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Driving performance

Porsche’s 918 Spyder is an eco-friendly hybrid that is at the same time a powerful sports car. One of its performance secrets: an ultra-thin coating on cylinder walls to help reduce engine friction.

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Riding in a Porsche 918 Spyder is the closest you might get to the experience of piloting a fighter jet. Sitting deep inside the car’s carbon fiber body, acceleration from the 887 horsepower (hp) system presses you back as the 918 jumps from 0 to 100 km per hour in just over 2.6 seconds. Less than five more seconds and the speed hits 200 km per hour. By the time you realize you lost your breath, it is hundreds of meters behind.

This raw powerhouse has already claimed at least one track record and yet remarkably is not actually a race car. Porsche designed the 918 for everyday utility and performance. Steerable front and rear axles improve stability and handling. Adjustable driving modes keep the car ready for anything from comfortable transportation to race-ready.

And yet, remarkably, the car uses a hybrid drive system, with a traditional V-8 engine that offers a maximum 608 horsepower power output and two electric motors that, between them, add another 285 hp, hence an impressive total output of 893 hp. Hybrids that combine internal combustion engines with electric power have become increasingly popular around the world for their eco-friendliness with the ability to increase mileage, which reduces carbon emissions and decreases the cost of operation. With all the performance the 918 offers,  the hybrid construction allows it to travel 100 kilometers on only 3.0 liters to 3.1 liters of petrol.

“Porsche has an obligation not only to provide our customers with the most thrilling automotive experience possible, but to find solutions that can increase the environmental friendliness of even our highest performance vehicles,” said Dr. Frank-Steffen Walliser, Vice President of Motorsport at Porsche AG.

The car is a miracle of refined design, intricate engineering and demanding production. But Porsche had an advantage that is invisible to the eye. The company used a special coating inside an innovative engine that weighs less and consumes less fuel. The material helps reduce friction and improve performance.

Coatings are chemical compounds and mixtures that bond to a surface, typically metal. A common example is steel that has been colored with a substance baked onto its surface, like the frame of a bicycle. In this case, the process is much more complex.

Most internal combustion engines are designed around a system of cylinders and pistons. Vaporized fuel and air are mixed and introduced into the cylinders, where they are ignited. The controlled explosion pushes the cylinder down, both allowing the gas to escape and pushing a linkage to the drive shaft so the vehicle’s wheels will turn.

The coating must be applied inside the 918’s eight cylinders. The process takes on a new order of difficulty because cylinders are precision machined into an aluminum engine block. After application, the coating is machined by honing to ensure the correct diameter, degree of roundness, and surface roughness needed by the cylinders. The coating must also bind permanently to the cylinder walls.

“We wanted to choose an available coating approved in mass production with minimal friction,” said Walter Buck, Porsche’s project leader for the 918 powertrain. By reducing friction, less of the engine’s energy is converted into heat, which would mean less efficiency. In addition, more friction and heat can put a heavier toll on the engine’s parts. Porsche wanted to “guarantee the functional reliability with regard to low blow-by and oil consumption,” according to Buck. (Blow-by occurs when wear on or deformation of cylinders and pistons allow the exploding gases in the engine to pass by the piston rings and move down into the crankcase, harming engine power and fuel economy.)

The use of cylinder coatings in vehicles is well established. For example, Oerlikon Metco, which was a technology partner for the special project team created to make Porsche 918 Spyder a reality, has developed such cylinder bore coating technologies for 20 years. Automobile and truck manufacturers in Europe, Japan and in the US rely on the products and systems.

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The cylinder bores of the V8 high-speed engine are coated with Oerlikon Metco’s SUMEbore technology.

Basic to the technology is a plasma depositing process called an atmospheric plasma spray, or APS. The plasma heats the coating powder materials above the melting point and turns them into a molten stream that can then be deposited through special spray equipment onto the interior surface of the cylinders. The process does not require the use of volatile chemicals as a carrier, and so is more eco-friendly than other coating technologies.

“The APS process offers the maximum variability in the coating composition and is best suited to optimally satisfy the engine specific challenges,” Buck said.

“Depending on the engine type, manufacturers have various things they’re looking for,” said Dr. Peter Ernst, head of SUMEBore technology at Oerlikon Metco. “Most are looking for low friction that will help them reduce emissions. Some of our customers have a corrosion problem, depending on fuel quality and the way the engine is run, especially in the trucking business. Some manufacturers want to increase the longevity of their engines by reducing the wear. It depends on the customer and the application what they are looking for.”

As one might expect, there is no single coating to fit all needs. Most address friction reduction, because greater efficiency ultimately translates into lower fuel consumption, necessary for economic and ecological reasons.

But deciding on exactly what coating to use is not necessarily easy. Different engines will vary in the metallurgical composition of their engine blocks and cylinders and the way the coatings can interact with the system’s operation. Porsche worked with Oerlikon Metco for nearly five years, trying different variations and testing the results to find a balance between pushing performance and yet helping the engine to run 100,000 kilometers to 200,000 kilometers without a problem.

Porsche eventually decided on an Oerlikon Metco coating called Metco F4301. The specific composition of F4301 with high amount of Molybdenum improves the coatings resistance to wear and scuffing and reduces the friction. Although molybdenum is often an alloy component in steel, here pure molybdenum is present as “islands” to create the unique properties Porsche is looking for.

But for all the technical details, at the end it is important to focus on what they enable. The research and development that helps allow a jet-like ride in one of the fastest cars on earth eventually find their way into mass market applications, or as improved engine longevity in a truck or reduction of emissions in a common passenger car. This is just another way that microscopically thin coatings make life better for everyone.

2 comments

  1. Sillu

    Good to know !!

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