The purpose of an expansion chamber is to create a shock wave in the reverse direction from the initial exhaust pulse when the exhaust port opens. This returning pressure pulse keeps the intake charge in the cylinder. All the magic is in how this is tuned. In a DI 2stroke, the fuel is injected under high pressure AFTER the port is closed on the piston's way back up to TDC. So, no shock wave needed. This is also why emissions are low. No big pipe needed but a high pressure injection system, ECU, etc. are.
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