ABSTRACT Engine blocks are the skeleton system of automotive engines. Due to the variety of functions they perform, the material choice is crucial in selection of block material to ensure the durability, strength & cost. Normally a block will be the last thing to be changed in any vehicle so it is expected that they last for all the life of a car. The stringent requirements for the block restrict the available materials to few options which are versatile and possess the following characteristics: Manufacturability Resistance to thermal expansion Good thermal conductor Strong Wear Resistant The selection is made after analyzing the requirements of customer and availability of manufacturing facilities. Whilst one material may seem better for performance, it may not be feasible considering the less expectations from the customer and the cost involved. The study of metallic structure at microscopic scale also comes into play when considering the phenomenon like creep and fatigue during engine operation. At the end of this study, we can develop a hierarchy that will help us to make decision when choosing the right material for engine blocks.
SELECTION CRITERIA FOR MATERIAL OF AUTOMOTIVE ENGINE BLOCK
SELECTION CRITERIA FOR MATERIAL OF AUTOMOTIVE ENGINE BLOCK Introduction Engine Block is the central component of any automotive engine. The composition of block has evolved greatly over the course of time due to the variety of challenges we face in making blocks reliable, durable and easy to manufacture. Reaching a tradeoff between various type of material considerations is a very crucial part of block manufacturing. Any inclination towards one feature or other would result in our final product not fulfilling our requirement in one way or another.
Function of Engine Block Before we go on analyzing various properties of materials, we need to figure out the stresses/loads/properties that an automotive engine has to bear. Following are some of the prominent points in this regard: 1. During the combustion stroke, a burst of heat waves hits the block. Thermal resistivity is therefore a major contributing factor towards the durability of engine. Furthermore, the block should distribute the heat it gets from the cylinder so as not to overheat a specific area of engine. Else those parts of engine would start ‘creeping’ 2. The cylinder houses the moving pistons as well which seal the chamber via piston rings. Proper seal of the chamber is must else the pressure would be lost while compressing the fuel. This requires engine block to have a good surface finish that won’t become rough destroying piston rings pretty often and losing engine performance
3. The microscopic structure of engine block should be able to sustain itself under repeated cycles of load i.e fatigue. This involves resistance to both thermal loads as well as the loads that arise due to other components being harnessed to the block 4. Time to time, engine blocks require to be machined for car service. During machining the cylinders are rebored and this requires block to have good machinability properties else after one run of optimum performance, block would be rendered wasted
5. Engine blocks are usually cast as a single component and not welded as the welded joints will eventually wear out and break. This requires the material chosen to have good castability to aid the mass manufacturing of blocks 6. Blocks can be immensely big and need to be as light as possible. Thus the weight of the block also becomes important when choosing materials.
A Quick Conclusion Analyzing the above mentioned points we can foresee that a single material will not fulfill our requirements rather a mixture of materials is to be used. The function of all those additives will be discussed later in this study.
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SELECTION CRITERION FOR BLOCK MATERIALS
SELECTION CRITERION FOR BLOCK MATERIALS 1. Manufacturability The foremost point in choosing a material is manufacturability. This is based on: 1. Abundantly available: Since the massiveness of the structure requires a lot of material easily available, abundant metal has to be used for this purpose. Generally, it’s the cast iron that is easily available compared to others like Aluminium etc. Metals like Aluminium, magnesium are not only hard to manufacture, expensive but also do not meet other requirements. 2. Castability: Cast iron is easily cast into any shapes compared to Aluminium which tends to oxidize and this can make casting process difficult although it can still be used by employing contemporary casting techniques. Cast iron can be used in the sand casting process – the cheapest of all casting processes ever existed. 3. Economy: Cast iron is much cheaper compared to other metals. For instance, in Pakistan it barely is above 300 Rs/kg whilst Aluminium goes upto 750 Rs/kg. For Aluminium, sand casting process cannot be used easily. Rather hot/cold processes are employed with steel dies operating at various pressures. For cast iron, an automatic machine prepares sand molds which are manually filled with molten iron. Whereas for Aluminium, a proper equipment with hot/cold chamber and die system is required, thus making manufacturing Aluminium blocks an expensive process. Some Aluminium processes are:
Pressure (MPa) Dimensional Accuracy Productivity1
Sand Casting
Gravity Die Casting
Lost foam
High Pressure Die Casting
0
0
0
100-150
Low Pressure Die Casting 20
Low
Medium
Medium
High
Medium
100
50
80
50-100
40
Conclusion: Thus if the manufacturing equipment for Aluminium is easily available, Al blocks are preferred due to their low weight. However for small scale production, cast iron is recommended and where vehicle performance is not much of an issue
1
Productivity is defined in comparison to high pressure die casting which produces 100 parts
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SELECTION CRITERION FOR BLOCK MATERIALS
2. Thermal Properties Thermal loads are considered to be loads of primary importance, unlike other structures where countering the mechanical loads is more of a concern than thermal loads. This leads us to demote the mechanical loads in our consideration and focus solely on thermal loads. Another important aspect in this regard is conductivity. Cylinders have to be cooled hence the material chosen should be a good conductor of heat. Two thermal properties are considered while choosing material: 1. Thermal Expansion: Due to temperatures crossing 1000°C, the block will undergo expansion due to heat that will temporarily deform the shape. Thus the material in for the block, especially cylinders, should have material that resists expansion as much as possible. In other words, it should have low coefficient of expansion. We see in table give at the right that cast iron has very good coefficient in this regard while still offering us strength, manufacturability & machinability. Pure Aluminium has a coefficient double as that of cast iron however Al alloys can be manufactured to match cast iron. 2. Thermal Conductivity: Engine block takes heat from combustion chamber and spreads it out across its structure uniformly to prevent overheating at any locality. This block material should have good coefficient of thermal conductivity. Silver gives maximum coefficient however it is ruled out due to not meeting other criterion. Cast Iron has a coefficient of 46.33 which is reasonable considering it is meeting our other demands. Aluminium becomes a better choice as it has coefficient of 136 and overshadows cast iron if other requirements are filled easily.
