ability of the fuel injection system to control the air/fuel ratio depends on
its ability to properly time the injector pulses with the compression stroke of
each cylinder and its ability to vary the injector "on" time, according to
changing engine demands. Both tasks require the use of electronic sensors that
monitor the operating conditions of the engine.
control the proportion of fuel to air in the air/fuel charge, the fuel system
must be able to measure the amount of air entering the engine. Several sensors
have been developed to do just that.
VOLUME AIRFLOW SENSOR The airflow sensor measures airflow, or air volume.
The sensor consists of a spring-loaded flap, potentiometer, damping chamber,
backfire protection valve, and idle bypass channel. As air is drawn into the
engine, the flap is deflected against the spring. A potentiometer, attached to
the flap shaft, monitors the flap movement and produces a corresponding voltage
signal. The strength of the signal increases as the flap opens. The signal
voltage is relayed to the electronic control module.
DAMPING CHAMBER The curved shape of the airflow sensor is the damping
chamber. The damping flap in this chamber is on the same shaft as the airflow
sensing flap and is also about the same area. As a result, the damping flap
smooths out any possible pulsations caused by opening and closing of the intake
valves. Airflow measurement can be a steady signal, closely related to
airflow as controlled by the movement of the flap.
BACKFIRE PROTECTION The airflow sensor flap provides for backfire protection
with a spring-loaded valve. If the intake manifold pressure suddenly rises
because of a backfire, this valve releases the pressure and prevents damage to
IDLE BY-PASS The airflow sensor assembly includes an extra air passage for
idle, bypassing the airflow sensor plate. When the throttle is closed at idle,
the opening and closing of intake valves can cause pulsations in the intake
manifold. Without the idle by-pass, such pulsations could cause the flap to
shudder, resulting in an uneven air/fuel mixture. The idle by-pass smooths the
flow of the idle intake air, ensuring regular signals to the electronic control.
Another design of airflow sensor, called a Karman Vortex sensor, works on a
different operating principle. Air entering the airflow sensor assembly passes
through vanes arranged around the inside of a tube. As the air flows through the
vanes, it begins to swirl. The outer part of the swirling air exerts high
pressure against the outside of the housing. There is a low-pressure area in the
centre. The low-pressure area moves in a circular motion as the air swirls
through the intake tube. Two pressure-sensing tubes near the end of the tube
sense the low-pressure area as it moves around. An electronic sensor counts how
many times the low-pressure area is sensed.
faster the airflow, the more times the low-pressure area is sensed. This is
translated into a signal that indicates to the combustion control computer how
much air is flowing into the intake manifold.
Air Temperature Sensor
air is denser (weighs more) than warm air. Cold, dense air can burn more fuel
than the same volume of warm air because it contains more oxygen. This is why
airflow sensors that only measure air volume must have their readings adjusted
to account for differences in air temperature.
systems do this by using an air temperature sensor mounted in the throttle body
of the induction system. The air sensor measures air temperature and sends an
electronic signal to the control computer. The computer uses this input along
with the air volume input in determining the amount of oxygen entering the
some early EFI systems, the incoming air is heated to a set temperature. In
these systems an air temperature sensor is used to ensure that this
predetermined operating temperature is maintained.
Mass Airflow Sensors
mass airflow sensor (MAF) does the job of a volume airflow sensor and an air
temperature sensor. It measures air mass. The mass of a given amount of air is
calculated by multiplying its volume by its density. As explained previously,
the denser the air, the more oxygen it contains. Monitoring the oxygen in a
given volume of air is important, since oxygen is a prime catalyst in the
combustion process. From a measurement of mass, the electronic control unit
adjusts the fuel delivery for the oxygen content in a given volume of air. The
accuracy of air/fuel ratios is greatly enhanced when matching fuel to air mass
instead of fuel to air volume.
mass airflow sensor converts air flowing past a heated sensing element into an
electronic signal. The strength of this signal is determined by the energy
needed to keep the element at a constant temperature above the incoming ambient
air temperature. As the volume and density (mass) if airflow across the heated
element changes, the temperature of the element is affected and the current flow
to the element is adjusted to maintain the desired temperature of the heating
element. The varying current flow parallels the particular characteristics of
the incoming air (hot, dry, cold, humid, high/low pressure). The electronic
control unit monitors the changes in current to determine air mass and to
calculate precise fuel requirements.
are two basic types of mass airflow sensor: hot wire and hot film. In the first
type, a very thin wire (about 0.2mm thick) is used as the heated element. The
element temperature is set at 100C to 200C above incoming air temperature. Each
time the ignition switch is turned to the off position, the wire is heated to
approximately 1000C for 1 second to burn off any accumulated dust and
second type uses a nickel foil sensor, which is kept 75C above ambient air
temperatures. It does not require a burn-off period. Thus, it is potentially
longer lasting than the hot wire type.
