Thanks for the shoutout! I just discovered this video and episode3. The youtube notification system doesn't work very well it seems. Anyways, good stuff!
Yes the notification system is bogus, it shows it to a few subscribers and if they don't view it then it doesn't show it to anymore. I've got subs from many years ago and they are not likely to view it so that doesn't help. I plan to do 2 or at least 1 around the weekend. Cheers.
This reminds me a bit of tuning a bass reflex speaker port for maximum low output based on the resonant frequency of the enclosure. Fascinating, thanks.
Not really Expansion chambers don't operate on resonance like a pulse jet or the like does. They shape of them reacts to a single compression wave as the exhaust port opens to produce the effect. Resonance (as in Helmholtz) applies to highly specific frequencies and harmonics of them where the expansion chamber exhaust operates over a wide range of frequencies over the range of engine RPM.
UPDATE: Was going to upload E3 but need to do another edit of it and its 3:30 am here so Ill get it done tomorrow. E3 covers calculating tuned lengths and shows two equations used in publications that are confusing, one for imperial and the other for metric. I will show a simpler self explanatory equation you can use that can use tuned length in inches or millimetres it doesn't matter. And shows why there are two tuned lengths values required not just one to the baffle cone and how to calculate both.
Hi Sean, might be an issue if the port inst chamfered right but otherwise they work fine. That is an IAME race piston from DAP or Parilla rotary disc 100 reed Kart Engine, revs to 20,000+ no problems.
Great videos that explain the science. I am an old mechanic that has stumbled into the two stroke world with a stock Aprilia RS125. I've lowered squish/raised compression and the bike has so much mid range torque and acceleration now. But... peak power is now down 1,000 RPM (11,500 down to 10,500 RPM), which obviously reduced the top speed. I am guessing the scavenging and pulse characteristics have changed? Would appreciate your thoughts on this. As I want to keep the bike as stock looking as possible, my next step was to experiment with some stinger inserts and other elements of the exhaust.
If you lowered the whole barrel to effect the changes then you will have reduced the port timing and that would favor low end power at the expense of peak power. If on the other hand if you have milled down the head to get higher CR and the barrel height is not changed then it may be the result of reducing the squish clearance is that the squish velocity is now too high at the higher RPM. When making changes to the squish clearance or angle to piston or percentage of the combustion chamber it is important to calculate what the squish velocity will be to ensure it remains within reasonable bounds. I will be doing a video on squish band and velocity soon.
@@AuMechanic thanks so much! I forgot to mention I shaved a bit off the head. I didn't want to affect the port timing. A squish video would be great! I still can't get over what 0.5mm achieved.
It is not just the clearance but also the angle of the squish area in relation to the top of the piston as well as what percentage of the chamber is taken up by squish band all of that and RPM determines what the squish velocity is. The simplest way to describe what squish velocity is would be that when you stamp your foot in a puddle of water causing water the shoot out from the gap between your foot and the ground, the speed that water shoots out sideways describes the squish velocity being the mixture shooting out of the squish gap into the combustion chamber bowl as the piston rises to top dead centre, the speed of that should generally not exceed the speed of the flame front heading in the opposite direction outwards from the spark plug when combustion occurs. There is an equation Blair provides to calculate squish velocity in which you can input a load of values measured and the resulting output value being squish velocity should be around 15 to 20 metres per second.
Hi. Thank you for these very interesting and in depth videos of some of the many different things going on all the time inside the 2stroke engine and exhaust system. I have a question well actually two about power valve systems, which I don't know if you have covered elsewhere? 1, I understand that the main types of power valve systems are the ones that control port height and area, and the other types that control exhaust resonance and effectively exhaust system volume. But why don't all more modern engines use both systems together which I assume would always be more effective than one strategy or the other? Is it because it's counter productive to try and control different things at the same time? 2, Do the resonance control types of power valve work by either effectively giving you a low rpm suitable expansion chamber, as well as a high rpm matched one all within one pipe design? Or is it that they don't really tune to pipe to work at lower rpm, but more nullify the effects of a pipe that's designed to work best at high rpm, by making the pipe so much bigger that it doesn't have any resonant frequency effects at all either good or bad, so that the engine behaves like it's got no expansion chamber at all until it reaches higher rpm?
