Wednesday, 28 September 2016
Bentley sheds kilos from Bentayga diesel
Bentley Motors is saving weight on its new diesel engine for the Bentayga sports utility vehicle (SUV).
As we reported yesterday, Bentley is using an APC process for coating the cylinder bores of the aluminium vee block.
Conventionally, APC normally refers to abradable powder coating and typically is used for pistons. But somewhat confusingly, the APC process used for Bentley Bentayga diesel engines is different to the APC related to pistons.
According to Bentley, the company is using the same technique used for NASCAR engines. Anodic Plasma Coating (APC) is a process of coating cylinder bores with a plasma that has been used to atomise and then project a metallic spray.
The result is a thin, uniform and smooth bore surface which can then be honed to a precise finish, and as there is no need for liners the finished block is lighter and the wall thickness between bores can be made thinner – leading to a more compact engine.
Volkswagen uses a similar process for the cylinder bores of its V18 engine as well as the five-cylinder diesel produced at 4,000 a day. VW uses Sulzer Metco’s SUMEBore technology.
In trials on NASCAR engines Sulzer Metco noted a power gain when the cylinder bore was modified as a system with both the Cr3C2 25 (Ni 20Cr) and the TiO2 plasma sprayed bore coatings – The TiO2 coating indicates a greater magnitude of friction reduction.
In addition to saving valuable engine weight, Bentley’s engineers can point to other benefits, such as important reductions in friction. Friction reduction optimization involves a number of variables in the combined cylinder block and piston ‘system’ These include bore coating and honing technique, piston skirt profile and clearances and, finally, the ring pack.
Bentley says it is not in a position to release information regarding either weight if the aluminium block or the weight saving.
Ford more yielding
Some clue to the potential weight savings from this and similar plasma coating processes can be gleaned from Ford Motor Company which is more yielding in releasing information.
Ford uses PTWA or Plasma Transfer Wire Arc, an advanced coating technology used for the cylinder bores of the GT500 Shelby Mustang 5.4-litre V8 engine.
The PTWA process enabled Ford to shed approximately 8.5 lb (3.85 kg) of steel cylinder liners in Shelby’s 5.4-litre aluminium block. Compared with the 2010 GT500 V8, which used a cast-iron block, the 2011 engine weighs 102 lb (46 kg) less, due mainly to the linerless 356-alloy aluminium block.
The PTWA process was co-developed by Ford and Flame-Spray Industries of Long Island, New York. It is widely used in aerospace gas turbines, where it provides an extremely durable surface coating for various high-stress/high-temperature components. It is also employed by Caterpillar in remanufactured heavy-duty diesel engine cylinders.
The process uses compressed air and electricity to create a plasma jet of 35,000°F (19,427°C), which melts a steel wire fed into a rotating spray gun. The pressurized air blows atomized droplets — 20 to 30 µm (790 to 1180 µin) in size — onto the cylinder walls, which have been specially machined to accept the coating.
The molten steel wire oxidizes and builds up a laminate structure on the bore consisting of a nanocrystalline material — iron and ferrous-oxide (FeO, known as Wuestite) — to a final thickness of 150 µm (5905 µin).
“The coating requires no curing; it solidifies in 10-6 seconds,” said David Cook, Vice President of Flame-Spray. Cook is a former Ford Research engineer who was part of the team that began investigating PTWA in the early 1990s.
After the coating process, the bore is diamond-honed to create the final production surface. “We have noted benefits in heat transfer and reduced internal friction, as the PTWA process creates micropores that help improve oil retention on the bores,” noted Matt Zaluzec, manager of Ford’s materials science & nanotechnology department.
“Durability tests have proven this is very durable. We have aggregated over 3 million miles of fleet testing, and we have test engines that have done 250,000 miles and they still have the cross-hatches on the bores—with no issues,” Zaluzec said.
The V8 blocks are cast in Germany by Honsel, a specialist casting firm that “embraced the technology from the start,” said Cook. “They see it as the future of aluminium cylinder blocks.”
Interlocking bore with coating
Honsel helped develop the mechanical interlock between the rough bore surface and the ferrous-oxide coating material. The company cuts a sophisticated groove into the bore prior to the plasma spray, which helps bond the material to its substrate.
At the time the SAE paper was written Honsel and Flame-Spray were working with Ford Research to reduce spray time per block. Zaluzec admits that time is running approximately 60 seconds per bore — slower than mechanical insertion of steel liners.
Cook and Zaluzec claimed PTWA is superior to Nikasil, which is an electrochemical “wet” coating process. PTWA, by comparison, is a dry process.
Although the 2011 GT500 is the first Ford vehicle to use PTWA-coated cylinder bores, Ford has licensed the technology to Nissan, which is using it on the bores of the GT-R’s turbocharged V6.
The Shelby V8 was derived from the supercharged DOHC, four-valve, all-aluminium engine that powered the Ford GT supercar. Rated at 550 bhp and 510 lbft (410 kW and 691 Nm, respectively), this offered a 10 bhp (7.4 kW) increase from the 2010 iron-block engine.
The engine produced 80 per cent of peak torque between 1750 and 6250 rev/min. Upgrades including six-bolt aluminium-billet main bearing caps and a larger twin-row intercooler (providing 40 per cent more cooling capacity) assist the power increase in all operating conditions.
(For more information, see “Thermal Spraying of Nano-Crystalline Coatings for Al-Cylinder Bores,” by Clemens Verpoort of Ford Research and Thomas Schlaefer of Aachen University, SAE Technical Paper 2008-01-1050.)