14 Valve timing

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Valve timing IVO : 10°-40°before TDC IVC : 40°-60°after BDC EVO : 40°-60°before BDC EVC : 10°-30°after TDC In the ideal cycles, valves open and close instantaneously at piston dead centre. In practice, they open and close in finite time (to maintain acceptable accelerations and velocities) and often quite far from the piston dead centre, for fluid and/or thermo-dynamic reasons. Usually the opening time periods for the inlet and exhaust valve are reported on two circular diagrams or in a linear diagram. EVO (Exhaust valve opening) Exhaust valves open 40°-60°before BDC to lower the pressure inside the cylinder, before the beginning of the exhaust stroke, but avoiding, on the other hand, a heavy reduction of the expansion work. When piston approaches BDC, the pressure inside the cylinder is quite low (0.3-0.6 MPa), the valve flow area increases slowly and the crank arm is smaller and smaller. For this reason, this early opening does not represent a significant loss in terms of expansion work, but it produces a spontaneous discharge (blow-down) of part of the gases into the exhaust manifold, due to the pressure head across the seat valve. In this way, the work done by the piston to expel the gases during the exhaust stroke is reduced. By advancing the EVO, the work lost in expansion increases, but the negative expulsion works decreases. The optimum EVO value, for a given engine speed, is the one which offers the best compromise between these two contrasting effects. This value can be initially estimated by a suitable computer model, and then refined by experimental tests. IVC (Inlet valve closing) Intake valve closes (IVC) quite late (40°-80°after BDC), to provide more time for fresh charge to enter the cylinder, when the pressure level in the inlet pipe is higher than…