Great explenation !!! As i understand, the goal is to create a counterforce of the exhaust gases to avoid the lost in exces of fresh air/fuel mixture in the scaveging process thru the exhaust port. Is this right???
There are 2 phases. The first is to aid scavenging by by having a lower pressure timed to arrive back at the exhaust port around BDC (which is the job of the first diffuser cone) and the second is to have a higher pressure timed to arrive at exhaust port just before the port closes (that is the job of the second baffle cone) to reduce fresh intake from spilling out in to the port.
G'day, thanks for your no-fuss explanation of what happens in an exhaust and why the length of it must be tuned. But what I'm stumped on is whether or not a bend in the exhaust would make a difference to the tuned length or to the behavior of the sonic waves and if not, why? Basically, does the exhaust pipe have to be straight or can it be bent?
Good question. In theory bends to not interrupt the sonic wave flows but only so long as the area (diameter) of the pipe does not change more than a few degrees because any such change will cause a reflection of a compression wave that will not return to the port at the intended time. Bends will effect bulk gas flows which is more of an issue the closer you get to the cylinder where the gas pressure is very high at the initial blown down after the port opens before it has had time to equalise in pressure farther down the pipe. This is a problem because generally the most acute bend is right up at the engine so for that reason the bend in the header must be clean, free of any interruption in diameter and certainly are progressive in change of angle and not a tight bend at one single point, preferably a short straight length (50 mm or so) right where it ends at the head before bending. As to sonic flows, in reality any sudden change in bend angle even done clean with no changes in diameter will create unwanted reflections so for that reason like above very important to have a pipe with a smooth bend and free of any defect internally like weld slag etc. If the header is made from straight tubing it need to be mandrel bent to ensure no pinch of crush point like normal tube bends. If it is made from welded sections each weld needs to be back ground inside with a die grinder to remove any signs of weld bead inside. And important to have it perfectly matching the port at the header flange. I might do a separate video on this issue.
you may have explained in another video.in a two stroke exhaust port with one large port and two boost ports does the pulse go thru the larger main port or does the pulse also go out the boost ports thatare much smaller.on my motor the boostports are 1 mm higher than main exhaust port.as i am going to build a drag race snowmobile engine and going to port it myself so i am trying to do some reaserch.
Whichever ports open first will begin blowdown and the compression wave. So tuned length will be based on the port timing of that. But for pipe diameter purposes re port area, include all exhaust ports in to area value as that is for bulk flow.
Thanks for the video! I have a question: can lowering the exaust port (due to closed valve) increase power at low rpm? Or will the effect of 'increased traped compression' not be noticable. In other words: on engines with adjustable exhaust valves, can power in low revs be increased by locking down the valve? I understand that this would depend on the pipe, but does the decreased diameter still matters much at low rpm? Thanks!
Both of those factors will increase mid range power by trapped comp and wave timing, at the expense of top end if you lock the valve shut. As to the reduced area of the port with the valve shut, that will effect the port to pipe ratio. Normally a larger port to pipe ration broadens the returning wave making it effective over a wider RPM range so that might tend to smooth the curve a bit in mid range which is not a bad thing.
Can u tell me the exaust chamber dimentions for my bike its a 100 cc 49x51.8 its a kb 100 rtz as its a rare bike here soo theres no specific chamber for this bike ill give u every information u want
Hi The latest video series I am doing shows how to design an expansion chamber in steps that are easy to follow. See the chamber design playlist on the main channel page
@@luist6635 Sorry but I got some stuff going on at the moment I have to get through (looking after a seriously ill friend) so wont be doing any work for some time. Maybe later in the year.
Thanks for the shoutout! I just discovered this video and episode3. The youtube notification system doesn't work very well it seems. Anyways, good stuff!
Yes the notification system is bogus, it shows it to a few subscribers and if they don't view it then it doesn't show it to anymore. I've got subs from many years ago and they are not likely to view it so that doesn't help.
