Start&Stop system: how does it affect the long-term life of cars?

More and more models come with the start&stop system as standard, but what is the long-term impact on the life of modern cars? This technology aims to save fuel and could reduce consumption, but is it really good for the engine’s operation?

As you probably already know, these stop-start assistants are very useful when the vehicle is used 100% in the city. What if this small benefit were linked to a shortening of your vehicle’s useful life? Would you still use it?

First, let’s look more closely at start&stop technology. It is a system that comes as standard on most modern cars and that can switch off the engine when the car is stationary in order to reduce fuel consumption and emissions. In hybrid cars, the same system can also switch off the combustion engine when the car is moving under low acceleration loads, when going downhill, and when decelerating from higher speeds. The engine restarts when the clutch is engaged, the brake is released, or even when the driver is ready to move off or accelerate again.

The system uses a control unit to detect when the car is stationary or when it is operating under low load conditions; at that point it cuts off the fuel injection and stops the spark in the engine. In the case of hybrid cars, the electric motor of the car can supply a limited amount of torque while the engine is switched off, although it is generally only sufficient to maintain speed on a level gradient or in the city. The engine starts again when the car begins to move, the clutch is pressed, or more power is needed.

Start&Stop: is it really good for your car?

The process happens automatically, but you can choose whether to activate it or not. To give you an idea, a conventional electric starter motor works through a small pinion gear meshing with a large ring gear mounted around the flywheel. The latest start&stop technology is very similar, but the motors are more powerful, faster-acting and more robust, and they are often necessarily backed by higher-voltage electrical systems with larger storage batteries. Some are designated ‘TS’ for ‘tandem solenoid’ and are designed to cope smoothly with situations where the engine is about to stop and the driver then accelerates again.

What drawbacks can the system cause in your vehicle?

When it comes to durability and service life, all the bases related to the starter gear must be covered, but the greater number of stop-start cycles leads to greater engine wear unless steps are taken to prevent it. “A normal car without automatic stop and start can be expected to go through up to 50,000 starts during its service life,” said Gerhard Arnold, head of bearing design at Federal Mogul. “But with automatic activation, every time the car stops, that figure rises dramatically, perhaps to 500,000 start cycles over the life of the engine.”

That is a big jump that poses major challenges for the durability and service life of the engine bearings. A fundamental component of the engine, and also one of the heaviest, is the crankshaft. It is supported as it rotates on a series of precision journals along its length that run in ‘plain’ main bearings. These are the main bearings, and the effect is greatest on the bearing at the rear of the engine immediately adjacent to the starter motor.

When the engine is running, the crankshaft and the main bearing surfaces do not 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 rotating crankshaft. This process is called ‘hydrodynamic lubrication‘, but when the engine stops, the crankpin settles onto the bearing and the two metal surfaces come into contact.

When the engine starts, there is a point before the two surfaces are separated by the oil film called the “boundary condition”, where the crankshaft turns but there is metal-to-metal contact between the bearing surfaces. This is when most of the wear occurs. Stop-start means the boundary condition could occur perhaps 500,000 times over the engine’s life instead of 50,000, and normal bearings would wear out much sooner.

Two things prevent that from happening. The first is that bearing manufacturers are developing new bearing materials with greater self-lubricating properties to resist wear on start-up. Federal Mogul has developed a new material called Irox with a polymer coating containing iron oxide (rust) particles, which in this microscopic form is surprisingly slippery. In fact, it is so slippery that the coefficient of friction of an Irox bearing is 50 percent lower than a conventional aluminium bearing and will easily last the life of an engine fitted with stop-start.

Does start&stop help save fuel?

Yes, in situations where you are stationary with the engine idling, such as in heavy traffic or waiting for traffic lights to change, it will save the amount of fuel the engine would have used while the car is stopped.

The amount of fuel saved is often debated and depends almost entirely on the type of driving done with the system. Obviously, more time stationary means more fuel savings. There are also occasions when stop-start will not activate, for example if the engine is cold, as the system is less likely to intervene, to allow the engine to fully warm up. It may also not switch the engine off if the battery is below a certain level, if, as with the Volvo system, the driver unbuckles their seatbelt, or if the air conditioning is switched on. Start&stop is also designed to reduce emissions in urban areas.