Electrical Systems

Modern day automotive electrical systems are complex with arrays of wiring, a power source and sometimes hundreds of sensors, also more computing power that was ever needed in the early days of cars. This can be viewed as as a blessing for the luxury of air conditioning, GPS navigation and an every increasing multitude of gizmo's and gadgets being added to the kit list- with no doubt an ever increase in comfort levels for both the driver and passengers alike.

When it comes to performance, the added weight and potential for faults does not lend it self to speed though. Some items no doubt have helped increase performance like electronic fuel injection, modern cpu chips and other neat ignition improvements. So why don't we explore the basics of a cars electrical system to gain an understanding of what helps your car work as efficiently as possible with modern day automotive technology.

Electrical Systems

Starting Systems

The starting system is a good place to begin with exploring the car's electrical systems as putting the key into the ignition is fundamental to using any of these technologies.

The starting system uses electrochemical stored energy in the battery (see charging system) and converts it into electrical energy to run the starting motor. The starting motto uses the electrical energy and converts it into mechanical energy (kinetic). The mechanical energy is than transferred from the starting motor to gears and drives, which rotate the flywheel.

Once the flywheel has enough kinetic energy (inertia) to start the combustion cycle, the starter motor will switch off as it has done its job. The flywheel will effectively rotate the crankshaft, connecting rods, pistons. Also as the camshaft is linked via a belt or chain to the crankshaft, it will also rotate along with the valves, inlet/outlet ports. Effectively simulating the combustion cycle through combustion, when air and fuel are ignited, we will then have a continuous combustion cycle, with out the need for the starter motor.

Starting System Components

  • Battery.
  • Ignition Switch.
  • Starter Motor.
  • Immobilizer or alarm system (if fitted).
  • Starter safety switch (if fitted).
  • Cabling and wiring

1. Ignition Switch

The Ignition switch is used to send a signal to engage the starter motor, sometimes via a starter solenoid or immobilizer/ alarm system depending on system design. If the immobilizer or alarm system is armed the car will not start. The ignition switch can be combined with the ignition barrel, basically this is where you put you key into to start the engine.

Another reason for the ignition switch is to stop electrical system draining the battery when the ignition switch is off. It is normal for the ignition switch to have up to four phases.

2. Ignition Switch Phases

  • Off - no electricity is drained from the system, although the sound system could be wired to by pass this.
  • Accessories - this allows the car accessories to be used.
  • On - all electrical systems are on but the starting is not engaged.
  • Start - the starter motor on signal is sent to activate the flywheel, notice how the ignition switch will go back to on after you release the key in the barrel.

3. Starter Motor

This device is essentially a DC motor, which converts electrical energy into mechanical energy. This could be viewed as the reverse of a alternator in some respects.

Originally internal combustion engines were hand cranked, normally by a lever at the front of the engine. As they can not provide any torque to rotate internal components at zero RPMĀ“s, to start the combustion cycle. This is why a starter motor is needed in modern car designs.

The starter motor might also incorporate a starter solenoid, which takes a charge from the battery on ignition switch activation signals. The starter motor uses electrical energy to spin an armature (rotor), this armature is surrounded by cooper wire and a metal casing. The armature has electrical bushes which make contact with the surrounding wire, electrical charge flows through them and to the wire, making a electrical-magnetic field. This is what creates the motion or mechanical energy to drive the flywheel.

The starter motor will have a series of low ratio gears and drives, which are connected to the flywheel, when in operation. These are what turns the flywheel to aid in the OTTO cycle.

4. Starter Safety Switch

Depending on if the device is fitted, essentially this device stops the engine starting if the car is in gear or the clutch is not depressed. Part of the manual clutch mechanism or torque converter system depending on vehicle specifications.

5. Cabling and Wiring

Cabling from the battery to the starter motor carries large amounts of electrical power. This is why these connection are thicker with increased capacity. The ignition switch to starter solenoid or immobilizer/ alarm system, only carries electrical signals, so is thinner with lower capacity.

Ignition System

The ignition system is critical in controlling the spark required in petrol engines to ignite the air and fuel ratio in the combustion chamber, which is required to create power and drive the wheels. This follows the OTTO cycle principals of Intake, Compression, Power and Exhaust, also known as the four stroke combustion cycle.

Ignition systems are not present in Diesel engines, as they rely on compression to ignite the air and fuel mixture, but may include a glow plug to heat the air and fuel mixture in cold starting conditions to aid in the combustion process. The process of compression actually creates enough heat/pressure to ignite the diesel and air, to create the power stroke.

The ignition system has therefore two critical jobs to achieve for maximum engine efficiency, power and longevity:

  • Distribute a high voltage spark to the correct engine cylinder (firing order).
  • Deliver the spark at the correct timing depending on its position on the OTTO cycle, when the engine is in its compression stroke (ignition timing).

1. Firing Order

Depending of the configuration of the engine in question, we have different firing order for different engine types and these can vary depending on manufactures. This is to achieve the best engine balance, reducing engine vibration and better efficient power delivery.

While different engine designs will effect the balance due to their design orientation, having an optimum firing order will improve the potential of a design to maximise piston rotations.

2. Example of Possible Engine Firing Orders


Possible Firing Order





In-line 6


In-line 4


The first numbered cylinder will ignite first as part of the OTTO cycle and the rest will follow depending on the actual configuration. The ignition timing firing order and combustion cycle piston strokes will always be linked to achieve optimum operating conditions.

