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Can somebody help me with this scavenging dilemma?
4

Can somebody help me with this scavenging dilemma?

Can somebody help me with this scavenging dilemma?

(OP)
I've made a short 48 second video to show the problem.

Youtube Link

I can see how scavenging would work in a system with very little back pressure, but how does it work in a system with a catalytic converter and mufflers?

RE: Can somebody help me with this scavenging dilemma?

Scavenging works because of pressure waves traveling through the system. You can't simulate this with a test that uses constant flow. You basically can't simulate this without an actual running engine- in the real world it happens much too quickly. Optimizing header designs (if that's what you are trying to do) is about doing some calculation to get close, and then testing a LOT.

RE: Can somebody help me with this scavenging dilemma?

(OP)
I agree. As I understand scavenging, it is the same as the effect experienced at the rear of a large truck, i.e. you can be "sucked" along if you get close enough. It is the tail of pulse A passing the opening of tube B that scavenges pulse B, not the Bernoulli effect of A's movement past the opening to tube B.

I think I am showing the Bernoulli effect.

Is this correct?

RE: Can somebody help me with this scavenging dilemma?

You're sort of correct and sort of not. A truck is not a great analogy. What you're describing is the result of several aerodynamic phenomena happening at the same time. Vortex shedding, boundary layer adhesion, bernoulli's principle, etc.

I'm at work right now so I can't view youtube. Search 'engineering explained exhaust backpressure'. There's a video called 'stop saying exhaust systems need backpressure'. Watch that video. It's imperfect but it's a pretty decent introduction to how exhaust scavenging works.

RE: Can somebody help me with this scavenging dilemma?

(OP)
It looks like you want the best compromise from
1) lowest back pressure [to increase differential pressure between cylinder and valve throat (the space between the valve and the header) when valve opens]
2) velocity [to increase inertial scavenging]
3) header tube diameter and length [to increase wave scavenging]

Each improvement in one will probably have a negative effect on the other two.

I'm going to try and find an experiment that will show wave scavenging.

RE: Can somebody help me with this scavenging dilemma?

Maybe using a blower to test the effect isn't representative of what really happens. Exhaust is a stream of pulses, not a continuous flow.

RE: Can somebody help me with this scavenging dilemma?

(OP)
Rod, I agree with that, but the effect is the same when I pulse the blower.

I've done a test using air and water and I've got the same result. I'll post the video later.

RE: Can somebody help me with this scavenging dilemma?

You cannot make pulses with your test rig that are even remotely close in frequency or pressure to what will be seen in a running engine.

Just because there is "average" back-pressure in the exhaust system as a whole doesn't mean there isn't momentary fluctuation widely above and below that average back-pressure when connected to a running engine.

RE: Can somebody help me with this scavenging dilemma?

Quote (Gary 321)

but the effect is the same when I pulse the blower.

You aren't going to be able to simulate this with a handheld blower of any kind.

You're talking about pressures that could be 50+ Bar and pulses that are a few milliseconds long.

RE: Can somebody help me with this scavenging dilemma?

A positive pulse happens from an opening exhaust valve at (say) cylinder 1.

This travels down that exhaust header until it reaches an expansion or a junction (which, to that pressure pulse, is an expansion).

The positive pressure pulse splits at the junction and travels BACK up the adjacent header pipes connected at that junction at the speed of sound and also down the collector and because it was an expansion, it gets reflected as a negative pressure wave back to the cylinder that created the positive pulse a few milliseconds earlier.

If that negative pressure pulse happens at a favorable time in that cylinder's exhaust stroke ... THAT is where the scavenging comes from.

Of course, the positive pressure wave that went back up the adjacent header pipes has to be accounted for, too. Hopefully when that gets to the exhaust valve for that cylinder, that exhaust valve will be closed. But it might not be, because you can't tune these things to work at all possible engine speeds. It could very well happen during THAT cylinder's valve overlap period, thus causing exhaust reversion in that cylinder.

Engines that have overlapping exhaust strokes (counting the whole exhaust valve opening time, not just the "nominal" exhaust stroke!) are prone to reversion caused by adverse pressure pulses. The traditional bent-crankshaft V8 is a disaster in terms of exhaust tuning. Inline-fours with aggressive cam timing can be trouble at low revs (I deal with motorcycle engines ... it's not uncommon to have a trouble spot somewhere near half of rated RPM / two-thirds of peak-power RPM). Inline-threes are pretty good, and sixes with split headers (as on your video) such that it acts like a pair of inline-threes are pretty good - the exhaust strokes are for all practical purposes non-overlapping.

