Designs change. Processes change. A new generation of engines made entirely from aluminum-silicon (AlSi) alloys is emerging. The goal: to reduce weight, fuel use, pollution, and friction.
In an attempt to make this happen, Sulzer Metco and Ford PowerTrain Research have joined forces to develop a cost-effective solution for coating engine block cylinder bores. They have devised a unique plasma thermal spray coating process as an alternative to traditional pressing, shrinking, or casting of cylinder sleeves, and the nickel-containing galvanic process, which has a high initial investment and operating costs. The galvanic process also produces environmental concerns regarding nickel emissions.
System applies powder spray. Sulzer Metco developed the RotaPlasma(reg)-500 integrated manufacturing system for plasma spray-coating AlSi engine-block cylinder bores.
The process involves injecting powdered material into a high-temperature plasma gas stream created by an internal arc. The plasma gas heats the powder and propels the semi-molten material to the surface at high speeds. The RotaPlasma-500 internal plasma spray torches operate with the necessary short spraying distances for cylinder bores of 70- to 100-mm diameters.
The test bed for this new technology was 20 1995 Ford Crown Victorias and Lincoln Town cars, equipped with Flex-Fuel System (FFS)-coated 4.6(liter) V8 aluminum engines.
In order to build the engines, engineers removed the blocks' cast-iron sleeves and replaced them with 6061 aluminum tube stock sleeving. Then they machined, grit blasted, cleaned, and plasma-sprayed the cylinders.
A single Sulzer Metco plasma spray coating system can spray up to 600 cylinder bores per shift with an iron or molybdenum-based coating. The resulting coatings are thin (140 to 200 (mu)m), uniform (±10% of the nominal coating thickness), and smooth (Ra = 8 to 12 (mu)m).
The process works like this:
- Install stock with a 35- to 40-mm interference by heating the block to 150C prior to installing the sleeve.
- Bore the cylinders to a diameter 300 mm larger than the finished bores size.
- Coat bores with a rotating plasma gun to a thickness of 200 mm.
- Chamfer blocks top and bottom, then hone to 0.2- to 0.3-(mu)m Ra surface finish using conventional honing equipment and diamond honing stones.
- Wash and prepare for assembly.
Ford tests the process. Test vehicles went into service at taxi companies noted for their well-maintained logs on oil usage. The engines were exposed to different weather, cold starting, and driving conditions.
Based on Ford's 100-hr Piston & Gasket Engine Dynamometer Durability Tests and Fleet Vehicle Durability Tests, the blocks performed as follows:
- Reduced friction torque up to 32%.
- Reduced brake-specific fuel consumption (BSFC) up to 4%.
- Improved peak power up to 3%.
- Reduced oil consumption up to 40%.
- Reduced top ring and bore wear by 50%.
Andrew Nicoll, an automotive applications specialist at Sulzer Metco, predicts that in the next decade the automotive industry will apply thermal spray coatings in a variety of ways to further improve automobile performance. Thermal barrier coated engines increase overall thermal efficiency by reducing heat rejection to the cooling system. "Thermal spray coatings improve the performance of critical components such as injection nozzles, piston rings, piston heads, valve heads, cylinder bores, and drive-train components," says Nicoll. Ford hopes to use the plasma spray coating process in future automobiles to reduce weight and costs.
What this means to you
Advantages of plasma coating technology include:
- Low operating cost
- Thinner, lighter engine blocks because of sleeveless walls
- Less friction, extended wear, and improved oil consumption
- Reduced environmental hazards by replacing nickel-based materials with a galvanic process
RotaPlasma-500 increases production
The RotaPlasma-500 plasma gun manipulator increases production rates by automating the process of coating internal surfaces where tight or demanding geometries are involved. The manipulator directly sprays all internal diameters individually without moving the component by continuously rotating the gun. Rotational speed ranges from 1 to 250 rpm, counterclockwise or clockwise. The maximum plasma current is 500A. A variable turning circle lets the user accurately set and maintain the spray distance. The smallest spray diameter is 35 mm, the largest 500 mm. Maximum spray length of gun motion is 1,200 mm. Applications for the RotaPlasma-500 include coating automotive engine-block cylinder bores, turbine-engine transition ducts, pump housings, flanges, pipe connections, heat-exchanger tubes/tubesheets, and bearing surfaces.