Engine block manufacturing process
Introduction
Cylinder block which is also called as engine block is the main structure of the engine which give the space for the cylinders, and it also give passages for the coolant, exhaust, and in take gases to pass over the engine and host for the crankcase and cam shafts. Engine block is the main housing of hundreds of parts found in modern engines. And it is the largest among the engine parts and it also constitute 20% to 25% of the total weight of the engine. The first successful internal combustion engine which can be used in an automobile was built by Siegfrid Marcus in about 1864. It was a upright single cylinder, two stroke petrol engine.
Today's engines has come to their maximum development and still being developed for the next years too. These developments have caused to increase the power, durability, resistance to wear, and efficient of the engine. Material used to build the engine block has being given the engine a higher strength with low weight which is more important for the power of the engine. For many years the engine block has being manufactured using cast iron alloys, it is due to its strength and low cost and its wear resistance. But as the engine become more complicated engineers found new materials to reduce its weight as well as to increase strength and wear resistance. A common alloy which is widely used is aluminum alloy, it is more popular due to its low weight but mostly within petrol engines.
Fig01: Finished engine block
Functional requirements of an engine blockFig01: Finished engine block
As the engine block is the main housing of the engine it has to include number of requirements. These requirements include the wear resistance, long lasting, maintenance, and withstand the pressure created when combustion take place. It also has to withstand high temperature, vibration when the engine is in the running conditions. For many of the requirements the main feature is its material used.
Material used in engine block casting
In order to meet the above functional requirements the material used for manufacturing the product should contain many properties. They are, the material should contain high strength, modulus of elasticity, wear resistance, ability to withstand vibrations, and corrosion resistance. High strength is mostly concerned in diesel engines because of their high compression ratios compared with petrol engines. In diesel engine its compression ratios are normally 17:1 or greater, but in petrol engine it is nearly 10:1. The material also should have low density to reduce its weight but with higher strength. It should also have a low thermal expansion under high operating temperatures and also a good thermal conductivity to give out the heat in minimum time. When it come to the manufacturing process the material should have good machinability and castability to reduce the time and cost consumed. As if the material is too hard the time and cost for manufacturing increases. When the engine is in running conditions it generates a higher vibration due to the motions in the internal parts like crank shaft and pistons, therefore the material has to be able to absorb the vibration energy with out fracturing.
Based on the above features the most widely used material are cast iron and aluminum alloys to manufacture the cylinder block. Cast iron alloys are used because they contain good mechanical properties, low cost, and availability compared with other metals. But certain aluminum alloys contain most of the characteristics of cast iron but with low weight. And also aluminum alloy casted engine block gives a good surface finish and high macinability compared with cast iron alloys. As the technology increases the engineers has found new materials such as graphite cast iron which is lighter and stronger than the grey cast iron mentioned above.
Grey cast iron alloys
Grey cast iron is the first and most material used for manufacturing of engine blocks. Though the aluminum alloy also contain many similarities with low weight, it is still used in the manufacturing of diesel engine blocks because their internal stresses are higher. Grey cast iron contains 2.5 – 4 % of carbon, 1 -3 % of silicon, 0.2 - 1% manganese, 0.02 - 0.25 % of sulfur, and 0.02 - 1 % of phosphorus. It has a excellent damping absorption, good wear and thermal resistance, and it is easily machinable and less cost due to its availability.
Aluminum alloys
Aluminum alloys main feature for its popularity is its low weight, this reduce the weight of the engine as well as in the vehicle. But the main disadvantage is their cost compared with grey cast iron. Aluminum alloy has a good machinability properties compared with grey cast iron. There are two aluminum alloys that are mainly used in manufacturing of engine blocks, they are 319 and A356.
319 aluminum alloy contains 85.8 - 91.5 % of aluminum, 5.5 - 6.5 % of silicon, 3 - 4 % of copper, 0.35% of nickel, 0.25% of titanium, 0.5% of manganese, 1% of iron, 0.1% of magnesium, and 1% of zinc. This alloy has good casting features, corrosion resistance, and good thermal conductivity. Under the heat treatment of T5 process, it generates high strength and rigidity for the engine block.
A356 aluminum alloy contains 91.1 - 93.3 % of aluminum, 6.5 - 7.5 % of silicon, 0.25 - 0.45 % of magnesium, 0.2% of copper, 0.2% of titanium, 0.2% of iron, and 0.1% of zinc. Although the mechanical properties are similar to 319, when it is under the heat treatment process T6 it gains higher strength than 319. But it has lower modulus of elasticity (72.4 GPa) than 319 with modulus of elasticity of 74 GPa.
Compacted graphite cast iron
Compacted graphite cast iron has a higher tensile strength and modulus of elasticity compared with grey cast iron. It is due to the compact graphite found on the microstructure of CGI. Similar to grey cast iron it has a good damping absorption and thermal conduction, but its low machinability has limited its wide usage.
