Different Strokes for different folks

[DangerousDean]

My friend asked me what the difference between a 2 stroke and 4stroke are, and i really couldnt answer him ... lol

I know some differnces, like the valves, and reeds..

but why 4"stroke" and 2 "stroke"

[416exmx]

one has a 2 stroke gas tank and the other has a 4 stroke gas tank i think........lol

[MILF_HUNTER]

im not 100% sure on this so anybody feel free to correct me if im wrong but a two stroke means that the fuel is injected every 2 revolutions of the pistons hence the name 2 stroke and on a 4 stroke feul is injected every 4 revolutions of the piston.

there for a 2 stroke makes more power but is less feul efficient. i think www.macdizzys.com has alot of info on 2 strokes.

[balla250ex]

Yea, that sounds about right. I've always wondered about this myself, and I thought I had a pretty good idea, but I was still confused. If, you can make an engine that only requires 2 strokes for the intake and exhaust strokes (if that's what you want to call it), then why make a 4. And also, what other 2 strokes are in a 4 stroke engine, you know what I'm saying? Like, what are the purposes of the 2 extra strokes in a 4 stroke compared to a 2 stroke? Maybe I'm confusing you all, I think I'm confusing myself.:cool:

[cheetah]

A two stroke takes two stokes of the piston to complete one cycle. (Intake, combustion, exhaust) A four stroke takes four. I'm not the best to try and explain it, but I can explain a lil about the 4 stroke motor. Starting from TDC the piston goes down on the 1st stroke. The intake valve opens and the fuel/air mixture is drawn into the cylinder. On the second stroke the intake valve closes and the piston compresses the mixture onto the spark plug causing compression. That pushes the piston back down on the 3rd stroke and the exhaust valve opens. The piston rises on the 4th stoke and pushes the exhaust gases out and starts all over. That's pretty much a 4 stroke in a nutshell, someone correct me if I have anything wrong. Now a two stroke does that cycle in 2 strokes, but it has things like reeds instead of valves. I'll leave that to someone else.

[VIC]

Heres a two-stroke

http://www.maximumsled.com/mountain/garage/2strokeprinciples/two-stroke_animation.gif

Here's a 4-stroke (http://science.howstuffworks.com/engine.htm)

[XANDADA]

Why does that 2 stroke gif show the gas mixture going into the crank area? I'm almost positive that intake ports (at least on 250r) are located above the piston at its bdc location which is when the air/fuel mixture enters the cylinder. Am I wrong about this or misinterpreting the gif image:confused: just curious...

[Bretmd94]

There is no oil in the crank area on 2 strokes. Thats why oil is mixed into the gas. Some of the fuel is sucked into the crank area to lube the rest of the motor. When the piston comes down It pushes the fuel back up into the cylinder.

So the 2 stroke kind of pushes out the combusted fuel while putting new fuel in at the same time in the same stroke. So the engine fires every time the piston goes to the top. On a 4 stroke the motor only fires every other time the piston reaches the top. because it uses one of those times to push out the exhaust and suck in more fuel.

Hope that helps a little.

[wilkin250r]

There have been several threads about how a two-stroke engine works. Most people are more familiar with a 4-stroke, so I'll start the explanation there. This will be just a general description, not a full in-depth analysis.

A "stroke" is when the piston travels through the cylinder, either up or down. Each revolution has two strokes. First the piston goes down, then it comes up.

4-stroke. We'll start at intake. The piston is at the top of the cylinder. The intake valve(s) open. The piston moves downwards, createing a vacuum in the cylinder that draws in fresh fuel/air through the carb. The piston reaches the bottom, and the intake valves close. This is one stroke, the intake stroke.

Then the piston moves upwards, but both the intake and exhaust valves are closed. The piston compresses the fuel air mixture. When the piston reaches the top, the fuel/air mixture is under very high compression. This is the second stroke, the compression stroke.

The spark plug fires, and ignites the fuel/air mixture, and it pushes the piston back down. This is the third stroke, called the power or combustion stroke.

Once the piston goes all the way to the bottom, the exhaust valve(s) open up. The piston travels upwards, pushing all the exhaust out the exhaust valves. This is the fourth stroke, the exhaust stroke.

Once the piston reaches the top, the exhaust valves close, and the intake valve opens, and we start at the first intake stroke again. It's called a 4-stroke because it takes for 4 stroke to complete the entire cycle from intake to exhaust.


Two-strokes are simpler in operation, but more complex to explain. The piston actually takes over the valve functions, so you don't have valves, or a camshaft.

You can see from the picture in the earlier post, a two-stroke actually has holes cut into the side of the cylinder wall. These holes are called ports. When the piston travels, it will cover and uncover these ports. Those are the valve functions.

So, to simplify things, we'll break this up into two sections, the cylinder and the crankcase.

