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In this tutorial we will be looking at the Electronic
Fuel Injection system, with particular focus upon the sensors and
actuators, their resultant inputs and outputs, to and from the vehicle's
ECM. The tutorial looks at the multi-point injection system, with
single-point being covered in a later tutorial.
Overview
Both the multi-point and the single-point systems operate in a very
similar fashion having an electro-mechanically operated
injector/injectors opening for a predetermined length of time. The
amount of injection period is determined by the engine's Electronic
Control Module (ECM). This time period will depend upon the engine's
temperature, the engine load and the information from the lambda sensor.
The fuel is delivered from the tank, via a filter and a regulator
determines its operating pressure. The fuel is delivered to the engine
in a precise quantity and in most cases is injected into the inlet
manifold awaiting the valve's opening, therefore being drawn into the
combustion chamber by the incoming air.
The Fuel Tank
This is the obvious place to start in any full system explanation.
Unlike the tanks on early carburettor equipped vehicles, it is a sealed
unit that allows the natural gassing of the fuel to aid delivery to the
pump by slightly pressurising the system. It may be noted that when the
filler cap is removed, pressure is heard to escape. The fuel filler caps
are no longer vented.
The Fuel Pump
This type of high pressure fuel pump (Fig 1.0) is denoted as a roller
cell pump, with the fuel entering the pump and being compressed by
rotating cells which force it through the pump at a high pressure. The
pump is capable of producing a pressure of 8 bar (120 psi) with a
delivery rating of approximately 4 to 5 litres per minute. Within the
pump is a pressure relief valve that lifts off its seat at 8 bar to
arrest the pressure if a blockage in the filter or fuel lines or other
eventualities cause it to become obstructed. The other end of the pump
(output) is home to a non-return valve which, when the voltage to the
pump is removed, closes the return to the tank and maintains pressure
within the system. The normal operating pressure within this system is
approximately 2 bar (30 psi) and at this pressure the current draw on
the pump is 3 to 5 amps. Fuel passing across the fuel pump's armature
will be subjected to sparks and arcing; this on the surface appears
quite dangerous, but the absence of oxygen means that there will not be
an explosion!
Figure 1.0
The majority of fuel pumps fitted to today's motor
vehicles are fitted within the vehicle's petrol tank and are referred to
as 'submerged' fuel pumps. The pump will invariably be located with the
fuel sender unit and both units can sometimes be accessed through an
inspection hole either in the boot floor or under the rear seat. Mounted
vertically, the pump comprises an inner and outer gear assembly that are
termed as the 'gerotor'. The combined assembly is secured in the tank
using a series of screws and sealed with a rubber gasket, or with a
bayonet type locking ring. On some models, there are two fuel pumps, the
submerged pump acting as a 'lift' pump to the external roller cell pump.
Figure 1.1
The waveform illustrated in Fig 1.1, shows the current for each
individual sector of the commutator. The majority of fuel pumps will
have 6 to 8 sectors and a repetitive point on the waveform can indicate
wear and an impending failure. In the illustration waveform it can be
seen that there is a lower current draw on one sector and this is
repeated when the pump has rotated through 720°. This example has 8
sectors per rotation.
Fig 1.2 shows typical current draw access to the fuel submerged pump.
The current drawn by the fuel pump is dependent upon the fuel
pressure but should be no more than 8 amps found on the Bosch K-Jetronic
mechanical fuel injection, which has a system pressure of 75 psi.
Fuel Supply
A conventional 'flow and return' system has a supply of fuel
delivered to the fuel rail and the unwanted fuel is passed through the
pressure regulator, back to the tank. It is the restriction in the fuel
line created by the pressure regulator that provides the system
operational pressure.
Returnless Fuel Systems
Have been adopted by several motor manufacturers and differ from the
conventional by having a delivery pipe only to the fuel rail with no
return flow back to the tank.
The returnless systems, both the mechanical and the electronic
versions, are instigated by emissions protocol. The absence of heated
petrol returning to the fuel tank reduces the amount of evaporative
emissions, while the fuel lines are minimised, thus reducing build
costs.
