TWO-STROKE
The term two-stroke cycle means that the engine develops a power im-pulse every time the piston moves down. The cylinder normally has two ports, or passages, one (called the intake port) to admit the air-fuel mixture, the other to allow burned gases to escape to the atmosphere. These ports are covered and uncovered by the piston as it moves up and down.
When the piston moves upward, the space it occupied in the lower part of the engine block becomes a vacuum. Air rushes in to fill the void, but before it can get in, it must pass through an atomizer called a carburetor,
where it picks up fuel droplets. The air pushes open a spring metal flapper over an opening in the crankcase and with the fuel enters the crankcase.
When the piston moves down, it pushes both against the connecting rod and crankshaft, and the air-fuel mixture as well, partly compressing it. At a certain point, the piston uncovers the intake port. This port leads from the
crankcase to the cylinder above the piston, permitting the compressed airfuel mixture in the crankcase to flow into the cylinder.
Now let’s look at an actual power cycle in 1-2, beginning with the piston in the lowest portion of its up-and-down stroke in the cylinder. The air-fuel mixture is flowing in and beginning to push burned exhaust gases
out the exhaust port, which also is uncovered.
The piston begins to move up, simultaneously completing the job of pushing the burned exhaust gases out of the exhaust port, and compressing the air-fuel mixture in the cylinder. When the piston reaches the top of the
cylinder, the piston is covering two ports, and the air-fuel mixture is highly compressed. At this point a spark plug, threaded into the combustion chamber, delivers a spark that ignites the mix. The greater the amount of compression, the greater the force of the explosion, and the greater the downward pressure on the piston.
The piston is forced downward and transfers the force through the connecting rod to the crankshaft, turning it. The downward moving piston also uncovers the exhaust port, then the intake port and again begins the
job of compressing the air-fuel mixture in the crankcase, to force it to flow into the cylinder above.
Although most two-cycle engines use the flapper valve, called a reed, in the crankcase, some engines do not. They have a third port, covered and uncovered by fhe piston, that permits the air-fuel mixture to flow into the
void in the crankcase created by the upward moving piston. See 1-3.
Post time: Jun-30-2023