Metallic Materials Used in Engines
Metallic Materials
Metals are characterized by having a crystalline structure, and the bonds between the atoms are such that the electrons in the outer shell are not bound, but are free to roam within the lattice. This explains their high electrical conductivity and good thermal conductivity. Metals are seldom used in pure form, because their qualities are enhanced by being alloyed with other substances. They’re grouped together as either ferrous (ie, they contain iron) or non-ferrous.
The strength of the finished parts depends to some extent on whether the parts have been cast, forged, or machined from a steel solid, and the various alloys are tailored to have the qualities needed for the relevant manufacturing process. The casting process involves pouring liquid metal into a mould, allowing it to set, but forging utilizes two or more dies that force red-hot solid material into the shape required.
Ferrous Metals
This group includes all the alloys that contain a high percentage of iron ore. They differ from one another mainly in the amount of carbon present. Most of these are refined from pig iron, which is obtained from smelting iron ore with coke (dehydrated carbon) and limestone (calcium carbonate).
Wrought Iron contains less than 0.1 per cent carbon and between 1.0 and 3.0 per cent finely divided slag (a mix of metal oxides) distributed throughout the material. It has a high resistance to corrosion, and in the past was used for water pipes and other products that were used in a corrosive atmosphere. It is easy to weld, very ductile and forms a good outer surface for protective coatings to adhere to. Most cast iron alloys are usually cast into a shape, hence the name. These days the term cast iron usually refers to grey cast iron, so named because fractures show a grey appearance. It contains between 2.1 and 4.0 per cent carbon, and 1.0 to 3.0 per cent silicon. It is fairly brittle, but has a low melting point, makes superb castings, and has excellent machining capabilities and wear resistance.
Steel contains at least 0.3 per cent carbon, and very small percentages of other elements. Most steels contain no slag, and are classified according to their carbon content. The amount of carbon added will increase the hardness and strength of the steel. There are so many different steel alloys so we will only mention the more important groups:
Plain-carbon steels consist mainly of iron and carbon, and are classified as low carbon (0 to 0.25 per cent C), medium-carbon (0.25 to o.55 per cent C) and high-carbon (above 0.55 per cent C). These steels vary mainly in hardness and therefore in tensile strength.
‘Carbon Increases the Strength of Steel’
Nickel steels contain between 5.0 and 35 per cent nickel. They offer increased resistance to low temperature impact, increased magnetic properties, and resistance to corrosion at high temp. Nickel chromium steels are tough and ductile and exhibit high wear and corrosion resistance. Molybdenum steels are easy to harden and stay hard at high temp. By adding molybdenum to low-carbon steel increases the tensile strength. Chromium steels have increased core toughness and wear resistance. Triple-alloy steels contain nickel, chromium and molybdenum. They offer a high strength-to-weight ratio and good corrosion resistance.
Non-Ferrous Metals
1. Aluminium
The most important of these is aluminium. It has sporadically been used for major engine components since the dawn of motoring. Its major attraction for design engineers is the fact that that the same part is two thirds lighter if made from aluminium instead of steel, but unfortunately it is also a lot less stiff by the same amount, and a lot softer. This means that any change from steel to aluminium can only be done after a complete redesign of the part. In practice, this means that some of the weight advantage of aluminium is inevitably lost when used in high stress situations. Aluminium alloys have improved beyond recognition in the last four decades. Aluminium’s great come back started in the 1960s, when aluminium cylinder heads started to replace traditional cast iron heads. Aluminium is often alloyed with copper, zinc, manganese, silicon or magnesium. Interestingly enough these alloys are not corrosion resistant as the pure metal. In the latter form aluminium tends to form a clear protective layer of oxide. On the other hand when aluminium alloy is in electrical contact with some other metals such as stainless steel, galvanic corrosion sets in quickly. Corrosion is a problem inside a water-cooled engine with an aluminium head and a cast iron block. One of the solutions is to use a cylinder head gasket fitted with stainless steel rings around the bore to prevent contact between these metals. American vehicles use on average about 157kg of aluminium, compared with 110kg a decade ago, and the metal is used to an even greater extent in Asia and Europe. In fact the only barrier to increased utilization of aluminium is the metal fluctuating price on world markets.