Manifold Absolute Pressure Sensor
EFI systems do not use airflow of air mass to determine the base pulse of the
injector(s). Instead, the base pulse is calculated on manifold absolute pressure
MAP sensor measures changes in the intake manifold pressure that result from
changes in engine load and speed. The pressure measured by the MAP sensor is the
difference between barometric pressure (outside air) and manifold pressure
(vacuum). At closed throttle, the engine produces a low MAP value. A wide-open
throttle produces a high value. This high value is produced when the pressure
inside the manifold is the same as pressure outside the manifold, and 100
percent of the outside air is being measured. This MAP output is the opposite of
what is measured on a vacuum gauge. The use of this sensor also allows the
control computer to adjust automatically for different altitudes.
control computer sends a voltage reference signal to the MAP sensor. As the MAP
changes, the electrical resistance of the sensor also changes. The control
computer can determine the manifold pressure by monitoring the sensor output
voltage. A high pressure, low vacuum (high voltage) requires more fuel. A low
pressure, high vacuum (low voltage) requires less fuel. Like an airflow sensor,
a MAP sensor relies on an air temperature sensor to adjust its base pulse signal
to match incoming air density.
EFI systems with a MAP sensor, the computer program is designed to calculate the
amount of air entering the engine from the MAP and engine rpm input signals.
This type of EFI system is referred to as a speed density system because the
computer calculates the air intake flow from the engine speed input and the
density of the intake manifold vacuum input. Many EFI systems with MAF sensors
do not have MAP sensors. However, there are a few engines with both of these
sensors. In these cases, the MAP is used mainly as a backup if the MAF fails.
When the EFI system has a MAF, the computer calculates the intake air flow from
the MAF and rpm inputs.
Other EFI System Sensors
addition to airflow, air mass, or manifold absolute pressure readings, the
control computer relies on input from a number of other system sensors. This
input further adjusts the injector pulse width to match engine operating
conditions. Operating conditions are communicated to the control computer by the
following types of sensors.
COOLANT TEMPERATURE The coolant temperature sensor signals the electronic
control unit when the engine needs cold environment, as it does during warm-up.
This adds to the base pulse, but decreases to zero as the engine warms up.
THROTTLE POSITION The switches on the throttle shaft signal the electronic
control unit for idle enrichment when the throttle is closed. These same
throttle switches signal the electronic control unit when the throttle is near
the wide-open position to provide full load enrichment.
ENGINE SPEED The ignition system sends a tachometer signal reference pulse
corresponding to engine speed to the electronic control unit. This signal
advises the electronic control unit. This signal advises the electronic control
unit to adjust the pulse width of the injectors for engine speed. This also
times the start of the injection according to the intake stroke cycle.
CRANKING ENRICHMENT The starter circuit sends a signal for fuel enrichment
during cranking operations even when the engine is warm. This is independent of
any cold-start fuel enrichment demands.
ALTITUDE COMPENSATION As the car operates at higher altitudes, the thinner
air needs less fuel. Altitude compensation in a fuel injection system is
accomplished by installing a sensor to monitor barometric pressure. Signals from
the barometric pressure sensor are sent to the ECU to reduce the injector pulse
width (or reduce the amount of fuel injected).
COASTING SHUTOFF Coasting shutoff can be found on a number of control
systems. It can improve fuel economy as well as reduce emission of hydrocarbons
and carbon monoxide. Fuel shutoff is controlled in different ways depending on
the type of transmission (manual or automatic). The ECU makes a costing shutoff
decision based on a closed throttle as indicated by the throttle position or
idle switch or on engine speed as indicated by the signal from the ignition
coil. When the ECU detects that power is not needed to maintain vehicle speed,
the injectors are turned off until the need for power exists again.
ADDITIONAL INPUT INFORMATION SENSORS Additional sensors are also used to
provide the following information on engine conditions. NOTE: This list does not
attempt to cover all of the sensors that are used by all manufacturers. It
contains the most common.
Timing of ignition spark
Gearshift lever position
Amount of oxygen in exhaust bases
Emission control device operation
ELECTRONIC CONTROL COMPUTER
heart of the fuel injection system is the control computer or electronic control
unit (ECU). The ECU is a small computer that is usually mounted within the
passenger compartment to keep it away from the heat and vibration of the engine.
The ECU includes solid state devices, including integrated circuits and a
ECU receives signals from all the system sensors, processes them, and transmits
programmed electrical pulses to the fuel injectors. Both incoming and outgoing
signals are sent through a wiring harness and a multiple-pin connector.
Electronic feedback in the ECU means the unit is self-regulating and is
controlling the injectors on the basis of operating performance or parameters
rather than on preprogrammed instructions. As ECU with a feedback loop, for
example, reads signal from the oxygen sensor, varies the pulse width of the
injectors, and again reads the signals from the oxygen sensor. This is repeated
until the injectors are pulsed for just the amount of time needed to get the
proper amount of oxygen into the exhaust stream. While this interaction is
occurring, the system is operating in the closed loop. When conditions, such as
starting or wide-open throttle, demand that the signals from the oxygen sensor
be ignored, the system operates in open loop. During open loop, injector pulse
length is controlled by set parameters contained in the ECU memory banks.