Introduction:
Every combustion engine (petrol/diesel) is equipped with a starter motor. This starter motor ensures that the engine is set in motion. The pinion of the starter motor engages with the ring gear of the flywheel, after which the flywheel is set in motion. An engine needs a certain cranking speed to initiate the four-stroke cycle (intake stroke–compression stroke–power stroke–exhaust stroke). The cranking speed must be high enough for the engine to start.
A starter motor is a direct current motor, which is connected directly to the battery with a thick positive cable. This is done because very powerful (high) currents (and therefore also heat) pass through it. The starter motor converts electrical energy into kinetic energy.
Operation:
The starter motor is always mounted at the flywheel. When the starter motor is engaged, the pinion (already rotating by means of the Bendix drive) meshes with the teeth of the flywheel. The flywheel then starts to turn. There are two electrical connections on the starter motor: one for the signal wire coming from the ignition switch (terminal 50) and one for the positive cable from the battery (terminal 30). Sometimes the connection from the ignition switch is designed as a plug connection instead of a screw connection (see image below).
When the key in the ignition switch is turned all the way to the start position, a voltage of 12 Volts is applied to the ignition switch connection (terminal 50) of the starter motor. This signal energizes the solenoid and the current from the 12 V connection flows through the windings. The operation of the solenoid is roughly the same as that of a normal relay; a small control current is used to switch a large main current. The solenoid is simply built much larger because very high currents pass through it. At the moment the solenoid is energized, the starter motor will start to turn.
Solenoid:
A solenoid is an electromagnetic component. When a current flows through the solenoid, it becomes magnetic. This magnetic action can be used to move a component mechanically, just like with a commonly used relay with which, for example, the lights and the windscreen wipers are switched on. In this case, the two connections of the starter relay are pulled towards each other by the magnetic force.
At the moment the engine is started, a small control current is applied to the “ignition switch connection”. In the solenoid, the voltage present at the “12 V (battery) connection” is then switched through. The carbon brushes pass the voltage on to the commutator. The north and south poles in the rotor ultimately create a powerful movement in the stator; the electric motor will rotate.
The lever is pulled to the left at the top by the solenoid. Due to the (blue) pivot point, the lower side of the lever will move to the right. This pushes the pinion, with a rotating motion, into the ring gear of the flywheel. The rotor will then start to rotate, which will eventually set the flywheel in motion. The combustion engine of the car will be driven at a certain speed (e.g. 400 rpm). During starting, the engine management system of the car ensures, among other things, the supply of the correct amount of fuel, so that the engine will fire up after a few crankshaft revolutions.
Bendix:
The assembly of the spring, freewheel and pinion is called the Bendix drive. In this Bendix drive a horizontal movement is made (in the illustration from left to right). The Bendix also ensures that the pinion makes a rotating movement as it moves outward. This prevents the teeth of the pinion from ending up exactly opposite the teeth of the starter ring, which would mean the teeth of the pinion could not engage with the starter ring but would instead butt against the teeth of the flywheel.
After the engine has started, the pinion is pulled back in again. This is sometimes accompanied by a shrieking sound. Cleaning and lubricating the Bendix can (temporarily) solve this problem. It is not harmful, just annoying.
Freewheel:
When, after starting, the combustion engine fires up, it will run at a higher speed (e.g. 800 rpm). This is twice the speed at which the starter motor runs. At the moment the engine fires, the pinion is still engaged with the flywheel of the engine. As a result, the pinion also starts to rotate at twice the speed. To prevent damage to the starter motor, a freewheel section is mounted in front of the pinion. The freewheel ensures that the engine cannot drive the rotor of the starter motor; the starter motor therefore cannot start rotating at the engine speed of 800 rpm.
The freewheel can be seen as two rings that are connected to each other with bearings. The outer ring can rotate in only one direction (for example, counterclockwise). When the direction of movement is reversed (clockwise), the freewheel will cause the outer ring to lock. The inner and outer ring are then locked together. Only when the outer ring is turned counterclockwise again will movement between the two be possible once more.
Planetary gear set:
Certain starter motors are equipped with a so-called planetary gear set, which is also used in an automatic transmission. This gear set provides a different gear ratio and is often placed in front of the Bendix drive. By reducing the speed of the pinion (relative to the rotor), torque multiplication is achieved. An electric motor has the highest torque at low speed. With a planetary gear set, a greater torque (thus more force) can be transmitted to the starter ring at a lower speed. See this page for an explanation of the planetary gear set.
Possible starter motor faults:
- Nothing happens when turning the key in the ignition switch, or only a clicking sound is heard:
This may be the result of a flat battery. Due to a voltage in the vehicle electrical system that is too low, the solenoid is energized (it then makes a clicking sound), but the voltage is too low to drive the rotor. When the battery voltage is in order (at least 10.5 volts) and the starter motor still does nothing, the solenoid may be “sticking”. In that case, the lever is not operated and nothing happens. By giving the starter motor housing a tap with a hammer (and possibly a screwdriver) (not too hard, of course), in most cases it will come loose again and the engine can be started. However, this is a problem that will certainly return in the future, so it is time to make an appointment with a garage or overhaul company. - A very loud shrieking noise is heard while starting:
The bearing bushes are defective and need to be replaced. Overhaul companies that mainly overhaul alternators and starter motors are specialized in this. - While starting, the starter motor “slips” briefly (accompanied by a grinding noise), then turns again for a moment and slips again:
The starter ring on the flywheel is probably damaged. If the teeth are worn down, the teeth of the starter motor’s pinion will slip over them. Possible causes are accidentally operating the starter motor when the engine is already idling, or very frequent starting (e.g. a driving school car or a courier’s car that has been started several times a day for years). - Additional noise after the engine has started:
The possible cause of this is a Bendix drive that keeps “sticking”. The noise can be heard for about one to two seconds after the ignition key has been released. The Bendix has been described above.