RE: Can somebody help me with this scavenging dilemma?

(OP)
Thanks Brian. Good stuff to read and absorb.


I'll post this because I said that I would. It's a 4 second video that shows scavenging. I now understand this to be "inertia scavenging". If you listen closely, you can hear a popping sound when I stop the flow. Is this popping the wave that you talk about?

Link

RE: Can somebody help me with this scavenging dilemma?

I watched your video- I don't think I can see enough of your setup to tell what is going on.

Can you describe the arrangement you're using?

RE: Can somebody help me with this scavenging dilemma?

(OP)
jgKRI - if you mean the 4-second video...

I've just noticed that you need headphones to hear the popping.

Here's a 58-second video of the setup...

Link

RE: Can somebody help me with this scavenging dilemma?

I can't view youtube at work, unfortunately. But I'll watch this tonight and see if I can understand.

What exactly are you trying to accomplish? Is this just an exercise in learning, or are you trying to translate this understanding into some design work?

RE: Can somebody help me with this scavenging dilemma?

I recall seeing (years ago, on a video somewhere) a set - or kit - of (very) adjustable length headers so that the engine tuner could fiddle with the effective header pipe lengths on the dyno. The pipes slide into each other and were then clamped for the next dyno run. They were only suitable for a dyno of course, because they were so unwieldy (they'd never fit into an engine bay). The resultant optimal lengths from the trials and tuning would then be used as the basis for the design of the actual headers.

I don't recall if they had any options for different diameters.

Disclaimer: this isn't my area. Just mentioning it.

RE: Can somebody help me with this scavenging dilemma?

(OP)
VE1BLL - That is a cracking idea if it could be made to work (it sounds like it would leak)

jgKRI - It started as a learning exercise because I like to learn about a subject before I try to modify it. Car forums are full of people who bolt on bits and say they have made an improvement. Usually all they have done is change the noise.

I am looking to improve the car's performance. I've measured (in lots of detail) the exhaust dimensions, valve lift (lift v crank angle) and the intake dimensions. I can then, hopefully, see areas for improvement.

I'm trying to prioritise what is my "biggest bang for the buck". For instance, the exhaust is "crushed" (to allow ground clearance) from 2463 sq mm to 2299 sq mm. If I can replace the crushed section with 56 mm pipe, I can reduce the back pressure in that area for almost no cost.



I've already insulated the intake and have measured improvement - that cost almost nothing.

As an idea of my thinking, I am considering dividing my plenum, so the intakes of cylinders 1, 2 & 3 are not influenced by cylinders 4, 5 & 6 - but that is a discussion for another thread.

RE: Can somebody help me with this scavenging dilemma?

Pipes don't have to be perfectly round... particularly at "crushed" sections.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Can somebody help me with this scavenging dilemma?

(OP)
Yes, but the cross-sectional area is 7% less.

I couldn't find any reference to this on the internet, so I did an experiment using different shaped orifices (all the same cross-section) and it proved what you say. The only problem that I noted was that some shapes promote vortices (which significantly reduced the flowrate).

RE: Can somebody help me with this scavenging dilemma?

Those shapes produce vortices because they are orifices, not because of the shapes.

Round is ideal, because you want to the flow area/surface area ratio, but you're talking veeeeeeeery small changes in power required to pump air through two tubes with the same cross sectional area but different shapes.

The effect of dents on exhaust flow has been studied, and found to matter very little. A 7% reduction in flow area would be noticeable if the entire header was changed, but for one dent in one of six tubes, and with the bulk of the path having the larger flow area, that dent matters very little.

RE: Can somebody help me with this scavenging dilemma?

jgKRI,

50+ bar ?! Really? I thought four-strokes were typically closer to 5 bar at the end of the expansion stroke when the exhaust valve opens.

Gary,

A lot of folks are into tuned pipes, but I'm not so much. Fixed tuned systems are peaky and will only operate ideally at a fixed engine speed (this is one reason why two strokes are so peaky). It makes a lot of sense to mechanically tune an engine that runs over a very narrow range of RPMs, but much less so to tune a typical car that operates over a wide range. Engines with close gear ratios, racing engines, etc. may all benefit noticeably, but I don't think a common road car does. Just my only slightly informed opinion. Perhaps I'm overestimating the sharpness of the mechanical resonance.

Rod

RE: Can somebody help me with this scavenging dilemma?

