% throttle the portion of throttle available would be biased to the top right (for aguments sake) section of the port and thus the right valve (when looking into the intake port). If the port devider were matched closely to the barrel then only this one valve would receive the intake charge, at (presumably) a greater velocity than if both were to receive the charge in the std barrel operation. As the barrel progressively opened then the optimal (or better than otherwise used in rotational opening) performance in one section of the port would be maintained for a period or at least until the other valve came into play (I hope that I am explaining my thoughts coherently here). I believe that there would be other issues with the intake valve not receiving the cooling of the intake charge, but perhaps nothing that could not be catered for?

 

[Marquis]

Hi Neil,If I understand what you're saying, is to use the PBT to double as a device to generate Barrel swirl within a cylinder rather then tumble-carefully matching up sections and port splitters for apprpriate/approximate sealing and ensuring there are no nasty wakes and /or 3-d effects that near to the port apart from the bias you're intending for.

What experience do you have on swirl on combustion at part load combustion and burn rates/stability?
What about tumble?
It's a difficult one and potentially a can of worms with conflicting info and many varied EDUCATED opinions!
I needn't go into it here, and indeed, don't KNOW all the answers only what I have seen. There's general agreement that the during the process of combustion the swirl motion will decay less as the piston comes up the bore where as the tumble will decay down into eddies and then break down further into even more localised eddies as the piston approaches. There's also timing to consider- as to which motion control regime introduces charge motion when in the induction and which is more favourable toward what we're looking for in combustion.I've heard seen a to much conflicting data and am at the moment too busy to enter into it all. Combustion can become such a black art.
In terms of one valve not getting the cooling of the flow of air, I don't think that would be too much of an issue as I've seen engines run with port deactivation and one side of the port totally blocked off-running fine in terms of that particular valves temperature.
Things to be aware of are the how and what kind of injectors are going to be run. If only one injector, effort must be put into targeting, because what you may find is that even a little bit of occasion dribble of fuel into the deactivated port is enough to have big droplets of fuel enter into the combustion chamber causing all sorts of atomisation and therefore HC emission problems!
The other thing that analysis has shown is that there's a definate compromise with the increase of swirl motion for good part load but the increase of pumping loss. Now this has yet to be confirmed with test but if the pumping work is compensated for by opening the throttle more you're altering the very thing you're trying to create!

You know Roger?I know him quite well too,he's been very helpful to me!
I wonder if you know me to?
I don't recognise your name!

 

[NeilRoshier]

Marquis I would like to be able to say that I know both Roger and yourself, but I am sorry to say that I do not. I was very fortunate to have Roger (and yourself in this forum) provide expertise and opinion on some things that are obviously even more complicated than I had imagined. I was very pleased that Roger gave his time so readily especially as I am sure that I could not afford to pay for it! Please pass on my thanks if you see him again soon (no doubt he will not remember me, but I appreciated his help nevertheless).
I had considered the issues of 'contamination' of the desired flow characteristics/mixture of the active section of the port and though that either the injector placement as you suggest or even a variable port devider that assisted the baising of the flow might have been possible, but in truth I recognise that this really is impractical (especially for me) and indeed may not work at all! (I had even thought of a split barrel with seperate throttling of each valve, i.e. one valve would be gradually throttled down with the other basically in WOT state...too much strong coffee that day perhaps!) I had not considered the pumping losses you mentioned, but now you mention it it seems to make sense. The barrel swirl at part throttle using one valve seemed a reasonable idea, perhaps mostly as I have to admit that I have no real experience on testing either swirl or tumble. My interest in such things is due to my desire to extend my knowledge further from that I have read (and my own very limited experience) in books as so often there appears to be gaps in the information presented which often raises more questions (hence my queries on this forum). My focus is the modification of engines for motorsport use (often for dual purpose i.e. road and motorsport hence the importance of combustion/emissions, and also vainly to become more knowledgable!) and given the newest emissions requirements and no doubt the future ones I feel that I should get a deeper understanding of the more technical issues surrounding the combustion (I hate wasting money and effort...especially my money!). Indeed an emissions test can cost $2,500 and if the engine fails then the next test will cost the same amount (no free retest!) so I hope you can understand my desire for more information.
I appreciate any guidance you can give, but please do not go to any great effort as I would not want to inconvenience you in any way
Regards Neil Roshier

 

[NeilRoshier]

I have spent part of this long weekend reading about swirl/tumble...I think that I see some of the errors that I have made. I unfortunately seem to have ascribed to the 'more is better' school of thought which is rather slow witted of me. However from what I have read the need to induce tumble/swirl (ie in cyl motion) will vary depending on a fairly wide range of variables, does this sound correct Marquis?

 

[Marquis]

Neil, yes it sounds about right.
But then don't believe everything you read either!
I have come across engines with alot of motion and a slower burn, or being MORE prone to knock.
It's far more complex then I can outline in a post and often there is no substitute other then following general guidelines and testing painstakingly or mimicking something that already works.
There are guidelines for tumble ratios and swirl indexs but conventions for these vary.

You probably already know that the energy you impart to the charge for motion is more often then not paid for in terms of outright flow coefficient ( except in the proposed case above!).