High performance, state of the art, engines are driven by the objective to reduce fuel consumption and exhaust gas emissions. This goal, however, is to be achieved while maintaining economic success (i.e. cost reduction). Countless measures that involved a great deal of development effort were developed and implemented in series production by manufacturers with support from the automotive suppliers.
In general, a distinction can be made between external and internal engine modifications. The continuous introduction of exhaust gas after treatment in the vehicle by means of a catalyst or diesel particulate filter is an important measure for improving exhaust gas emissions. Intelligent control loops, such as the automatic start-stop system and the partial cylinder shutoff of a twelve-cylinder engine in the premium category, are very much in line with the trend. Internal engine modifications can generate a change of the crankcase, from which changed requirements for the production method of the foundry as the automobile manufacturer’s supplier are derived.
Since the crankcase is the largest engine component, there is a strong focus on it with regard to reducing consumption by means of weight saving. The rule of thumb is: If the mass of the passenger car is reduced by 100 kg, the fuel consumption decreases by 0.3 liters per 100 km. The material development of a crankcase is therefore of great importance. The material characteristics of density and dynamic strength are major influence factors for the component mass. Cast iron as a material for crankcases is relatively outdated.
However, the series production of crankcases made of aluminum alloys, which started at a much later point in time, represents a turning point for the passenger car. In principle, the crucial disadvantage of standard version cast iron as compared to aluminum is its higher density of 7.2 g/cm³. Crankcases for trucks are generally made of cast iron to this day. The two materials are currently competing to be selected as the material for the crankcase of large series passenger cars.
A subsequent turning point has become known as "downsizing." Downsizing involves replacing large-volume engines with engines that have a smaller cubic capacity but at least the same power. It is possible to reduce fuel and exhaust gas by having more power with the same engine weight or more power with the same cubic capacity. The power-to-weight ratio and the power output per liter are therefore the characteristic values for downsizing. These values are improved by means of engine supercharging.
A gradual evolution from the naturally aspirated engine to supercharging and twin-charging can be noted. The associated units are the supercharger and the turbocharger. For an appropriate vehicle, a high-performance 4-cylinder in-line engine with twin-charging represents an alternative to a naturally aspirated 6-cylinder engine. This is also referred to as cylinder downsizing. One fewer cylinder means less friction, a smaller crankcase, a smaller cylinder head, fewer bearing caps, etc., and as a result, less total weight. In general, increasing the ignition pressure by means of supercharging increases the load on the crankcase. In order to compensate this, the dimensions of the walls would have to be thicker. However, thicker walls mean an increase in weight and higher fuel consumption. Materials with increased strength are in demand, since their use does not lead to a loss of the advantage of supercharging. Downsizing has thus broadened its restricted view of density to include material strength. This is a large field of work for the development in the foundry and its suppliers. ASK, too, contributes to this: The development work on increasing strength by means of improved heat dissipation through the mold material. In addition to the chemical composition of the material, the component strength is also determined by the thermophysical properties of the mold material.
New approaches in lightweight design development
- Crankcase hybrid made of a complex cast iron structure and aluminum alloy (Gießerei Halberg Guss together with VAW mandl & berger)
- Crankcase hybrid made of aluminum alloy and magnesium alloy (BMW AG, Gießerei Landshut)
- Low-density cast iron - < 6.8 g/cm³ (Gießerei Halberg Guss)
System solutions in engine block casting
The surface topology of the cylinder barrels, threads, very thin oil ducts and areas with very small dimensional tolerances cannot be achieved in a single production step in casting. Mechanical processing is used for this, but near-net-shape casting is a crucial requirement for the casting type so that the processing costs can be kept to a minimum. Standardized casting tolerances, which also depend on the casting method, form the basis.
- Flexibility of the size: All dimensions of crankcases can be produced using the sandcasting method; from the small 3-cylinder crankcase for passenger cars to the largest crankcase for trucks.
- Flexibility of the geometry: The freedom of design for the cavities in the crankcase is much higher as compared to the pressure die-casting method.
- Flexibility of time: Changes to the geometry and also prototypes can be implemented much quicker and in a more cost-effective way as compared to the die-casting method.
When considering the entire production, the "birth" of the crankcase begins with the primary molding process of casting. A distinction is made between casting into a metallic permanent mold and into expendable molds. Casting into expendable molds made of bound sand – sandcasting – is widely spread in the production of crankcases, as it is the casting method that unites the highest level of flexibility with excellent economic efficiency.