Quote (Gary_321)

Yes, but the cross-sectional area is 7% less.
I can make a square pipe with a cross-sectional area twice that of a round pipe... you're making the incorrect assumption that I was talking about a square pipe that would fit within a round pipe. That's a bad assumption. A 2" diameter round pipe has a cross-sectional area of pi... but I'm sure you could come up with a rectangular pipe that would meet your ground clearance requirements while still having a cross-sectional area of pi.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Can somebody help me with this scavenging dilemma?

(OP)
MacGyver - I think we are talking at cross purposes. The crushed pipe that I am referring to is part of the muffler section...





The crushed section is 200mm long. I made the reasonable assumption that the cross-section is oval.

I measured the wall thickness as 1.5mm, the major width as 71mm and the minor width as 46mm. This gives a cross-sectional area of 2299 sq mm.

The pre-crushed pipe is 59mm OD. It's cross-sectional area is 2463 sq mm.

My modification would be to cut out the crushed section and replace it with a 200mm section of 56mm ID pipe.

RodRico & jgKRI - Here is the data that I have been using for my calculations...

RE: Can somebody help me with this scavenging dilemma?

(OP)
jgKRI - One of my headers (#6) has a 55mm crushed section that reduces its cross-section from 962 to 800. There's not much that I can do about this, unless I want to convert the car to left hand drive 3eyes.




Re the bucket test, I found that a triangle created a vortex most easily, but if I suppressed the vortex, it flowed the quickest (marginally). Every triangle test created a vortex. Circle created a vortex the least number of times.

Just for fun, here's the data from the test...

Orifice Shape (mm) - Time (no vortex) - Time (vortex)
Circle (15.6) - 36 - 42
Square (13.8) - 36 - 52
Triangle (18.2) - 33 - 52
Oval (25.6 x 8) - 38 - 48
Oval (37.6 x 5.25) - 34 - 50

RE: Can somebody help me with this scavenging dilemma?

Yes, and my point was why replace the crushed section with more round... if you feel you need more cross-sectional area, just replace it with a rectangular section with the desired area (46mm x "as big as you want"). I think we've established it's not really going to gain you much, but my point still stands.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Can somebody help me with this scavenging dilemma?

(OP)
That's an option - and a good one, as it will maintain the ground clearance thumbsup2.

RE: Can somebody help me with this scavenging dilemma?

Gary,

I had a moment to do a bit of research, and it only reinforces my view that you likely aren't going to get much gain for all your trouble.

The website www.epi-eng.com is a *wonderful* resource for all things related to engines. They walk through header theory at http://www.epi-eng.com/piston_engine_technology/ex... . Note, however, that every application cited is a high RPM race engine that flows a *lot* of air, does not employ a catalytic converter or muffler, and certainly doesn't use Exhaust Gas Recirculation (EGR) to limit emissions of NOx.

The article at http://www.hotrod.com/articles/header-basics/ does a pretty good job of explaining how pipe length, diameter, and collector design shift the torque peak around. They even go a bit into how one can shift peaks from different tubes to get a small gain over a wider RPM range. The article very specifically brings up the purpose of the headers, scavenging, and mentions that catalytic converters and mufflers will have the expected detrimental result. The article *doesn't* mention that modern cars can *never* attain anything close to ideal scavenging as the intake charge is *intentionally* corrupted with exhaust via EGR to aid in reducing combustion temperature and NOx.

Finally, there are some free calculators at http://www.speed-wiz.com/calculations/engine/heade... , and what appears to be some moderately comprehensive software for $75 at http://www.maxracesoftware.com/PIPE395.htm . As the epi-eng article notes, however, professionals use much more complex CFD/thermal analysis to design their exhaust systems, so I can't say whether these calculators are really worth much effort.

I have always been highly skeptical of headers on a comparatively stock engine with EGR, catalytic converters, and mufflers; there's a lot of money being made selling headers into street legal cars (read emission compliant with mufflers), so I suspect few want to mention it. If you must proceed, I hope you find the references above to be useful.

Rod

RE: Can somebody help me with this scavenging dilemma?

(OP)
RodRICO - I've done a lot of research in the last six months and got a copy of one of the "bibles" (Four-Stroke Performance Tuning" by A. Graham Bell). I've seen epi-eng and hotrod. I think that it was hotrod where I read that flow should be measured between "0.050in after open" and "0.050in before close" and not from "just open" to "just close".

I've not seen the speed-wiz calculator, but I have seen many others. I'll look at the page source and reverse engineer the calculations to see what they have done. I always like to go back to first principles so that I can understand the theory.

