Follow the explanation
Back to the model ↑How the parts work together
Power and exhaust
From top dead centre the burning charge expands and drives the piston down. Near 100° the exhaust port opens, allowing pressure to fall before the transfer port opens.
Scavenge
Around the bottom of the stroke, fresh mixture enters through the transfer port and sweeps across the cylinder toward the exhaust. Real engines tune this flow carefully to limit short-circuiting.
Close the ports
On the return stroke the transfer port closes first, then the exhaust port. The rising piston compresses the trapped fresh charge while the crankcase draws in the next charge.
Ignite and repeat
A spark near top dead centre starts the next expansion. Shift the timing control to see the marked ignition event move; this lesson does not calculate flame speed or efficiency.
The main parts
- Cylinder
- The bore guides the piston and contains the changing charge above it.
- Piston
- Its skirt uncovers the ports; its crown receives the expanding gas pressure.
- Connecting rod
- A rigid link that transmits the piston motion to the crank pin.
- Crankshaft
- The eccentric pin turns the rod's reciprocating motion into rotation.
- Crankcase and transfer passage
- The sealed lower space is the pump chamber in this schematic. It directs a fresh charge through the green passage toward the transfer port; real passage shapes and pressure waves are not modeled.
- Exhaust port
- Opens first near the bottom of the power stroke so spent gas can leave.
- Transfer port
- Opens after exhaust and admits a fresh charge from the crankcase.
- Spark plug
- Ignition is shown near top dead centre; timing is a control, not a combustion solver.