The two tuned lengths of the 2 stroke expansion chamber, photo of compression wave formation. E3
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- Опубліковано 6 вер 2024
- There are 2 tuned lengths in a 2 stroke expansion chamber not only the port to baffle cone taper.
This video explains why that is and how the timing of both compression wave and expansion wave is critical to peak power.
Also showing the image of the compression wave formation at the end of an open exhaust pipe.
I provide a simpler calculation to that found in the works of Gordon Jennings and Graeme Bell that does not require conversion to Metric or Imperial and is self explanatory.
Freeware 2 Stroke Expansion Chamber Exhaust calculator program
Download at birdcagesoft.com
(Page link on sidebar at site)
#2Stroke #ExpansionChamber
I'm loving these videos mate! I'm a mechanic myself and getting into 2 stroke and i love learning this stuff! Awesome knowledge mate!!
Thank's Jimbo, are you in the Bike or Car trade ?
I spent most of my years in car trade simply for the higher pay, but working as mcycle mechanic had benefits when I was racing and enjoyed it far more.
Working for small bike shops it was better than car workshops, free shop bike to ride was another benefit.
AuMechanic that’s awesome mate. I’m in the car trade, i’m a senior technician at a Toyota dealership but worked around a lot in the last 20 years. I don’t have a bike but have mates that do. I have 2 stroke rc cars and am about to put a tuned pipe on a big 1/5th scale petrol 2 stroke truck. It’s extremely interesting learning what you’re teaching as it fills the gaps on my knowledge of 2 stroke performance!!! Amazing stuff. Very well presented videos mate I’m thoroughly enjoying them!!
Glad you're interested.
Thanks a lot für These Great explaning Videos, easy to understand even für non english speakers. Best greatings from Germany!!
Thank you for the comment.
Hi! I've been watching your videos on 2-stroke exhaust calculations with my full attention, to try to understand the variables and how they relate / effect each other.
In this video there is something that I don't understand:
Where, inside each cone do you consider the end of the tuned length? Is it in the middle of each cone's length?
Thanks. Keep up the good work!!
(I'm actually surprised that you don't have many views, because your content is much better than most I've seen in UA-cam on this subject and there are so many 2-stroke fans out there - their loss, I guess)
Hi Jose, thanks for comments
The tuned length is taken from ex port window to half the length of the full taper. That is basically half the straight line length of the complete cone shape.
So with the baffle cone you would draw a cone that ends in a point with no outlet pipe, and then measure the straight line length from to tip of the cone to the base, and half of that length is where to take tuned length from.
I mention this again in latest video E7 ua-cam.com/video/n5AvG-hDvY8/v-deo.html at 1:47
Re views in, YT doesn't like tech lecture videos like this, it only promotes entertainment type videos as far as I can tell over the last year or so.
the length starts at the piston side of the port correct? thats where i always went by. I got into pipe building for a while. i made about 5 different pipes for my kx. i think by the fourth or 5th using a 3 stage diffuser i finally surpassed the pro circuit it originally had. probably only because the pro circuit i designed for stock port timing and i was aiming for full race. if i remember correctly my port timing was 135in 205ex and i made power around 10000rpm. i used a few software programs RacingSM software was a great program.
Yes the tuned length is from the port window itself where the primary compression wave starts at blow down period.
There are some basic starting numbers to use and wave timing is not hard to calculate, but to accurately calculate things like diameters of mid section, header and tail pipe requires software as the calculation to arrive at that are very lengthy so software is needed to design a pipe accurately for sure.
Re the port timing, was the KX a reed intake, was just wondering if the 135 was the transfer.
ya transfers at 135 and the rear boost port was 140.
thank you for your reply
arvo
Cheers
For some reason video E4 and E5 are missing form your page. Thanks Rick .good info I use to build adjustable pipes. The stinger could move to change length.
Those other vids were mostly just covering the software project which is has changed since and Ill be doing a downloadable version.
Outboards use a "sliding" expansion chamber. I believe they shorten the pipe for torque, lengthen for RPM. Can you explain this?
