khodges

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A 2-stroke engine has a power stroke every revolution of the engine's crankshaft, so each time the piston reaches the top of its stroke, it fires. As the piston travels down in the power stroke, it also exhausts the spent fuel/air charge through the exhaust port and begins to take in a new fuel/air charge through the intake and transfer port, which is coming from the crankcase, which is under pressure from the descending piston. As the piston passes bottom dead center and starts up on a new compression stroke, the pressure in the crankcase is reversed, making a partial vacuum and drawing fuel and air from the carburetor into the crankcase. As the piston reaches the top of the stroke, it fires again, and the cycle repeats. SO--- one stroke up, and one stroke down, there's your 2 stroke cycle.

A four stroke engine has a power stroke every other revolution of the crankshaft. Since it does not depend on crankcase pressure to move the fuel/air mix, it has to draw the mix in through a valve, controlled by a camshaft which is geared to the crank at a 1 : 2 ratio ( the camshaft rotates once to two turns for the crank). As the piston starts down, the cam opens the intake valve, and the fuel/air mix enters the cylinder from the carburetor. As the piston reaches the bottom of its stroke, the cam closes the intake valve, and the piston starts back up again, compressing the mixture, which fires at the top of the stroke from compression heat and catalytic reaction of methanol and platinum (the glow element), and then you have the power stroke as the piston is pushed down by the burning mix. As the piston reaches the bottom of the power stroke, the rotating cam then opens the exhaust valve, and the rising piston pushes the spent charge out the exhaust. As the piston reaches the top, the intake opens and the cycle repeats. SO-- one stroke up, compression, one stroke down, power, one stroke up exhaust, one stroke down, intake, and there's your 4-stroke cycle.

Part of the reason there's so much difference in how 2- and 4- strokes make power lies in the combination of strokes in the 2- stroke cycle. Since the intake and exhaust ports are about half way down the cylinder, as the piston is moving down on the power stroke, as soon as the exhaust port is uncovered, the pressure pushing the piston is released through the port, so only about half of the downward travel of the piston actually produces power. A 4-stoke, on the other hand, has the piston "pushed" almost its full travel on the power stroke before the exhaust valve opens and releases the pressure, so more power is made for each downward motion of the piston. I compare it to the difference between having someone shove you in the chest, vs someone punching you in the chest. Both are going to produce motion (power), but the 2-stroke is dependent on the more frequent power pulses and a generally higher rate of making them (RPM), where a 4-stroke gets more energy per stroke but makes them less frequently and at a generally lower rate. This translates into the greater torque at lower rpm's in a 4-stroke vs a 2-stroke, but the overall higher horsepower at higher rpm's in a 2-stroke.

This is a fairly simplified explanation, because there are many variables to consider, such as port size and timing(2-strokes) and valve size and overlap (4-strokes), ignition advance, fuel mix, etc. But these are the basics for 2- and 4-stroke operation