Tooling required for casting engine block
The main tool needed for sand casting is the mold, the mold is generated by a mixture of sand, clay, and water. The pattern is the main tool required to form the mold, it is normally machined by wood or aluminum which can be easily machined. The pattern is kept on the wood or metal frame and the sand mixture is poured in to it, then vibrations are applied for the mixture to get free from air bubbles. After the mould has being hardened it can be used for the casting process.
After the casting process is over the casted engine block is passed through few machines to get the surface finish and correct dimensions. Computerized milling machines and boring machines are used in this operations.
Manufacturing process of engine block
Manufacturing of engine blocks are mainly done using sand casting, although die casting also used it is more cost effective as the die wear out easily due to the high temperature of the molten metal. The casted engine block is then machined to get the surface finish and coolant passages.
In the sand casting processors the widely used in engine block casting is green sand mould casting. The term green denotes the present of moisture in the sand mold. A combination of silica sand, clay, and water are poured in to the one half of the aluminum block pattern with wood or metal frame. The mould is then compacted by applying pressure or vibrating on the metal frame. This process is repeated for the other half of the mold. Then both halves of the mould are removed from the pattern.
Fig02: Patterns
The core shown below provides the space for water jackets around the cylinders. The core has being painted to seal the gas formed during the casting process within the core. And the pink colored ends are not painted to let the gas escape to the out side. Aluminum reinforcing rods are used to give more strength to the core. These rods get melted due to the molten metal poured during casting.
Fig03: core shown above provides the space for water jackets around the cylinders
Then the water jackets and cylinder molds are arranged in the main mold as a one cube. The mold is then tightened using clamps to withstand the pressure of gravity when pouring molten metal.
Fig04: main mold as a one cube
Fig04: main mold as a one cube
Now the mould is ready for the casting. The molten metal is poured in to the mold through the smaller front center hole which fills the mold from bottom back up to the top through the risers, which can be seen as 8 large holes. When the casting is cooling down the molten metal in the riser is drawn back down in to the casting. The risers act a main part in the casting process by supplying required molten metal during shrinkage.
Fig05: Just removed cast from the mold
Fig05: Just removed cast from the mold
The rough aluminum block casting is shown above after the removal of the sand mold. the sand is removed by applying vibrating on the casting. The casting has to be machined to get correct dimensions and smooth surfaces of the engine block.
Fig 06: Finlay machined to get correct dimensions and smooth surfaces
The rough aluminum cylinder block is done with surface grinding to get smooth surfaces in the head gasket face and the faces where other components are fitted. Then the block is ready for the line boring of the main bearing bores. Bearing caps are fitted temporally for the line boring of the main bearing bores. Then in to the line boring of the crank and the cam shaft bearing housings. The boring bar contains multiple tools so in one operation all the boring operations are done. Therefore the boring bar is carefully positioned in the mold. After the boring has being finished the crank and cam shafts are fitted temporally to check the clearances at the bearings. Now the engine block is ready for the further fittings of crank, cam, cylinders, connecting rods, and valves.
Theory behind casting
Casting is a solidifying process which means solidification phenomena controls the most of the properties of casting. And most of the casting defects occur during solidification. Solidification occurs in two steps, they are nucleation and crystal growth. In nucleation stage solid particles are formed within the liquid and these solid particles have lower internal energy than the surrounded liquid. There for they go below the freezing temperature because of the extra energy required. Then again it get heated up to form crystal structures.
Quality consideration during the production
The quality of the sand used widely affects the surface finish of the engine block. The sand should contain these features to get the required finish.
- Strength of the sand has to be high to maintain a rigid shape.
- Permeability is the size of the sand grains. Higher permeability can reduce the porosity of the mold, but a lower permeability would let to have a good surface finish.
- The thermal stability of the mold should be high to resist the damages such as cracking due to the molten metal.
- Ability of the sand to compress during solidification has to be high, unless the casting will not be able to shrink freely in the mold and it may result in cracking.
- The sand has to be reusable for next sand molds to be formed, because one sand mold can be only once used.
- The sand mixture must be well compressed around the pattern to get a higher strength, unless it will get cracked during the casting or when the molds are set on each other.
- The risers has to be well planed to make sure they does not get solidified until the whole block has being solidified.
- The contains in the molten alloy must be up to the standard to over come the defects.
- The clearance in the cylinder bores, crank and came bearings has to be up to the correct standard measurements.
- The cooling rate has to be up to the standard. The cooling rate is mostly controlled by the molten metal and the surrounding temperature, therefore the casting should be done in its certain thermal conditions.
Possible defects during the production
Any defect will reduce the strength of the engine block, as the engine block is running under higher temperatures small defect can be a reason for any failure of it.
- If the permeability of the sand used for casting is high, the strength and the surface finish of the mold will be reduced.
- If the thermal stability of the sand is low, the mould may crack due to the molten metal.
- If the compression of the sand is low the casting would not be able to shrink and will end up with cracking.
- If the risers get solidified before the other parts of the casting, it would give a engine block with less strength.
- If the molten alloy is not up to standard it will failure in high running conditions.