When a cold engine starts, some of the fuel injected by the main injectors condenses on the cold intake port, or the cylinder walls. Less fuel stays mixed with the air, which weakens the mixture. To overcome this and ensure a rapid start, an extra supply of fuel must be provided.
In some cases, during engine cranking, extra injection pulses in each revolution can provide the extra fuel. It depends on engine temperature, and there is a time limit to prevent flooding. The cranking period is followed by an after-start enrichment. Over about 30 seconds, this slowly reduces to normal warm-up. The engine then responds steadily, immediately after releasing the starter.
More air comes from an auxiliary air valve or bypass air control valve. This bypasses the throttle valve, to raise the engine’s idle speed when it is cold, and during warm-up.
Extra fuel can also come from a separate cold-start injector, normally mounted centrally on the plenum chamber. It’s supplied with fuel under pressure from the fuel rail, and only operates when the engine is cranking. A switch called a thermotime switch, immersed in engine coolant, completes the electrical circuit. It controls the operation, according to engine temperature. This ensures the injector operates under cranking conditions only when the engine is actually cold.
The control unit can help cold-starting and provide cranking enrichment by increasing the pulse width of the injectors. This is in addition to the cold-start injector operation, and is again, temperature-controlled.
Some sequential systems use a pre-injection of fuel. This means all injectors open simultaneously, to provide an initial injection of fuel. This happens only during cold cranking, and there is a time delay, to prevent pre-injection occurring again within a certain time if the engine does not start. The system reverts to sequential injection when the engine starts.
Alternatively, simultaneous injection may be used during cranking, and also in the warm-up period. Once a pre-set temperature is reached, the control unit changes to sequential mode.