- In order to optimise the exhaust valve operation over the normal load range, the closing point of the exhaust valve is changed over 70-85% x MCR. During VEC operation the exhaust valve is closed EARLIER which means that the compression stroke of the piston is increased. This will lead to higher compression pressures and temperatures.
- For instance if the normal closage of the exhaust valve is 130o BTDC, and this gives a compression ratio of 10.6, then by closing the valve 10o earlier, the compression ratio will increase to 11.2, and the compression pressure will rise by 4 bar, for the same scavenge air/load setting.
- The unit achieves this by leaking some of the oil contained in the high pressure pipe when the valve is in the OPEN position. This will cause the valve to slightly close, but will mean that the exhaust valve will be fully closed EARLIER than normal. Hence the piston can start to pressurise the cylinder contents earlier. The amount the valve closes is dependant on the position of the VEC shaft and eccentric mounted on the side of the exhaust valve actuator.
Any opening in a pressure vessel is kept to a minimum and for a man entry an elliptical hole is lesser in size than the corresponding circular hole. More over it is prime concern to have a smoothed generous radius at the corners to eliminate stress concentration. Hence other geometrical shapes like rectangle and square are ruled out. To compensate for the loss of material in the shell due to opening, a doubler ring has to be provided around the opening. The thickness of the ring depends on the axis length along the dirrection in which the stresses are maximum and the thickness of the shell. It is important to align the minor axis along the length of the vessel, as the stress in this direction is maximum. Longitudinal stress: Pd/2t where P= pressure inside the vessel, d= diameter of the arc, t= thickness of the shell plating Circumferential stress: Pd/4t More over a considerable material and weight saving is achieved as minor is along the direction of maximum stress.