The working of a double acting steam engine is a shown in Fig. 5 3. As the piston reaches the inner dead centre (IDC) position A, the D-slide valve clears the inlet port C and admits the steam into the cylinder. This high pressure steam pushes the piston on the forward direction and performs work by rotating the crank and eccentric. The steam continues to enter into the cylinder maintaining a constant pressure inside. With the further rotation of crank and eccentric, D-slide valve is moved backwards and closes the inlet port C. The steam supply to the cylinder is stopped and this is known as cut-off of steam. After the cut-off the steam in the cylinder expands pushing the piston in the forward direction till the piston reaches the outer dead centre (ODC) position B. Just before the ODC position, the valve connects the cylinder through the port to exhaust E. Therefore the pressure on this side of cylinder falls to the atmospheric pressure or condenser pressure. After the ODC position the piston reverses its direction of motion and the return stroke starts. The steam is admitted through port D and exhausted on the IDC side. Just before the piston reaches the IDC position, the port D closes and low pressure steam in the cylinder is compressed and acts as a cushion to the piston at the end of the stroke. As the piston reaches the IDC position, the D-slide valve opens the port C and the cycle is repeated. When admission and expansion take place on head end side, exhaust and compression takes place on the crank end side.
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