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.)
No comments:
Post a Comment