A two-stroke engine completes its power cycle in a single revolution of the crankshaft (two piston strokes), so there is no time for separate suction and exhaust strokes as in a four-stroke engine. Instead of poppet valves worked by a camshaft, the two-stroke design typically uses ports — openings cut into the cylinder wall that are alternately covered and uncovered by the moving piston itself.

In a two-stroke diesel engine, as the piston travels down towards BDC (bottom dead centre) it first uncovers the exhaust ports, letting the high-pressure burnt gases blow down and escape. Slightly later the scavenge (inlet) ports are uncovered and fresh air, usually delivered under slight pressure by a blower or scavenge pump, sweeps in and pushes the remaining exhaust gases out — a process called scavenging. Thus the exhaust gases leave through ports, which is the correct answer.

• Spark plug is used only in spark-ignition (petrol) engines to ignite the charge; a diesel ignites by compression and has no spark plug, and in any case a spark plug does not vent exhaust.
• Carburettor is a fuel–air mixing device found on petrol engines, not a diesel; it has nothing to do with removing exhaust.
• Valves are the exhaust route in four-stroke engines (and in some large uniflow two-strokes an exhaust valve is used), but for the standard port-scavenged two-stroke described here, exhaust leaves through ports rather than conventional valves.

Hence exhaust gases in a two-stroke diesel engine are removed through ports.