4-stroke power

[Racer#213]

I was just thinking, why does the 4-strokes need the extra displacmetn to be equal to the 2-stroke? like why isn't a 250 4 the same as a 250 2? is it from how the valves work, exhaust ports, piston. what is it that makes them need this extra displacement

 

[MDA]

Here is the condenced version as I understand it:

In a two stroke, every time a piston reaches the top, there is a detonation and hence a power stroke on the way down. A four stroke only detonates every second time it reaches the top of its stroke. This means that the four stroke has hitorically needed almost twice the displacement to achieve the same horsepower.

There are many other differences in the engines (when exhaust gases are vented, when air/gas is injected into the cylinder, etc, etc), but the major difference is how often the fuel in combusted that differentiates the two engine types.

I hope I got this right. If I didn't I'm sure someone will make darn sure to correct me.

-Mark-

 

[Racer#213]

oh yea, i think your right. i forgot about that. that does sound right though

 

[kevinkdx]

actually a 2 stroke only fires every second time the piston comes to tdc hence the name two stroke. and a four stroke fires on every 4th stroke hence the name 4 stroke.

 

[jmics19067]

Not quite Kevin. 2 stroke would be one up on down firing once every revolution.

4 stroke would be compression <up> firing<down> exhaust <up> intake <down> the firing once every 2 revolutions.

one stroke is either up or down

 

[mxneagle]

The way I make logic out of allowing twice the displacement for a 4 stroke is that they only run half the time ;) Interpret that as you wish

 

[dirtfreak]

Two strokes fire every revolution (or every time the piston is at TDC). Four strokes fire every other revolution (or every other time the piston is at TDC). The reason they are called "two strokes" and "four strokes" is b/c a "stroke" is half of a revolution of the crankshaft. That would be why a two stroke fires every revolution, b/c there are two strokes involved in that one revolution (up and down). Same principle for a four stroke. That is why four strokes need roughly twice the cc's to compete with a two stroke, b/c two strokes fire twice as much as four strokes do, therefore making roughly twice the power of an equal sized four stroke. Granted, like MDA said, the engine design (the flow of air/fuel mixture) plays a big part in the performance as well. Hope I didn't come across as confusing.

 

[yzbluebeast]

When the piston moves from TDC to BDC that is one stroke and vice-versa.
In a 4 stroke motor, because of the intake and exhaust valves and the lack of cylinder ports, it takes 4 strokes of the piston to complete one cycle(intake, compression, combustion and exhaust)
Per every 720 degrees of crank rotation a 4 stroke motor only has one combustion stroke.
Because on a 2 stroke motor intake and combustion happen simultaniously and exhaust and transfer happen simultaniously, there is a combustion stroke every 360 degrees of crank rotation.
This is why 4 strokes need almost twice the displacement to equal a 2 stroke.

 

[VinnyTAMU]

This will explain everything.

2-stroke
http://science.howstuffworks.com/two-stroke2.htm

4-stroke
http://science.howstuffworks.com/engine2.htm

 

[VinnyTAMU]

Also here is some good information about how expansion chambers work on a 2-stroke.

http://auto.howstuffworks.com/framed.htm?parent=question635.htm&url=http://www.stannakajak.com/skalmanmc/rd350/tuning/expan1.htm