I have done calculations based on the measurements of my engine and my calculated tuned-RPMs are too high (by a factor of 10). I'll look at the speed-wiz calculator to see if I can get any sensible results before I start another thread about calculating header lengths (if I need to).

Thanks for the steer cheers

RE: Can somebody help me with this scavenging dilemma?

(OP)
I think the original question has been answered. I originally thought that there was only one type of scavenging (and that was inertia scavenging).

In the original video, inertia scavenging is possible with the header removed, because the back pressure is low and the flowrate is high. The back pressure in the full system is too high for the 15psi blower to generate a high enough flowrate to promote inertia scavenging.

I think that the wave produced when the valve is opened in the 4-second video can be heard using headphones. Unfortunately, this pressure wave would pass too quickly (and be too low a value) to be measured using a home-made manometer.

I was under the wrong impression that inertia scavenging was what determined header length. I think that I understand wave scavenging, but I've still got some reading to do.

Many thanks to all.

[edit] I've downloaded Speedwiz. It cost $30 and looks to be worth the money, but you need to input a lot of data. Luckily, I've been busy with my crank angle gauge, DTI and micrometer!!

RE: Can somebody help me with this scavenging dilemma?

I like this statement on EPI's main page:

Quote:

NOTE: All our products are ORGANIC, GLUTEN-FREE, CONTAIN NO GMO's, and will not upset anyone's precious FEELINGS or delicate SENSIBILITIES.

2thumbsup

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Can somebody help me with this scavenging dilemma?

(OP)
I've done some calculations and discovered that the pressure in the cylinder drops from around 125 psi to atmospheric in less than 50 milli-seconds at idle, and around 6 milli-seconds at max rpm.

I was planning to pulse some air from a compressor to test the full exhaust system, but there is no way that I could set up a rig that could open and close a valve in tens of milli-seconds.

I would then have to find some very sensitive pressure measuring equipment that could measure the reversion pulse wave in the other header tubes.

It's the sort of experiment I would love to have done for my thesis, but that was over thirty years ago. Unfortunately, I don't have access to that sort of equipment today.

RE: Can somebody help me with this scavenging dilemma?

Gary,

How about putting air at exhaust pressure behind a rotary valve? That doesn't seem *too* hard. Alternatively, you could use open source CAD programs and OpenFoam (https://www.openfoam.com/) to simulate what's going on. It would take some learning, but at least it would be free.

Rod

RE: Can somebody help me with this scavenging dilemma?

Or you could, you know, just actually use an engine for your testing. A bit noisy and smelly, but sometimes we have to suffer a bit to get to the final result.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Can somebody help me with this scavenging dilemma?

(OP)
I don't think that I would be able to source the equipment for a reasonable cost. I could have a go at making a rotary valve and use an electric drill and compressor. I don't think that I would be able to sense the reversion wave, let alone mesure it.

I've been busy since I downloaded Speed-Wiz. It's a fun tool. I recommend it as a tool for many aspects of car design, not just engine. I can't see the source calculations, but the answers seem reasonable.

There are some problems with it. The answers are imperial and some of the conversions to metric are obviously wrong, so I don't have 100% confidence that the inputs aren't similarly affected. All the inputs are based on you having access to a stripped-down engine and rolling road data, so some of the inputs have to be estimated.

It is very good for adjusting inputs to see their effects.

RE: Can somebody help me with this scavenging dilemma?

2
If you have some (significant) spare money, or access to a good library, excellent books for this subject would be the pair written by Winterbone & Pearson:


Design Techniques for Engine Manifolds: Wave Action Methods for IC Engines Hardcover – 28 Jul 1999
by Desmond E. Winterbone (Author),‎ Richard J. Pearson (Author)

Theory of Engine Manifold Design: Wave Action Methods for IC Engines Hardcover – 29 Sep 2000
by Desmond E. Winterbone (Author),‎ Richard J. Pearson (Author),‎ & 1 more


If you want to capture the effects of exhaust design and layout in a simulation, you will need to at least get hold of a 1-D simulation tool. There are some free options. A google search using terms like "1d" "free" "engine" and "simulation" produces many useful leads. Good luck!

Steve

RE: Can somebody help me with this scavenging dilemma?

(OP)

Quote (SomptingGuy)

Theory of Engine Manifold Design: Wave Action Methods for IC Engines

PDF requested from ResearchGate.

Thanks for the steercheers

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