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Old 05-22-2013, 10:34 AM
nerobro nerobro is offline
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Join Date: Dec 2009
Location: Chicagoland
Posts: 25
Default Re: difference in a 4 stroke and 2 stroke motor

Originally Posted by dumpstercrusher View Post
whats the difference between the 4 stroke motors and 2 stroke besides the less moving parts on the 2 stroke? more reliable? i know 4 strokes can be a little harder to put on a bike due to it being a larger motor. would need a wider pedal crank
You're mixing up a few things here. But ... lets see if I can't make it make sense.

Piston engines run on the Otto cycle. That is the classic suck, squish, bang, blow.

4 stroke engines, typically have two sets of valves, and a geared camshaft to ensure all of those functions happen at the right time, and right order. So, on a 4 stroke, the piston travels down, with the intake valve open, and the exhaust valve closed. This sucks in air and fuel. At the bottom of the stroke, the intake valve closes, and the piston travels up, compressing the air fuel mixture. At the top, you get the bang, when the spark plug does it thing and ignites the fuel air mixture. This heated air pushes down on the piston and this is where you get the engines power from. At the bottom of the stroke, the exhaust valve opens, and the piston travels up again, pushing the burnt air out. "Blow."

This segregated, and carefully timed process means you get to burn most of the air in the cylinder EVERY OTHER rotation.

Two strokes... are a lot more funny. They manage to squeeze all four processes into two strokes. We'll start the description with the piston at the top of the stroke, with compressed fuel and air in the cylinder.

The spark plug fires.. and the piston starts traveling downwards. About halfway down, a hole opens in the side of the cylinder wall, and the burnt gasses start rushing out. This air has momentum... this is important to remember.

3/4 of the way down, a hole opens on the opposite side of the cylinder. That hole leads to the crankcase. The air and fuel is under a little bit of pressure, and assisted by the rush of exhaust gets blown, and sucked, into the cylinder.

This is a messy process, so most of the time not all of the air really gets exchanged.

The piston starts moving up again. Closing off the hole in the cylinder wall. (and in our engines..) The skirt, as it travels up, also opens the passage to the intake manifold, and carburetor. The piston traveling up, sucks fuel and air into the crankcase.

As the piston travels, it also closes off the exhaust port. Now the cylinder is sealed, and the engine is compressing the air and fuel mixture, getting ready to spark the spark plug and start the next cycle.

So... why did I explain all of that? 2 stroke motors are "tuned." Like a tuba, or trombone. They work really well at certain speeds, and throttle positions, but when they're not operating at their ideal rpm range (which is pretty short on most motors) their torque is very low.

Between disk valves, or reed valves, a good tuned pipe, and a few other tricks, 2 stroke motors can get absolutely clean air in their cylinder, and make lots and lots of power, at certain RPM. Sadly, they don't operate there much. The classic 2 stroke sound is actually the motor 4, 6 or 8 stroking, because they're not getting enough clean air in the cylinder to burn. So the motor skips a few cycles as good air cycles in.

4 stroke motors make good torque at very low rpm, and their torque curve falls off in a smooth fashion. 2 stroke motors make low torque at low rpm, good torque at their operating rpm, and very low torque when you exceed their operating rpm.
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