A conventional electric starter motor works by engaging a small pinion gear with a large ‘ring’ gear fitted around the outside of the engine flywheel.
The latest stop-start technology looks much the same but the motors are more powerful, faster acting and more robust. Some are designated ‘TS’ for ‘tandem solenoid’ and designed to cope more smoothly with scenarios where the engine is about to stop and then the driver accelerates again.
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Such a moment may come when the driver has decided to stop, but for whatever reason has a change of mind, such as when the traffic moves off unexpectedly.
At that moment the engine might be ‘committed’ to stopping but is still spinning, so to avoid crunching, one solenoid fires up the starter motor to synchronise its speed with the engine before the second smoothly engages the gear.
Disadvantages of stop-start technology:
Does stop-start wear out my engine?
When it comes to durability and long life, all the bases relating to the starter gear itself should be covered, but the higher number of stop-start cycles lead to increased engine wear unless steps are taken to prevent it.
“A normal car without automatic stop-start can be expected to go through up to 50,000 stop-start events during its lifetime,” says Gerhard Arnold, who is responsible for bearing design at Federal Mogul.
“But with automatic stop-start being activated every time the car comes to a standstill, the figure rises dramatically, perhaps to as many as 500,000 stop-start cycles over the engine’s life.”
That’s a big jump and one that poses major challenges to the durability and life of the engine’s bearings.
A fundamental component of the engine and also one of the heaviest is the crankshaft. It’s supported as it spins by a number of precision ground journals along its length running in ‘plain’ main bearings (no ball bearings or rollers, just smooth metal). These are the main bearings and the effect is greater on the bearing at the back of the engine immediately adjacent to the starter motor.
When the engine is running, the crankshaft and main bearing surfaces don’t actually touch, but are separated by a super-thin film of oil, fed under pressure and pumped around the bearing surfaces by the action of the spinning crankshaft. This process is called ‘hydrodynamic lubrication’ but when the engine stops, the crank settles onto the bearing, the two metal surfaces coming into contact.