During the intake stroke, the piston is pulled downward in the cylinder by the rotation of the crankshaft. This decreases the pressure in the cylinder and causes air under atmospheric pressure to flow via the carburetor which meters the correct amount of fuel. Then, the fuel-air mixture makes its way through the intake pipes and intake valves into the cylinders.
In order to induce a greater quantity of the fuel-air charge into the cylinder and increase horsepower, the intake valve is opened before the piston reaches the TDC (Top Dead Center) on the exhaust stroke. The extent to which the valve opens is limited by several factors, including the possibility that hot gasses remaining in the cylinder from the previous cycle may make their way back into the intake pipe and induction system.
Meanwhile, the intake valve is timed to close somewhere between 50 and 75 degrees past the BDC (Bottom Dead Center) on the compression stroke, depending on the engine in question. Because of the large volume of the cylinder above the piston when the piston is near the BDC, the slight upward travel of the piston during this time does not have a major effect on the incoming flow of gasses.
Once the intake valve is closed, the continued upward movement of the piston compresses the fuel-air mixture to acquire the necessary burning and expansion characteristics. The charge is fired by means of an electric spark as the piston approaches the TDC. Typically, the time of ignition varies from 30 to 35 degrees before reaching the TDC based on the requirements of the specific engine to obtain complete combustion of the mixture before the piston passes the TDC position. Keep in mind that many factors affect ignition timing; thus, manufacturers must spend a considerable amount of time determining the best setting.
As the piston moves through the TDC position at the end of the compression stroke and begins the power stroke, it is pushed downward by the rapid expansion of the burning gasses in the cylinder head with great force. As the piston moves downward during the power stroke, the downward movement of the connecting rod is changed to rotary movement by the crankshaft. From here, the rotary movement is transmitted to the propeller shaft to drive the propeller.
As the burning gasses expand, the temperature drops to a safe limit before they flow out the exhaust port. The timing of the exhaust valve opening is determined by the extent to which expansive force is needed and how quickly the cylinder is scavenged. Thorough scavenging is critical as it ensures any exhaust product remaining in the cylinder dilutes the incoming fuel-air charge at the start of the next cycle.
The piston travels through the BDC at the end of the power stroke, starts upward on the exhaust stroke, and pushes the burned exhaust gasses out of the exhaust port. The speed at which the gasses escape creates low pressure within the cylinder. This reduced pressure speeds the flow of the fresh fuel-air mixture into the cylinder as the intake valve is beginning to open. It is worth noting that the intake valve opens between 8 and 55 degrees before the TDC on the exhaust stroke in a number of engines.
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