Electronic Gasoline Injection system

L Jetronic

© Dr. Amr Ibrahim, 2010, all rights reserved. The commercial use of these slides withoutpermission is strictly prohibited.

Principle of electronic injection systems:The electronic injection systems control the amount of injected fuel electronically

Electronic injection system consists mainly of:

• Sensors

• Electronic Control Unit (ECU)

• Actuators (e.g. solenoid operated fuel injection valves)

Schematic of L Jetronic

Fuel supply system

Solenoid operated fuel injection valves:

when the injector opens for longer time, more fuel is injected

• when the injector opens for shortertime, less fuel is injected

• the valve opening duration (injectionperiod) is controlled by the ECU

• all injectors inject the fuel once everycrankshaft revolution

Fuel pressure regulator:

• the regulator maintains thepressure difference between fuelpressure and intake manifoldpressure constant

Electronic control unit:

• the ECU, which consists of electronic components, receives electricsignals from all engine sensors in order to determine the fuelinjection duration

Air flow sensor:

• the air flow sensor reports the amount of engine air flow to the ECU

• this results in a specific angularposition for the sensor flap (point A)

• this leads to a specific voltage to beproduced by the potentiometer (pointB)

• the ECU receives the voltage signaland calculates the basic injected fuelquantity (point C)

• the basic injected fuel quantity (pointC) must be equivalent to thetheoretical injected fuel quantity (pointD) as determined by the required air-fuel ratio

• assuming flowing a specific air quantity via the air flow sensor (point Q)

• Information about engine speed is calculated from the number ofelectric signals induced by the ignition system to produce sparks

• assuming 4-stroke-cycle 4-cylinder engine, 4 spark signals areproduced for 2 crankshaft revolutions

Calculating engine speed:

• that means f=2n, where f=number of sparks produced persecond, n= number of crankshaftrevolutions per second

Basic injection fuel quantity:

• Both the air flow and engine speed signals are the mainvariables to determine the basic fuel quantity injected for eachcrankshaft revolution. For a 4 stroke cycle engine:

)(cycle/min N/2(kg/min) m(kg/cycle)m a

a

AF (kg/cycle)m(kg/cycle)m af

• the basic injection duration, Tp, is proportional to the basic amountof injected fuel calculated from the main variables

2(rev/min) N)(cycle/min cycles amic thermodynof no

2/ (kg/cycle)m(kg/rev)m finj f,

Total injection duration:

• Tm = the pulse duration extension as determined according toengine operating conditions such as:

o engine cold start

o engine warm up

o engine load condition (idling or full load)

• Tm is a measure of fuel enrichment

• The total injection duration (the duration of electric currentflow in the injector solenoid winding) determined by the ECU, Ti,is calculated as follows:

Ti = Tp + Tm + Tu

• Tu= the pulse duration extension according to engine batteryvoltage

• the pick up time of fuel injection valves depends very much onthe battery voltage

• the lower the battery voltage, the longer the time which isneeded to build the magnetic field to open the injector

• without voltage compensation, low battery voltage would resultin the injector opening duration to be too short

• the battery voltage is fed to the ECU in order for the Tu to becalculated

Engine temperature sensor:

• it reports the engine temperature tothe ECU

• it consists of an NTC (negativetemperature coefficient) resistor

• when the temperature increases, theelectrical resistance of the NTC resistordecreases

Throttle valve switch:

• it is mounted on thethrottle body and connectedto the throttle valve shaft

• it reports to the ECU whether the throttle valve is closed (idling)or fully open (full load)

Engine cold start fuel enrichment:

• fuel enrichment during cold start can be achieved by either:

o extending the injection duration , or

o using a cold start valve & a thermo time switch

warm up fuel enrichment:

• fuel enrichment is needed duringengine warm up to compensate for fuelcondensation

• the amount of fuel enrichment dependson the engine initial temperature

Atmospherictemp=22o C

Auxiliary air device:

• it is used to increase the amount of air fuel mixture during idling when the engine is cold to overcome engine friction

Engines fitted with a 3-way catalyst:

• the engine must operate at almost stoichiometric mixture in orderfor the catalyst to operate efficiently

• lambda sensor is used in order to measure the oxygenconcentration in the exhaust gas

• the value of λ is fed to the ECU

• the ECU controls the fuel injection duration in order to keep λ=1

Lambda sensor:

Overrun fuel cutoff:• the ECU cuts off fuel injection when the throttle valve is closedwhile the engine speed is higher than the idling speed duringbraking or downhill driving

• the fuel injection starts again when engine speed reaches to theidling speed value

• this reduces engine fuel consumption and emissions

Supplementary functions

Maximum engine speed limitation:• the ECU cuts off fuel injection when engine speed increases abovethe maximum speed limit specified by the manufacturer

• fuel injection starts again when engine speed is reduced below themaximum limit