CHAPTER 4. Powerplants

Two-Stroke Process

The two-stroke process begins with the fuel entering the engine and concludes as it exits as exhaust.

Crankcase Vacuum Intake Stroke—Piston Moving Up

The upward stroke of the piston [Figure 4-6A] creates a vacuum in the crankcase and pulls the fuel/air/oil mixture into the crankcase through the intake valve system from the carburetor. [Figure 4-6B] This can be a pressure-actuated reed valve, a rotary valve, or a ported inlet system where the lower piston skirt provides an opening for the fuel/air/oil mixture to fl ow in when the piston is reaching its highest point of top dead center (TDC). At this point, the greatest portion of the fuel/oil/air mixture has fi lled the crankcase. [Figure 4-6B]

Crankcase Compression Stroke—Piston Moving Down

During the downward stroke, the pressure valve is forcibly closed by the increased crankcase pressure, the mechanical rotary valve closes, or the piston closes off the fuel/air oil mixture intake port as shown. The fuel/oil/air mixture is then compressed in the crankcase during the downward stroke of the piston. [Figures 4-6B to 4-6D]

Crankcase Transfer/Exhaust—Piston at Lowest

When the piston is near the bottom of its stroke, the transfer port opening from the crankcase to the combustion chamber is exposed, and the high pressure fuel/air mixture in the crankcase transfers around the piston into the main cylinder. This fresh fuel/oil/air mixture pushes out the exhaust (called scavenging) as the piston is at its lowest point and the exhaust port is open. Some of the fresh fuel/oil/air mixture can escape through the exhaust port, resulting in the higher fuel use of the two-stroke engine. [Figure 4-6D]

Cylinder Start of Compression Stroke—Piston Initially Moving Up

As the piston starts to move up, covering the transfer port, the tuned exhaust bounces a pressure wave at the precise time across the exhaust port to minimize the fuel/air/oil mixture escaping through the exhaust port. [Figure 4-6E]

Cylinder Compression Stroke—Piston Moving Up

The piston then rises and compresses the fuel mixture in the combustion chamber. [Figure 4-6E to 4-6F] During this piston compression process, the crankcase vacuum intake process is happening simultaneously, as described earlier. This is why four processes can happen in two strokes. [Figures 4-6B and 4-6C]

Cylinder Power Stroke—Initial Piston Moving Down

At the top of the stroke, the spark plug ignites the fuel/oil/air mixture and drives the piston down as the power stroke of the engine. [Figures 4-6F and 4-6G]

Cylinder Power Stroke—Final Piston Moving Down

As the piston passes the exhaust port, the exhaust exits the combustion chamber. As the piston continues down, the transfer port opens and the swirling motion of the fuel/ oil/air mixture pushes the exhaust out of the exhaust port. [Figures 4-6H]

Piston Reverses Direction From Down Stroke to Up Stroke

As the piston reverses direction from the down stroke to the up stroke, the process is complete. [Figures 4-6H and 4-6A]

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