Each year, cars seem to get more and more complicated. Cars
today might have as many as 50 microprocessors on them.
Although these microprocessors make it more difficult for you
to work on your own car, some of them actually make your car
easier to service.
Mouse over each item in the list of parts to view
its location and description.
Some of the reasons for this increase in the number of
The need for sophisticated engine controls to meet
emissions and fuel-economy standards
Simplification of the manufacture and design of cars
Reduction of the amount of wiring in cars
New safety features
New comfort and convenience features
edition of HowStuffWorks,
we'll take a look at how each of these factors has influenced
the design of your car.
Sophisticated Engine Controls Before
emissions laws were enacted, it was possible to build a car engine
With the enactment of increasingly stricter emissions laws,
sophisticated control schemes were needed to regulate the
air/fuel mixture so that the catalytic converter could remove
a lot of the pollution from the exhaust. (See How
Catalytic Converters Work for more details.)
The computer from a Ford
Controlling the engine is
the most processor-intensive job on your car, and the
engine control unit (ECU) is the most powerful computer
on most cars. The ECU uses closed-loop control, a
control scheme that monitors outputs of a system to control
the inputs to a system, managing the emissions and fuel
economy of the engine (as well as a host of other parameters).
Gathering data from dozens of different sensors, the ECU knows
everything from the coolant temperature to the amount of
oxygen in the exhaust. With this data, it performs millions of
calculations each second, including looking up values in
tables, calculating the results of long equations to decide on
the best spark
timing and determining how long the fuel
injector is open. The ECU does all of this to ensure the
lowest emissions and best mileage. See How
Fuel Injection Systems Work for a lot more detail on what
the ECU does.
The pins on this connecter interface with
sensors and control devices all over the
A modern ECU might contain a 32-bit, 40-MHz processor. This
may not sound fast compared to the 500- to 1,000-MHz processor
you probably have in your PC, but
remember that the processor in your car is running much more
efficient code than the one in your PC. The code in an average
ECU takes up less than 1 megabyte
(MB) of memory.
By comparison, you probably have at least 2 gigabytes (GB) of
programs on your computer -- that's 2,000 times the amount in
The processor is packaged in a module with hundreds of
other components on a multi-layer circuit board. Some of the
other components in the ECU that support the processor are:
converters - These devices read the outputs of some
of the sensors in the car, such as the oxygen sensor. The
output of an oxygen sensor is an analog voltage, usually
between 0 and 1.1 volts (V). The processor only understands
digital numbers, so the analog-to-digital converter changes
this voltage into a 10-bit
High-level digital outputs - On many modern cars,
the ECU fires the spark
plugs, opens and closes the fuel
injectors and turns the cooling
fan on and off. All of these tasks require digital outputs.
A digital output is either on or off -- there is no
in-between. For instance, an output for controlling the
cooling fan might provide 12 V and 0.5 amps to the fan relay
when it is on, and 0 V when it is off. The digital output
itself is like a relay. The
tiny amount of power that the processor can output energizes
in the digital output, allowing it to supply a much larger
amount of power to the cooling fan relay, which in turn
provides a still larger amount of power to the cooling fan.
Digital-to-analog converters - Sometimes the ECU
has to provide an analog voltage output to drive some engine
components. Since the processor on the ECU is a digital
device, it needs a component that can convert the digital
number into an analog voltage.
Signal conditioners - Sometimes the inputs or
outputs need to be adjusted before they are read. For
instance, the analog-to-digital converter that reads the
voltage from the oxygen sensor might be set up to read a 0-
to 5-V signal, but the oxygen sensor outputs a 0- to 1.1-V
signal. A signal conditioner is a circuit that adjusts the
level of the signals coming in or out. For instance, if we
applied a signal conditioner that multiplied the voltage
coming from the oxygen sensor by 4, we'd get a 0- to 4.4-V
signal, which would allow the analog-to-digital converter to
read the voltage more accurately (see How
Analog and Digital Recording Works for more details).
