Fuel Injection
Fuel Injection
The carburetor, despite all its advances: air bleeds, correction jets, acceleration
pumps, emulsion tubes, choke mechanisms, etc., is still a compromise. The limitations
of carburetor design is helping to push the industry toward fuel injection.
Direct fuel injection means that the fuel is sprayed directly into the combustion
chamber. The fuel injected nozzle is located in the combustion chamber.
Throttle Body injection systems locate the injector(s) within the air intake cavity,
or "throttle body". Multi-point systems use one injector per cylinder, and usually
locate the injectors at the mouth of the intake port.
The fuel injector is an electromechanical device that sprays and atomizes the fuel.
The fuel injector is nothing more than a solenoid through which gasoline is metered.
When electric current is applied to the injector coil, a magnetic field is created,
which causes the armature to move upward. This action pulls a spring-loaded ball or
"pintle valve" off its seat. Then, fuel under pressure can flow out of the injector
nozzle. The shape of the pintle valve causes the fuel to be sprayed in a cone-shaped
pattern. When the injector is de-energized, the spring pushes the ball onto its
seat, stopping the flow of fuel.
Mechanical Fuel Injection
Mechanical fuel injection is the oldest of the fuel injection
systems. It uses a throttle linkage and a governor. It is now used mainly
on diesel engines. Hydraulic fuel injection is used by some of the imports.
Hydraulic pressure is applied to a fuel distributor as a switching device
to route fuel to a specific injector. The fuel from the tank is carried
under pressure to the fuel injector(s) by an electric fuel pump, which
is located in or near the fuel tank. All excess is returned to the fuel
tank.
Electronic Fuel Injection
The principle of electronic fuel injection is very simple. Injectors are
opened not by the pressure of the fuel in the delivery lines,
but by solenoids operated by an electronic control unit. Since the fuel
has no resistance to overcome, other than insignificant friction losses,
the pump pressure can be set at very low values, consistent with the limits
of obtaining full atomization with the type of injectors used. The amount
of fuel to be injected is determined by the control unit on the basis of
information fed into it about the engine's operating conditions. This information
will include manifold pressure, accelerator enrichment, cold-start requirements,
idling conditions, outside temperature and barometric pressure. The systems
work with constant pressure and with "variable timed" or "continuous flow"
injection. Compared with mechanical injection systems, the electronic fuel
injection has an impressive set of advantages. It has fewer moving parts,
no need for ultra-precise machining standards, quieter operation, less
power loss, a low electrical requirement, no need for special pump drives,
no critical fuel filtration requirements, no surges or pulsations in the
fuel line and finally, the clincher for many car makers, lower cost.