I plan to do 2 or at least 1 around the weekend.
Cheers.
AuMechanic Of course... Had to be an "algorithm" for that too. Ok, then I'll remember to check in on the weekends!
This reminds me a bit of tuning a bass reflex speaker port for maximum low output based on the resonant frequency of the enclosure. Fascinating, thanks.
Not really
Expansion chambers don't operate on resonance like a pulse jet or the like does.
They shape of them reacts to a single compression wave as the exhaust port opens to produce the effect.
Resonance (as in Helmholtz) applies to highly specific frequencies and harmonics of them where the expansion chamber exhaust operates over a wide range of frequencies over the range of engine RPM.
UPDATE: Was going to upload E3 but need to do another edit of it and its 3:30 am here so Ill get it done tomorrow.
E3 covers calculating tuned lengths and shows two equations used in publications that are confusing, one for imperial and the other for metric. I will show a simpler self explanatory equation you can use that can use tuned length in inches or millimetres it doesn't matter.
And shows why there are two tuned lengths values required not just one to the baffle cone and how to calculate both.
I use those L shape rings in my kawasaki 125 rotary valve engine and was told they woukd be trouble but so far so good
Hi Sean, might be an issue if the port inst chamfered right but otherwise they work fine. That is an IAME race piston from DAP or Parilla rotary disc 100 reed Kart Engine, revs to 20,000+ no problems.
Great videos that explain the science. I am an old mechanic that has stumbled into the two stroke world with a stock Aprilia RS125. I've lowered squish/raised compression and the bike has so much mid range torque and acceleration now. But... peak power is now down 1,000 RPM (11,500 down to 10,500 RPM), which obviously reduced the top speed. I am guessing the scavenging and pulse characteristics have changed? Would appreciate your thoughts on this. As I want to keep the bike as stock looking as possible, my next step was to experiment with some stinger inserts and other elements of the exhaust.
If you lowered the whole barrel to effect the changes then you will have reduced the port timing and that would favor low end power at the expense of peak power.
If on the other hand if you have milled down the head to get higher CR and the barrel height is not changed then it may be the result of reducing the squish clearance is that the squish velocity is now too high at the higher RPM.
When making changes to the squish clearance or angle to piston or percentage of the combustion chamber it is important to calculate what the squish velocity will be to ensure it remains within reasonable bounds.
I will be doing a video on squish band and velocity soon.
@@AuMechanic thanks so much! I forgot to mention I shaved a bit off the head. I didn't want to affect the port timing. A squish video would be great! I still can't get over what 0.5mm achieved.
It is not just the clearance but also the angle of the squish area in relation to the top of the piston as well as what percentage of the chamber is taken up by squish band all of that and RPM determines what the squish velocity is.
The simplest way to describe what squish velocity is would be that when you stamp your foot in a puddle of water causing water the shoot out from the gap between your foot and the ground, the speed that water shoots out sideways describes the squish velocity being the mixture shooting out of the squish gap into the combustion chamber bowl as the piston rises to top dead centre, the speed of that should generally not exceed the speed of the flame front heading in the opposite direction outwards from the spark plug when combustion occurs.
There is an equation Blair provides to calculate squish velocity in which you can input a load of values measured and the resulting output value being squish velocity should be around 15 to 20 metres per second.
Hi. Thank you for these very interesting and in depth videos of some of the many different things going on all the time inside the 2stroke engine and exhaust system. I have a question well actually two about power valve systems, which I don't know if you have covered elsewhere?
1, I understand that the main types of power valve systems are the ones that control port height and area, and the other types that control exhaust resonance and effectively exhaust system volume.
But why don't all more modern engines use both systems together which I assume would always be more effective than one strategy or the other? Is it because it's counter productive to try and control different things at the same time?
2, Do the resonance control types of power valve work by either effectively giving you a low rpm suitable expansion chamber, as well as a high rpm matched one all within one pipe design?
