Car starter-How it works?

car starting system

Do you know what happens inside the car engine compartment once you turn on the car/vehicle ignition key? You simply supply the battery power to the car starter to turn on the engine. This blog is about the car starter, functions of a car starting system, car starter installation.

How the starter of a car works?

To start the engine, the crankshaft must be rotated at 1000-1500 RPM by an external driving mechanism (at ideal engine speed). The car starting system (starter) acts as an external driving mechanism.

The engine starter rotates the flywheel, flywheel (as it’s mounted on the crankshaft) rotates the crankshaft for a few seconds. Once the crankshaft starts rotating it actuates the fuel system / pump, sparks plug (petrol engine), for every two rotation of the crankshaft engine completes suction, compression, ignition, and exhaust strokes. After a successful power stroke (ignition) engine starts auto-rotating. Once the engine started starter disconnected.

How engine starter rotates the engine?

Engine flywheel equipped with spur gear (which is also known as ring gear) and the engine starter equipped with the pinion gear. Once the key is turned on for ignition, power from the battery is connected to the engine starter via a solenoid. Solenoid transfer power from the battery to the starter. Once power is transmitted to the starter motor, it converts electrical energy into two types of mechanical energy.

Please refer following circuit diagram of the car starting system (this system is applicable for motorbike, car, and commercial vehicles also)

Car starter electrical layout

The above diagram describes the car starting system electrical layout. Battery +Ve terminal connected to starter solenoid (not directly to starter). Battery +ve terminal connectors are also known as the power cable. Power cable transmits power to a solenoid switch. Battery +ve terminal is also connected to the ignition key and relay.
The output of the relay connected to the starter solenoid switch.

In almost all cars, a 12V battery is used. Battery rating varies depending on the electrical and electronics requirements of the vehicle. Typically for a 1.2 liter to 1.6-liter engine, a 40 AH battery is used.

AH= Ampere Hours

Battery negative terminal is connected to the car/vehicle body to serve as a ground connection. Battery +Ve terminal covered with cover to avoid sparking inside the engine compartment.

As indicated in the following image, the +Ve terminal serves as a terminal for the starter power cable and other electrical systems. Power cable draws maximum current from the battery during the ignition. That’s why all other electrical / electronics systems of the vehicle turned off for the fraction of seconds during ignition.

car starting system - car battery connection system

Once the ignition key (switch in a modern car) turned / switch on, electrical current flows from the ignition switch to the relay.
Relay work as safety features. If a car is in neutral gear, the relay allows current flow from the battery to the solenoid. If the car is not in neutral gear, no current flows from battery to solenoid. In a modern car, the relay can still be activated even though the car is engaged in the gear, this is done by pressing the clutch. Pressing clutch turns on the relay (neutral switch) which activates solenoid.

Once battery current passes to the solenoid, solenoid connects battery power cable to starter armature through carbon brush and slip rings. As soon as the current passes through the armature winding its starts rotating. As pinion and overrunning clutch mounted on armature shaft it starts rotating too.

engine car starter construction details

Solenoid switch is equipped with a lever system. A lever slides pinion and overrunning clutch on the splined section of the armature shaft. Spline on armature shaft provides a positive drive to starter pinion. Engine starter works on DC (Direct current) power.

car starting system - starter construction details

Why Solenoid switch required to operate a car starter system?

  • Solenoid works as a switch to transmit high current from battery to starter armature winding.
  • Solenoid works as a safety switch which protects the vehicle from an electric spark. Electric spark generates when battery +Ve power cable terminal connects with the starter armature winding.
  • Solenoid switch engages and disengages engine starter with engine flywheel by a lever. The engagement of starter pinion is done through a lever system. Disengagement of starter pinion is done once battery current stops to solenoid via the ignition switch.

Why a small pinion gear required to drive a flywheel?

To turn the engine from 1000 RPM to 1500 RPM with compensating the engine compression, higher torque is required.

Torque is inversely proportional to the speed. It means, to crank the engine (to rotate the engine crankshaft) power is transmitted from the starter pinion at 5500 RPM, with 5:1 gear reduction (as pinion diameter is smaller than flywheel) flywheel (crankshaft) rotates at 1000 RPM.

Why the overrunning clutch is essential in a starter motor?

Cranking the engine can be done at idle speed 1000 to 2000 RPM by the starter motor. Once the engine starts it can rotate higher than its ideal speed. Eg. Diesel engine RPM ranges from 0 to 6000 RPM and whereas the petrol (gasoline) engine rotates from 0 to a maximum 8000 RPM.

Without an overrunning clutch, the starter armature shaft rotates at extremely high speed which will damage its bearing and other components.
Eg. If the crankshaft is rotating at 8000 RPM with a 1:5 speed ration (speed increment) starter shaft will rotate at 40,000 RPM. No starter manufacturers design starter to withstand at 40,000 RPM.

How to select a car starter motor?

Be rest assured starter motor selection is done by the car/vehicle manufacturer. For starter motors replacement (if the starter motor is no longer working), get it fixed at an authorized service center.

Starter motor selection is a technical matter, lots of technical specifications are considered for the selection of a right starter motor. Following specification considered,

  1. Application – Lightweight vehicle (car / SUV), commercial vehicle, or heavy-duty engine (off-road vehicles).
  2. Electrical Rating: Voltage (12V or 24V), pull in voltage eg. 8V in 12V electrical system.
  3. Thermal Rating: Operating temperature, cold-cranking requirement.
  4. Mechanical: Power output, speed & torque of the starter.
  5. Rotation : Clockwise / anticlockwise.
  6. Drive Type: Inertia drive or pre-engaged starter drive with over-run clutch.
  7. Types of engine – Petrol engine or diesel engine

We hope you have learned about the car starting system. Stay tuned, very soon we will share more information about engine starter testing, statutory requirements.

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