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Making the Invisible Smooth

In water, the amount of drag greatly increases. It comes as no surprise, then, that the body surfaces of animals that can swim at high speed underwater are devised to reduce water resistance.

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In water, the amount of drag greatly increases. It comes as no surprise, then, that the body surfaces of animals that can swim at high speed underwater are devised to reduce water resistance.

Take sharks. They have a special skin with tiny vortices to help reduce drag. Or in the case of tuna, the whole body is covered by a smooth coating. With its low water resistance, tuna are believed to be able to swim at speeds of over 100km/h (62mph).

An automobile has many parts that can move at high speeds and so reducing drag is still very important. In this edition of Nissan Technology Magazine we will be looking at one of the technologies that cut down on friction in the engine when it is moving at high speed.
Making a coating to reduce drag
A very simple explanation of how an engine works would go something like this: The up-and-down motion of a piston is converted into rotational movement and this power is then used to move the wheels. The piston moves inside a tube-shaped space called the cylinder bore that is set into the cylinder block. Recently carmakers have been reducing the weight of the cylinder block so they use aluminum. But this creates a problem for the cylinder bore since when the piston ring moves up and down inside the aluminum bore, you will always get friction and abrasion.

A cast iron cylinder is inserted into the cylinder bore. This is called a cylinder liner. The piston oscillation then takes place inside this cylinder. Instead of inserting a cylinder liner, though, Nissan’s mirror bore coating technology sprays molten iron onto the surface of the cylinder bore and makes an iron coating layer.

But this is more than just a layer of coating. As the name suggests, it turns the surface into a mirror.

Normally the surface of a cylinder bore needs to be fitted with very fine cross-hatching. Oil is stored in the furrows and this retains an oil film. But the mirror bore coating utilizes minute holes on the coated layer to maintain the oil, meaning cross-hatching isn’t needed and the surface can be given a mirror finish.

With its smooth and shiny surface, there is less drag when the piston is moving up and down. Combined with other associated benefits, there is up to a 1.2% improvement in fuel consumption. Well, you might not think that’s such a big difference. But fuel consumption technology today is a bit like wringing water out of a dry rag. In this way, this kind of innovation is very unusual since it can reduce drag without needing to resort to expensive extra devices or forcing the driver to operate the car in a certain way.
A shark's skin feels rough from its tail right through to its head. But the small bumps on the surface are actually helping the fish move smoothly through the water. The shark skin has even inspired special swimsuits designs for competitive swimming that greatly increase swimmers' speeds.
Adopting racing car technology
Carmakers have used iron film coating technology instead of a cylinder liner before. But due to problems with cost and the effort required to implement it, until now it had only been adopted mainly for racing cars or very high-end sports cars.

Nissan, though, uses the technology in the Nissan GT-R. The Nissan GT-R used a unique technology that pretreated the aluminum before spraying on molten iron. Now Nissan has upgraded the technology further so it can be used in mass-produced engines. By taking advantage of the attributes of the coating and treating the surface, it has realized performance that is even more improved.

While the major merit of this technology is that it reduces drag, there are other benefits too, such as reducing the weight (as now there is no cylinder liner), and improving thermal conductivity by the cohesion of the piston and cylinder bore. Needless to say, fuel consumption is also related to improved drive performance too. Mirror bore coating can actually be applied very widely, not only in vehicles with gasoline engines but also hybrid cars.

In the future engines are set to become smaller and lighter. This mirror bore coating technology turns what you normally cannot see into something very smooth and shiny, and surely signals how the potential of engines is expanding.