Adjusting the length changes to "tuned length" as covered in the videos.
A shorter tuned length will be tuned to higher RPM
thank you dave for your reply
i realise that making the pipe longer you will loos top end power as the volume and distence won't be accurate but would it increase power at a lower RPM?
thanks again for taking the time to reply
i always though that if i took a standard pipe and increased the length of the middle section it would change the power to a lower RPM
No, the pipe will just be mismatched to the engine.
The length has to be whatever the engine exhaust port timing dictates.
if i won't more bottom end is the is the pipe longer then for top end power
Hi Arvo
The power characteristic of the engine is largely determined by the port timing and other factors like compression ratio etc.
Once that is established the chamber is design to match that.
Tuned length matches the RPM range the engine is design to operate in.
So having a longer pipe for low RPM on an engine with peak at high RPM engine will just result in mismatch and less power.
The shape of the pipe likewise has to match the range of power the engine is designed for.
hi dave to achieve more bottom end power the shape of the exhaust port end taper is narrower as shown in your drawings is the tuning length from exhaust port to stinger shorter or grater ?
Tuned lengths using Blairs equation is taken from port window at the piston to the end of the baffle cone.
For Jennings from port window at piston to half the full length of the baffle cone.
Bell simple copies Jennings.
The disparity between the two is something I will cover later in the pipe design series.
Those are the tuned lengths of the baffle cone position relevant to the port window, the baffle cone dictating port plugging at peak that should chop power off over then RPM if length is tuned correctly.
The other tuned length not covered by either equation is the tuned length of the first Diffuser cone that determines the arrival time of the expansion wave or negative pressure pulse.
The equations assume the phasing of both diffuser and baffle cone to be fixed, this is something I do not agree with and believe the equations should embrace both tuned lengths.
But at this stage I am covering the equations as per Blair specifies and not my own opinions.
What goes into determining the correct length and size and shape of the whole pipe?
Watch the rest of the pipe series that covers all of that.
Hello, I wanted to ask you a "purpose only" styled question. I'm not sure I would do this but wanted to ask the theory behind it.
In the enduro world the expansion pipes eventually get dented and damaged unless you guard them. My question is if you built a pipe that had a longer header that was close to the bike and "up" out of the way then added the diffuser and the rest of the system also up out of the way. Would you agree that the bike would run sufficient enough for low speed technical riding. Also would this style of system cause hardship in anyway to the engine ? Thank you in advance for any input ... much appreciated :)
As long as the pipe dimensions and by that I mean the diameters and lengths of each section were right then there would be no problem, and if you are not worried about power the only part that can cause an issue for the engine is the outlet pipe if it is too restrictive.
Making a longer header you could just take some out of the straight neutral chamber section to ensure the pipe has the same tuned length and not suffer a huge power loss.
I have a later series that covers the equations used to design a pipe and am about the upload a free pipe design software shortly that makes the job a bit easier.
Hello @@AuMechanic thank you ... If I fabricate this style with longer header I'm under the understanding it will increase low speed torque output. I would utilize a slightly more sharp than slender diffuser angle and keep the tuned length as close as possible. But as I understand there is no exact approach, test and tune to achieve your desired result as is part of the fun. Your reply has helped quite a bit cheers
@@outbackenduro7418
You can leave the diffuser as is and just cut out some of the flat neutral chamber to offset the extra header length. Making a sharper angle on the diffuser will make it more peaky which is counter to the longer header.
That was the basic approach using Gordon Jennings equation from the 70's
Blair equations from 1996 differ a bit in that the mid point of the diffusers timed returning expansion wave the same (around BDC) regardless of the other variables.
See this video ua-cam.com/video/5fojBHZNAWM/v-deo.html
Hi @@AuMechanic a wealth of knowledge, thank you. I will start my research now appreciate your input
I saw in another video you said something about using a lower rpm for the diffuser than the baffle for targeting tuned length?
That's right, the baffle cone dictates power at peak, the diffuser OTH has to provide the effect over the an RPM band and up to the peak over it length.. So the far end of the diffuser is the lower end of the range and the nearest end to the engine is the top of the effective range.
