The Production, Characteristics And Use Of Aluminium And Its Alloys In Engine Applications Essay, Research Paper

Aluminium is a strong, durable and lightweight metal. In today’s energy-conscious society, these three basic properties combine to make the metal the preferred material of construction for transport applications, where weight saving to reduce fuel consumption and to increase load carrying capacity is vital. Products like motor cars, is an obvious example. It is not surprising therefore, that the use of aluminium in all its various forms – plate, sheet, extrusions, castings and forgings – is increasing across the whole range of transport applications. The use of aluminium for automobile engine blocks and cylinder heads, heat exchangers, transmission housings, engine parts and automobile wheels has risen steadily over the last decade.

The reasons for the extensive use of aluminium lies in the diverse range of properties (mechanical, physical etc.) exhibited by aluminium and its alloys:

h A key property is low density. Aluminium is only one-third the weight of steel.

h Aluminium and most of its alloys are highly resistant to most forms of corrosion. The metal’s natural coating of aluminium oxide provides a highly effective barrier to the effects of air, temperature, moisture and chemical attack.

h Aluminium is a superb conductor of electricity. This property allied with other intrinsic qualities has ensured the replacement of copper by aluminium in many situations.

h Aluminium is non-magnetic and non-combustible, properties invaluable in advanced industries such as electronics or in offshore structures.

h Aluminium is non-toxic and impervious, qualities that have established its use in the food and packaging industries since the earliest times.

Other valuable properties include high reflectivity, heat barrier properties and heat conduction. The metal is malleable and easily worked by the common manufacturing and shaping processes.

Another area that is becoming increasingly important is the economic disposal of used vehicles using materials recycling. Aluminium offers the best prospects from all of the materials currently used in vehicles, approximately 95% of the aluminium in old vehicles is regained. New technologies will continually increase this, the low energy requirement for processing scrap into new semi-finished products is an important contribution to solving ecological problems.

All the above properties can be found in an impressive array of commercially available alloys. The composition and logic of those alloys are regulated by an internationally agreed classifications system for wrought alloys and by various domestic nomenclature schemes for the casting alloys.

Aluminium alloys with great durability and high strength, some with a tensile strength greater than that of constructional steels, are available to the designer in the form of extruded profiles, rolled sheet and plate, castings and forgings. The majority of these alloys consist of aluminium with carefully controlled additions of magnesium, silicon, manganese or a combination of magnesium and silicon, although some high strength alloys with additions of copper and zinc and recently, lithium, have been developed for industry.

The following table shows the average use of aluminium castings and production methods in the engine of a production vehicle:

Examples Die

cast Chill

cast Sand

cast Typical weight in vehicle in kg

Piston X 1-3.5

Cylinder head X 10-20

Rocker cover X X 1-2.5

Engine block X 14-45

Engine cover X 1-3

Oil sump X X 3-5

Induction system X 2-7

Gearbox system X 8-15

Injection system/ carburettor X 1-3

Water pump casing X 0.5

Petrol pump casing X 0.5

Oil pump casing X 0.5

Engine block mount X X 1.5-2.5

Engine bearing X 0.5

Differential casing X X X 5-8

Looking at the most significant components where aluminium is used:

h Cylinder head, The cylinder head is required to be made from a material that can be readily cast with complicated internal shapes for coolant passages and inlet/exhaust ports. It must be able to operate under continuously fluctuating gas pressures and temperatures. Also good thermal conductivity is required so that lubrication remains effective, and combustible petrol-air mixture does not overheat and ignite. Aluminium alloys are the ideal materials for this application as they offer a good combination of the required properties.

The use of aluminium alloys however is not without its shortcomings, it is relatively expensive, and it has a high thermal expansion.

The two most common aluminium alloys are:

i) 3.0% copper, 5% silicon, 0.5% manganese in a matrix of aluminium

ii) 4.5% silicon, 0.5% manganese, 0.5% magnesium in a matrix of aluminium

The addition of the copper and silicon reduce the thermal expansion and improve fluidity and castability of aluminium. Copper added to aluminium hardens and strengthens the structure over a period of time (age-hardening), and silicon improves the abrasion resistance. Both manganese and magnesium improve the strength of the alloy.

