Currently reading: Stop-start systems: is there a long-term impact on my car’s engine?
It's designed to save fuel and reduce emissions, but what effect does the technology have on engine life?

In urban situations especially, stop-start should be making a real-world difference, but will the durability of engines be affected in the long term?

What is stop-start technology?

Stop-start is a system on most modern cars that cuts the engine when the car is stationary in order to reduce fuel consumption and emissions. In hybrid cars, the same system may also shut down the combustion engine when the car is cruising at low throttle loads, when descending gradients and when decelerating from higher speeds. The engine starts again when the clutch is engaged or the brake is released, or when the driver is ready to move or accelerate again. 

How does stop-start work?

The system uses a computer to detect when the car is stationary or out of gear, or when it's running in low-load conditions; at which point it halts fuel delivery and spark to the engine. In the case of hybrid cars, a limited amount of torque can be supplied by the car's electric drive motor while the engine is off, although it's usually only enough to maintain speed on a level grade or around town. The ignition starts again when the car begins moving, the clutch is pressed or more power is needed.

The process happens automatically, but drivers can choose whether the system is active or disabled by pushing their car’s stop-start button; a capital A with an arrow circling clockwise. 

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, and often they're necessarily backed by higher-voltage electrical systems with bigger storage batteries. 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.

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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.

Read more: Winter car maintenance tips

How rust helps to prevent wear

When the engine starts, there’s a point before the two surfaces become separated by the oil film called the ‘boundary condition’, where the crankshaft is spinning, but there’s metal-to-metal contact between the bearing surfaces.

This is when most wear takes place. Fitting stop-start means the boundary condition (and metal-to-metal contact) could exist perhaps 500,000 times in the life of the engine instead of 50,000 and normal bearings would wear out long before that.

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Two things prevent that happening. The first is that bearing manufacturers are developing new bearing material 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 particles of iron oxide (rust), which in this microscopic form is surprisingly slippery.

In fact it’s so slippery that the coefficient of friction of an Irox bearing is 50 per cent lower than a conventional aluminium bearing and will easily last the life of an engine equipped with stop-start.

Low friction oils help prolong engine life

The second is improvements in lubricating oils. A modern engine oil contains an additive package comprising a complex chemical cocktail. The technical director of UK company, Millers Oils, Martyn Mann, says the formulation of these packages are critical: “We’ve reduced friction with our oils and improved durability of the oil film and we think that has to be the way forward with stop-start systems.”

Millers began researching low-friction oils in its laboratories back in 2006. “We put a formulation together, tested it on a friction rig and found we could reduce the sliding friction between typical components like pistons and liners by 50 percent,” says Mann.

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Generally, this reduces heat, power loss, fuel consumption and wear but Miller’s new triple ester nano-technology, known as Nanodrive, goes further. Tiny nano-particles like microscopic ball bearings exfoliate under high pressure, the polymer ‘flakes’ adhering to the engine surfaces.

So far the technology is available only in Miller’s high-end racing oils, but in relation to stop-start, it could also reduce wear during each re-start when the most wear takes place.

With low-friction bearing and lubrication technology in place the potential threat to engine life by stop-start systems should theoretically be overcome. But the current technology is still relatively new and only time will tell whether every car manufacturer has got it right.

Does stop-start help save fuel?

Yes - in situations where you’re stationary with the engine idling, such as in heavy traffic or waiting for traffic lights to change, it will save however much fuel would have been used by the engine while the car is stationary. 

How much fuel is saved is often disputed and depends almost entirely upon the type of driving undertaken with the system. Obviously, more stationary time means more fuel saved. There are also occasions when stop-start will not kick in, for example if the engine is cold, the system is less likely to intervene, to allow the engine to warm up fully. It may also not shut off the engine if the battery is below a certain level, if, like Volvo’s system, the driver unfastens their seatbelt, or if you turn the air conditioning on. 

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Stop-start is also designed to decrease emissions in urban areas where traffic is more likely to be stationary for longer, so despite the benefit to drivers’ fuel consumption, there are more benefits to the systems than monetary ones. 

If you want to find out more about how much fuel your car uses, visit What Car? and try out the True MPG calculator.

Join the debate

Add a comment…
Phil R 27 March 2017

Update the story please!

This story first came out in 2014 and I can't tell which part of it has been updated. What would be good though is with at least 3 years of data, and surely there is at least 5 available is a review of if stop start has caused any extra failures or not.
Fasteddie 9 February 2017

Don't Panic!

This story's warning is probably unneeded to car makers. Yes, start-stop system create new durability challenges, but these will be mastered. The same warning has been repeated since the 1970s as we added a long list of add-ons to make engines cleaner, including from my recall:

Positive crankcase ventilation (PCV)
Exhaust gas recirculation (EGR)
Vacuum amplifiers
Electronic ignition
Lead-free gas
Oxidizing catalytic converters
Air injection systems
Electronic Engine Controls
Fuel injection
Reducing catalytic converters
Sealed fuel tanks
Vapor recovery systems
Vapor recovery fuelers
Cam and crank timing sensors
Knock sensors
Mass airflow sensors
Manifold Absolute Air Pressure Sensors
Free oxygen sensors
Electronic throttle control
Variable valve timing
Low sulfur fuels
Diesel particulate traps
Hybrid ICE/battery electric vehicles
Multi-speed transmissions
Continuously variable transmissions
Multi-clutch transmissions
Urea injection and selective reduction catalysts
Direct fuel injection

...and most recently:
48V mild hybrids with start/gen units
LiIon batteries plus electric turbochargers
Continuously variable compression ratio crankshafts

...and, soon, camless valve systems?

With every add-on, stories like this one surfaced. But today's cars are more reliable than ever.
I should add that that long list of add-ons suggests we are nearing a tipping point for internal combustion engines. The work to create today's ICE cars has been fantastic....yet all are far more polluting than a simple electric vehicle.

xxxx 27 March 2017


All of the above improve ICE engines in some way. Stop Start doesn't, it can only make the engine heavier and more prone to break downs. Can it save the driver money, NO.
I worked it out once and it'll take over 7 years to pay for itself, and that's if the battery doesn't fail, have you seen the extra cost of stop start batteries! let alone the cost of a stop start starter motor.
The first thing me or the wife does after starting the car is turn it off! Apparently BMW's can be reprogrammed to do this by default, true BMW owners????
ricequackers 9 February 2017

TL;DR: The engineers have

TL;DR: The engineers have thought about this, and designed and developed their engines accordingly. It's designed to be used (and abused), just use it.