A mode of injection describes the timing, and the sequence, of injecting fuel.
Simultaneous injection means, every injector opens at the same time. Fuel sprays into each intake port, where it stays, until the inlet valve opens. During each engine cycle, the injectors open twice, and each time they deliver half the fuel needs of each cylinder. This happens regardless of the position of the intake valve. The injectors are triggered by the ignition system. So, for a 6-cylinder engine, the control unit triggers the injectors on every third ignition pulse.
Sequential injection means injection for each cylinder occurs once per engine cycle. It is timed to each individual cylinder in the firing order. Fuel spray stays in the intake port until the inlet valve opens. Grouped injection divides the injectors into 2 groups. A 6-cylinder engine can have injectors 1, 2 and 3 in group 1, and injectors 4, 5 and 6 in group 2. The control unit operates the groups in turn, to spray fuel once per engine cycle. Group 1 injects, then, 360°, or one crankshaft rotation later, so does group 2. This happens, regardless of the position of the intake valve. Just one injection provides the full quantity of fuel for each cylinder during that engine cycle.
In some applications, different modes of injection are combined, so that the mode changes according to the operating conditions. Sequential mode may be used for low engine speeds, changing to simultaneous mode at high speeds. The same principle is used in changing from light loads to heavy loads. Similarly, the mode may change from group injection, to simultaneous.
Using different modes for different operating conditions makes the most of how the fuel is used, and that improves power output, fuel economy and emission control.