Nissan’s Decherd assembly line is using a mirror bore process in the all-new 2019 Nissan Altima 2.5-liter engine. (At WCX SAE World Congress Experience this year in Detroit, Nissan presented a paper on the development of and technologies applied in the new 2.5L engine for the Altima, including the mirror bore coating.)

This will mark the highest volume application of the technology within Nissan. The team in Decherd assembles an engine every 19 seconds and is capable of building about 1.4 million engines a year.

The mirror bore treatment, first used in the Nissan GT-R and applied to other specialty models such as the Sentra NISMO, eliminates the need for bulky cylinder liners and raises energy efficiency by reducing the friction inside the engine.

Many engines today use lightweight aluminum materials for the cylinder block. However, since aluminum cannot endure the friction and heat that arises in the cylinder itself, designs also use a cast iron cylinder liner. Rather than inserting a cylinder liner, mirror bore coating technology sprays molten iron onto the surface of the cylinder bore and forms an iron coating layer on the walls inside. By giving this a mirror-like finish, the drag that arises when the piston is operating can be reduced.

The main purpose of this technology is to reduce drag, though there are other merits as well. First, by removing the cylinder liner, the design realizes a lighter engine.

Second, compared to the roughly 2mm-thick cylinder liner, the 0.2mm mirror bore coating is extremely thin, making for better heat conduction. This results in better cooling performance and less engine knocking, and the efficiency of the engine as a whole is improved. The leeway gained can go towards increasing fuel economy and engine power.

In the second stage, a special drill bit is used containing tiny bits of diamonds that were unfit for jewelry. The bit is spun at high speed inside the cylinder to polish the metal until shiny and smooth—giving the process its “mirror bore” name and the cylinder walls a reflective surface.

Water is applied during this stage is intended to keep the machinery and metals at optimum temperatures and reduce friction.

Through unique technology that pretreats the aluminum surface by spraying molten iron, Nissan achieved a large reduction in the cost of the technology, allowing it to be employed in mass-produced engines without an increase in cost.

Advanced technology like the mirror bore process only makes sense for Nissan if we can offer it to as many customers as possible. The end result is an innovative and efficient engine made with manufacturing technology usually not available at this volume or price point.

—Jay Boyte, director of engineering at Decherd Powertrain Plant, Nissan North America

2.5L engine. The new 2.5L 4-cylinder direct-injection engine (PR25DD) was developed for use on the new 2019 model year Altima as a successor to the QR25DE engine mounted on the previous model. The development concept defined for this new 4-cylinder engine was to achieve acceleration, fuel economy and noise, vibration and harshness (NVH) performance at the highest possible levels by incorporating the latest technologies, including a world’s first application.

The PR25DD engine continues Nissan’s new engine concept of recent years with regard to the basic engine systems, including the use of direct injection, an electrically operated valve timing control (VTC) system, cooled exhaust gas recirculation (EGR), an integrated exhaust manifold, mirror bore coating and a variable displacement oil pump.

In addition to these features, it also adopts a resin intake port. The resin port is inserted into the intake port cast in the cylinder head, thereby forming an air layer between the intake air passageway and the head inner wall so as to suppress the rise in intake air temperature. This world’s first application of a resin intake port improves anti-knock performance, thus contributing to improvements in combustion efficiency and power output.

Nissan Decherd currently assembles engines for Nissan Altima, Maxima, Rogue, Pathfinder, Frontier, TITAN and INFINITI QX60, as well as motors for the all-electric Nissan LEAF.

Resources

• Yoshida, N. (2019), “Development of New I4 2.5L Gasoline Direct Injection Engine,” SAE Technical Paper 2019-01-1199 doi: 10.4271/2019-01-1199