Mechanical Returnless Fuel Systems
The 'returnless' system differs from the norm by having the pressure
regulator situated within the fuel tank. When the fuel pump is
activated, fuel flows into the system until the required pressure is
obtained; at this point 'excess' fuel is bled past the pressure
regulator and back into the tank.
The 'flow and return' system has a vacuum supply to the pressure
regulator: this enables the fuel pressure to be increased whenever the
manifold vacuum drops, providing fuel enrichment under acceleration.
The 'returnless' system has no mechanical compensation that effects
the fuel pressure, and it will remain at a higher than usual 44 to 50
psi. By increasing the delivery pressure, the ECM (Electronic Control
Module) can alter the injection duration to give the precise delivery,
regardless of the engine load without fuel pressure compensation.
Electronic Returnless Fuel Systems
This version has all the required components fitted within the one
unit of the submersible fuel pump. It contains a small particle filter
(in addition to the strainer), pump, electronic pressure regulator, fuel
level sensor and a sound isolation system. The electronic pressure
regulator allows the pressure to be increased under acceleration
conditions; additionally the pump's output can be adjusted to suit the
engine's fuel demand. This will prolong the pump's 'life' as it is no
longer providing a larger than required output delivery.
The Electronic Control Module (ECM) supplies the required pressure
information, while the fuel pump's output signal is supplied in the form
of a digital squarewave. Altering the squarewave's duty cycle will
effect the pump's delivery output.
To facilitate the changing viscosity of the fuel with changing fuel
temperatures a fuel rail temperature sensor is installed. A pulsation
damper may also be fitted prior to, or within the fuel rail.
Injectors
The injector is an electromechanical device, which is fed by a 12
volt supply from either the fuel injection relay or from the ECM. The
voltage will only be present when the engine is cranking or running, due
to the voltage supply being controlled by a tachometric relay. The
injector is supplied with fuel from a common fuel rail. The length of
time the injector is held open will depend on the input signals seen by
the ECM from its various engine sensors. The held open time or 'injector
duration' will vary to compensate for cold engine starting and warm-up
periods, i.e. a large duration that decreases the injection time as the
engine warms to operating temperature. The duration time will also
expand under acceleration and contract under light load conditions.
The injector will have a constant voltage supply while the engine is
running and the earth path will be switched via the ECM. An example of a
typical waveform is shown below in Fig 1.3.
Figure 1.3
Multi-point injection may be either sequential or simultaneous. A
simultaneous system will fire all 4 injectors at the same time with each
cylinder receiving 2 injection pulses per cycle (720° crankshaft
rotation). A sequential system will receive just 1 injection pulse per
cycle; this is timed to coincide with the opening of the inlet valve. As
a very rough guide the injector durations for an engine at normal
operating temperature, at idle speed are around 2.5 ms for simultaneous
and 3.5 ms for sequential.
Anything electro-mechanical will of course take a small amount of
time to react, as it will require a level of magnetism to build before
the pintle is lifted off its seat. This time is called the 'solenoid
reaction time'. This delay is important to monitor and can sometimes
equate to a third of the injector's total duration. A good example of
the delay in opening can be seen in the example waveform shown below in
Fig 1.4.
It can be clearly seen from the example waveform that the waveform is
clearly 'split' into two easily defined areas. The first part of the
waveform is responsible for the electromagnetic force lifting the pintle,
in this example the time taken is approximately 0.6 ms. At this point
the current can be seen to fall before rising again as the pintle is
held open. With this in mind it can be seen that the amount of time that
the injector is held open is not necessarily the same as the time
measured. It is not however possible to calculate the time taken for the
injectors spring to fully close the injector and cut off the fuel flow.
This test is ideal for identifying an injector with an unacceptably
slow solenoid reaction time. Such an injector would not deliver the
required amount of fuel and the cylinder in question would run lean.
Figure 1.4
Fig 1.5 shows both the injector voltage and current
displayed simultaneously.
Figure 1.5
All the example waveforms used were recorded using a
Pico automotive oscilloscope .
Other manufacturers' equipment will have different voltage ranges but
the resultant picture should be very similar. Please remember that using
a higher voltage range will result in the waveform appearing to have a
lower amplitude, although the overall voltage will be the same.
In the next tutorial we will be look at the input signals to the ECM
that control the injector duration.
This tutorial was first published by
The Institute of the Motor Industry
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