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Why exhaust valve opens before inlet ports are uncovered

  • When the exhaust valve opens before the inlet ports open, the exhaust pressure is slightly higher than the atmospheric pressure so it automatically drives the exhaust gas out from the cylinder. This is called blowing down effect. 
  • This is highly important because it is the time when maximum amount of the exhaust gas is removed from the liner. When the piston is moving upwards, it is necessary that the exhaust gas pressure should be dropped sufficiently down so that energy is not exerted by the piston to remove the exhaust gas. This helps to avoid the pumping loses and the backpressure.
  •  If sufficient exhaust gas pressure is still existing when the piston is moving upwards, then this exhaust gas has to be driven by the upcoming piston which causes the pumping loses as power required for this is taken from the engine. At the same time, it reduces the backpressure.
  •  If there is backpressure on the upward moving piston, it will have the same effect i.e. it will have more resistance for the upward movement. To overcome this resistance, energy is required which is taken from the crankshaft. Ultimately, the total power output of the engine is reduced.
  •  At the same time, the specific fuel oil consumption will be reduced as energy is required for removing the exhaust gas from the cylinder. 
  • It is therefore necessary to open the exhaust valve much early before BDC so that the blowdown effect is useful in removing maximum amount of exhaust gas from the cylinder and thus reduce the pumping loses and backpressure on the piston.
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