The baffle cone OTH plugs at peak to increase peak power and then effectively does the reverse and stops it revving any higher, it is the rev limiter in effect.
With the baffle cone it will not have a sudden cut off up top, but will also have a lower peak power.
Keep in mind the baffle cone is up to double the angle of the diffuser cone so a very short duration.
Ill be doing some more detail in the diffuser timing, I'm just adding some bits to the model software to display it visually so you can see where the reflection points are at given RPM and how the centre point of the wave moves its alignment as RPM rises.
AuMechanic thanks you ..cant wait to get off work to watch your new video and learn more....if u could please edit your last comment...clean it up a bit it confuse me a bit...just check it over please. Thanks for all you do I learn more from you than any other source
I have only put basic points in the comments that does not fully explain it and yes may be a bit confusing.
It is probably best to cover it in the video in some detail so it is much clearer to understand.
This new video I have just done is 3 equations for measuring the taper angle of the cones that may be useful to measure a chambers tapers of unknown angles.
Thanks mate, just the sort of info I was looking for.
Do the same calculations apply to using different fuels, like methanol/nitromethane in radio control aircraft?
Yes John the same equations do apply to any 2 stroke with piston port exhaust and crank case induction.
The difference with fuels is in the tuned length calculations where the sonic velocity in exhaust gases varies from gasoline but only slightly.
@@AuMechanic
Thanks for that.
I just converted my RC helicopter 15cc glow ignition (methanol/nitromethane) to petrol with a CDI spark ignition and was hoping I could use the same pipe.
If the exhaust port timing and port sizes (area) is different then you will need a pipe to suit that, tuned lengths is not the only parameter.
See some of the later videos in which I cover the equations used to design a pipe and I also have a software available that does the same equations.
Hi, thank you for this videos , they are really helpfull. As a general rule i undersand that having a longer pipe between exhaust port and the divergent cone help for low rom . How will look like an exhaust for a minareli am6 with 80cc cilinder 22mm carb for as much torque as posibile and as wide as posibile power band ? I agree to sacrifice all the power above 9k-10k rpm If needed . I want to get rid of that motocross feeling and get a more usuable power band between 5k-10k .
Regarding the header pipe length, what you stated there about longer headers equals more low end, it is from very early book and pipe equations by Gordon Jennings from the 1970's and was created by him measuring numerous pipes and some reverse engineering to arrive at his theories on pipes..
If you watch the series of videos here they are based on later more scientific SAE published studies done by Professor Gordon Blair published around 1996 that provides pipe equations used in the free software I have available.
In that, more low end or wider power spread requires a few factors in the whole pipe design more than just the header pipe, as you will see if you use it.
I have a older jet boat. Inline 4 cyl 2stroke an they all share a single manifold an just feel like it’s a horrible design an possibly a waste of fuel an more so power. I am limited on space a little bit in comparison to say 4 single 400cc dirt bike exspansion chambers to be able to fit in there so I’m looking into making my own. I’ve gotta pretty good at welding an tig welding. Now looking for more info on the actual leaghths to see if with my limited space if I’m able to achieve performance gains with four short chambers or if I’m best off with the current no chambers design it came with in 1999 an also all 2stroke boat engines fer that matter. Still makes no sense why they did that. I run 3,000 rpm when I hit throttle an up to 5,500rpm but most the time I’m at 4,500-5,200rpm so it’s still a very narrow window witch wat I heard is they don’t run them because they are for a narrow window of rpm we’ll boats are in a narrow window of rpm. I’m wrkn with 1,000 rpm so wouldn’t chambers be a great way to add some usable power
Ive replied to this in the other video comment (here ua-cam.com/video/4_DmKcJieNg/v-deo.html&lc= ) but just to reiterate that noise and emissions are other reasons for the stock exhaust.
An expansion chamber can be designed to provide broad power but it come as the expensive of peak power.