There has been an increasing trend over the years, towards the use of aluminium for the cylinder heads. Before the introduction of aluminium cast iron was used. Cast iron does have its relative merits but the properties of the aluminium alloys mentioned above are perceived to be more important.

h Cylinder block, The cylinder block should be made from a material that when cast in a monobloc form, will have sufficient strength and rigidity in compression, bending and torsion. This very important so as there is the necessary support against the gas loads and various other forces acting on it. There is once again the necessity for a casting with good impressions, easily machined, good abrasion and corrosion resistance, low thermal expansion, high thermal conductivity and lightweight. There has once again been a shift towards the use of aluminium alloys from cast iron mainly for the weight saving capabilities and the high thermal conductivity compared to the cast iron. To compensate for the reduction in strength, aluminium alloy blocks are cast with thicker sections and additional support ribs. A typical aluminium alloy composition would be 11.5% silicon, 0.5% manganese, and 0.4% magnesium, with the balance aluminium. The high silicon content reduces expansion and improves castability, strength and abrasion resistance, while the other two elements strengthen the aluminium structure. With the advent of the aluminium alloy block came the renewable cylinder liners. The liners are essential as aluminium is soft and cannot be used directly for wear resisting

h Cylinder Liners (sleeves), Although very rare at the moment some manufacturers make use of low-friction silicon-aluminium alloy cylinder liners. In 1998, Mercedes-Benz the company who first introduced these cylinder liners claims that their silicon-aluminium cylinder liners provide a low friction surface, which allows piston-ring spring tension to be reduced by 50%. As a result of this, the internal drag (friction) of the engine is reduced by 45% compared to similar engines.

h Piston, The piston material should meet certain requirements such as good castability, high hot strength, high strength-to-mass ratio, good resistance to surface abrasion, and good thermal conductivity. At high speeds the reciprocating forces created by pistons reversing their direction must be minimised. The obvious choice here would be an aluminium alloy where the aluminium alloy is considerably lighter than a cast iron version of a piston and the small percentages of other materials in the alloy will improve the strength over the operating temperature range, and also improve resistance to abrasion. Aluminium is a much better conductor of heat than cast iron therefore reducing the maximum piston-crown operating temperature. The main problem with aluminium as seen with other cases is the thermal expansion, this is a major problem in the case of a piston as the piston could become tight and seize under operating conditions. The development of low-expansion aluminium alloys has helped to reduce this problem. Aluminium alloys with a high silicon content are commonly used for piston applications, as the silicon content increases, the thermal expansion is reduced to grades used are: 12% silicon and 22% silicon with aluminium.

h Crankcase sump, The crankcase sump may be made from a single sheet-steel pressing or an aluminium alloy casting with cooling fins and sometimes strengthening ribs. The preference is for the cast aluminium alloy to be used due to its weight saving capabilities and it is better at dissipating heat. There is no general requirement for the sump to contribute to crankcase rigidity, but with some transverse front-wheel drive engines the final-drive housing and sump form an integral unit. In this case an aluminium alloy casting would normally be used, which has been designed to withstand transmission loads and therefore must have structural stiffness report.

In all the above cases where aluminium alloys are used, the aluminium alloy is described as the preferred material compared to what was previously used. This well may be the case when we are talking about production vehicles where keeping costs low is a main objective of the design task and therefore the selection of materials. Another material that is surfacing as a substitute for aluminium alloys in engine applications are Metal Matrix Composites (MMC s). Metal Matrix composites have similar advantages to the aluminium alloys, but also enjoy the following:

h Physical and mechanical properties remain stable at high operating temperatures compared to aluminium alloys

h Lower coefficient of expansion

These characteristics make MMC s an ideal material for such applications, the only drawback being that it is expensive. For this reason MMC are used in applications where costs are not of great importance relative to design requirement. However, MMC s are a material of the future when production techniques improve and the costs come down.

Bibliography

1. Heisler H, Vehicle and engine technology, 1997, Arnold

2. Heisler H, Advanced engine technology, 1998, Arnold

3. Baker A, The Component Contribution, ENGINE PROGRESS THROUGH THE SPECIALIST MANUFACTURERS, 1979, Hutchinson Benham

4. Stone R, Introduction to INTERNAL COMBUSTION ENGINES Second edition, 1992, Macmillan

5. European Aluminium Association V www.eaa.net

6. The Aluminium Association V www.aluminium.org

7. World Aluminium V www.world-aluminium.org

8. Innovation, Mercedes-Benz V www.mercedes-benz.com

9. Ostermann F, Aluminium materials technology for automobile construction, 1993, Mechanical Engineering Publications