**2.4 Exhaust stroke**

The upward movement of the piston again from BDC to TDC pushes out the burnt gases through the fully open exhaust valve. As the piston approaches the TDC, the intake valve also opens. This allows admission of fresh air into the cylinder, the density of which is greater than that of burnt gases, displace them and fill up the cylinder again. The pressure drop created by early opening of the exhaust valve also reduces the work done by the piston in pushing out the burnt gases. The cycle then continues.
