Why do large fuel injectors give poor exhaust emissions at idle speed?
The fuel injector is basically an electronically controlled tap & spray mechanism. Larger size injectors have larger sized nozzles. Injectors have only two positions, fully open, or fully closed, it is not possible to only half open an injector. To control the quantity of fuel provided, the injector is held open for a number of milliseconds by the engine management system. It is a basic fact that a known quantity of fuel is required for a certain level of horsepower, and the injector size (or flow rate) should be determined to provide this in the time allowed. For example, at 6000 RPM, there are only 20 milliseconds (0.020 seconds) in-between the ignition of each cylinder. If the fuelling requirements require a large injector to give high power at high engine speeds, this will require a very short opening duration at low engine speeds. In effect, a large, thin, heavy mass of fuel will be injected into the inlet air. As this will be unable to atomise fully within the air, not all of the
