Thread: When 2 Strokes 4 Stroke

We usually consider a 2 strokes mixture is correct when it is partially 4 stroking at full throttle no load. What is actually occuring to cause the engine not to fire every stroke. I have never heard a good explanation of what causes this.

Frank,

I think that you are merely assuming that the engine isn`t firing on every stroke when it four strokes. Four stroking is a term related to the sound of the engine.

Russ

could this be that?

As the mixture is increased above the stoichiometric ideal, the buring velocity of the fuel decreases. for example the burning velocity of most common hydro carbons drops by about 25% if the fuel air ratio is increased to 1.1 times the stoichiometric ideal.

So if the burining velocity of the fuel drops, this may lead into a chain of events; with less compleate combustion more unburned mixture is left in the clyndar, then when the next charge is pushed in the resulting mixture is richer yet, this process self propogates until the mixture becomes too rich (above the explosive limits). At that point there is a partially or compleatly missed stroke (hang fire), and the unburned charge is partially pushed out into the exaust and partially burned during the proceding stroke returning the process to start over.

It seams that the 4 stroking does not hapen every second stroke but rather regularly once every few strokes proportionaly with the degree to wich the mixture is richened.

Just some fire protection theory, not sure if it works for chainsaw theory.

Then why does it perfectly 2 stroke under load with the settings the same?

2 thoughts come to mind:

1 the reduced RPM in the cut drops the velocity of air in the carb and hence also less fuel is drawn in by the venturi. and therfor the mixture leans slightly

2 As smokless powder required containment to produce the explosive wave front so does the mixture, could the 4 stroking be akin to the piston speed approching the velocity of the expanding wave front. then once loaded / slowed piston with counter force transferd through from the wood the piston slows to well below that of the expanding chage giving the charge something to push against (containment) and hence restores compleate combustion and hence the 2 stroking.

HEAT my friend when the motor is under load it builds heat when not under load the intake charge cools down and runs fat again

heat may be a factor, because a richer mixture can be ignighted if heat is increased (UEL is increased). but I am not sure that heat is the answer here because if the saw 2 stokes in the cut and is heated up, then pulled form the cut it still breaks to 4 stoking immidiatly.

More load, more pressure, more heat, complete combustion. Makes sense. So the mixture that was just lean enough to 2 stroke under no load full throttle would be too lean under load.
It seems with modified saws it is a little harder for my ears to pick up the different note and tuning down to the factory recommended tach reading would be a way too rich. Wouldn't it be nice to have a head temperture readout like aircraft engines.

Some interesting carb dynamics going on here, I think.

when the air velocity in the carb increases the fuel that is drawn in by the venturi increases in a proportional fassion. However the increased velocity through the venturi drops the air preasure and consiquentialy air density.

If proprorionaly more fuel is added to a proporionaly increased air volume the mixture does not change, but because the air becomes less dense the fuel ratio actually increases. At least unless there are compensation mechanisums engeneered into the carb to limit the problem in the form of multipul jets (ie. L/H or more), maybe even the shape of the venturi, position of jets, or restrictions in the air filter other parts of the intake, ect.?

This said lower rpms = a leaner mixture, given that the engeneered compensation mechanisums are not perfect or over compensating at that point.

How come then when you open the muffler and allow more flow thru the system, that you usually have to richen the mixtures?

because your muffler will build presure and push air back inside the motor when the piston is starting its way back up and this will hold the mix in the cylinder So if you make your muffler holes larger it wont hold the mix back in the cylinder

Then why is no adjustment typically needed to lean the saw when switching to a pipe?

If the saw is holding a higher RPM in the cut the mixture would be richer, and also if less unburned fuel is wasted out of the exaust the mixture should be far too rich?

or are the increased volume of mixture jammed ito the cylendar and higher temps making up for the added fuel?

or is the effect of the pipe felt all the way back to the carb?

because a pipe tuned correctly will push alot more air and unburnt fuel back in the cylinder

I can't see that the pipe can push air into the cylinder, as the stinger is a blead down for the expansion chamber the flow is only in one direction out. is it not?

I think that there is only exaust inside the pipe. (and maybe air with the O2 burnt out of it)

But if there was more fuel and no additional O2 there should be a need to lean the mixture?

The pipe should effect velocity all the way to the air filter by creating an optimal pressure in the cylinder at the desired RPM. By my own admission I'm weak on two stroke single piston setup, but V8's use headers to control the exhaust pulse from one cylinder to create a vacuum at the exhaust port of another cylinder as the valve opens. Same way with an X pipe vs H pipe setup. The pulses on one side of the X pulls a vacuum on the other header discharge effecting the powerband of the motor. Pretty nifty stuff if you can figure out how to make it work for you and fit the application.

I agree with timberwolf except I figure with a tuned pipe the motor would rev clean.

Jamie