The fuel injector was invented to increase the performance of the engine.
The electronic fuel injector, which has replaced the carburetor, provides
a better distribution of fuel into the combustion chamber.
There are three generations of the fuel injector. The first generation
injectors are mechanical actuated by a pulsating the pressure from the
high pressure fuel pump. The injectors are spring opposed and when the
critical pressure from the pump is achieved, the power of the spring is
overcome and the needle opens allowing gas into the intake manifold. The
second generation is the most common and is in most of today's cars. It
its electronically controlled using a computer and a solenoid to open a
needle valve. When the solenoid is energized the spring is overpowered
and the needle moves from the closed position to the open position. When
the power is cut off, the spring returns the needle to the closed position
closing the flow of gas to the intake manifold. The third generation is
the same principal but the gas is injected directly into the cylinder and
is still being refined. There are many mechanical and thermal obstacles
that have to be refined before the full potential of this generation is
reached. There are electronic fuel injectors for diesel engines but we
will be examining gasoline engine injection systems in this analysis.
Functions
Inject Even distribution of Fuel into Cylinder to prevent excessive wear
and optimize power output from engine.
Correct ratio of Fuel to Oxygen
Adjust for Temperature and Speed of car to optimize engine performance
Reduce Emissions
General concepts
The amount of fuel administered is determined by the rpm, throttle position,
and oxygen remaining in exhaust. Engine speed is relayed to the Electronic
Control Unit (ECU) from the distributor triggering contacts. Engine load
information is taken from the intake air pressure sensor. This all has
an influence on how long the injectors stay open. When the engine is cold,
additional gas is injected into the intake manifold when the starter is
on. During acceleration the length of injection is increased and when the
engine is decelerating the gas is turned off and when the revolutions drop
below a set level, around 1000 rpm's the injectors open again to allow
the engine to idle.
Carburetor vs. Injected
Injected gas allows for a more accurate amount of fuel being used. This
provides for better fuel economy and better emissions and more power. This
allows for the optimum fuel to air ratio of 1:14.7 respectively.
Advantages
With the precise control from the Electronic Control Unit, a varying pulse
can be sent to all the injectors
Length and therefore the amount of gas injected, can be precisely controlled
Electronic Fuel Injection ensures Uniformity of:
The gas being administered between piston and piston can be controlled.
Timing
Amount
This is all especially important over the short periods of time that the
fuel that is injected
Techniques to increase accuracy of the correct amount of gas used and the
proper timing;
By stepping the voltage from the cars electrical system up from 12v to
100v, one is able to minimize the open/close delay. A Inductive coil is
used to step up the power and is then stored in a capacitor until it is
needed to activate the solenoid, and then discharged.
Higher voltage prevents unnecessary current that heats the solenoid
This also reduces the closing delay since it doesn't have to decay from
a higher level allowing the needle to move downward sooner.
The driver from the electronic control unit is a square wave, (i.e.
on or off). The goal is to reduce the lag between when the pulse of electricity
is received and when the solenoid is fully open. This is achieved by increasing
the pressure of the fuel coming into the injector and the reducing the
air gap. The air gap is the distance between the fully closed position
and the open position of the needle. Typically this is around 6mm. The
increased pressure reduces the time it takes to administer the correct
amount of gas and reduces the size of the hole there need to be to allow
the gas into the manifold.
Sensors
Gas Rail Pressure Pressure sensors so that the computer knows how much gas in going into
the intake manifold for a given amount of time. This is taken from the
common rail so that the computer Electronic control unit knows how fast
the gas is being administered.
Exhaust gasses Measures the amount of oxygen left in the exhaust to better adjust
the amount of gas going into the intake manifold. The ideal ratio is 14.7
oxygen to 1 fuel.
Mechanical aspects
Materials that are less prone to ware and heat. With higher ware comes
leakage of gas into cylinder. Reduce leakage with stronger springs to hold
the needle in place and put the needle into the seat sooner after the coil
is deactivated. Higher power in the solenoid is needed to over come the
high strength of the spring. Problem of the needle bouncing. With the needle
being forced back to the seat faster, it will bounce back. To eliminate
this problem in newer designs of the injector, a thin wall pipe has been
placed between the solenoid and the needle. This absorbs the energy as
the needle stops in the seat and "slowly" stops the inertia of the solenoid
so that it doesn't bounce back and breaks the seal allowing more gas to
go prematurely into the cylinder. Keep the solenoid cool during operation.
Gas is injected around the coil to keep it cool.
