The position of the ports is important for how mixture enters the cylinder, and for how burned gases leave it. Removing exhaust gases is called scavenging. If they aren’t fully removed, it reduces engine efficiency.
Some engines use a deflector on the piston crown. It directs air-fuel mixture up towards the spark plug, and exhaust gases down towards the exhaust port. This reduces the amount of incoming mixture escaping through the exhaust port and helps remove exhaust gases.
How scavenging occurs depends on the design of the cylinder and ports. Crossflow scavenging occurs when mixture enters on one side of the cylinder, and exhaust gases exit on the other side. No valves are used, and mixture flows across the cylinder.
Loop scavenging means scavenge mixture enters the cylinder across the top of the piston and it is caused to flow upwards in a loop before moving in the direction of the exhaust port.
