(Engine) turbulence?

[st3v3]

On another forum, someone is going on about 'turbulence' typically engine based, in relation to 2stroke tuning, never before have I heard anyone, mechanic or even guys in the know - talk about turbulence in the same sentence as an engine.

Aircraft, yes. Motorcycles (or cars), no.

Google brought nothing back.

I guess members like Kickstart or Stinkwheel would be among my top repliers for this ideally, but i'm not being picky.
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[Johnser]

Turbulence is just a word used to describe the discorded flow of air (or any other fluid) that is typically very unpredictable.

Every engine inlet, combustion chamber, exhaust, will have turbulent regions.

Engine designers work very hard to understand the types of turbulence in an engine with regards flow (for exhaust/inlet) as well as combustion (which further complicates things) in the cylinder.

It is beyond most mechanics to worry about the finer details of combustion within a cylinder and is usually left to Engineers with big glasses and lightspeed computers to attempt to eek out another 0.01% efficiency/power from an engine design.

That said, some old boys who have a lifetimes' experience (especially tuning 2-strokes where trying to control the exhaust pulse in an expansion chamber is a dark art) seem to have an instinctive feel for how air will flow and how modifications will affect things.

[FirebladeRuss]

As above. Good post

This is why it always makes me chuckle when you see people taking off their standard air boxes on their cars and strapping a K&N 57i filter directly onto the air flow meter.
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[Kickstart]

Hi

Some turbulence in the intake air is meant to be beneficial (and polishing intakes and ports can be counter productive because of this). However beyond helping the mixture stay, err, mixed I am not sure of the mechanics behind it.

[Rowey]

I'm not entirely sure on combustion engines. Although jet engines I am.

In a jet engine, in the combustion stage you have a device called a swirler (or swirl vanes) these are small aerofoils angled to make the air and fuel mixture swirl. This makes the fuel swirl in a low velocity recirculating pattern, taking the form of a toroidal vortex, similar to a smoke ring. This has the effect of thoroughly mixing the air and fuel, stabilising the flame and making the fuel droplets burn quicker and more complete by consolidating and rapidly bringing them to their ignition temperature.

So my guess would be to essentially aid in air/fuel mixing and increase ignition efficiency.
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[jjdugen]

Turbulence can be a performance killer. I'm in the process of putting a VTR1000 Firestorm back on the road. Around town it was all fine, higher speeds and it hunted and lurchedn so badly it was unridable.
Turns out that a cheap bit of plastic that fits at the front of the frame was more than a splash guard, as I thought. It acts to stop the turbulent air from the fairing area upsetting the airflow in the airbox. Without it, lurching, with it in place, smooth as butter. And this with a good airfilter and the box properly fitted and sealed.

[Teflon-Mike]

The simple 'piston ported' two stroke engine; in theory shouldn't work.

Charge sucked IN on the up stroke of the piston, ought to be squashed back OUT on the down stroke. Fact that it DOES work, is exploiting the fact that the real world is 'imperfect', and the science goes all 'fuzzy' in the chaos of competing effects.

And, the reason the piston ported two stroke works, is because air, has mass. Having mass, when it moves it has momentum, which wants to keep it moving. Newtonian physics; a body in the absence of an applied force a body in motion will remain in motion....

So piston rises, creates a vacuum in the crankcase; that pressure difference gives rise to a 'force' and that makes air 'move' and start filling the space in the crankcase left by the piston.

So far so good. But now piston reaches top dead center, and STOPS moving......

No change in force or displacement to cause air to move or stop moving, BUT no more 'space' for air to fill... BUT column of air, with MOMENTUM.....

So, the air carries ON moving, into the cylinder, under momentum alone, even though there's no 'suck' to drag it in.

Now piston turns round starts coming back down the bore.

THIS is where, in theory, with nothing to stop whats just been sucked in, being squashed back out...... the engine shouldn't work!

But this is where we get 'clever' with momentum, and 'turbulance':

Charge is being sucked in through a hole in the wall of the cylinder; on the outside of that wall, is a long tube.

About 2/3 the way along that tube is the carburetor; strategically placed, for reasons I wont go into right now; and then up-stream of the carb, a long bell-mouth induction trumpet.

Purpose of the trumpet, is to smooth the flow of air, into the induction tract. Again in 'Theory' the air surrounding the engine is 'still', so the trumpet spreads the mouth of the tract over a wider area, and gives a nice funnel to direct it in, and get it all moving 'smoothly' in the same direction down the tract.

