Exhaust Port Dyno Test At 203° - Will It Work? 100mph Moped Challenge

Hi, welcome back everyone. Back with another video so a little bit different. I got back into the theory of two-stroke exhaust pipes so I thought I'd like to show this. There's a wave viewer feature in Engmod 2T which actually shows the pressure waves as they travel down an expansion pipe or any pipe and I thought I'd show that. Do some simulations with a straight pipe to show why then modify it obviously not this pipe but to simulate something like this where you just have a diffuser section so you should still get that depression at bottom dead centre and then finally look at a full pipe and actually how that really really works really well and show the performance differences but also to look at the waves and hopefully explain how these waves work within an expansion pipe and that's what actually gives us that extra power that supercharging effect. Also check out AU Mechanic I've re-watched his videos getting myself back into the theory I'll leave a link in the description he does a really good series of how pipes work. Two-stroke stuffing's got a good one as well with some water in a bottle which is actually really good if you want to visualize it. Hopefully though the wave viewer I'm going to show today should help people visualize what's going on inside these very simple or very effective pieces of metal. Right a bit of an explanation it shows you basically the waves that are produced at any given point throughout the expansion pipe for a given piston position and you'll see this piston move up and down. We're really interested in the area where the exhaust ports open and also the transfer ports are open and we've got the pressure ratio down this axis so one is atmospheric pressure 0.5 and zero is obviously below atmospheric pressure and obviously above that is above atmospheric pressure. First of all you have three waves you have the superposition wave and that is green so in this case let's draw it there but it's made up of two waves a wave that goes right and a wave that goes left. So the rightward wave if for example this wave here which is going right and then conversely a wave that would come back down the pipe would be the left and again these waves are just a snapshot in time. These left and right waves are added together and they make up the superposition wave. The superposition wave is what you would be able to measure with a pressure transducer. The actual left and right wave themselves are in theory theoretical and they can only be calculated in simulation software such as EngMod2T which is what's happened here. However these constantly move and you'll see throughout the simulation and we'll look at where we get our negative pressure or low pressure area which helps draw on the exhaust port draw on the transfers to help scavenge the cylinder and pull that fresh mixture in and also get rid of the spent exhaust gases. But conversely at the end of the cycle you'll see where we get a higher pressure here which is effectively pushing fresh charge back into the cylinder. Okay a bit more about wave flow and direction so obviously we've got our left and right waves they're expansive and compressive waves so to start off with we're just going to look at compressive waves. So these compressive waves are always greater than one atm so you can have two types you can have a rightward moving wave so that's away from the piston towards the exhaust and it looked like that and you can have another one that goes leftward and again like that. Now in terms of direction of the wave direction so the wave direction going to the right would induce a gas particle movement to the right so the flow would be to the right and conversely a wave that's moving to the left would also induce a leftward flow in the gas particles. So that's the compressive wave and again they're always greater than one however conversely less than one as we saw before on here that is known as an expansion wave. So as I've already stated that's below one so you can have a rightward moving expansion wave would look like this and a leftward expansion wave would look like that. So in terms of direction so the wave is going that way however the flow it induces into the gas particles is the opposite direction so when we've got a leftward wave we get a rightward flow. So that's a little overview of what you see in the wave viewer and what it all comes down to and normally it's this green bit that we can actually measure so if you actually instrument an exhaust which I would actually like to do at some point this is what we'll see but bear in mind it's made up of the right and left would wave. This is the blue screen shown when you're doing a simulation in NGMOD2T so the green line is the instantaneous pressure at the exhaust port the red line is the in-cylinder pressure so as you can see through the blowdown phase with high peak amplitude here which then reduces down where it goes below atmosphere that's where it's sucking on the transfers and then again it increases in value as you can see greater than cylinder pressure so forcing some of that mixture back into the cylinder. So for the sim I used the file that came with NGMOD2T for a TZ250 it's putting out just under 45 horsepower note this is for one cylinder so now we'll have a look at the three different pipes and how the different shapes of the pipes affect the pressure waves within that. Right first up four-stroke pipe so pistons uncovered the exhaust port you've got the right compression wave traveling down the pipe there's no reflection so there's no leftward moving wave until it gets to the end of the pipe it then gets reflected back up as a left expansion wave you can see the superposition pressure start to drop and match that leftward wave as there's not a great deal of right compressive wave there and as the piston gets down to bottom dead center it's still above atmosphere it's now just dropping down and there is some below atmosphere pressure so there is a lower pressure there it should be helping to scavenge the cylinder however as the exhaust closes as you can see there's no um no higher pressure there to push some of the fresh mixtures back in and we should see the results of that when we compare all the paragraphs at the end. Next up we've just got a diffuser section so half an expansion pipe effectively so let's see what happens so we're going to get the blow down as we did before so we've got a rightward moving compression wave it's traveling down but immediately see this time the left moving wave there is starting to be starting to move below atmospheric pressure and that is as the diffuser action starts to act as a open pipe not quite as abrupt as the previous pipe and then when it gets to the end of that pipe you can see the left moving expansion wave moving up and it's pretty much pretty well time for bottom dead center so it'll be sucking out quite a bit of fresh mixture helping scavenge the cylinder but the problem we're going to have this time is again there is no return wave at a higher pressure to then reverse from that mixture and help trap it in the cylinder with better trapping efficiency and more power. Okay and thirdly we've got the full expansion pipe this time so blow down rightward moving compression wave travels down the pipe at the exhaust port down the header and continues down you can immediately see there is a left hand expansion wave starting to generate a lower pressure at the exhaust port to help scavenge as the exhaust as the piston descends to bottom dead center but this rightward wave compression wave still going down meets the baffle cone gets reflected and you can start seeing that the left moving wave now becomes a compression wave and starts to move back down leftwards back to the exhaust port where it should be able to stuff some extra mixture in so all the suction's done now and we're now starting to build up pressure from the left moving wave and you can see the position wave as well and that is stuffing some of that fresh mixture back into the exhaust increasing trapping efficiency increasing power and here we can see the results in the paragraph so red is your four stroke straight pipe black is just the diffuser section and the green is the full expansion pipe so as you can see even with a diffuser section it's more effective than just the plain pipe but the full pipe gives the best power. Okay I hope that's been of some use to some of you it's helped me some of the reference material design simulation two-stroke engines there's also a few SAE papers that I've had these are Blair's work they go into the right or left waves the pressure wave the superposition wave there's even one on four strokes that also refers to it just a bit of reading to get back into it so hopefully you'll all agree with me these are well clever very very very clever very simple but effective piece of equipment for increasing power if anyone's got any points any questions please let me know I'll try and answer them respond to you in the comment section as always and if you like this today please give us a like consider subscribing and give us a share so next time I'll be continuing doing expansion pipes so I'll continue with my pipe build and hopefully get the AR93 back on the dyno with a pipe