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Two Stroke? Four Stroke? What's the Diff?

 
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ericalm
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PostPosted: Wed Mar 09, 2011 4:53 pm    Post subject: Two Stroke? Four Stroke? What's the Diff? Reply with quote

I almost included this in my Stella 4T review, but that was already getting very long and I figured it might make a good reference on its own.

This is a pretty basic, overly simplified explanation. That also describes my depth of understanding of these things, ha.

Some engine basics: Internal combustion engines comprise one or more pistons housed in cylinders. The piston is attached to a crankshaft, which rotates as the piston moves up and down the cylinder. (Up and down are relative here, but they make it easier to envision this.) Your transmission takes that rotation and transfers the power to your wheels as needed.

A "stroke" is the action of a piston traveling the length of the cylinder it's housed in. Bottom to top is one stroke, top back to bottom is the next. Combined, these actions cause the crankshaft to rotate fully once (see illustrations below). The combustion cycle consists of drawing air and fuel into the cylinder, compressing the fuel and air, combustion (igniting the fuel with a spark), expelling exhaust. The engine's power comes from the combustion phase; this controlled explosion provides the force for the motion of the piston.

The basic difference between a two-stroke and four-stroke is how many times the piston will move up and down the cylinder to complete the cycle.

In a two-stroke (2T) engine, the piston will travel the length of the cylinder twice (once up, once down) to complete the cycle. Fuel is ignited every time the piston is at the top of the cylinder. More fuel is drawn into the cylinder and exhausts are expelled in the same stroke. So, in a 2T, the ignition of fuel and gas only has to power one up/down motion of the piston.


The two-stroke cycle. Air and gas in, ignition, exhaust and gas out.

In four-strokes, the intake of air and fuel, compression, combustion, and exhaust actions all occur in separate strokes. This means that combustion occurs every other time the piston is at the top of the cylinder, not every time. As such, combustion is powering twice as many strokes as in a 2T.


The four-stroke cycle. Air and gas in, compression, combustion, exhaust out—all in separate strokes.

So 2T engines, in general, should generate a lot more power because the piston works half as much, right? Kind of. In general, yes, 2Ts are more powerful, but 4T engines are engineered to be as efficient as possible, maximizing the force of combustion. Other factors (transmission, exhaust, fuel, cooling) contribute to how much power is ultimately available to get your scooter moving. Many 4Ts are as fast or faster than 2Ts with equal displacement.

The benefits of the 2T are that they're simpler in design, contain fewer parts, and benefit more from tuning and modification.

The problem with 2Ts (in general) is emissions. To lubricate the engine, oil is mixed with the fuel and burned. In vintage scooters, oil was added directly to the gas ("pre-mixing"). In newer 2Ts, oil is fed from a separate reservoir. This is cleaner than pre-mixing, but the emissions are still dirtied by the burning of oil. Additionally, the 2T cycle just produces more exhaust.

This is why the new 2Ts we have today are all in smaller engines. Lower displacement means smaller cylinders and pistons, shorter strokes, less exhaust and emissions.

In some countries, 2T scooters with catalytic converters are available. These reduce harmful emissions, but have the unfortunate effect of hampering performance.

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Last edited by ericalm on Wed Mar 09, 2011 7:57 pm; edited 1 time in total
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superseagulls
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PostPosted: Wed Mar 09, 2011 7:46 pm    Post subject: Reply with quote

Great write up Eric!

I am concerned how you manage to make a living, considering the work you put in around here, AND finding time to do 3000 miles on the new scoot! Very Happy
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mike_cor
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PostPosted: Wed Mar 09, 2011 7:50 pm    Post subject: Reply with quote

superseagulls wrote:
Great write up Eric!

