From the above sketch,

• 1-2 piston is moving upwards, scavenging the cylinder
• 2-3 Scavenging ports are shut, exhaust closing
• 3-4 Compression
• 4-5 Fuel injection and combustion cause rapid rise in pressure
• 5-6 Expansion: Piston forced down by expanding gases
• 6-7 Exhaust opens, cylinder blowdown, rapid pressure drop
• 7-1 Scavenge ports open, scavenging commences