TURBULANCE.....

School Out time! Hoards of kids all rush out of classrooms into corridors and start heading for the door....... get the picture in your head, this is air-flow!

Corridor is a uniform width, and you can have wall to wall kids, rushing along it.....

This is 'Lamina' flow..... all going in the same direction, at the same speed....

ONLY, the kids near the walls, are bumping into the wall, and back off it... they jostle the kids next to them, who jostle the kids next to THEM a bit less and so on....

So the kids by the wall travel a bit slower; kids in the very middle a bit faster, and you get whats called a 'velocity profile', with a 'high speed stream' down the middle and some 'drag' down the edges..... I'll come back to this........

NOW: imagine that ONE really annoying kid, reaching up and grabbing a coat peg.........

He stops.....

He now causes an obstruction to every-one behind him, who have to squeeze around him.

The flow is no longer 'laminar'... as the kids all change direction to squeeze around, and then fan back out as they go past, possibly turning to cuff him round the ear-hole if they are close enough.....

you get a swirling pattern, and THAT is 'Turbulance'...

The 'dicontinuity' (kid on the coat-peg') causes a disruption making the flow spill and tumble in all directions, and the result is, the traffic slows down....

OK.... that lamina velocity profile..... and whats called TEAR EFFECTS.

Imagine lining the walls of the corridor with coat pegs..... kids now stay AWAY from the corridor walls, because bumping coat pegs HURTS, or they get caught between the coat pegs, and cant fight thier way back into the main stream of traffic.

The main flow down the middle of the corridor now flows less people; because you have taken one 'moving' person off either side of the front row.... they are now stood still......

BUT, those stationary people are no longer jostling the rest, slowing them down, so the remaining people all move at the same speed, and faster.....

& Traffic flow can actually increase, as on average more people are moving more quickly....

This is like leaving a rough surface finish on a port wall; it traps a bit of air..... and acts like a buffer, letting the rest of the column flow more smoothly.

(Effect was first recorded, in the 1920's on the Norton 'International'; the road going customer version of the works 'Manx'. Rushing to get the race bike ready for the Island, the works bikes never had thier ports polished; it was a 'sales feature' on customer bikes; so after the races; they set about building the customer engines, and duly polished the ports and were surprised that rather than making MORE power than the race bike with rough ports... actually made LESS.... due to this tear effect)

TEAR is NOT strictly turbulence; though you do often get a turbulent 'layer' or boundary effect, in a tear layer.

AND as mentioned; some turbulance can be helpful;

For good cylinder filling; we want as little impediment to 'flow' as possible, so we can cram as much charge in the engine as possible, and hence get as big a bang as we can.... which would suggest we want as smooth and lamina flow with as little turbulance or disruption as possible.

Back to imperfect world and competing effects;

But our CHARGE is not a nice homogomiouse gas, like say Carbon Di-Oxide or Argon, or 'something'.......

We have 'Air' which is a mixture of gasses, of different densities.... and we have made it into a charge, by adding liquid petrol, as a 'vapour'...

Back to our nice long inlet tract, with the carburetor bunging the petrol vapor into the air stream about 2/3 the way to the engine.....

If we have nice smooth flow, that vapour sprayed in and mixed in nicely, will try and 'dew out' of the air stream again.....

Variouse mechanisms; one is condensation on cold port walls, another is actual 'dewing' where the fine droplets bump into each other, combine to make bigger droplets and at some point like rain become too heavy to be carried along by the air stream..

So; our nice smooth lamina flow in the inlet tract is great for full throttle max revs 'power'... BUT... when gas is moving more slowly can rather unhelpfully leave the petrol behind!

Here; a 'little' turbulance' to beat the mixture a little and stir it up a bit, can help keep the petrol in the charge and the droplet sizes convenient for good combustion.

Right; BACK to our piston ported two stroke engine…. Or a class room full of kids!

Piston has gone up, and sucked charge down, long smooth funnelled corridor, and inertia has seen it carry on going ‘in’ even though the piston has stopped moving, due to inertia….

NOW; as piston starts to FALL, we rapidly put a pressure on the charge in the crank case that deters any more from coming in, even through inertia; but worse, with the only thing controlling flow in and out of the engine, holes in the piston skirt and cylinder wall, which will be as far open on the way down for as long as they were open on the way UP, we risk squashing all that charge BACK out the crank case…..

