November 23, 2005
By Jim Kerr
BMW 6-cylinder gasoline engine. Click image to enlarge
BMW has proclaimed a breakthrough in engine production technology: the 3.0 litre inline six cylinder engine design that has almost become a trademark for BMW has changed dramatically – although you probably wouldn’t know it to look at it from the outside. By using magnesium for a significant portion of the engine block, BMW is able to reduce engine weight, reduce engine noise transmission and add strength.
Why magnesium? According to BMW, there are no more weight reductions possible through the use of aluminium in their engines. The engine block is the heaviest single component of an engine and by using magnesium, the BMW composite block weighs 24% less than an aluminium block. For drivers, that lighter weight translates into faster acceleration, shorter stopping distances, better cornering and improved fuel economy.
Magnesium is already in widespread use where lightweight and high structural strength are needed. After seeing use in the aerospace industry, it is now being used in common items such as laptop computers, cordless power tools, and automotive components. At one time it was used to make racing wheels, hence the name “mag wheel”, but now most wheels are aluminium alloys. Other automotive components such as transfer cases, transmission components, and body structural braces may be made of magnesium to reduce vehicle weight and increase strength.
Magnesium is easy to machine and process and has a melting point and expansion rates virtually identical to aluminium. It has excellent acoustic qualities so it can dampen vibrations from internal engine components. While it can be mined, most magnesium is extracted from sea water, ensuring an abundant supply for the future.
Magnesium-aluminum-compound crankcase. Click image to enlarge
The disadvantages of magnesium are that it is susceptible to corrosion when wet and in contact with other materials such as iron. It cannot be used to carry coolant. It also has a tendency to creep under high loads, so it is not suitable for engine locations such as piston running surfaces or main bearing housings. These disadvantages can be overcome by BMW’s new composite casting process.
BMW makes the block by combining an aluminium insert with a magnesium shell. The aluminium is a central casting that contains the engine cylinder bores, coolant passages, upper main bearing housing, and bolt and stud anchors. The magnesium shell surrounds the aluminium, core and carries accessories and oil passages. A magnesium bedplate forms the
lower crankcase and supports the integral steel main bearing caps that hold the crankshaft in place.
Construction of magnesium-aluminum-composite crankcase and aluminum insert on toolset. Click image to enlarge
The two block components are fused together in a high-pressure die at 700 degrees C and 4000 tonnes pressure. The mould weighs 60 tonnes! In 6/100 of a second, the liquid magnesium is diecast around the internal aluminium insert. Ten seconds later, the magnesium turns solid and the composite structure is bonded into one unit.
During engine assembly, special gaskets are used with multiple sealing areas to ensure separation of liquids from the magnesium. If you look at machined surfaces such as the block deck or crankshaft bedplate support structure, you will be able to distinguish the slightly different colour of the magnesium part from the aluminium part, but they form one solid structure.
Fully automated production of magnesium-aluminum-composite crankcase. Click image to enlarge
This engine construction took four years of development and has a large Canadian connection. BMW worked with Noranda Canada to develop the technology. Researchers from Noranda/Falconbridge, McGill University, and the National Research Council were key to enabling this process.
Improving engine technology helps keep BMW engines at the forefront. For example, in BMW’s 330i, the engine produces 30 more horsepower, is 20 kilograms lighter and uses 12% less fuel. Now that is performance.
The magnesium/aluminium composite engine design will be found in all 2006 North American 3-series cars, the majority of 5 series cars and in many BMWs worldwide.
Next week, we will take a look at the changing technology under the skin of Honda’s new Civic Hybrid.