Let's pretend that the fresh fuel/air is already in the cylinder. The piston travels upwards, it covers both the intake and the exhaust ports, so the cylinder is sealed. The piston compresses the fuel/air mixture. This is one stroke.

At the top of the stroke, the spark plug fires, the fuel ignites, and the piston travels downwards. Towards the bottom of the cylinder is the exhaust port. Again, remember that the port is an actual physical hole in the cylinder wall. When the piston uncovers the exhaust port, all the exhaust gasses rush out of this hole. A little further down, the piston uncovers the intake port on the opposite side of the cylinder. For reasons I'll explain in the crankcase section, the fuel air is pressurized, so it comes RUSHING into the cylinder, forcing the rest of the exhaust out. Some of the fresh fuel/air also excapes out the open exhaust port. This is the second stroke.

Now, the cylinder is full of fresh fuel/air mixture again, so the cycle begins again. It only takes two strokes to complete the whole cycle.

So, let's look at the crankcase. When the piston travels upwards, in the cylinder, it creates a vacuum inside the crankcase. The carb (and reeds) are actually attached to the crankcase, so the vacuum inside the crankcase actually pulls air through the carb into the crankcase. When the piston begins traveling downwards in the cylinder, it compresses the fuel/air inside the crankcase. The reeds act as a one-way only valve, so air can come IN, but it can't go OUT.

So, combustion is happening in the cylinder, the piston traveling downwards, so the crankcase is getting pressurized. The crankcase has a small tunnel to the intake port of the cylinder. When the piston uncovers the intake port, the pressurized fuel/air mixture inside the crankcase comes rushing into the cylinder through the intake port.

So why doesn't everybody use two-stroke engine? Remember that both intake and exhaust ports are open at the same time, some some fresh fuel/air actually escapes out the exhaust port. Also, since the fuel/air comes through the crankcase, you must mix oil with the fuel to provide lubrication to the crank. When you burn the fuel/oil, you get more smoke. The tree huggers have raised a fit about this, and have all but banned two-stroke motors because they pollute much more than four-stroke motors.

[wilkin250r]

That turned out a little longer than I expected. Maybe I should have split it into two separate posts...

[stupid driver]

well explained wilkin250r.

[SGA]

http://science.howstuffworks.com/two-stroke1.htm

[Dupontster]

All I know is that two storkes don't have a cam 4 strokes do and two strokes have a power band and they sound different.

[lil400exman]

Originally posted by Dupontster
All I know is that two storkes don't have a cam 4 strokes do and two strokes have a power band and they sound different. they dont have a band that makes power its where the engine makes a good load of power and it usually has a hard hit when you get into the begining of the power.a powerband is where the negine makes peak hp too

[wilkin250r]

The powerband is created by pressure waves in the pipe, and is a comples topic in itself.

[lil400exman]

Originally posted by wilkin250r
The powerband is created by pressure waves in the pipe, and is a comples topic in itself. yeah lol i kept it simple for them btw i am finding out how to change the waves with heat tape..........its very very touchy but can really customized to you and your riding!;)

[speedy400]

:huh :huh :huh
I guess ya gotta learn somewhere....-darren

[wilkin250r]

Originally posted by lil400exman
yeah lol i kept it simple for them btw i am finding out how to change the waves with heat tape..........its very very touchy but can really customized to you and your riding!;)

How? Is it just subtle little changes, or are they drastic changes?

[MOFO]

I think that picture summed it up... LOL.

[Bad Habit]

Originally posted by wilkin250r
A "stroke" is when the piston travels through the cylinder, either up or down.

(In my best Beavis & Butthead voice) hehe, You said "stroke".

Good explanation, what kept your fingers from cramping up?:D

[lil400exman]

Originally posted by wilkin250r
How? Is it just subtle little changes, or are they drastic changes?
well it depends on how much you use and hwere you put it. It kind of works along with the pipe to perfect the power where you want it it holds in heat that specific area of the pipe creating a different peak Exhuast point instead of the one your pipe came with. I was talking to sparks about it and thats the way he explained it and told me a good starting point and its made a difference for my woods riding......:) hope i didnt lose you it still doesnt make 100% sense to me but it does work!

[Honda Jay]

two strokes take one revolution to complete a cycle, hense this means combustion and exhaust are the 2 strokes (u, down). the intake and the exhaust are mixed together at once and that is why the power is so unpredictable (having different air/fuel ratios every time)

where as a 4stroke takes two revolutions to complete a cycle, hense the intake, compression, combustion, exhaust. all the strokes have there own purpose this way, and creates a steady power band.

My motor shop teacher easily put it (4stroke)
suck, squeeze, bang, blow
not to be offensive

just my .02
Jayson

:D

[SGA]

Heres the invention of the expansion chamber. Its alot of reading!
How Two-Stroke Expansion Chambers Work, and Why You Should Care.
http://www.motorcycle.com/mo/mcnuts/em-pipes.html