The molding base material and the binder are of great importance in the sandcasting method. The naming was also adapted due to this great importance. In the core package method, the entire geometry is represented by sand cores. The surface of the core is coated depending on the viscosity behavior of the melt, the pressure of the melt, the filling rates, and the casting temperature range. Coatings, a dispersion of refractory particles and water, are used for molten cast iron, whereas a powdery core coating is applied for molten aluminum.
In the molding sand method, at least one main contour is represented by bentonite-bound sand. However, this can also involve cavities with undercuts for auxiliary and ancillary equipment, such as a water-pump housing, due to additional cores that were introduced into the bentonite-bound mold. Both casting types can be found in the cast iron area.
The dominant method for making crankcases out of aluminum alloys is the core package method. Increased hydrogen content in molten aluminum causes casting defects. The mold heats up due to the melt cooling off. The binder it contains is heated up and emits gas that finds its way through the mold material’s porous body to the boundary between the mold and the melt. If the gas contains hydrogen, it can find its way across the boundary and into the mold. Since molten aluminum is sensitive to hydrogen, one requirement for the binder is that it should not emit gases that contain hydrogen. As water vapor is released when using bentonite-bonded molds, the core package method is used for molten aluminum as a precaution.
Besides the technical requirements, such as sufficient mold strength and resistance against abrasion, requirements for environmental protection are increasingly pushing themselves to the forefront. Gas emissions should be more environmentally friendly in terms of their composition, amount and odor. ASK Chemicals has defined the milestones of developing highly reactive binders and inorganic binders.
As the following examples demonstrate, the first rough parameters for the casting process emerge from the features listed above, i.e. design, cubic capacity, number of cylinders, number of cylinder banks, angle of the cylinder banks, "open deck" or "closed deck." V-engines with a 90° cylinder bank angle always require the core package method. The problem of undercuts that otherwise occurs is solved by a "heart core" in the core package, which represents the gap between the cylinder banks. In contrast to this, a 6-cylinder V-engine with an extremely narrow angle between the cylinder banks can be cast horizontally using cast iron and without a core package since this design does not involve cavities between the cylinder banks. The relation between the cubic capacity and the size of the mold shall serve as the third example: Appropriately large mold boxes and tools are necessary for producing cores for a truck crankcase for a V-engine with a 12-liter cubic capacity. Four crankcases for a 1.4-liter passenger car engine could be cast simultaneously in the same mold box.
The rough parameters of the casting process include the casting position. If the cylinder barrels are largely parallel to the horizontal mold partition, the casting position is referred to as horizontal. If the cylinder barrels are at an angle of 90° to the horizontal mold partition, the casting position is referred to as vertical. A vertical casting position is only possible with the core package method.
Requirements for the material
The crankcase takes on a local complex load that must be born reliably for several hundred thousand kilometers and many years. This results from the complex of loads that act on the component and that are composed of gas forces in the combustion cycle, of reaction forces in the power cycle and of bends and screw joints. There are also the interior forces from the thermal expansion, internal stresses, the weight force of the component's own mass, the weight force of screwed on components, such as the crankshaft and the cylinder head, and the forces from the thermal expansion of the add-on parts that are transmitted via the screw connection.
The highest thermal stress is usually found in the web area of the cover plate. The fact that water and lubricants are routed through the component poses the requirement of pressure tightness, cleanliness and permeability of the liquid-carrying ducts in the crankcase. As a consequence of the task of piston guidance, the cylinder barrel must be part of a tribology system. This is the cylinder barrel/lubricant/piston ring system. The surface film of the cylinder barrel must therefore display tribologically appropriate properties. Low friction loss, wear and tear, and lubricant consumption are the goals.
The requirements for the material are derived from the loading cases, the production operations of the automobile manufacturer and the environmental aspects:
- CO2 equivalent of material production – low
- Machinability – high
- Fatigue strength under reversed bending stresses – high
- Fatigue strength under alternating stresses at operating temperature – high
- Density – low
- Pressure tightness – high
- Compression strength – high
- Damping of natural vibration – high
- Elasticity modulus – high
- Tolerable surface pressure – high
- Overall cost of the production process – low
- Corrosion resistance – high
- Creep resistance – high
- Shrink hole tendency and inclusion tendency – low
- Recyclability – high
- Thermal expansion coefficient – as similar to the crankshaft material as possible
- Wear resistance – high
- Thermal conductivity – high
There is no material that fulfills the aforementioned requirements 100%. Aluminum alloys and cast iron alloys are therefore competing against each other, and this competition is increased due to ever new approaches in lightweight design development.