Re lengths, have a look at the later videos covering "tuned length"
How do you calculate the 2nd(diffuser) tuned length? Whats the formula
You dont need to using the caclulator software.
Showing there are 2 tuned lengths is just to demonstrate the principle of the 2 differring wave reflections in the pipe being the expansion wave from the diffuser and the compression wave off the baffle cone and there different arrival times back at the exhaust port.
@@AuMechanic so you cant calculate the 2nd tuned length without the software? Just like how the 1st tuned length can be calculated with jennings and bell's formulas
@@wrongchannelsearchfordeank6834
You can calculate it just the same way, but for the purpose of designing a pipe it is not needed.
Bells equations are a metric derivative of Jennings for the late 1970's
What I use in the software is Blairs equation from the late 90's that was created using far more scientific methods than Jennings.
Basically Jennings specifies lengths of the sections where Blair provides ratios to determine the lengths that you enter into the software.
@@AuMechanic thank you i truly appreciate your explaining
Great stuff mate. I’m in the process of designing a pipe for a vintage MX 125 and I’d like to verify my design with these concepts. What formula should I use to arrive at the tuned lengths for these two tuned lengths ? For the port blocking do I use the degree open of ex port to closure of transfer port ? In my case it would be half of 190 (ex) plus half of transfer 130 equaling 160 degrees.
Have a look in the description section for the link to my free Pipe Calc software that can do the calculations.
Transfer timing is not part of the tuned length calc, it is based on Ex port timing only.
Sorry , night shift haze . Port plugging is ex port open to port close . 190 degree in my case however expansion wave from BDC to TFR close I take it . Question 2
If Approaching BDC piston movement is Deminishing for every degree of rotation. This would mean crankcase compression is also reducing. Maybe the inertia of the gas flow counters what I’m about to say but wouldn’t it be good to get pipe suction just before BDC ?
Donated to buy you a drink , Cheers
@@thirdstone4107
Thank you, much appreciated.
Hope you find the software useful.
@@thirdstone4107
Re question 2. That is the case, the expansion wave (AKA suction wave) that comes from the diffuser cone should arrive around BDC. See this video here ua-cam.com/video/5fojBHZNAWM/v-deo.html
3 min 44 you say "sometime between that point". do you go into it in more detail on any of your videos? also at what point of the combustion stroke do the gases exiting the exhaust port not carry a pulse? cos they only fire out of the port for so long.
and when exactly do you want the suction pulse to stop in relation to the transfer . just as its closing or a tad before? also i would love to see the demonstration with numbers that you refer to that you say on the next one but i cant find it. great videos.cheers
I cover this a bit more in other videos but specifically or at least in the case of the Blair Design Chamber the peak of the Expansion wave needs to return around BDC and continue to through the period of upstroke while the transfer is still open to a fair degree to aid transfer.
When the transfer is nearly closed there is little served by having the pulse timed to move only a small amount through the remaining small opening, keeping in mind the pulse of the expansion wave only last for so long.
The exhaust "pulse" you refer to from exhaust gases occurs only at the short moment the exhaust opens at the blow down stage, being created by the initial pressure differential causing the gas to pass through the opening at very high speed which causes the compression wave (you can think of as a sound pulse) to travel at the local speed of sound down the pipe which is a lot faster than the bulk of the exhaust gas.
See later video named "Active Demo of Wave" might explain bit more.
so once the piston is opening the transfers on its descent that's basically the end of usable pulses coming FROM the exhaust port?so that's the point in crank rotation that I have to work out how fast the pulses travel and are sent back to do their correct jobs through cone convergence or divergence?I know I could just enter the details into your calc but I still would like to know whats happening. initially my first thought is, having a longer blowdown (till the transfers open) gives me a longer pulse to play with. and if the pipe is good enough it could actually outweigh the disadvantages of a longer blowdown re less scavenging.?
There is one "pulse" coming from the exhaust at the moment it opens being the initial compression wave that then travels down the pipe, the second one being the expansion wave and it is timed to be reflected back from the diffuser taper back into the exhaust port, the peak of it arriving around BDC.
