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Four Strokes versus Two Strokes
2-Strokes vs. 4-Strokes
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If you spend any time in the dirt bike or motocross world at all, you’ll probably catch a debate about 2-strokes compared to 4-strokes. Or if you’re thinking about buying a new or used dirt bike for the first time, you’ll wonder if you should go with a two-stroke or four-stroke. Here are a few basics on the two types of engines.

2-Stroke Operation

1) The fuel/air mixture is pulled into the crankcase (not the cylinder) by the vacuum created during the upward stroke of the piston.

2) During the downward stroke, the valve into the crankcase closes. The fuel mixture is then compressed in the crankcase during the remainder of the stroke.

3) Toward the end of the stroke, the piston exposes the intake port that allows the compressed fuel/air mixture in the crankcase to get past the piston into the main cylinder. On the way in, the fresh fuel/air mixture pushes the exhaust gasses out the exhaust port, usually located on the opposite side of the cylinder. The only way to make sure the cylinder is full of the fuel/air mixture, is to push “too much” through the cylinder, so some unburned fuel/air gets into the exhaust system.

4) The piston then rises, driven by flywheel momentum, and compresses the fuel mixture. It's a little confusing, but at the same time, another intake stroke is taking place in the crankcase -- underneath the piston -- as in step #1.

5) At the top of the stroke the spark plug ignites the fuel mixture. The burning fuel expands, driving the piston downward, to complete the cycle. And again, a little confusing, but underneath the piston, the fuel/air mixture ie being compressed.

You'll find an EXCELLENT animation of a working 2-stroke engine, as well as diagrams,
here.

4-Stroke Operation

1) During the intake stroke, the piston moves down, pulling fresh fuel/air mixture through the open intake valve into the cylinder. An exhaust valve in the cylinder is held closed at this stage.

2) With the intake and exhaust valve both closed, the piston moves up in the cylinder, propelled by the momentum of the flywheel. As the piston moves up, the fuel/air mixture is compressed.

3) At the top of the compression stroke the spark plug fires, igniting the compressed fuel/air mixture. The expanding gasses resulting from this controlled explosion push the piston downward.

4) At the bottom of this power stroke, the exhaust valve is opened (the intake valve remains closed) and, as the piston rises again, exhaust is pushed out of the cylinder.

You'll find an EXCELLENT animation of a working 4-stroke engine, as well as diagrams,
here.

2-Stroke Engine Advantages

The spark plug fires once every revolution of the crankshaft.

Basically, they produce twice the power of a four stroke engine of the same size.

The two-stroke engine is much simpler than four strokes, with lighter construction and fewer parts.

2-Stroke Engine Disadvantages

You have to buy two-stroke engine oil, measure it, and mix it with your fuel.

Technically, they wear out faster because lubrication is not as efficient as in a four stroke engine with heavier oil.

Because unused fuel is exhausted with each cycle of the engine, they use more fuel.

Because unused fuel is exhausted with each cycle of the engine , they pollute more.

4-Stroke Engine Advantages

They generally last longer than two stroke engines because of the more efficient lubrication of moving parts.

Four-strokes use less fuel.

Four-stroke engines pollute less than two-strokes.

4-Stroke Engine Disadvantages

Construction/assembly/operation of the engine is more complicated. More moving parts means more things that can go wrong (Although I personally have had very little trouble in this respect. How about any of you?).

Four-strokes are half as powerful as two stroke engines of the same size.





20 comments:

  1. 4 strokes also have smoother torque then 2 strokes because 2 strokes have to much power for their size which makes them free spin more. Which i think is more fun to ride :)

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  2. Not all 2T are created equally. Fly wheel weight, pv design, and pipe design all play a part in 2t performance. My GG ec300 will lug down as good as any 4t I have had with out the need to touch the cluch in tight woods trails. And have found that I can get real good fuel milage. 10lt to 100km under hard riding condition. More if I milk it.

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  3. i have a yamaha blaster 200 two stroke it does great i can do anything with it from creeping through the woods to racing it also has great power for hills and for those muddy areas.

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  4. my friend has a yamaha yz125 that out runs my yz250 and he weighs more than why is that? + my 250 is newer than his and in better shape did i get a lemon?

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  5. yz 250f? The 125 could be bored. I had a RM 125 and when racing next to kids lighter then me on 250f's, I would run right next to them. The four stroke has more bottom end pwr. But once you get out of that there about the same. It comes down to preference on how you want to ride. Theres a huge difference in riding and control between a 4 stroke and 2 stroke. Naturaly a 4 stroke is going to be a little heavier. But gives you more on the bottom because of valves. Either way there both a blast to ride.

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  6. 2 strokes are way better

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  7. i have a kx 250 and the power band is amazing i race a buddy that has a yamaha 250f that was decked out with performance parts and bored out we raced down a dirt road my tire wouldnt stop spinning and i still beat him 2 strokes are better!

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  8. 4 strokes are better

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  9. I have a 4 stroke and all my friends want me to get a 2 stroke and I want one but my dad thinks i will get hurt and it will have a ton of troubles cause when he was a kid he had one and always had trouble

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  10. Dude get a 2 stroke they are more fun easier to work on and you can get hurt just as easy on a 4 stroke

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  11. Yeah my friend has a two ktm 2 stroke and I have a yamaha 4 stroke and I like to ride his bike better than mine, now I'm trying to get my dad to get me one.

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  12. 2 Wheels or NOTHING!!! Sometimes 1...September 17, 2010 at 9:07 AM

    I have a Stock 2005 KX 250 and a few of my family members have; Pro Circuit(ed) out Yz 450's and the only time that they can even get close to catching me is right out of a turn on a sharp berm, or the starting line if I do not use the clutch perfectly... Because I am spinning for a while, but then again half the time I am roosting them so hard that they can't concentrate enough to pass me. Other than that I leave them everywhere: Strait aways, Whoops, Scrubbing/Whips(cuz 2 strokes are lighter... And BETTER!!!!), Sweeping Turns(cuz you can start the Drift whenever you want, where-as the 450's have to either pop the clutch or down-shift, the 2 stroke 250 just has to be given a little more gas). What really trips me out is in the long run, I still leave them, they have a ton of after-market parts and a larger engine... I have Stock Kawasaki parts. Anyone wanna tell me why this happens!?!?! OOOOOoooo Yeah! I know! 2 Strokes OWN 4 stroke thumpers!!!

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  13. I can say 2 strokes are good. I feel the ride when I'm using 2 stokes.

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  14. i have a 2000 xr100r, and next spring im getting a yz125,will it be hard to adjust from 4 to 2 stroke?

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  15. ktm 250 sx has the bottom end like a 4 stroke and a sick top end 52 rear tooth acceleration is off the charts 2 strokes for life

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  16. Is that the only reason 2 strokes are no longer in production is because of EPA?

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    Replies
    1. they still make them !!!

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  17. i have a 2001 yamaha 125 cc, very fast and very powerful and very high. The thing i hate about is it takes a lot of mixed gas

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  18. 2 stoke has more accleration

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