I am concerned how you manage to make a living, considering the work you put in around here, AND finding time to do 3000 miles on the new scoot! Very Happy


AND live in LA! Laughing
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Skootz Kabootz
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PostPosted: Wed Mar 09, 2011 7:53 pm    Post subject: Reply with quote

Excellent Eric. Really nicely explained. Thanks.
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ericalm
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STELLA FOUR STROKE FURY! + Vespa LX 150/190 + '87 Honda Helix CN250

PostPosted: Wed Mar 09, 2011 8:00 pm    Post subject: Reply with quote

mike_cor wrote:
superseagulls wrote:
Great write up Eric!

I am concerned how you manage to make a living, considering the work you put in around here, AND finding time to do 3000 miles on the new scoot! Very Happy


AND live in LA! Laughing

Uh… make a living…? Smile

Much of this is condensed from Wikipedia. That's where I got the images!

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JHScoot
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PostPosted: Wed Mar 09, 2011 9:05 pm    Post subject: Reply with quote

i'm gonna go out on the limb here and say the difference is two strokes! Razz

ROFL

no but seriously that is some nifty info and some great illustrations. i mean, they move Shocked

i learn so much at MB, what a board!

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mike_cor
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PostPosted: Wed Mar 09, 2011 9:31 pm    Post subject: Reply with quote

So here's a question, if can be explained with some ease. I'm kicking around the idea of getting a Prima pipe/exhaust installed. How does that pipe change the overall performance of the bike with regards to a slightly higher top end? In other words, what does the Prima pipe do that the stock pipe doesn't?

I understand installing it voids the warranty which is why I haven't moved forward on the idea.
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ericalm
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STELLA FOUR STROKE FURY! + Vespa LX 150/190 + '87 Honda Helix CN250

PostPosted: Wed Mar 09, 2011 9:48 pm    Post subject: Reply with quote

mike_cor wrote:
So here's a question, if can be explained with some ease. I'm kicking around the idea of getting a Prima pipe/exhaust installed. How does that pipe change the overall performance of the bike with regards to a slightly higher top end? In other words, what does the Prima pipe do that the stock pipe doesn't?

On a 4T, the last stroke is the exhaust stroke: The exhaust valve opens and the gases are forced out by the piston. From what I understand, performance exhausts for 4Ts work by reducing resistance in the system, meaning the piston won't have to work as hard to push it all out. All of the power comes from the third stroke, so anything that eases the resistance and the amount of force the piston needs in the other three strokes helps create more power. This is also one benefit of fuel injection.

That's about all I know about that, though, so maybe someone more familiar with the inside of an exhaust can provide some details. Smile

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laxer
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PostPosted: Wed Mar 09, 2011 9:56 pm    Post subject: Reply with quote

"Gold jacket, green jacket, who gives a sh**?"
Though I won't be owning a 4T Stella anytime soon, I'm excited to check them out, and maybe to see if I can get a 2T on the cheap.


Last edited by laxer on Wed Mar 09, 2011 9:58 pm; edited 1 time in total
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mike_cor
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PostPosted: Wed Mar 09, 2011 9:56 pm    Post subject: Reply with quote

Ah thats perfect, all I was looking for. Just a basic overall kind of idea on how they all go together, and it makes perfect sense. Thanks!
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BuddyRaton
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2 - many

PostPosted: Thu Mar 10, 2011 1:54 am    Post subject: Reply with quote



Also a decent representation of how expansion chambers work for a 2T and will do nothing for a 4T

As the exhaust gasses expand (expansion chamber..get it! Mr. Green) it creates a lower pressure...pulling more air fuel into the crank case. Then a shock wave is sent back towards the exhaust port acting kind of like a valve to keep the air fuel oil mixture in the cylinder until the piston passes the exhaust port. This results in more air fuel and a higher compression (hence often the need for higher octane fuel). Yeah they can be loud...but loud for a reason..you need that shock wave to kick back!

Most expansion chambers have a sweet spot. On my GT with malossi top end and cut crank (cut to get even MORE air fuel in!) its at high revs...hit that spot and it takes off like a rocket!

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