Four cycle Two cycle ?

[pearl111]

Is the reason a four cycle motor is called that because the piston goes up and down for intake valve then up and down for the exhaust valve, which is four times total. Meaning four cycle or four stroke?

Also when I changed the gaskets on a 49cc motor I didn't noticed any valves. So I'm guessing the piston goes up and down once to make a complete cycle? Meaning two stroke or two cycle motor.

Could this be correct?

 

[JTSpeedDemon]

Yup! Pretty much exactly!

Technically the terms are four-stroke cycle and two-stroke cycle, although 2 stroke, 4 stroke, 2 cycle, and 4 cycle are all commonly used.

A stroke is a movement of the piston up or down.

With a 4 stroke, on the first intake stroke, the piston goes down, and the intake valve is open, drawing gas & air into the combustion chamber.
Then the intake valve closes, sealing the combustion chamber, and the piston goes up, compressing the fuel air mixture.
Then around top dead center of the piston, the spark plug sparks and ignites the highly combustible fuel/air. Since both valves are closed and the expanding gases have nowhere else to go, they push the piston down, creating rotational power.
Then the exhaust valve opens as the piston goes up, pushing the resulting exhaust out the muffler/exhaust pipe.

The strokes are named Intake, Compression, Power, & Exhaust, respectively. a.k.a Suck, Squeeze, Bang, Blow.

Hope that helped clear a few things up!

 

[Flyinhillbilly]

Two strokes suck, squeeze, bang and blow also, they just do the suck and squeeze in one stroke, and the bang and blow in another. The piston moving down displaces the air and fuel in the crankcase causing it to move up through the intake port, at higher rpm, this action, combined with the reed valves cause a supercharging effect, more commonly known as a “powerband”

 

[jmaack]

never seen this video. But I like it. explains very well also.

https://youtu.be/Z6YC3I54so4

 

[itsid]

Indeed.. ALL engines (2,4,5,6 stroke doesn't matter) take air in, compress, combust and exhaust.
the difference is in fact that a four stroke
does so in four distinct piston stokes
and a two strokes only needs two distinct piston strokes from one combustion to the next.
(and the random five and six and whatever stroke engines respectively 5, 6 or whatever)

And all two strokes also use valves.. just usually different type of valves (and fewer)
the simplest form would be a piston port (where the piston itself acts as a slifing valve)
then there are reed valves as fhb mentioned and rotary valve just to get the three most common types for two strokes in here.
and usually they only act upon the intake,
the exhaust more often than not is backpressure controlled
or in case of small weedwhacker /chain saw engines.. not [much] at all

'sid

 

[JTSpeedDemon]

Here, this'll help it make sense a little more.

The first picture is a 4 stroke, the second one is a piston ported 2 stroke. On reed valve 2 strokes, a reed valve regulates the fuel/air entering the crankcase instead(reed valves are desirable).

Most reed valve 2 strokes have the carburetor mounted to the crankcase.

 

[pearl111]

Thanks everyone there's a lot of good information here.

Anyway here's a crazy thought probably more trouble then it's worth. But I wonder if some planning and ingenuity if a 4 stroke say a Pred. 212 could be turned into a 2 stroke? Also I wonder if the clutch could handle the intense high RPMs? Probably a dumb idea anyway.

 

[Kartorbust]

It wouldn't be worth the time, money, and effort to convert a Predator into a 2 stroke engine. You'd be miles ahead buying a non-runner 2 stroke and rebuilding it. 2 strokes are generally cheaper to rebuild than 4 strokes.

 

[Flyinhillbilly]

If I’m not mistaken the 212 crankcase would have way too much volume to have any hope of running well as a two stroke. If memory serves me, the displacement and crankcase volume should be pretty close on a two stroke.

