An indirect injection diesel engine pumps fuel into a prechamber, where combustion starts and then spreads to the main combustion chamber. The prechamber is made to ensure proper mixing of the atomized fuel with the heated air.
The purpose of indirect injection is to speed up combustion to increase the power output by increasing the engine speed. Indirect injection systems have air that moves fast, mixing the fuel and air. This usually simplifies injector design and lets use smaller engines and less tightly toleranced designs. This allows designing of engines that are more powerful yet less costly to manufacture.