Electronic Engine ECU and Sensor Operation
vijay tharad
Director Operations at Corporate Professional Academy for Technical Training & Career Development
ECU basics
The Engine Control Unit is used to control the operation of internal
combustion engines. Typically this involves the control of fuel quantity and
spark timing as well as other ancillary controls. The ECU is a microprocessor
based electronic circuit that is capable of executing its code at very high
speeds and thus able to monitor and control the engine to crank angle
resolution.
The ECU operates off look-up tables to determine the appropriate value of
fuel quantity and spark timing. The look-up tables would usually be
determined through experiment on the same engine.
ECU, Sensing
The ECU requires knowledge on the engine status in regards to its
Crank angle
Engine rpm
Engine load (determined through Manifold Absolute Pressure or Throttle Position Sensor),
Coolant temperature
Air temperature
Exhaust Oxygen (Lambda) sensor
The sensors used are not unique as there are many manufacturers of sensors.
Crank and Cam Sensors
The function of the crank and cam sensors is to provide knowledge of angular
position and speed of the engine to the ECU. The ECU requires knowledge
of angular position of the engine crank so that spark and fuel are generated at
the desired crank angle.
Usually these sensors are inductive type, two wire (or three wire) and operate
on the principle that a voltage is generated in a coil when iron (a tooth) goes
past the sensor at some speed. Other types of position sensing is sometimes
used such as optical triggering or hall effect (hall effect requires use of
magnets).
Manifold Absolute Pressure (MAP)
The MAP sensor is used to provide intake manifold pressure measurement
which can be used as an engine load indicator. Sometimes this is also
referred to as Manifold Air Pressure, however the use of the word Absolute is
more descriptive as it has to be appreciated that the pressure being measured
is not gauge but absolute. Note that gauge pressure refers to pressure
quantity above atmospheric pressure. Ambient pressure is 100kPa (14.7 psi)
in an absolute scale and not zero. MAP sensors are typically three wire
(ground, signal and supply) and vary in their pressure measuring range
depending on application. Naturally aspirated engines typically utilise 100kPa
sensors while turbocharged (or supercharged) engines utilize 200kPa or
300kPa sensors.
Throttle Position Sensor (TPS)
Usually a potentiometer directly connected to throttle body’s butterfly shaft.
The overall electrical resistance of the potentiometer can vary from one
sensor to another. However the overall resistance has practically no effect on
the throttle position measurement. The ECU reads the voltage at the wiper
which is a function of the orientation (angular position) of the shaft.
Coolant and Air temperature
The coolant and air temperature sensors are usually thermistors. Thermistors
are resistors whose resistance changes with temperature.
conjunction with a pull-up resistor, the thermistors and pull-up resistor make a
potential divider whose voltage output depends on temperature. The voltage
is read by the ECU to provide temperature measurement. The thermistor has
two electrical terminals and therefore two connections to the harness,
however sometimes the coolant temperature sensor has one side of the
thermistor grounded to the engine and hence the sensor will have only one
electrical terminal.
Oxygen (Lambda) sensor
This sensor has seen a lot of evolution over the years. The fundamental
principle is based on the production of a voltage by zirconium dioxide element
when exposed to fresh air and exhaust gas. The most basic sensor is the
one-wire sensor. The single wire provides a voltage that changes in relation
to exhaust oxygen. The output signal of the single wire sensor referenced to
chassis ground. The two-wire sensor provides two electrical connections one
for ground and the other for signal. Therefore the two-wire has better signal
quality compared to the one-wire (note that the single wire’s ground
connection to the chassis is through the possibly rusted exhaust system ).