3. Ignition Timing

Ignition timing is critical for performance, fuel economy and engine wear rates, if this is not correct we could have misfires, increased engine vibration, poor drive ability and an effect on emission outputs.

Ignition timing is used to control this process and has to be dynamic in nature in order to deliver the spark for maximum combustion pressures. Depending on engine speed, intake manifold pressures , combustion chamber design and temperatures, fuel and air ratio, compression ratio, ignition timing changes (advance or retard) to maximise the power stroke efficiency. The spark is delivered later at lower engine speeds (static) and sooner at higher engine speeds (advanced), during the compression stroke. How this is controlled depends on the type of ignition system which is in place.

Common Ignition Components

Lets take a more in depth look at some of the components which make up the ignition system in a standard system.

1. Coil

Stores and delivers ignition energy, transforming low voltage to high voltage, which surges through a HT cable to the distributor. Connected to the battery, ignition switch, distributor.

2. Ballast Resistor

Shorted (by-passed) during the starting of the engine, for starting voltage requirements.

3. Condenser

Prevents premature burn off, of the breaker points as a result of sparking when the points open (reduces arcing), increases spark duration. Breaker-triggered coil ignition systems cannot function correctly without an ignition condenser, as they help to accelerate and collapse the LT magnetic field.

4. Contact Breaker

Used for energy storage and voltage conversion purposes, by opening and closing the ignition coil primary circuit.

5. Centrifugal Advance System

This mechanical governed mechanism, is connected to the distributor shaft and has two weights on it. As the shaft increases in speed, due to centrifugal forces, move outwards and adjust the cam relative to the distributor shaft. This is what advances the spark needed with increased engine speed. Held in place by two light springs, which are under tension, this tension dictates advanced ignition timing.

6. Distributor

Inside the distributor cap there are up to 30,000 volts between the live parts. The spark discharge between the distributor rotor and the HT lead contacts generate heat and a high proportion of aggressive nitrogen oxides. Extreme demands are therefore made of the chemical resistance and tracking resistance of the distributor cap. Many Bosch distributor caps are made of glass fibre reinforced polyester with an additional special surface seal.

7. Distributor Rotor

Made of the same material as distributor caps, distributors rotors must therefore also have high tracking resistance. There are two main types of distributor rotors with and without engine speed limitation. In the case of distributor rotors with engine speed limitation the ignition voltage is short circuited depending on the speed by means of centrifugal force providing reliable protection against over revving the engine.

8. HT Cables

Responsible for carrying HT voltages, consisting of one wire (conductor) which is insulated to stop the current going to earth. Insulation is normally silicone in construction, which protects the conductor from heat, water.

9. Ignition and Starter Switch

A switch which transmits a signal when the key is turned in the ignition barrel (sometimes combined units), the signal is sent to a primary circuit in the coil. See starting systems.

10. Igniter

Used to control the coil primary circuit control, also communicates with the ECU in regards to ignition confirmation, dwell angle control, lock prevention circuit, voltage prevention control, current control, tachometer signals. Features of the unit depend on the ignition system specifications.

11. Spark Plug

Seals of the combustion chamber and is the critical component containing the electrodes for creating the spark for ignition.

12. Vacuum Advanced System

A diaphragm which is connected via a pipe to the inlet manifold on one side, and the contact breaker plate on the other side. Depression which is dynamic in the inlet manifold, due to engine speed and throttle position, moves the diaphragm. As it is connected to the contact breaker plate, this advances or retards the ignition timing, which in turn controls the spark. A spring is designed into the unit which allows a degree of control.

Charging System

The charging system in a car is used to supply electrical power to the car and also recharge itself. The charging system of most road cars have three primary components: battery, alternator and regulator.

1. Battery

When a vehicle is not running or is being started by the ignition system, the battery is the source of all electric power in a car. It is a device which uses electrochemical reactions to provide electricity. It changes stored chemical energy and transforms into into electrical energy.

It is used by an array of different components in cars including the ignition, charging and starting systems. When the car is running at higher speeds, excess power produced by the alternator recharges the battery. The battery may also contribute to the electrical system at low engine speeds when the alternator is not producing enough electrical power.

The battery in car are normally of the lead acid type, with a combination of lead and sulphuric acid. It is 12 volt with positive (+) and negative (-) connections, there is normally a earth connection to the battery and the vehicle chassis to dissipate electrical charges. It is critical to have the right positive and negative terminals connected, otherwise there is a risk of damage to components. Fuse boxes are a safety net and will normal short out as a result.

2. Alternator

The alternator can be seen as the heart of the electrical system in a car when the engine is running. It is used for all the electrical systems power source, when the car is running at higher speeds and excess power is transferred to the battery to recharge it.

It uses electromagnetism to convert kinetic energy from the engine (normally via a belt), and transforms it into electrical power. At lower speeds the battery may help supplement the alternator, especially when first starting out on a journey.

The alternator has magnets located in its body, which spin around a metal coil to produce electricity. The magnet fields generated by the magnets are controlled via a regulator, which affects electrical power output levels.

3. Regulator

The regulator is a device in the charging system used to keep the rate of the alternators power generation charge to pre-defined voltage levels. It monitors battery voltage and system requirements It does this by altering the alternators field current which affects it magnetic field strength. Regulators can be external to the alternator or incorporated in the overall design of the unit.

If the alternator is producing too much electrical power for the systems and battery, it will reduce the field current and decrease electrical power output. If there is not enough electrical power for the electrical systems and battery, the field current will be increased and the alternator will produce more electrical power output.