Automotive
Electronic Fuel Injection
Having been proven to be the most reliable, precise and cost-effective means of delivering fuel to
the combustion chambers of today's engines, the automotive electronic fuel injection systems are
computer controlled and constructed to provide the correct air/fuel ratio for all engine temperatures,
loads and speeds.
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Types of Automotive Electronic Fuel Injection
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Throttle-Body Injection (TBI)
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- The TBI system is a system in which the fuel was injected above the throttle plates. The TBI system
was used as stepping stone from carburetors to more advanced port fuel injection systems.
- TBI systems were used on many engines during the 1980's and early 1990's.
- The throttle body unit is similar in size and shape to a carburetor, and , like a
carburetor, it is mounted on the intake manifold.
- The injector(s) spray fuel down into a throttle body chamber leading to the intake
manifold.
- The intake manifold feeds the air/fuel mixture to all cylinders.
- Four cylinder engines have a single throttle body assembly with one injector and
throttle.
- Six and eight cylinder engines have dual injector's and two throttles on a common
throttle shaft.
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Automotive Port Fuel Injection (PFI)
- Used exclusively after 1995 in most applications.
- The PFI system has injector's located in the intake ports of the intake manifold.
- PFI systems use one injector at each cylinder.
- They mount in the intake manifold near the cylinder head, where they can inject a fine, atomized fuel
mist as close as possible to the intake valve.
- A fuel rail assembly is used to deliver fuel from the fuel pump to the fuel injector's.
- The throttle body assembly for a port fuel injection system controls the amount of air that enters the
engine. It allows the driver to control the amount of air that enters the engine, thereby controlling the
speed of the engine.
- The throttle body assembly also contains the idle air control valve (IAC) to control Idle speed of the
engine and the throttle position sensor (TPS).
- The throttle body assembly is bolted to the engines intake manifold.
The 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 accordingly to changing engine demands. Both tasks require the use of electronic
sensors that monitor the operating condition of the engine.
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Automotive Power Train Control Module
The central part of the fuel injection system is the computer or powertrain control
module (PCM). The PCM receives signal from all the system sensors, processes them, and transmits
programmed electrcal pulses to the fuel injectors. The PCM is controlling the injectors based on
operating conditions rather than on preprogammed instructions.
Automotive Electronic Input Sensors
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Mass Airflow Sensor
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- The mass airflow sensor measures air mass. the mass of a given amount of air is calculated by
multiplying its volume by its density.
- The denser the air, the more oxygen it contains. Monitoring the oxygen in a given volume of air is
important, because oxygen is a prime catalyst in the combustion process.
- From a measurement of mass, the Powertrain Control Module (PCM) (computer) adjusts
the fuel delivery for the oxygen content in a give volume of air.
- The accuracy of air/fuel ratios is greatly magnify when matching fuel to air mass instead of
fuel to air volume.
- The 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.
- The electronic control unit monitors the changes in current to determine air mass and to calculate
precise fuel requirements.
- The signals from the exhaust gas oxygen sensor, are used by the PCM to monitor the air/fuel
mixture.
- The signal from an oxygen sensor is based on the amount of oxygen in the exhaust gas.
- The O2 sensor is threaded into the exhaust manifold or into the exhaust pipe near the
engine.
- Late model vehicles have two oxygen sensors in each exhaust system, one before the catalytic converter
and one after it.
- The readings from the two sensors let the ECU know how efficiently the catalytic is working.
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Coolant Temperature
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- The coolant temperature sensor signals the PCM when the engine needs cold enrichment, as it
does during warm-up.
- The ignition system sends a tachometer signal reference pulse corresponding
to engine speed to the PCM. This signal advises the PCM to adjust the pulse width of the
injector's for engine speed.
- 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
- 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 PCM to reduce the amount of fuel injected.
Additional Automotive Electronic Input Information Sensors
Detonation
Crankshaft Position
Camshaft Position
Air Charge Temperature
Air Conditioner Operation
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Gearshift Lever Position
Battery Voltage
Vehicle Speed
EGR Valve Position
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