Communication chips - These chips implement the
various communications standards that are used on cars.
There are several standards used, but the one that is
starting to dominate in-car communications is called
CAN (controller-area networking). This communication
standard allows for communication speeds of up to 500
kilobits per second (Kbps). That's a lot faster than older
standards. This speed is becoming necessary because some
modules communicate data onto the bus hundreds of times per
second. The CAN bus communicates using two wires.
In the next section, we'll take a look at how communication
standards have made designing and building cars easier.
Advanced Diagnostics Another benefit of
having a communications bus is that each module can
communicate faults to a central module, which stores the
faults and can communicate them to an off-board diagnostic
The diagnostic port from a Toyota
This can make it easier for technicians to diagnose
problems with the car, especially intermittent problems, which
are notorious for disappearing as soon as you bring the car in
page lists the fault codes stored in the ECU for various
carmakers. Sometimes, the codes can be accessed without a
diagnostic tool. For instance, on some cars, by jumping two of
the pins in the diagnostic connecter and then turning the
ignition key to run, the "check engine" light will flash a
certain pattern to indicate the number of the fault code
stored in the ECU.
Let's take a look at how microprocessors and communications
standards have made cars easier to build.
Easier Design and Manufacturing Having
communication standards has made designing and building cars a
little easier. A good example of this simplification is the
car's instrument cluster.
The instrument cluster gathers and displays data
from various parts of the vehicle. Most of this data is
already used by other modules in the car. For instance, the
ECU knows the coolant temperature and engine speed. The
transmission controller knows the vehicle speed. The
controller for the anti-lock
braking system (ABS) knows if there is a problem with the
All of these modules simply send this data onto the
communications bus. Several times a second, the ECU will send
out a packet of information consisting of a header and the
data. The header is just a number that identifies the packet
as either a speed or a temperature reading, and the data is a
number corresponding to that speed or temperature. The
instrument panel contains another module that knows to look
for certain packets -- whenever it sees one, it updates the
appropriate gauge or indicator with the new value.
Most carmakers buy the instrument clusters fully assembled
from a supplier, who designs them to the carmaker's
specifications. This makes the job of designing the instrument
panel a lot easier, both for the carmaker and the supplier.
It is easier for the carmaker to tell the supplier how each
gauge will be driven. Instead of having to tell the supplier
that a particular wire will provide the speed signal, and it
will be a varying voltage between 0 and 5 V, and 1.1 V
corresponds to 30 mph, the carmaker can just provide a list of
the packets of data. Then, it is the carmaker's responsibility
to make sure that the correct data is output onto the
It is easier for the supplier to design the instrument
panel because he doesn't need to know any details of how the
speed signal is generated, or where it's coming from. Instead,
the instrument panel simply monitors the communications bus
and updates the gauges when it receives new data.
These types of communications standards make it very
uncomplicated for carmakers to outsource the design and
manufacture of components: The carmaker doesn't have to worry
about the details of how each gauge or light is driven, and
the supplier who makes the instrument panel doesn't have to
worry about where the signals are coming from.
Smart Sensors This
technique is starting to be used on a smaller scale for
sensors. For instance, a traditional pressure sensor contains
a device that outputs a varying voltage depending on the
pressure applied to the device. Usually, the voltage output is
not linear, depends on the temperature and is a low-level
voltage that requires amplification.
Some sensor manufacturers are starting to provide a smart
sensor that is integrated with all the electronics, along with
a microprocessor that enables it to read the voltage,
calibrates it using temperature-compensation curves and
digitally outputs the pressure onto the communications bus.
This saves the carmaker from having to know all the dirty
details of the sensor, and saves processing power in the
module, which otherwise would have to do these calculations.
It makes the supplier, who is most up on the details of the
sensor anyway, responsible for providing an accurate reading.
