4.5.5.b Exhaust valve
The exhaust valve opens before the piston reaches BDC. In this case, the gases that are under high pressure inside the cylinder, quickly start exhausting into the muffler, although the piston is still moving down. Then the piston, upon passing the BDC and moving towards the TDC, will push out the gases remaining in the cylinder. The exhaust valve closes when the piston goes over TDC. Despite the fact that the piston will start moving down a little, the gases will continue to leave the cylinder by inertia and due to the suction effect of the flow of gases moving in the exhaust pipeline. Thus, the opening time of the exhaust valve is longer than the time during which the shaft half turn occurs, and the cylinder is better cleaned of exhaust gases.
Note
The angle of rotation of the crank, which corresponds to the position at which the intake and exhaust valves are simultaneously open, is called the valve overlap angle. Due to the insignificance of this angle and the negligible size of the gap between the valves and sockets, the possibility of leakage of the combustible mixture is excluded. Overlapping of valves is necessary for additional purging of the cylinder in order to better fill it with fresh charge.
A slight decrease in the pressure of gases on the piston, which occurs during the working stroke due to the early opening of the exhaust valve, and the loss of part of the work of the gases. In this case, it is compensated by the fact that the piston, which is moving upward during the exhaust stroke, does not experience much resistance from gases remaining in a small amount in cylinder.
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