In a piston-ported two-stroke engine however, the situation is more complicated due to the need to prevent unburned fuel/air mixture from passing right through the cylinders into the exhaust. During the exhaust phase of the cycle, back pressure is even more undesirable than in a four-stroke engine due to the shorter time available for exhaust and the lack of pumping action from the piston to force the exhaust out of the cylinder. However, since the exhaust port necessarily remains open for a time after scavenging is completed, unburned mixture can follow the exhaust out of the cylinder, wasting fuel and increasing pollution, and this can only be prevented if the pressure at the exhaust port is greater than that in the cylinder.