“We are very excited about the future prospects of this technology—we think there is a lot more opportunity for its use,” said a beaming Gerhard Schmidt, as he pointed to the new linerless aluminum cylinder block on display.
Ford’s Chief Technical Officer was talking about PTWA (Plasma Transfer Wire Arc), an advanced coating technology that debuts next year on the cylinder bores of the GT500 Shelby Mustang 5.4-L V8.
The PTWA process enabled Ford to shed approximately 8.5 lb (3.85 kg) of steel cylinder liners in the new Shelby’s 5.4-L aluminum 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 Al block.
Ford engineers indicated that moving to linerless aluminum blocks on other engine families can save the company significant mass, at a time when every kilo counts toward improved fuel efficiency.
The PTWA process was co-developed by Ford and Flame-Spray Industries of Long Island, NY. 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 that is 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.
(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.)
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 aluminum cylinder blocks.”
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.
Currently, Honsel and Flame-Spray are working with Ford Research to reduce spray time per block. Zaluzec admits that takt time is running approximately 60 seconds per bore—slower than mechanical insertion of steel liners.
Cook and Zaluzec claim PTWA is superior to Nikasil, which is an electrochemical “wet” coating process. PTWA, by comparison, is a dry process.
The first hints of the achievement came last year when its inventors were honored by the Intellectual Property Owners Education Foundation with the 2009 National Inventor of the Year Award. Ford holds more than 25 patents related to the technology, according to Zaluzec, and licenses it to Flame-Spray Industries for use in various industry sectors.
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 new Shelby V8 is derived from the supercharged DOHC, four-valve, all-aluminum engine that powered the Ford GT supercar. It will be rated at 550 hp and 510 lb·ft (410 kW and 691 N·m, respectively), a 10-hp (7.4-kW) increase from the 2010 iron-block engine.
The new engine produces 80% of peak torque between 1750 and 6250 rpm. Upgrades including six-bolt aluminum-billet main bearing caps and a larger twin-row intercooler (providing 40% more cooling capacity) assist the power increase in all operating conditions.
Also helping to deliver the additional power is a new, throatier 2.75-in (70-mm) exhaust.
Shelby Mustang owners will delight in the fact that the 2011 GT500 is the first contemporary Shelby vehicle that does not carry a gas guzzler tax. Its EPA estimated fuel economy numbers are 23 mpg highway/15 mpg city, up from 22/14 mpg for the 2010 car. Fuel-economy gains come from the new aluminum-block engine that is 102 lb (46 kg) lighter, electronic power steering, and various aerodynamic changes to the vehicle.