Oxygen sensors require an operational temperature above 300°C to function
properly. The three-wire senor has an embedded heater that heats up the
sensor quickly on start-up thus enabling a much faster knowledge of exhaust
oxygen. In a three-wire sensor, usually two wires are for the heater (typically
two white wires) and the third is signal (referenced to chassis ground). A fourwire
sensor has two wires for heater (typically two white wires) and the other
two wires are signal and signal ground. One, two, three and four wire sensors
provide a voltage ranging from zero to 1Volt. A voltage of approximately 0.45
volts indicates stoichiometric condition, voltages lower than 0.45 imply lean
combustion while voltages higher than 0.45 imply rich combustion. The
measured voltage cannot provide knowledge on the Air to Fuel Ratio AFR but
only knowledge whether rich or lean. Five-wire sensors do provide a voltage
that provides knowledge on the AFR. Five-wire sensors are also referred to
as wide- band sensors. Wide band sensors have signal conditioning circuitry
and provide a linearized voltage output with AFR.
ECU, Electronic Control
The ECU controls the engine through fuel injection and spark timing. For
spark ignition engines, the quantity of fuel required is in direct proportion to
the quantity of air inhaled by the engine. The mass of Air to mass of Fuel
ratio (AFR) for ideal operation is stoichiometric. When a three way catalytic
converter is used in production vehicles, the AFR is cycled (through closed
loop control) between rich and lean in order for the catalyst to be able to
perform both oxidizing and reduction reactions. In racing applications the
AFR is typically maintained rich (that is AFR smaller than AFR stoichiometric)
because this produces more power and is safer for the engine.
Fuel Injection
Spark ignition engines operate at AFR close to stoichiometric. The quantity of
fuel required to obtain the required AFR is controlled by the amount of time
the injector is left open, and is referred to here as Duration Of Injection (DOI).
The DOI required at any condition depends mostly on Volumetric Efficiency
which in turn is very dependent on engine rpm. The DOI required is also
dependent on engine load which is determined through the MAP or TPS
sensors. It is noted here that the logical consumption of much more fuel at
higher rpm is due to the fact that the DOI applicable is injected every
revolution (or every other revolution). Fuel injectors are very quick-acting onoff
valves capable of being cycled (that is opened and closed) in the order of a
millisecond. Injectors are available in a variety of flow rates and are also
divided into low impedance and high impedance injectors depending on their
electrical resistance. Peak-and–hold drivers can drive both low impedance
and high impedance injectors while saturation drivers can drive high
impedance injectors only.
Spark Generation
The timing of the spark is critical for optimal engine operation. Typically spark
timing has to be advanced with increasing engine rpm. This is due to the fact
that spark has to be generated in an earlier crank angle if the flame front is to
travel across the combustion chamber at higher rpm while still fully
combusting all gases just several degrees after top dead centre. The optimal
spark timing is also dependent on engine load. Lighter engine loads require
more advanced spark due to a slower moving flame in lower density
combustion gases. In older mechanical systems this spark advance at low
engine loads was achieved by the vacuum advance system. Various types of
spark generation and delivery are available, namely, one coil with distributor,
a coil every two cylinders (wasted spark) and an individual coil for each
cylinder. The spark, as with the older contact breaker setup (make and break)
is generated by the switching-off of current to the coil. This is so because the
coil (inductor) cannot allow the magnetic flux to vanish immediately and
therefore a high voltage is produced which is capable of producing an
electrical discharge across the spark plug gap. The Capacitive Discharge
Ignition (CDI) delivers a quantity of electricity to the coil at a very high voltage
on the primary side of the coil (can be 300V). This high voltage in CDI
systems charges the coil a lot faster and leaves enough time to recharge and
spark the plugs more than once per engine cycle (multi spark).
Using the ECU
The ECU is an electronic circuit using state of the art microprocessor,
memory, signal conditioning and power transistors. The wiring diagram
should be well followed before connecting power to the system. Damage to
the ECU can be done if wiring is not correct or not following the wiring
suggestions. This applies most of all to making sure that ECU pins that are
supposed to be connected to power are correctly connected to the relevant
power, while pins that are not supposed to be supplied with power aren’t
connected to power. It is also worthwhile mentioning that high voltage spikes
(around 350V) are generated by the spark plug coils even on the low voltage
side (that is ECU side). These high voltage spikes are properly handled by
the coil drivers but should not be connected to any other ECU pins other than
the coil drivers.