 

[itsid]

should be fairly equal, yes;
but not because it wouldn't run too well otherwise,
but because it wouldn't run too well _for long_ otherwise
(the missing lubrication would cause a major hiccup eventually the rest can be dealt with usually)

two strokes are far more complicated than one might think at first..
while you don't have to scratch your head about cam timing lift and duration,
you will have to take all that and far more into account setting the rest up.

reed valves are fairly simply.. as soon as the crankcase is under low pressure (piston moving up)
it'll open and you have your mix taken in (halfway)
but then the piston drops down again and that mix has to find it's way into the combustion chamber,
therefor you need a transfer port and that then has to be perfectly timed to
get all the mix up into the combustion chamber without letting it escape back into the crankcase (the problem a large crankcase volume is not exactly helping with less compression ratio means less gas speed)

And the exhaust porting isn't any easier.. on it's way down the piston need to open the exhaust
just when there's no more noteworthy power to be extracted from the combustion itself,
while at the same time allowing the fresh mix in on the other end.

And just because you're making it a two stroke doesn't mean your engine will rev twice as high really..
it only does so if your powerband is set perfectly (and you might find one of Brets old threads [desertduler]) where he explains how to adjust the powerband by altering the exhaust geometry .. and what else is needed in order to do so properly.
in short: it's far from a quick and easy task!

Like say the common misconception of the powerband beeing some kind of supercharging effect due to intake geometry.
it's just not that..
the change in volume above and below the piston is perfectly identical
(very few exceptions with counterbalanced crankcase compressors)
So at every stroke the exact same amount of combustible gas is forced through the transfer port into the combustion chamber.
The point is.. that amount is always "too much"
in order to expell and dilute the exhaust fumes.
(also piston travel dictates the bigger than necessary amount that's why bore to stroke ratio of the piston itself is important)

The "supercharging" effect is based on the resonating hot gasses in the exhaust's resonator pipe;
at the ideal rpm (set by the geometry of the exhaust) the last bit is forced back into the combustion chamber when the transfer port is already closed up again
and if the geoemtry is perfect, that bit is also nothing but combustible gas
that's where the nice smack we all know and love comes from.

Talking about supercharging...
Bret had a great idea for a self supercharging two stroke .. last I know he hadn't made a prototype yet but that's been quite a while now.. if he ever made one..
I'd love to see that thing in action. (honestly I'd love to drive that...)

Anyways.. the proper porting of the piston (for exhaust expelling and crank case to combustion chamber filling) in relation to bore and stroke dictates where the engine likes to have it's ideal power and then you still have to make the matching exhaust for that
to get the best possible result and not waste most of the power.
and that alone is tedious enough

No frankly... converting a 212 to a two stroke isn't exactly easy,
and the result will be more than likely extremely disappointing.

If you have a trashed engine laying around, and some time to waste..
sure, give it a go.. I bet there's something to be learned in such attempt.

But personally, I think it'd be even easier if you get a set of wheedwhacker enignes
and merge them into a multicylinder two stroke instead

'sid

 

[pearl111]

Thanks again everyone makes a lot sense now. And itsid great educational information. I don't think I'll attempt the conversion I'll just use the scrap motor for parts.

 

[JTSpeedDemon]

Well, there's the transfer port type 2 stroke, and then there's the TRUCK type 2 stroke. 2(EDIT - there are usually 4) normal exhaust valves, oil filled crankcase, and a roots-type blower to shove the air and fuel in! No mixing oil into your fuel, no transfer port, just 2 stroke goodness!

I'd say go for that! Probably the biggest things needed would be a modified ignition system, and a custom cam setup that spins at the same speed as the crankshaft, opening both exhaust valves at once(or even just one and leave the other one closed for simplicity).

PS - And a transfer port 2 stroke Predator probably wouldn't get enough lube to the con rod journal, causing it to seize anyways. (most 2 strokes use needle bearings on the con rod journal)

I think the clutch would hold up, but IDK.

 

[Kartorbust]

There are 4 valves in the most common 2 stroke diesels.

 

[JTSpeedDemon]

Well then, hmmm......................................

It would still be fun to try!

 

[jmaack]

Detroit diesels need super charger or turbo to run. This is because of the traditional valves and being 2strokes. When you hear one run on youtube or in large forklifts ar work you will tell that they also spin at a fairly high rpm compared to diesel 4 stroke counterpart.