2. Magnesium
Most people are familiar with magnesium because “mag” wheels as well as beverage cans are made from such alloys. It is the third-most commonly used structural metal after steel and aluminium, and has two-thirds the density of aluminium, and shares with the latter metal the propensity to develop a thin layer of oxide that prevents against further tarnishing. From there conception VW Beetles and Porches were designed with magnesium-alloy cylinder heads and block crankcase units. At the time the alloy was relatively inexpensive, and its lightweight was needed to curb the rear end mass would have on stability and handling. When the Beetle died so did the use of magnesium alloy. During the Beetles peak production years the Volkswagen Company used 50% of all magnesium consumed in Germany, amounting to nearly 42,000 tons a year.
Magnesium is highly flammable in powder form, and even from small chips arising from machining can catch alight. The resulting flame cannot be extinguished with water or carbon dioxide: sand should be used instead. Currently, most European companies use magnesium under the trade name Electron, for items such as transmission casings, intake manifolds and cylinder head covers.
3. Titanium
This metal is names after the Titans of Greek mythology, and has been called a space age material because it has the highest strength-to-weight ratio of any metal. It is as strong as some steel alloys, but weighs 45% less in pure form, and only slightly more as an alloy. It is also highly corrosion resistant.
Engine Parts
The materials used for various engine parts are:
1. Cylinder Bock / Crankcases
During much of the pre-1930 era it was common to build up the engine using a two-piece cylinder block. The part housing the cylinders was cast iron, either in unit with or separate from, the cylinder head. The lower part housing the crankcase was an aliminium casting, and so was the sump. For many years cast iron has been used for engine blocks, but at present it is considered too heavy for petrol units, but is still the material of choice for most diesel engine blocks. Modern petrol engine cylinder blocks are cast using aliminium because the reduction in mass is desirable. The recent BMW straight-six engine gained a two-piece magnesium/aluminium cylinder block to reduce mass on the front wheels by nearly 14kg. This helps to promote a near 50:50 weight distribution.
The cylinder bores are either loose cast iron sleeves or the bare aluminium treated with a coating or chrome spots are introduced using a special process. Failing to do this would result in excessive wear, as aluminium is not wear-resistant.
2. Cylinder Heads
For many years cylinder heads were made from cast iron and still is used for most diesel engines. However, in the 60’s aluminium took over, initially for it’s superior heat conductive properties, but these days also to reduce engine mass.
3. Pistons
Until the early 1920’s most engines were fitted with cast iron pistons, but these were later replaced by aluminium items. The latter offer a reduction in reciprocating mass as well as superior heat conducting properties. Most production pistons are cast from alloys having a high silicon content, but racing pistons are usually forged. Forging is a superior but also a more costly process, and a stronger alloy can be used.
4. Connecting Rods
High strength steel is used for most ‘rods, although a special aluminium alloy was tried in the past but doesn’t appear to have been very successful. Modern high performance engines tend to use titanium alloy because of its superior strengths.
5. Crankshaft
Until the early 1930’s most cranks were forged from high strength steel, but Ford started the fashion for casting cranks from a special mix of cast steel alloy. This saved time and money, and the process is used extensively these days. High performance crankshafts are either forged from special alloys if a large quantity is needed, or machined from billet if only a few are required.
6. Sump
Cast aliminium is used on expensive engines, but on most mass-produced units, pressed mild steel is the most inexpensive choice.
7. Manifolds
Exhaust manifolds are usually iron castings, but intake manifolds are either cast iron or aliminium castings. Plastic intake manifolds are also becoming popular with vehicle manufacturers.
