Summary

Shielding Metal

Shields, such as the ones used in breaker point distributors, block the waves from components that create RFI signals. The circuits of onboard computers are protected to some degree from external electromagnetic waves by their metal housings.

Ground Straps

Ground or bonding straps between the engine and chassis of an automobile help suppress EMI conduction and radiation by providing a low-resistance circuit ground path. Such suppression ground straps are often installed between rubber-mounted components and body parts, Figure 4-36. On some models ground straps are installed between body parts, such as the hood and a fender panel where noelectrical circuit exists, Figure 4-36. In such a case, the strap has no other job than to suppress EMI. Without it, the sheet-metal body and hood could function as a large capacitor. The space between the fender and hood could form an electrostatic field and couple with the computer circuits in the wiring harness routed near the fender panel. For more information about ground straps, see the section on “Diagnostic Strategies” in Chapter 4of the Shop Manual.

EMI Suppression

Interference suppression is now a critical automotive engineering task because the modem automobile has an increased need for EMI suppression. The increasing use of cellular telephones, as well as onboard computer systems, are only two of the factors that have made interference suppression extremely important.

Even small amounts of EMI can disrupt the operation of an onboard digital computer, which operates on voltage signals of a few millivolts (thousandths of a volt) and milliamperes (thousandths of an ampere) of current. Any of the interference transmission modes discussed earlier are capable of creating false voltage signals and excessive current in the computer systems. False voltage signals disrupt computer operation, while excessive current causes permanent damage to micro-electric circuitry.

As the complexity and number of electronic systems continues to increase, manufacturers are using multiplex wiring systems to reduce the size and number of wiring harnesses, which also reduces EMI. Multiplexing is a method of sending more than one electrical signal over the same channel.

Figure 4-36. Ground straps are installed in many areas of the engine compartment to suppress EMI.

(Reprinted by permission of Robert Bosch GmbH)

SUMMARY

Electricity can be generated in several ways. The most important way for automotive use is by magnetism. Magnetism is a form of energy caused by the alignment of atoms in certain materials. It is indicated by the ability to attract iron. Some magnetic materials exist in nature; others can be artificially magnetized. The magnetic properties of some metals, such as iron, are due to electron motion within the atomic structure. Reluctance is resistance to the movement of magnetic lines of force: iron cores have permeability and are used to reduce reluctance in electromagnetic fields.

Lines of force, called flux lines, form a magnetic field around a magnet. Flux lines exit the north poleand enter the south pole of a magnet. Magnetic flux lines also surround electrical conductors. As current increases, the magnetic field of a conductor becomes stronger. Voltage can be generated by the interaction of magnetic fields around conductors.

The relative movement of a conductor and a magnetic field generates voltage. This process is called induction. Either the conductor or the magnetic field may be moving. The strength of the induced voltage depends on the strength of the magnetic field, the number of conductors, the speed of the relative motion, and the angle at which the conductors cut the flux lines. Electromagnetic induction is used in generators, alternators, electric motors, and coils. Magnetomotive force (mmf) is a

measure of electromagnetic field strength. The unit of measure used for mmf is ampere-turns. The principle of a transformer can be summarized by describing it as flowing current through a primary coil and inducing a current flow in a secondary or output coil.

Electromagnetism can also generate electromagnetic interference (EMI) and radiofrequency interference (RFI). Such interference can disrupt radio and television signals, as well as electronic system signals. Many devices are used to suppress this interference in automotive systems.