Blow down period is not directly related to the pipe function, blown down is the period between exhaust open and transfer open, and blow down has to be long enough to ensure enough pressure blows down out the exhaust before the transfer opens or an excess of exhaust will be pushed back down the transfer when it opens against the fresh intake coming up the transfer to the cylinder
really only one pulse? so when is the exact time of this pulse for working out when it needs to return? I cant get my head round when this happens. it cant happen as the port opens because its only a fraction open, or does the pulse have a duration? ie starts as the port opens and ends a few degrees later?
@@aljaxon69
One positive pressure pulse is created, but you end up with 2 pulses, one is a reflection form the first cone and the other is the original pulse that bounces of the second rear cone.
"does the pulse have a duration? ie starts as the port opens and ends a few degrees later?"
Yes that is pretty much it.
The positive pressure pulse (compression wave) moves at the local speed of sound inside the pipe which is predominately determined by gas temperature in the pipe it is travelling through.
As the initial compression wave passes the first diffuser cone an expansion wave (you can think of as suction pulse although that is not technically correct) is reflected back arriving at around BDC to help draw fresh mixture into the cylinder from the transfers..
The total length of the pipe (tuned length) is set to ensure the pulse (compression wave) travels down the pipe to the end cone and back again within the exhaust port open period at a given RPM.
It's job is to push intake mixture that has spilled out the exhaust port back into the cylinder, called port plugging.
Would expansion exhaust be benifital for a rotary engine ?
In short, no.
The long answer as to why it wouldn't help is that an expansion chambers relies in a pressurised intake such as a 2 stroke has, the crank case creating a compression of the intake mixture that is then fed to the cylinder via transfers.
A rotary or any normally aspirated engine lacks that.
It would work with a super charger and has been used but as the super charger pressure can be dialed up it negates the need for an expansion chamber that would only work over a limited RPM range anyway.
@@AuMechanic do you know of any rotary exhaust tunning science, like matching extractor lengths to engine port sizing?
@@shaunessex3095
Never built an exhaust for a rotary only 4t and 2t piston engines so I don't have the experience to give advice on it.
A general guideline to matching ex port to header is aim to have a the same or slightly large header than port, but never smaller header.
And always use mandrel bent sections for bends to maintain the cross sectional area of the pipes.
I've always been in the understanding rotary exhaust requires a sertain amount of back pressure but not many people mention it though, rotaries are essentially 4 cycle 2stroke simular animals ,but the gas tem
What demonstrations
Which demonstrations are you referring too ?
Gas;ikrd
Traduza para portugues!
Can I reduce the header length by 90% & compensate in My baffle cone & chamber dimensions?
Not if you want to pipe to work correctly.
A small change in the header is ok but not 90%
@@AuMechanic
So why is a motocross pipe header so much shorter than a header on a street machine like the H1?
I just wonder if the software will let Me start the divergent cone within 3" of port instead of 12" before diverging, if I overcompensate in other areas to possibly increase overall performance.
One thing is certain, I will be moving redline from 9000 to 12,000 rpm, so many factors at work here.
Thanks for taking time to answer.
What would be the broad effect?
A total loss of power or merely narrowing the power band?
I thought of taking an 01 CR250R pipe apart, & reverse engineering 3 prototypes & just bolt em on to see what changes good or bad occur, then rechunking the figures in the software to establish a baseline.
@@AuMechanic
Not much for kicking ideas around eh?
Answer Me this then, all things being equal what would be the effect of a 250cc pipe on a 125cc & vice versa?
@@Not1Edit "So why is a motocross pipe header so much shorter than a header on a street machine like the H1?"
Because the header is quote tapered compared to a street or race pipe for example that operates in a more narrow power band.
In short, longer cones equals wider power band at lower peak. shorter straighter cones are shorter power band at higher peak
@@Not1Edit Got as full dance card these days.
As to 250 125, the pipe dimensions are in proportion to the port window area which itself is in proportion to the engine capacity, so a smaller bore and stroke will have a smaller port window which will result in a smaller pipe using a pipe calculator.