FUEL INJECTORS | DIESEL INJECTORS
Engine Fuel Injectors dispense fluid such as petrol and diesel under high pressure which is converted into a high-velocity jet at the throat of the convergent-divergent nozzle which creates a low pressure at that point. The low pressure draws the suction fluid into the convergent-divergent nozzle where it mixes with the motive fluid. The Venturi effect, applies to the operation of this device.
This means, the pressure energy of the inlet motive fluid is converted to kinetic energy in the form of velocity head at the throat of the injectors nozzle. As the mixed fluid then expands in the divergent diffuser, the kinetic energy is converted back to pressure energy at the diffuser outlet. The optimum amount of injected fuel depends on conditions such as engine and ambient temperatures, engine speed and workload, and exhaust gas composition.
Central to an EFI system is a computer called the ECU or Engine Control Unit which monitors engine operating parameters via various sensors strategically placed throughout an engine. The ECU interprets these parameters in order to calculate the appropriate amount of fuel to be injected, among other tasks, and controls engine operation by manipulating fuel and/or air flow as well as other variables.
The electronic fuel injector is normally closed, and opens to inject pressurized fuel as long as electricity is applied to the injector's solenoid coil. The duration of this operation, called the pulse width, is proportional to the amount of fuel desired. The electric pulse may be applied in closely controlled sequence with the valve events on each individual cylinder, or in groups of less than the total number of injectors called a batch fire system.
Since the nature of fuel injection dispenses fuel in discrete amounts, and since the nature of the four-stroke engine has discrete induction events, the ECU calculates fuel in discrete amounts. In a sequential system, the injected fuel mass is tailored for each individual induction event. Every induction event, of every cylinder, of the entire engine, is a separate fuel mass calculation, and each injector receives a unique pulse width based on that cylinder's fuel requirements.
It is necessary to know the mass of air the engine "breathes" during the air flow of each induction event. This is proportional to the intake manifold's air pressure/temperature, which is proportional to throttle position. The amount of air inducted in each intake event is known as "air-charge", and this can be determined using several methods.
The three elemental ingredients for combustion are fuel, air and ignition or “spark”. However, complete combustion can only occur if the air and fuel is present in the exact ratio which allows all the carbon and hydrogen from the fuel to combine with all the oxygen in the air in exact increments or air mass flow. Oxygen sensors monitor the amount of oxygen in the exhaust, and the ECU uses this information to adjust the air-to-fuel ratio in real-time and this is referred to as stoichiometry.
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