Nissan has announced that its Decherd, Tennessee, assembly plant is using a fascinating cylinder-coating technology in the Altima's 2.5-liter, four-cylinder engine, a technology that the company points out has been used in the high-performance GT-R. This isn't the first application of what Nissan calls its "mirror-bore" technology; it has used the coating process in a number of vehicles. But it's worth taking a minute to examine the interesting advantages being implemented in the Altima's PR25DD engine.
Most engines that feature an aluminum block use a pressed-in iron cylinder liner for durability. Aluminum simply isn't very tough, and a piston (and its steel rings) moving around in an uncoated aluminum cylinder bore would destroy it fairly quickly. Iron is much more durable, yet still much lighter than an entirely iron engine block.
Several companies have explored using a thinner liner on aluminum engines, but this concept has a bad reputation among auto enthusiasts due to the Nikasil fiasco. This silicon carbide-nickel coating proved to fail when exposed to certain fuels, leading to a much-publicized recall of BMW and Jaguar cars. But Nissan's "mirror bore" process isn't much like Nikasil at all.
Instead, it's basically a very thin coating of metal, applied using a process called plasma-transferred wire arc (PTWA). An incredibly thin layer of metal wire is superheated and sprayed onto the cylinder bore — the specific metal used can vary depending on the application, but it seems like Nissan is using iron wire. The result is a coating that is measured in tenths of an inch — in the PR25DD's case, it's 0.2 mm. The coating is then highly polished to reduce friction.
The result is many of the advantages of an iron liner with very little of the weight and reduced friction. After all, the friction of the piston sealing rings against the bore is a major component of overall engine friction. Reducing friction, so long as you preserve durability, is an excellent way to increase efficiency. So is reducing weight. It seems that the technology can also improve cylinder cooling, since there's no thick iron liner soaking up heat. That also makes it easier to control detonation.
PTWA does all that, and probably more. According to several sources, it can also reduce manufacturing costs, since the process can coat different size bores without different tooling. Think about machining iron liners for different bore sizes to precise tolerances, compared to just spraying a liner in. There are also claims that it can reduce CO2 emissions. Ford, which developed the technology and was the first to use it commercially, in the 2011 Mustang GT500 where it saved a full 8.3 lbs, is even exploring a way to use it to help remanufacture engines that would otherwise be scrapped due to defective liners.
That said, it's neat to see this technology reach the Altima and more consumers, since it's cost-effective and should result in increased efficiency — although Nissan hasn't quantified that yet, we'll let you know if it does. Cumulatively, every little bit helps, since as you know manufacturers' total fleet emissions are regulated. Perhaps it's the beginning of the end for heavy iron cylinder liners in more vehicles.