How Cars Start

ignition_keyThe engine is the powerhouse of a vehicle that is self-sustaining once it’s started—thanks to inertia. The starter motor gets the engine up to speed so inertia can take over, but the starter motor also needs a start, which is provided by the solenoid. The starter and solenoid are buddies attached at the hip and are typically replaced together as a unit.

There are four distinct tasks accomplished by four components that make up the vehicle’s starter system. The components include:

  • Starter Motor
  • Solenoid
  • Battery
  • Ignition Switch

When the key is turned or the button is pushed in the ignition switch, a weak electrical signal, thanks to the battery, is sent through the switch to the solenoid. This weak current is sufficient to engage two large contacts and force them together. These contacts draw a much larger electrical current directly from the battery. The strong current requires heavier wiring for the flow of electricity to the starter where it initiates two distinct actions.

First, the current activates a lever that forces a small gear outward on a spring shaft. This gear, known as a pinion, extends in contact with a toothed starter ring gear on the outer rim of a large flywheel at the end of the engine’s drive shaft.

Second, the current passes through the starter motor and is transformed from electrical energy into mechanical energy, which causes the central shaft to rotate. This transformation occurs when the electrical energy passes through a large coil of wire creating a magnetic field that rotates the shaft.

When the spinning shaft reaches its top designed speed, the pinion disengages from the ring gear. At this point the engine runs on its own using fuel and inertia. The spring brings the pinion safely back to its resting position and the job of the starting system is completed.

The key switch also has a spring that will return it from the start position to the on position. Since it’s a switch, it only needs to be turned long enough for the weak current to reach the solenoid. When the key switch is held in place, not allowing the spring to return it to the on position, the switch sends another signal to the solenoid to once again engage the pinion.

The grinding noise you might hear is the pinion trying to reengage the ring gear while it’s already at speed. Thankfully a safety mechanism stops it from actually reengaging at the wrong speed, which could break the shaft. However if this behavior continues for long lengths of time, the pinion will wear down much faster than designed.

Here is a quick review of activities:

1. Ignition switch is turned to start and sends weak current to solenoid.
2. Solenoid’s large contacts come together and draw heavy power from battery and send it to the starter.
3. The pinion is engaged into the ring gear.
4. The electrical energy is transformed into mechanical energy and spins the shaft.
5. After the top speed is met the spring pushes the pinion back to its original position.
6. The spring in the ignition switch returns the key to the on position.
7. The engine runs on its own.

© 2017 Hi-Tech Automotive Specialists, Inc.

About CJ Powers

CJ is an author and speaker.
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