Another advantage of the smart sensor is that the digital
signal traveling over the communications bus is less
susceptible to electrical noise. An analog voltage traveling
through a wire can pick up extra voltage when it passes
certain electrical components, or even from overhead power
Communication buses and microprocessors also help simplify
the wiring through multiplexing. Let's take a closer
look at how they do this.
Simplified Wiring Multiplexing is a
technique that can simplify the wiring in a car. In older
cars, the wires from each switch run to the device they power.
With more and more devices at the driver's command each year,
multiplexing is necessary to keep the wiring from
getting out of control. In a multiplexed system, a module
containing at least one microprocessor consolidates inputs and
outputs for an area of the car. For instance, cars that have
lots of controls on the door may have a driver's-door module.
Some cars have power-window, power-mirror, power-lock and even
power-seat controls on the door. It would be impractical to
run the thick bundle of wires that would come from a system
like this out of the door. Instead, the driver's-door module
monitors all of the switches.
Doors with lots of switches are becoming more
Here's how it works: If the driver presses his window
switch, the door module closes a relay that provides power to
the window motor. If the driver presses the switch to adjust
the passenger-side mirror, the driver's door module sends a
packet of data onto the communication bus of the car. This
packet tells a different module to energize one of the
power-mirror motors. In this way, most of the signals that
leave the driver's door are consolidated onto the two wires
that form the communication bus.
The development of new safety systems has also increased
the number of microprocessors in cars. We'll talk about this
in the next section.
Safety Systems Over the last decade, we've
seen safety systems such as ABS
bags become common on cars. Other safety features such as
traction-control and stability-control systems are starting to
become common as well. Each of these systems adds a new module
to the car, and this module contains multiple microprocessors.
In the future, there will be more and more of these modules
all over the car as new safety systems are added. At the 2001
American Auto Show, we saw Volvo's
Safety Concept Car (SCC), which showcased some of these
upcoming safety features.
The Volvo Safety Concept
New technology developed for this car allows the vehicle's
interior to adjust to the driver's body size and eye position.
Sensors scan the precise position of the driver's eyes and
then adjust the driver's seat to offer the best possible
vision. The steering wheel, floor, pedals and center console
also adjust to a more comfortable position for the driver.
The SCC includes active rearview mirrors and rear bumper
sensors that alert the driver to approaching traffic in the
car's blind spot. Rear-facing cameras also add to the driver's
field of vision. Adaptive headlamps monitor the car's speed
and steering wheel movements and adjust lighting accordingly.
For example, at high speeds, light beams are given a longer
reach. The car is also equipped with an infrared light
enhancer to improve night vision.
Drivers moving outside of their lane will be warned by the
SCC's remain-in-lane technology. Forward-facing cameras
monitor the car's position in relation to the road's
centerline and side-marker lines for 20 meters ahead of the
car. If the car begins to veer out of the lane, a warning is
Each of these safety systems requires more processing
power, and is usually packaged in its own electronics module.
But it doesn't end there. In addition to more safety features,
concept cars at the 2001 auto show were bursting at the seams
with new convenience features, meaning still more
Comfort and Convenience In coming years,
we'll have all kinds of new features in our cars, and each of
these requires more electronics modules containing multiple
microprocessors. The Dodge
Super8 Hemi concept car showcases some of these
The Dodge Super8
This concept car has such features as wireless
Internet access and voice control of many car functions,
including audio, climate-control, phone and even e-mail!
The rear passengers have individual LCD touch screens so
they can watch movies or access the Internet. The car has satellite
radio and can play MP3 music that
you transfer from your home stereo to the car while it is
parked in the driveway or garage.
It seems that there is no limit to how much technology
carmakers are going to pack into our cars. The addition of all
these electronic features is one of the factors driving
carmakers to increase the system voltage on cars from the
current 14-V system to a 42-V system. This will help provide
the extra power these modules require.
For more information, check out the links on the next