Typical casting defects and suitable remedial action
Here are some characteristic examples to highlight this:
Problems in the area of the cylinder ridge can occur when designing a crankcase to achieve smaller gaps between cylinders. Undesired cavities are exposed after the cylinder barrel has been processed. In the case of horizontal casting, this occurs predominantly at the level of the core clasp of the water jacket. In combination with an unfavorable flow of the melt, the core clasp probably acts as a formation aid and obstacle for the gas bubble, which is made of core gases or entrapped air from mold filling. A solution would be to define a premature concept regarding the position and type of core clasps as early as the product development stage. The question is whether it is possible to move the core clasp downward in the cylinder barrel, so that the position of the defect is below the critical piston travel zone. A further question would be whether it would be possible to introduce the core package method in a vertical casting position during the development phase of the component. Core clasps in the water jacket are not necessary for this casting position. A further advantage of the core package method is that it can be easily traced due to the automated labeling of the core, which is transmitted to the cast part. Process monitoring and the analysis of the causes of the cast defect can be done at a deeper and broader level in the core production parameters. However, it must be checked whether the uncontrolled seepage is restrained in the area of the cast iron in the core package method.
Typical casting defects found in ducts are:
- The duct is not sufficiently permeable
- The residual dirt content is too high
- The oil duct and water jacket are not sufficiently pressure-tight
The undesired deviation in shape in the form of veining in oil ducts can range from limited permeability that is no longer tolerable to complete closure. Oil ducts twisted in increasingly complex fashions are the trend in downsizing, so that it is becoming increasingly difficult to check and rework the ducts in the casting cleaning room. Suitable remedial action includes changing the characteristics of the grain for the core, additives and also adapted coatings.
The casting process is not completed after solidification. The boxing-out process and the removal of the core should not be underestimated. For example, openings may become necessary for removing the sand in the case of very thin and deep water jackets. They are closed with sheet-metal covers in a further production step after mechanical processing. The size of the openings in the cover plate should also be questioned when the concept is drawn up, in order to take core removal capability into account in the design. Core removal capability in particular is an important area of development in the binder system. This is accompanied by compliance with the amount of residual dirt. The amount of residual dirt refers to the percentage of material from the completed primary molding process that remains in the cavities of the metal body after it has passed through the full process. Loose particles of residual dirt could find their way into the oil while the engine is running and, in the worst case, very large particles could reduce the bearing life. 500 mg are therefore required for a crankcase for a 2.0-liter passenger car as a precaution. ASK Chemicals makes a contribution in the form of coatings with a reduced formation of compounds that are developed together with the respective foundry.
Analyzing the causes is important for defining remedial action. For this, ASK Chemicals has its own in-house simulation software as a tool. Solidification simulation is available for analyzing the causes of shrink holes. But what about casting defects caused by gas bubbles? ASK Chemicals is doing intensive pioneering work on this issue. The milestone here is to introduce appropriate simulation software that provides orientation for questions concerning the amount of gas that forms locally and the local gas pressure at the boundary. Furthermore, the development toward highly reactive binder systems should be mentioned. Due to the high reactivity, the required amount of binders decreases, and a lower binder content means less gas pressure at the boundary between the mold and the melt. A decision aid provided by the simulation tools and the favorable conditions of a modern binder system takes the place of coincidence. If these are applied early on in the product development phase, they can help to prevent the defect from being carried over into the later phases. The later the defect is eliminated, the greater the economic damage that is caused.
The surface topology of the cylinder barrels, threads, very thin oil ducts and areas with very small dimensional tolerances cannot be achieved in a single production step in casting. Mechanical processing is used for this, but near-net-shape casting is a crucial requirement for the casting type so that the processing costs can be kept to a minimum. Standardized casting tolerances, which also depend on the casting method, form the basis.Requirement for the mold, the core, the material, and the casting type
The quality management of the foundry provides data on internal, externally detected, obvious and hidden casting defects for the crankcase. In general, undesired cavities and undesired deviations in shape that prevent the component from functioning reliably are considered to be defects. The focus in the sandcasting method is on the function groups of the cylinder barrel, the oil duct and the water jacket. Saving material by means of downsizing and reducing the engine installation space results in increasing requirements for the foundries.Two explanations are often used to analyze the causes of undesired cavities, and these explanations are summarized here in strongly simplified terms. First, there is the undesired cavity caused by shrinkage, the shrink hole, which is a result of the volume deficit that occurs during the transition from the liquid aggregation state to the firm aggregation state of a metal. The second undesired cavity is the gas bubble that is trapped during solidification and surrounded by the metal. The mold heats up due to the melt cooling off. The binder it contains is heated up and emits gas that finds its way through the mold material's porous body to the boundary between the mold and the melt. If the pressure of the gases is high enough, they can find their way into the melt and turn into a gas bubble. In practice, there are many transitional forms between shrink holes and gas bubbles.