Which is where turbulence gets HELPFUL…. Remember; to get the charge ‘in’, we gave it a long straight ‘trumpet’ and funnelled it through small door in the crank case door.

NOW the charge is trapped in the crank case, and only a small amount of charge can ‘see’ that port, and as pressure builds up, all of it rushing at that small door, it will ‘clog’, like half a dozen kids getting ‘jammed’ in the doorframe when the school bell rings, three hitting the door together, the two only half making it, squashing a third who has to squeeze through really slowly…..

This ‘sudden’ discontinuity creates a LOT of turbulence AGAINST reverse flow…..

And THAT is ALL that makes a plain piston ported two stroke ‘work’!

A ‘small hole’…..

Which of course is a limiting factor to flow for full throttle and high rpm power; and it would make an AWFUL lot of sense to have some kind of ‘door’ in there to stop reverse flow, rather than relying on a small door and turbulence to close it against back-flow….

THIS is of course the ‘key’ to the hi performance two stroke engine; and evolved in the 1950’s; significantly by Walter Kaaden, who gave his name to ‘Kaaden’ effects, about how turbulence and shock waves effect flow paterns and practically INVENTED the high-performance two stroke, working for MZ! Utilising Disc valves to put a door on the inlet tract at close to TDC to increase trapping efficiency, and developing the expansion chamber exhaust to utilise shock waves and harmonics to influence trapping efficiency above the cylinder. Meanwhile under the piston, Yamaha came to a similar conclusion using a ‘flap’ or ‘Reed’ valve.

SO! lets look at whats going on ABOVE the piston!

As piston falls some time before bottom dead centre; piston passes a hole in the culinder that lets exhaust gasses out.

The exhaust gasses are probably still burning, but they are hot and just made the ‘pressure’ to push the piston down, and at the point the exhaust port opens, the gasses are STILL under pressure.

Like the small door on the crank case, though; even with a lot of pressure behind them; the exhaust gas will get ‘jammed’ in the port until the piston has exposed a fair bit of it to let the gasses get moving…..

But as port opens; more gas can get out; and under pressure happy to do so; ONLY as it leaves, reduces both volume and pressure in the cylinder, so that the last to leave are loath to go, and you end up with a cylinder full of stagnant smoke.

This is NOT good to have in the cylinder you want to burn another charge in.

It will get in the way of new charge coming in; and lots of ‘inert’ CO2 in the mixture it will ‘damp’ and inhibit combustion of the fresh charge.

But it is there, as the transfer ports open, towards bottom dead centre, letting in the fresh charge.

At this point, the exhaust port is still open, and by all rights, the new charge ought to rush in the bottom of the cylinder, and head straight out the open exhaust port……

Which is where Walter Kaaden was such a genius….. he arranged two transfer ports directly opposite each other over an almost flat topped piston, and instead of trying to ‘smooth’ the angle of the port so that the flow of charge was pointing up towards the combustion chamber, directing a stream of charge ‘through’ the stagnant exhaust gasses, he angled the ‘jets’ from the transfer ports directly at each other over the top of the piston.

If the ports directed charge ‘up’ towards the combustion chamber, the two streams would have a long diagnonal path through the exhaust gasses, with which they would ‘mix’ where the fresh charge at the edge of the stream pushed it out of the way, causing turbulence and mixing, creating a very poor contaminated charge; though some would get to the combustion chamber at the top, and start ‘washing’ stagnant exhaust ‘down’ towards the exhaust port. BUT, you got a LOT of mixing, and very poor ‘scavenging’; a lot of stale exhaust remaining in the fresh charge and a lot of fresh charge escapeing down the exhaust port.

Water Kaadedn’s arrangement over a flat topped piston; directed the streams of gas directly at each other, over the top of the piston, where they only had a short distance and three sides to risk mixing with exhaust gasses; then they met, like two trucks at speed, head on, colliding, with the only direction they can go being straight up.

THIS created a high speed ‘fountain’ of fresh charge, from the centre of the picton straight up to the middle of the combustion chamber. The two streams combined providing a very high speed, high momentum ‘flow’ inside the cylinder, with a minimal area for turbulent mixing on its boundtries.

Hitting the hemispherical combustion chamber, the ‘fountain’ divides, and fills from the top, again, providing a very small area of interference with stagnant exhaust gasses, washing them straight down the cylinder towards the exhaust port….

