Automotive Electrical / Electronic Systems

Automobiles have many circuits that carry electrical current from the battery to individual components.

The total electrical system includes such major sub systems as the ignition system, starting system, charging system, and the lighting and other electrical systems.

Ignition System

After the air-fuel mixture has been delivered to the cylinder and compressed by the piston, it must be ignited. A gasoline engine uses an electrical spark to ignite the mixture. Generating this spark is the role of the ignition system.

The ignition coil generates the electricity that creates this spark. The coil transforms the low voltage of the battery into a burst of 30,000 to 100,000 volts. This burst is what ignites the mixture.

The mixture must be ignited at the proper time in order for complete combustion to occur. Although the exact proper time varies with engine design, ignition must occur at a point before the piston has completed its compression stroke.

On most engines, the motion of the piston and the rotation of the crankshaft are monitored by a crankshaft position sensor. The sensor electronically tracks the position of the crankshaft and relays that information to an ignition control module. Based on input from the crankshaft position sensor, and, in some systems, the electronic engine control computer, the ignition control module then turns the battery current to the coil on and off at just the precise time so that the voltage surge arrives at the cylinder at the right time.

The voltage surge from the coil must be distributed to the correct cylinder because only one cylinder is fired at a time. In earlier systems, this was the job of the distributor. A distributor is driven by a gear on the camshaft at one-half the crankshaft speed. It transfers the high-voltage surges from the coil to spark plug wires in the correct firing order. The spark plug wires then deliver the high voltage to the spark plugs, which are screwed into the cylinder head. The voltage jumps across a space between two electrodes on the end of each spark plug and causes a spark. This spark ignites the air-fuel mixture.

Today's ignition systems don’t use a distributor.

Instead, these systems have several ignition coils- one for each spark plug or pair of spark plugs. When a coil is activated by the electronic control module, high voltage is sent through a spark plug circuit. The electronic control module has total control of the timing and distribution of the spark-producing voltage to the various cylinders.

Starting and Charging Systems

The starting system is responsible for getting the engine started ( |||| 5). When the ignition key is turned to the start position, a small amount of current flows from the battery to a solenoid or relay. This activates the solenoid or relay and closes another electrical circuit that allows full battery voltage to reach the starter motor. The starter motor then rotates the fly wheel mounted on the rear of the crankshaft. As the crankshaft turns, the pistons move through their strokes. At the correct time for each cylinder, the ignition system provides the spark to ignite the air-fuel mixture. If good combustion takes place, the engine will now rotate on its own without the need of the starter motor. The ignition key is now allowed to return to the "on" position. From this point on, the engine will continue to run until the ignition key is turned off.

The electrical power for the engine and the rest of the car comes from the car's battery. The battery is especially important for the operation of the starting system. While the starter is rotating the crankshaft, it uses a lot of electricity. This tends to lower the amount of power in the battery. Therefore, a system is needed to recharge the battery so that engine starts can be made in the future.

The charging system is designed to recharge and maintain the battery's state of charge. It also provides electrical power for the ignition system, air conditioner, heater, lights, radio, and all electrical accessories when the engine is running.

The charging system includes an AC generator (alternator), voltage regulator, indicator light, and the necessary wiring. Rotated by the engine's crankshaft through a drive belt, the AC generator converts mechanical energy into electrical energy.

The AC is converted into direct current (DC) within the alternator. When the output or electrical current from the charging system flows back to the battery, the battery is being charged. When the current flows out of the battery, the battery is said to be discharging.

|||| 4 An ignition module and coil assembly for four cylinders.

|||| 5 A typical starting system. Battery; Ring gear; Ignition switch; Pinion gear; Starter motor

Electronic Engine Controls

Nearly all vehicles on the road have an electronic engine control system. This is a system comprised of many electronic and electromechanical parts. The system is designed to continuously monitor the operation of the engine and to make adjustments that will cause the engine to run more efficiently. Electronic engine control systems have dramatically improved fuel mileage, engine performance, and driveability and have greatly reduced exhaust emissions.

Electronic control systems have three main types of components: input sensors, a computer, and out put devices. The computer analyzes data from the input sensors. Then, based on the inputs and the instructions held in its memory, the computer directs the output devices to make the necessary changes in the operation of some engine systems. Electronic control systems have fewer moving parts than old-style mechanical and vacuum controls. Therefore, the engine and other support systems can maintain their calibration almost indefinitely.

As an added advantage, an electronic control sys tem is very flexible. Because it uses computers, it can be programmed to meet a variety of vehicle engine combinations or calibrations. Critical quantities that determine an engine's performance can be changed easily by changing data that is stored in the computer's memory.

|||| 6 The major components of a late-model AC generator.

|||| 7 Late-model electronic engine control systems are made up of many different sensors and actuators and a central computer or control module.

|||| 8 A typical automotive computer.

On-Board Diagnostics

Today's engine control systems are on-board diagnostic (OBD II) second-generation systems. These systems were developed to ensure proper emission control system operation for the vehicle's lifetime by monitoring emission-related components and systems for deterioration and malfunction. This monitoring includes also a check of the tank ventilation system for vapor leaks. The OBD system consists of the engine and transmission control modules and their sensors and actuators along with the diagnostic software.

The computer can detect system problems even before the driver notices a driveability problem because many problems that affect emissions can be electrical or even chemical in nature.

When the OBD system determines that a problem exists, a corresponding "diagnostic trouble code" is stored in the computer's memory. The computer also illuminates a yellow dashboard light indicating "check engine" or "service engine soon" or displays an engine symbol. This light informs the driver of the need for service, not of the need to stop the vehicle.

A blinking or flashing dashboard lamp indicates a rather severe level of engine misfire. When this occurs, the driver should reduce engine speed and load and have the vehicle serviced as soon as possible. After the problem has been fixed, the dashboard lamp will be turned off.

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