please explain four stroking to me in laymans terms

[scotclayshooter]

When i posted that no one has come up with an explanation i didnt mean on this thread i was meaning even the experts cant fully agree.

Im away to find a post by TimberWolf back soon

 

[scotclayshooter]

I don't think it is a misfire, that is a much more pronounced event.

What I think, though could well be wrong is going on is that as soon as the engine gains enough RPM for the given mixture the piston actually out runs the expanding charge and there is not the same pop when the port opens. As a result full burn and scavenging is hurt and the saw slows ever so slightly though it would not be a total misfire. The next cycle the incoming charge volume will be slightly lower due to losses in scavenging and RPM leading to a slight leaning of the mixture, and with the lower RPM you get a full burn the next time. The cycle then finds a balance of partial burns on 3rd, 4th, 5th... cycles.

Heres TWs take on it.

 

[WoodViking]

listen to this

I don't know what happens when the saw is 4-stroking but for those that have trouble hearing it try this video:

http://www.youtube.com/watch?v=4PxVYud58L4

I think the 2171 is 4 stroking especially in the beginning of the second cut at approx. 18 seconds.

 

[scotclayshooter]

TWolf is saying the saw has the hiccups!

My take on what he said is this: because of high rpms, the piston moves past the exhaust port before the charge is completely burned, i.e., there is not enough time for complete burning. Because the burn is incomplete, pressure is lower, and so less of the the exhaust makes it way out.

Because there is more exhaust left, the next charge is diluted, and so takes less time to burn. In addition, the engine slows a hair, so that charge has more time to burn. The result is a complete burn, and maximum pressure for that charge, so the saw speeds up again, and a higher proportion of the exhaust leaves the cylinder, because the scavenging is more efficient.

This puts the engine back at square 1 with a dense charge that takes longer to burn than it takes for the piston to clear the exhaust port.

The load on the saw in the wood drops the rpms enough to prevent the overrun, giving more time for a complete burn.

Edistos take on it.

 

[scotclayshooter]

It's firing every stroke, but there is not enough O2 in the fuel-air mix to support complete combustion, somewhere in or around 10 : 1 .

What I believe your what we're hearing is the ragged sound of an incomplete exhaust pop, commonly called 'burbling' ?

It would take a very fast pressure/vacuum sensor, but my bet is that the exhaust reading would go very rapidly from an exhaust pressure pulse to a slight negative reading as the exhaust flow was incomplete.

Basically, the fire in the cylinder is going out, before all the fuel is burned.

Yet another!
I like this one

 

[drkptt]

Flame speed is highest when the air/fuel mixture is set near the point of best power. Flame speed is slower for both rich and lean mixtures, but the lowest flame speed of all is measured for mixtures near the lean misfire limit.

So if these theories about 4-stroking being caused by slow-burning rich mixtures are true, then a saw would 4-stroke when set lean as well.

 

[scotclayshooter]

Flame speed is highest when the air/fuel mixture is set near the point of best power. Flame speed is slower for both rich and lean mixtures, but the lowest flame speed of all is measured for mixtures near the lean misfire limit.

So if these theories about 4-stroking being caused by slow-burning rich mixtures are true, then a saw would 4-stroke when set lean as well.

I think rich burns slow like Diesel and lean explodes. The amount of Oxegen is less with a rich mix.
Harder to burn without O2.

Clive Cussler fan????

 

[drkptt]

I think rich burns slow like Diesel and lean explodes. The amount of Oxegen is less with a rich mix.
Harder to burn without O2.

Clive Cussler fan????

Lean mixtures burn the slowest. I pulled two sources off the shelf:

Edward F. Obert, Internal Combustion Engines and Air Pollution, pg 99.

Charles Fayette Taylor, The Internal Combustion Engine in Theory and Practice, Volume 2, pg 23.

The test engines had a window in the cylinder head and photographic film was dragged across the window at a constant speed. You could see when the fire started and when it was extinguished as a function of crank angle.

Rich mixtures just burn until all the O2 is consumed; partially-burned fuel (CO) and unburned fuel ends up in the exhaust.

Was a fan of Cussler; haven't read anything of his in a while. Good pickup on the name with the missing vowels.

 

[scotclayshooter]

Lean mixtures burn the slowest. I pulled two sources off the shelf:

Edward F. Obert, Internal Combustion Engines and Air Pollution, pg 99.

Charles Fayette Taylor, The Internal Combustion Engine in Theory and Practice, Volume 2, pg 23.

The test engines had a window in the cylinder head and photographic film was dragged across the window at a constant speed. You could see when the fire started and when it was extinguished as a function of crank angle.

Rich mixtures just burn until all the O2 is consumed; partially-burned fuel (CO) and unburned fuel ends up in the exhaust.

Was a fan of Cussler; haven't read anything of his in a while. Good pickup on the name with the missing vowels.

Ok cool were all learning!
I thought detonation was a result of too lean a mix as it explodes rather than burns (Just like using too small a charge when handloading bullets)

The Dirk Pitt books are all the same now but The Oregon files are good

 

[drkptt]

Ok cool were all learning!
I thought detonation was a result of too lean a mix as it explodes rather than burns (Just like using too small a charge when handloading bullets)

An engine is more prone to detonation when the combustion pressure and temperature increase. Lean mixtures do not inherently detonate, but may trigger detonation due to higher temperature.

The Dirk Pitt books are all the same now

Funny you should say that--that's exactly my complaint.