HUGELY increasing two stroke engine efficiency, through reducing charge contamination and improving ‘scavenging’…

SOME fresh charge though is washed down the exhaust port, chasing the exhaust gasses….. BUT as his engines became more and more highly tuned, Kaadedn discovered he could use musical ‘harmonics’ and a reverse shock wave from a cleverly shaped resonance chamber exhaust to provide a reverse pulse back up the exhaust pipe, JUST before the exhaust port closed, in order to push charge that had gone down the pipe BACK into the cylinder…..

OK!

We have now managed to get charge into the crank case, and not pumped it straight back out again, instead, using clever flow phenomina to get it through transfer ports and into the cylinder, and even more clever fountain flow paterns to get it to scavenge the cylinder of spent exhaust and not bugger off down the exhaust pipe! We now have a chance of making a bang in the pot!

And HERE is the LAST bit of flow science in the process, and you MAY have come across it called ‘Squish’.

Piston, is not ‘quite’ flat topped, its slightly domed. Combustion chamber is similarly slightly domed.

The ‘perfect’ combustion chamber would be a ball, with a spark-plug electrode right at the centre, so when the spark ignites charge a flame front rushes out in all directions burning at equal speeds creating a nice even pressure across the piston.

On many early engines therefore they had half domed combustion chambers and half dished piston crowns to create a perfect spherical combustion chamber…

Back to science and imperfect world and competing phenomenon.

If you have a 125cc engine, of traditional ‘square’ 54x54mm bore and stroke dimensions, and cut a ‘sphere’ into the piston and cylinder head, 54mm diameter, you get a combustion chamber volume of 82cc, which gives a compression ratio of around 1.5:1. This is NOT good for combustion efficiency, where the higher the compression ratio the better, and THAT is far more significant than combustion chamber shape.

However; in the early days this was not realised, and it was one of the reasons engines were so often so ‘under square’ with a bore much smaller than their stroke; because that reduced combustion chamber volume and increased compression ratio.

More modern engines, simply have ‘hemi-spherical’ combustion chambers; (Immortal Chysler ‘Hemi’ engine) with a flat topped piston to halve combustion chamber volume, and later realised that you don’t actually need a FULL hemisphere in the combustion chamber, you can use just the top section, and you can get combustion chamber volume down even MORE making the dish smaller than the bore….

THIS is what gives rise to ‘Squish’ Imagine flat topped piston, and the combustion chamber ‘roof’ almost flat, for about ¼ of it’s diameter, and only dishing in the middle.

If you can run a very tight tolerance between piston and cylinder head, so the piston wont hit it, as the piston approaches top dead centre, any charge in that thin flat ‘ring’ around the edge of the dome, is going to get ‘SQUISHED’ out of the ring……

Remember Kaaden’s ‘Fountain’ putting port jets across the top of the piston top?

Right, same deal, that ‘ring’ of charge squished into the middle of the piston creates another little fountain inside the charge in the combustion chamber, colliding at the centre of the piston and fountaining ‘up’ right in the middle to jet at the spark plug… which has either just ignited or is just about to ignite some charge…

Creating a flow over or through ‘the flame front’ propagating the burn, spreading it right around the combustion chamber, but importantly getting the charge right at the top, against the combustion chamber roof, normally the last to burn, burning early….

SO!

Long winded and rambling as usual…… we are talking about gaseous ‘FLOW’ or the science of fluid-dynamics.

TURBULANCE is semi ‘random’ or ‘uneven’ flow paterns around usually a ‘discontinuity’ or in a situation where the flow is ‘unmanaged’.

Turbulance can be helpful or unhelpful; depending on what you want to achieve.

The streams and fountains deliberately created in gas flow within the two stroke engine, often rely on turbulence, and often the flow patterns created, may be described as ‘turbulant flow’… mainly because they are not straight forward smooth ‘lamina’ flow, but they are not necessarily ‘turbulence’, berely ‘non liniar’ flow patterns!

It’s a complicated subject; confused by terminology used curiousely. BUT; explaining the two stroke and where and how ‘gas flow’ is important and where and when some of these more curiouse flow phenomina are actually used to positive effect, ought to explain where and how any-one using the work ‘Turbulance’ to describe any of it, might be getting it from.
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[st3v3]

Wow.

Cheers dude.
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[N cee thirty]

the YEIS thing on yamaha's is ment to smooth out intake pulses
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