Or is it that they don't really tune to pipe to work at lower rpm, but more nullify the effects of a pipe that's designed to work best at high rpm, by making the pipe so much bigger that it doesn't have any resonant frequency effects at all either good or bad, so that the engine behaves like it's got no expansion chamber at all until it reaches higher rpm?
Great explenation !!! As i understand, the goal is to create a counterforce of the exhaust gases to avoid the lost in exces of fresh air/fuel mixture in the scaveging process thru the exhaust port. Is this right???
There are 2 phases.
The first is to aid scavenging by by having a lower pressure timed to arrive back at the exhaust port around BDC (which is the job of the first diffuser cone) and the second is to have a higher pressure timed to arrive at exhaust port just before the port closes (that is the job of the second baffle cone) to reduce fresh intake from spilling out in to the port.
G'day, thanks for your no-fuss explanation of what happens in an exhaust and why the length of it must be tuned. But what I'm stumped on is whether or not a bend in the exhaust would make a difference to the tuned length or to the behavior of the sonic waves and if not, why? Basically, does the exhaust pipe have to be straight or can it be bent?
Good question.
In theory bends to not interrupt the sonic wave flows but only so long as the area (diameter) of the pipe does not change more than a few degrees because any such change will cause a reflection of a compression wave that will not return to the port at the intended time.
Bends will effect bulk gas flows which is more of an issue the closer you get to the cylinder where the gas pressure is very high at the initial blown down after the port opens before it has had time to equalise in pressure farther down the pipe.
This is a problem because generally the most acute bend is right up at the engine so for that reason the bend in the header must be clean, free of any interruption in diameter and certainly are progressive in change of angle and not a tight bend at one single point, preferably a short straight length (50 mm or so) right where it ends at the head before bending.
As to sonic flows, in reality any sudden change in bend angle even done clean with no changes in diameter will create unwanted reflections so for that reason like above very important to have a pipe with a smooth bend and free of any defect internally like weld slag etc.
If the header is made from straight tubing it need to be mandrel bent to ensure no pinch of crush point like normal tube bends.
If it is made from welded sections each weld needs to be back ground inside with a die grinder to remove any signs of weld bead inside.
And important to have it perfectly matching the port at the header flange.
I might do a separate video on this issue.
you may have explained in another video.in a two stroke exhaust port with one large port and two boost ports does the pulse go thru the larger main port or does the pulse also go out the boost ports thatare much smaller.on my motor the boostports are 1 mm higher than main exhaust port.as i am going to build a drag race snowmobile engine and going to port it myself so i am trying to do some reaserch.
Whichever ports open first will begin blowdown and the compression wave.
So tuned length will be based on the port timing of that.
But for pipe diameter purposes re port area, include all exhaust ports in to area value as that is for bulk flow.
Thanks for the video! I have a question: can lowering the exaust port (due to closed valve) increase power at low rpm? Or will the effect of 'increased traped compression' not be noticable. In other words: on engines with adjustable exhaust valves, can power in low revs be increased by locking down the valve? I understand that this would depend on the pipe, but does the decreased diameter still matters much at low rpm? Thanks!
Both of those factors will increase mid range power by trapped comp and wave timing, at the expense of top end if you lock the valve shut.
As to the reduced area of the port with the valve shut, that will effect the port to pipe ratio.
Normally a larger port to pipe ration broadens the returning wave making it effective over a wider RPM range so that might tend to smooth the curve a bit in mid range which is not a bad thing.
Thank you very much!
Can u tell me the exaust chamber dimentions for my bike its a 100 cc 49x51.8 its a kb 100 rtz as its a rare bike here soo theres no specific chamber for this bike ill give u every information u want
Hi
The latest video series I am doing shows how to design an expansion chamber in steps that are easy to follow.
See the chamber design playlist on the main channel page
Do you port motors for customers?
Not for some time now.
But my main job has ceased to be so I wont rule it out in future.
@@AuMechanic what's your email? Here's mine luistamayo0001@gmail.com contact me.i need some work done ASAP
@@luist6635
Sorry but I got some stuff going on at the moment I have to get through (looking after a seriously ill friend) so wont be doing any work for some time.
Maybe later in the year.
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