In urban situations especially, stop-start should be making a real world difference, but the durability of engines be affected in the long term?
What is stop-start technology?
Stop-start is a system in most modern cars that cuts the engine when the car is stationary, with the aim of reducing the fuel consumption and emissions. Tthat the engine starts again when the clutch is engaged or the brake is released, or when the driver is ready to move again.
How do stop-start work?
The system uses a computer to detect when the car is stationary or the car is out of gear, at which point it stops the supply of fuel and spark in the engine. The ignition starts again when the car starts moving or the clutch pressed.
The process happens automatically, but drivers can choose whether the system is on or off by pushing his car stop home button; a capital with a arrow circling clockwise.
Conventional electric starter motor works by engaging a small pinion gear with a large ‘ring’ gear mounted around the outside of the engine flywheel.
The last stop-start technology looks much the same, but the engines are more powerful, faster and more robust. Some are designated ‘TS’ for ‘tandem solenoid’ and designed to cope more easily with scenarios where the engine stops and then the driver accelerates again.
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The time may come when the driver has decided to leave, but for some reason, has a change of mind, as when the traffic moves off unexpectedly.
At that time, the engine may be ‘committed’ to stop but is still spinning, so to avoid crunching, a solenoid activates the starter motor to synchronize the speed with the engine before the second smoothly engages the gear.
Disadvantages of the technology of on-off:
Makes stop-start wear out my engine?
When it comes to the durability and long life, all the bases relating to the starter gear should be covered, but the greater number of stops and starts of the cycles lead to increased engine wear unless you take steps to prevent it.
“A normal car without automatic start and stop you can expect to go through up to 50,000 stop-start events during its entire life”, says Gerhard Arnold, who is responsible for bearing design in Federal Mogul.
“But with automatic start and stop is activated every time the car comes to a standstill, the figure rises dramatically, perhaps to as many of the 500,000 stop-start cycles of the engine’s life.”
That is a big jump and that poses major challenges to the durability and useful life of the motor bearings.
A critical component of the engine and also one of the more heavy it is the crankshaft. It is compatible tour for a number of precision ground journals along its length that is executed in ‘plain’ main bearings (no ball bearings or rollers, just smooth metal). These are the main bearings and the effect is greater in the bearing in the rear of the engine immediately adjacent to the starter motor.
When the engine is running, the crankshaft and the main bearings surfaces do not contact, but are separated by a super-thin film of oil, fed under pressure and is pumped around the bearing surfaces by the action of the spinning of the crankshaft. This process is called “hydrodynamic lubrication”, but when the engine stops, the crank sits in the bearing, the two metal surfaces that come in contact with.
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How rust helps to prevent wear
When the engine starts, there is a point before the two surfaces that are separated by the oil film called the ‘boundary condition’, where the crankshaft rotates, but not metal-to-metal contact between the bearing surfaces.
This is when the most wear takes place. Adjustment of start and stop means that the boundary condition (and metal-to-metal contact) could be maybe 500,000 times in the life of the engine instead of 50,000 and normal bearings wear out long before that.
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Two things prevent that from happening. The first is that bearing manufacturers are the development of new bearing material with greater self-lubricating properties to resist wear in the boot.
Federal Mogul has developed a new material called Irox with a polymer coating that contains particles of iron oxide (rust), that in this microscopic shape is very slippery.
In fact is so slippery that the friction coefficient of a Irox bearing is 50 percent smaller than that of a conventional bearing made of aluminum and will last the life of an engine equipped with the stop-start system.
Low-friction oils help prolong the life of the engine
The second is improvements in lubricating oils. A modern engine oil contains an additive package which comprises a complex chemical cocktail. The technical director of the company in the uk, Millers Oils, Martyn Mann, says that the formulation of these packages are critical: “We have reduced the friction with our oils and the improvement of the durability of the oil film, and we think that has to be the way to go with start & stop.”
Miller began to investigate low-friction oils in their laboratories in 2006. “We put a formulation together, tested on a rig of friction and found that we could reduce the friction between the typical components such as pistons and liners by 50 percent,” says Mann.
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In general, this reduces the generation of heat, power loss, fuel consumption and wear and tear, but Miller’s new triple ester nano-technology, known as the Nanodrive, it goes beyond that. Tiny nano-particles such as microscopic ball bearings exfoliate under high pressure, the polymer ‘flakes’ adhere to the surfaces of the engine.
Until now, the technology is available only in Miller high-end racing oils, but in connection with stop-start, it might also reduce the wear and tear during each re-start when the wear takes place.
With low-friction bearing and lubrication technology in place of the potential threat to the life of the engine start-up and shutdown of the systems should theoretically be exceeded. But the current technology is still relatively new and only time will tell if each car manufacturer has the right.
Makes stop-start to help save fuel?
Yes – in situations where you are stationary with the engine idling, such as in heavy traffic or waiting for the traffic lights to change, it will save you however, the quantity of fuel that would have been used by the engine when the car is stationary.
The amount of fuel that is saved is often controversial and depends almost entirely on the type of driving done with the system. Obviously, the more stationary time means more fuel saved. There are also times in 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 completely. Also you can not turn off the engine if the battery is below a certain level, if, as Volvo’s system, the driver unfastens his safety belt, or if you turn off the air conditioning.
Stop-start is also designed to reduce the emissions in urban areas, where traffic is more likely to be stationary for more time, despite the fact that the benefit to the drivers in the consumption of fuel, there are more benefits to the systems of money.
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If you want to know more about how much fuel your car uses, please visit What Car? and to test the True MPG calculator.