Magnesium And Aluminum In Lightweight Supercar Engine

BMW's latest 3.0 liter in-line six-cylinder engine is the first production engine in the world with a cylinder block made from a magnesium outer shell and an aluminum core. This method produces the lightest cylinder block practical, and is ideal for supercars and sports cars.

Many cylinder blocks of engines are still made from cast iron, because it is cheap, but to reduce gas mileage and pollutants many manufacturers now use aluminum cylinder blocks and heads, the two main housings of the engine.

BMW's innovation does not end there. The engine also has variable valve timing, called Valvetronic, which enables the valve lift to be varied from nothing to full lift according to the conditions. When the engine is idling, the valves hardly lift at all and open for a short period, increasing efficiency, and there is a medium amount of lift and opening period at low speeds, such as when cruising on the highway. When you press the accelerator right down, though, you get full lift and longer valve opening periods and maximum power.

Needless to say this is a very powerful engine, producing 258 bhp and yet it weighs only 355 lb. This is about the same as a normal 2.0 liter engine, and helps give BMW supercars sparkling performance.

24 per cent lighter

BMW says that the cylinder block weighs 24 per cent less than an aluminum block, and 43 per cent less than a cast iron one.

With these developments, BNW has increased the power by 12 per cent and reduced fuel consumption by 12 per cent at the same time – quite an achievement. The reduced weight helps improve gas mileage and reduce CO2 emissions, but the new valve gear is more important in this respect.

Although magnesium is 30 per cent lighter than aluminum, it is not suitable for a complete cylinder block because it is easily corroded by the water and anti-freeze used in the cooling system. Also, it is less stable than aluminum. For this reason, BMW uses an aluminum insert, which includes the cylinders, the water jacket and top deck of the block. This casting ii used as an insert in the main casting.

The magnesium outer is cast around the aluminum casting, bonding the two materials together so that they are in one piece. The magnesium outer casting includes the oil galleries and brackets. Normally these brackets are separate from the block, but as they are cast into the block, weight and assembly time are reduced.

Separate ladder frame

In addition, there is a separate ladder frame which contains the main bearing caps. The use of a separate frame, attached to the bottom face of the block makes the cylinder block very rigid where stiffness is needed. This ladder frame is also made from magnesium, with steel inserts to reinforce the bearing caps. The cylinder head cover is also magnesium. This advanced technology, combined with Valvetronic variable valve timing, gives the driver the best of both worlds – more power and lower gas mileage. The technique is used only by BMW at present, and is fairly expensive, but is ideal for supercars.

John Hartley is editor of www.fast-autos.com, an online magazine devoted to fast cars and supercars. He has written from many of the world's top auto magazines, and has written about 10 books about cars and the auto industry, including 'Suspension and Steering Q&A' and 'The Electronics Revolution in the Motor Industry'. He has been writing about fast cars and testing them for over 20 years, and has been involved in race car design, and has also raced small sports cars and sedans.