Thursday, July 2, 2020

Variable Exhaust Valve Closing (VEC)

  • 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.

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