Flow management through reeds

[James W.]

Flow management?

So what's goin' on 'ere then, eh? https://www.youtube.com/watch?v=FwcCd9mAZeY

EGR?

[oldjohnno]

That looks very much like a current Stihl technology that uses two carb throats in a staged arrangement. The primary is conventional while the secondary is just a butterfly and is air only. It's probably not quite accurate to call it a stratified charge but the idea is that the porting is arranged so that the part of the scavenging flow that is likely to be lost out the exhaust port is air-only, the remainder being from the primary throat being air and fuel. I think its primary purpose is emission reduction.

[2TInstitute]

Jeepers, 15 or 20 cycles to get mixture up the transfers? Really?

No. You can build a fresh engine without a drop of fuel in it, bolt on the carb, kick it twice and it's running. The cylinder has enough mixture in it to fire and run after 3 or 4 revolutions.

Must be time for some maths revision , most kick starts turn the engine over at 300 rpm and about 500 rpm with a man kick. A 1 second kick will have the engine turn over 50 to 70 times.

Jeepers, 15 or 20 cycles to get mixture up the transfers? Really? WTF is hsppening down there in that time perod?
No wonder there used to be talk of direct injection.
(Here's where I'm told I've completely missed the point...!)

There is simply not enough time to move any more regardless of rpm

[2TInstitute]

Flow management?

So what's goin' on 'ere then, eh? https://www.youtube.com/watch?v=FwcCd9mAZeY

EGR?

Mind you Stihl can engineer emissions across their entire range the little less powerful saws make up for the 'performance' saws. The secondary intake acts just like an air bleed

[oldjohnno]

Jeepers, 15 or 20 cycles to get mixture up the transfers? Really?

No. You can build a fresh engine without a drop of fuel in it, bolt on the carb, kick it twice and it's running. The cylinder has enough mixture in it to fire and run after 3 or 4 revolutions.

Must be time for some maths revision , most kick starts turn the engine over at 300 rpm and about 500 rpm with a man kick. A 1 second kick will have the engine turn over 50 to 70 times.

Jeepers, 15 or 20 cycles to get mixture up the transfers? Really? WTF is hsppening down there in that time perod?
No wonder there used to be talk of direct injection.
(Here's where I'm told I've completely missed the point...!)

There is simply not enough time to move any more regardless of rpm

50 to 70 revolutions from one kick? You must have the worlds biggest flywheel.

[James W.]

Here's a recent local variation on the flow management theme, by dint of a 'sliding cylinder'.

https://www.youtube.com/watch?v=2p0dyPu7Gcs

[AusMecham]

2 cycles to fire up thats all, magneto CD ignition more but battery powered induction ignition that provides full energy from 1 RPM upwards such as I had on my racing RD350LC moded ignition, 3rd compression hump is go on a racing push start. from engine off, not primed.

Discovered this was a benefit racing at Amaroom in the 80s, crazy bastards had push start races was worried about getting mowed down on the grid.
2 races went from mid field to front row at the start since my engine fired after 2 or 3 revolutions and got off a lot quicker then expected and nearly took off without me, legs out the back.
Pity we didnt have push starts in Vic, I would have creamed every other LC off the line.

But anecdotals aside.
How fast the mixture ends up at the combustion chamber is also determiendf by the speed at which the piston is movingf during those strokes, more so in a 2 stroke being more reliant on gas inertia.

Move the crank at 1 RPM and itll take many revolutions to get mixtures to the cylinder.
Next
Drop the bike off the back of a moving trailer in first gear at about 60 kmph and see how fast she fires up.
A lot less revolutions to start.
You get the idea
Gas inertia is the result of the pistons speed at starting,
During running, even faster.

[James W.]

Funnily enough, I was mowing the lawn with the good ol' Victa 2T a couple of days ago,
& the electronic ignition cut postion - on the throttle control lever - decided not to cut,
so I turned the fuel tap off..

When the carb ran out of juice, it momentarily gained revs, then cut out, but caught again for a bit,
& repeated it, once more - until it finally chuffed out..

I wondered if it was due to a high vacuum draw on the carb dregs,
or was it residual crankcase mix - being pulled up?