Before using the ECU, the wiring strategy must be developed.
Usual Wiring Information and Commonalities
ECU’s are powered from battery voltage, nominally 12V. The battery voltage
is not actually 12V all the time as during cranking voltage will surely drop,
while during charging voltage would be around 13.8V. The spark plug coils,
injectors, oxygen sensor heater, relays, dashboard indicator lights and other
ancillaries will typically run off 12V supply. The ECU internal electronics will
typically run at lower voltage.
This voltage was 5V until recently and now is 3.3V. Sensors will also typically be powered by a lower voltage, typically 5V, however some sensors do get powered by the battery 12V.
Sensor signals are typically between 0 and 5V, one exception is the two wire inductive pickup (used for crank and cam sensors) whose output voltage increases from less
than a volt at low rpm but can reach as high as 20V depending on application.
Due to the fact that ECU electronics and power electronics have a common
ground but a different high side voltage as described above, switching of the
power circuits by the ECU electronics is achieved by closing or opening the
connection of the power circuits to ground. That is, coils and injectors would
have a continuous 12V supply (battery voltage), the ECU would then turn on
the coils and injections by supplying a ground connection to them. Turning-off
of the power is achieved by breaking the connection to ground. Such a
strategy was also used in the past on mechanical contact breakers systems.
At this stage it is appropriate to note that due to the fact that all current from
coils, injectors and other power circuits flows into the ECU through the low
voltage side (ECU side) of these power consumers, the ground current
flowing out of the ECU is very high when compared to the much smaller
current flowing into the ECU from the battery positive supply to power the
ECU electronics. This fact needs to be appreciated to recognize why there
are typically many more ground connections compared to the 12V positive
supply connections. It is advised that all these ground connections are
connected so that there is ample current handling capability.
Another word on grounds, different types of grounds are cited, namely battery
ground and analogue ground. Battery ground is the ground that is directly
connected to battery, its main feature is its huge current carrying capacity, the
current flowing from coils and injectors would be routed to this ground inside
the ECU. The analogue ground is the ground that is used by analogue
sensors, analogue meaning voltage that can vary continuously between
ground and supply voltage. Examples of analogue sensors are TPS, MAP
and temperature sensors. The voltage output of these sensors varies in direct
proportion to the measured parameter. Therefore the ground voltage level of
these sensors has to be very stable otherwise a slight shift in the voltage level
of the ground would be erroneously translated into a change in the measured
parameter value. It should be noted that battery ground would have discrete
shifts in ground voltage level due to the turning on and off of coils and
injectors and turning on and off of other digital electronics. A filter to cancel
these shifts in ground level is typically employed to produce a clean analogue
ground. The supply voltage to the analogue sensors (typically 5V) would also
be a clean voltage, that is it would also be without any voltage shifts due to
switching. Appreciating the differences between these ground and supplies is
important so that connections are made to the appropriate terminals and not
just by whatever happens to seem the easiest physical connection on the
vehicle.
Heat dissipation: Electronic circuits do need to get cooled and cannot operate
at high temperatures. The ECU heats up in part due to the microcontroller
and associated electronics but mostly due to the power transistors associated
with switching on and off of the coils, injectors and other auxiliaries. The
reason behind the heat generated by power transistors is due to the fact that
when switched on, the power transistors would have a voltage drop across
them say of 0.8V. Therefore if a coil draws 5Amps in saturation, it would
translate in 4W (P=IV, P=5*0.8=4) of heat generated in the transistor that has
to be dissipated into the surroundings. Therefore ECU’s typically have there
case that functions as a heat sink for the internal electronics. To make sure
the heat sinking is effective, the ECU should be mounted in a relatively cool
location and if possible have air current or mounted to heat sinking (and cold)
metal parts.
Associate Professor at B N COLLEGE OF ENGINEERING, Pusad
7 年Great information sir