hello again two stroke fans. Previous video I showed episode five, I'll put links to
previous video up in the corner and the playlist covers all the videos. The
reason we're doing short ones is so you can watch one or two,
if you can't handle any more just stop at that position and carry on to the rest later
on. Previous video we did timings of the relationship from the exhaust board as
to the distance of the diffuser cone and the baffle cone, now we're going to do
the shape or the angle, you'll see on some expansion chambers they have a
steep angle on the diffuser baffle cone and in some we have a shallower angle,
Now how do we work out what's going to occur with those angles, well there's a
standard computational method used to do that and as we know when the compression
wave comes along and is turned into mirrors when the compression wave comes
along and mirrors an inverted expansion wave going back in the other direction
there is periods at each point where the wave comes through that the mirrored
expansion wave begins to evolve as it moves through the diffuser cone this one
it will be rapidly this one it will be over a longer period but not so rapidly
and the difference in the wave reflected is this will have one with a
higher amplitude over shorter distance and this one will have a lesser pressure
amplitude or vacuum over a longer period, so what we can do is use a computational
method to calculate each individual increment and map how the expansion wave
creation is evolving through the diffuser cone. That is done because we
have already studied data and equations that can tell us what happens when we
have a small pipe compression wave exit into a larger pipe and with that in mind
and all the conditions being equal then we can continue doing the same
thing in a simple computational step so we've got an equation here between this
to this, and then another equation here for this to this, and we can map that
continually all the way, as you see there's little steps in these angles you
know from digital to analog now that's a computational model it doesn't mean that
there's a individual expansion wave small expansion waves created all the
way down it it just means you've got a computational model to gradually evolve
the creation of the expansion wave and likewise the reflection off the baffle
cone in a way that you can have a straight-line cone and you can also have
multiple angles through the diffuser cone as well and that's using the same
computational method because each step is calculated there may be a variance in
these steps as it goes down and progresses through the diffuser cone
so each formula as it goes through the mesh is adjusted accordingly
until you get the output which is the expected pressure and amplitude, which
we'll get to next. Here's our pressure wave moving out of the exhaust port,
compression wave, as it moves down friction against the walls, now that's
just the shape of the graphical representation of the pressure at any
one point in the pipe, the wave isn't shaped like a that its dome shape as you
know from earlier photos in the previous videos as it moves down friction
overcomes that the sonic wave moves ahead okay, and then it gets to the
diffuser cone and it's reflected as an expansion wave, now if we've got a really
shallow angle and that bottom cone we get a long wave, low amplitude if you've
got a steep angle on the diffuser we get a shorter wave length and we get a
higher peak, likewise from the baffle cone if we have a shallow baffle cone
get a longer wave in the compression positive okay, steeper angle we get a
sharper way of coming back okay, now as for the exact position timing of that
wave if you can refer the previous video about where that wave arrives, but as you
can see the pressure before the wave and after the wave is the same so if the
wave passes through at a different time to the event we want it's not going to do
anything it has to be on the front of the wave where the pressure rises or at the
front of this wave where the pressure drops, it has to happen, that has to
happen when it meets the port at exactly the right rpm so as you can see at a
low rpm to a high rpm that's very little time available whereas this one
we've got much longer time available when the shallower thing but we don't
get as much boost so this would be for a shallow diffuser for an engine that's
got to perform long rpm range under power and if you've got a multi ratio
gearbox like a motorcycle we only need to operate under a couple of thousand
rpm range we can have a stronger peak so for the stronger peak we have steeper
cones and for the shallower peak we have that and you might find you no longer
steeper ones like this in engines with no gears like a go-kart and this one you
sharper one your motorcycle but that also explains how the multiple tapers
our modelled and that'll do it for this video and I'll get more into that in the
next one tomorrow so bye for now