Table showing thermal properties2
3. Machinability, Surface Finish & Lubrication Cylinder blocks need to have high dimensional accuracy with tolerances allowed only in order of μm. To improve surface finish, a process called ‘honing’ done on special honing machines is carried out. During honing process, the honing tool carves patterns on the inner surface of cylinder which holds oil for lubrication. Some facts about metals in this regard are:
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The traces of graphite in cast iron act as solid lubricants. MoS2, WS2, tin and lead all are good solid lubricants. Hence they can be lined inside the cylinder to provide good amount of lubrication in case of less oil/ overused oil. Since the honing of cast iron exposes graphite on its surface, an alternative idea is to line the inside of cylinders bored in a block of any metal such as Aluminium. Since aluminium is light and only lining of cast iron inside is needed, this meets both of our requirements of less weight while giving good surface finish, solid lubrication.
http://www.engineersedge.com/properties_of_metals.htm
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SELECTION CRITERION FOR BLOCK MATERIALS
3Microscopic
view of Cross-hatch pattern after honing
4How
graphite works as solid lubricant
Among all solid lubricants known, graphite is predominantly used because it is naturally present inside cast iron. This leads us to explore the possibilities of other material which have the structure to provide solid lubrication. This lubrication basically emerges from the less friction between basal planes of crystalline solids during slip. Hence, microscopic examination of potential materials can provide us more options to use in applications where solid lubrication is needed. The type of finish honing process produces also resists ‘scuffing’ of the cylinder walls. Thus materials preventing scuffing become important as well.
4. Wear Resistance Some elements are known to provide surface hardness giving a good resistance against wear and tear due to piston motion. These are called Anti-wear additives (AW additives). Phosphorus is one of the additives that increase the resistance to wear. Thus many phosphorus compounds are added during manufacturing of metals that are to be used in metal-to-metal contact situations. Often the high-phosphorus gray cast iron is used to provide durability. The phosphorus crystallizes in the space between iron crystals as Fe3P. This alloy gives good wear resistance but has poor machinability so it is used in cases where the cylinders are not expected to distort any sooner or subject to high loads. Both Zinc and Phosphorus compounds are used in applications that require wear resistance. For more of their uses, please read http://www.polarislabs1.com/test-explanations/additive-metals.php Another method to prevent wall wear is to electroplate the lining with chromium metal. Chromium plating has a good hardness of 800 HV. However, due to complexity of the process, this method maybe employed only in high performance cars. A cheaper process would be to dip the entire block into a bath containing electrolyte however this becomes time-consuming and costly as the parts which do not need electroplating also get electroplated. Cast-in Composite were introduced by Honda in which carbon fibres in a pre-form are pressed by a medium pressure die into the bore wall. This forms a Metal Matrix Composite (MMC) on the wall with a wear resistance equal to that of cast iron lining in a aluminium block
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Materials of Automotive Engines by Hiroshi Yamagata, page 31 Materials of Automotive Engines by Hiroshi Yamagata, page 32
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SELECTION CRITERION FOR BLOCK MATERIALS
5. Weight To increase power-to-weight ratio, car’s total weight needs to be reduced. No doubt most of the components’ weight comes from the engine itself in which block is heavier than other engine components. Efforts have been made to reduce engine weight without compromising the strength, thermal and wear resistance. Aluminium is the lightest metal used for big applications in the industry. While cast iron has been in use extensively, it is now being replaced by Aluminium on following grounds: 1. 2. 3. 4. 5.
Aluminium manufacturing systems are getting advanced Aluminium offers light weight With increase in use of Al for other applications, it is becoming easily available. Very good thermal conductivity compared to cast iron Aluminium is near to iron in strength
Although Aluminium alone doesn’t offer a all-in-one package like cast iron. Therefore, Aluminium blocks are manufactured and modified using some other metals. One case as discussed earlier is the cast iron lining in aluminium block. This allows for better wear resistance as given b cast iron while allowing for the rest of the block to be made by aluminium. The table below shows some key points about two main materials: Cast Iron and Aluminium. The selection is based on:
Product Requirement: Sports car, normal use Durability: Where decline in performance over time isn’t prominent, Al block maybe used Power: For heavy duty applications, thermally and mechanically strong material would be preferred.
Weight
Cost
Cast Iron
Heavy
Cheap
Aluminium
Lighter
Expensive when including manufacturing cost
Cutaway of Al block with cast iron lining
Wear Resistance Good, little addition required Additives are must
Thermal Properties Less distortion, bad conductivity More distortion, high conducting
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CONCLUSION
CONCLUSION Based on above discussion, following hierarchy can be developed for selection of material (in order of decreasing preference).
1st
• Manufacturability • Thermal & Mechanical Strength
2nd
• Wear Resistance • Machinability
3rd
• Cost • Product Requirement
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