The cylinder head of an engine determines the engine's properties in terms of operating behavior, e.g. power output, torque and exhaust gas emission behavior, fuel consumption and acoustics, like almost no other component assembly of the engine. It contains the key elements for mechanically controlling the gas exchange or combustion. Valve control is of particular importance here.
The casting method for the cylinder head should be determined early on. It is recommended to take the expertise on casting and patternmaking into consideration when creating the basic design of the cylinder head. The desired geometries cannot be implemented with all casting methods. The shape and position of the intake and exhaust ducts as well as the shape of the combustion chamber in particular determine the overall geometry of the cylinder head. Furthermore, the cylinder bore and the distance between the cylinders have great influence on the basic geometry.
As the combustion of fuel causes high temperatures – also in the cylinder head – an appropriate cooling concept is of great significance. The coolant is generally fed into the underside of the cylinder head via the cylinder head gasket of the crank case and via multiple openings. From all the possible cooling systems (e.g. cross-flow cooling, longitudinal-flow cooling or a combination), the optimum cooling system is determined using appropriate simulation models, and possible critical areas are detected at an early stage.
The ducts for the water cooling system and the oil supply are often very fine and pose the greatest challenge to the caster in cylinder-head manufacturing nowadays, since even minor changes in the process can lead to rework or rejections of the component.
The casting methods
Cylinder heads for combustion engines place high requirements on the mechanical properties of the materials above 150°C. The complexity of the shape and also the level of the stress that occurs during operation have increased considerably in the newly developed cylinder heads for direct injection diesel engines in particular.
Different materials are used to produce cylinder heads depending on the requirements profile of the engines and the casting method used. In addition to aluminum, it is also possible to use cast iron materials for large engines and commercial vehicles. With only few exceptions, aluminum is the material that is used in the area of passenger car engines. In the case of ignition pressures → 150 bar, specific alloys are needed that must meet highest requirements in terms of
- High tensile strength and high creep resistance at temperatures between room temperature and 250°C
- High thermal conductivity
- Low porosity
- High ductility and elasticity with high thermal shock resistance
- Good casting properties and low susceptibility to hot cracks
Simulation tools, e.g. for mold filling and solidification behavior during the casting process, are already used intensively during the planning phase of a new component in order to find an optimum between the different target parameters.
Nowadays, the following casting methods are widely spread:
- Permanent mold casting
- Lost foam method
- Pressure die-casting method
In sandcasting, both the mold and the core are manufactured on the basis of silica sands or special sands. In general, the mold is based on bentonite-bonded binders, whereas the cores are based on chemical binders.
Complicated component geometries – even with undercuts – can be achieved easily using chemically bonded mold materials in the core package method. A further advantage of the methods is that cost-effective production is possible even in the case of low quantities and that changes can be implemented relatively quickly.
The advantage of the core package method, where all component contours are represented by sand cores, is that the temperature of the cast parts generally does not fall below 500°C between casting and solidification. This results in casting that is low in stress to the greatest extent possible, and this yields high dimensional accuracy.
The low-pressure sandcasting method is recommended for the production of prototypes and small series. The melt finds its way into the mold via a standpipe and is exposed to a pressure of approx. 0.1 to 0.5 bar. This results in a structure of very high quality since the pressure is also maintained during solidification.
The permanent molds made of gray cast iron or hot-work steels are used to produce light-metal alloys. As in sandcasting, the cores are placed into the casting mold. In permanent mold casting, a distinction is made between gravity die-casting and low-pressure die-casting.
The lost foam method is basically a special form of sandcasting. The various layers of the cylinder head are formed via foaming polystyrene material and then glued together. In this method, two cylinder head models are joined together with the gate system and the risers to form what is known as a cluster. This cluster of models is then dipped into ceramic coatings repeatedly and dried by means of airflow. The cluster is cast after it has been placed into the gooseneck and surrounded by loose sand. During mold filling, the polystyrene recedes and turns to gas. One advantage of this method is that boreholes with a wall thickness of up to 4 mm are also cast in the process. Furthermore, it is possible to create oil ducts of any shape and to achieve significantly more accurate tolerances in the combustion chamber, while the necessary machining effort is reduced to a minimum.
Permanent molds made of heat-treated hot-work steel are used in the pressure die-casting method. The molds must be treated with a release agent before every "shooting" process.
Prospects of cylinder head technology
The further development of cylinder head technology will head in the direction of lightweight design, more high-strength materials and more cost-efficient production processes. The use of multiple-valve technology, also for diesel engines, will make it possible to achieve further optimized gas exchanges and a higher specific cylinder output. The properties of the engines are also continually refined with regard to favorable consumption and emission behavior. Constantly facing these new requirements is a major challenge for the designers.
System solutions for cylinder head casting
In addition to the exact geometry of the component, the possible casting and molding methods must also be taken into account in advance in order to obtain an optimum cast part under series production conditions, too. Besides various changes to the construction, initial experiences with regard to future component properties, rejections and rework are also gained during pilot production. It is extremely important to have the rejections and, ultimately, all factors that restrict the output under control by the start of series production at the latest. This is the only way to achieve high productivity and low costs at the same time in order to be able to maintain the specified price for the component.
As already described in detail above, the cylinder head is the component that essentially determines the properties of the engine. Downsizing ensures the constant decrease of the weight of the cylinder head – both in iron and in aluminum casting. At the same time, the requirements for the component are continually increasing. Close cooperation between the patternmaker, the caster and the designer at the beginning of the new development is the only way to counteract this conflict of aims.
The reagents used in the process are also becoming increasingly important in order to ensure the component's reliability throughout the entire lifecycle of a vehicle. Nowadays, software for simulating the mold filling process and also solidification has become state of the art. In addition to clearly defined processes, the melting and melt treatment of the casting metal require the cleanest charge materials and treatment agents.
In mold production and especially in core production, it must be ensured above all that the molding base materials and binding agents used guarantee good dimensional consistency of the cores or core packages. In iron casting, it is possible that additives and refractory coatings are also used. The fact that the wall thicknesses of the water jacket cores and also the oil duct cores in particular were reduced to 4 mm requires the core sand binders to guarantee high initial strengths in order to ensure that the core can be handled without core fracture.
The binder used must have high hot strength during the casting and solidification processes, so that the risk of core fracture and a lack of dimensional stability can be ruled out even in this highly critical condition of the filigree cores. However, the binder is to enable optimum shake-out performance during the core removal process at the same time. This balancing act becomes particularly clear when casting aluminum alloys. Bubble defects and microporosity in the component cause leaks and usually lead to rejections. In order to keep this risk to a minimum, it is important to invest in mold filling that is as calm as possible. Furthermore, it is important to take core gas extraction into account and to implement it in a suitable way. Overall, the binding agent must be designed in such a way that gas and other emissions are kept to a minimum. The burst of gas from the binding agent during casting must be coordinated to the casting metal and the solidification interval. The same applies to the additives. If coatings are used, the product's gas permeability must also be suitable for the process here, too.
In addition, scabbing can then occur in iron casting. The sand expansion behavior of the molding base materials, the gas burst behavior of the binders, the gas permeability of the coating, and the strength level of the mold material mixture have great influence here, but can only compensate defects in the mold filling process and a corresponding heat-up of the section of the mold to a limited extent. A major challenge in iron casting is to prevent metallization and/or penetration and veining. Metallization and penetration can be prevented primarily by selecting the correct coating, which must be coordinated to both the mold material and the casting metal in order to prevent unfavorable chemical reactions.
Veining, especially in water jacket cores and oil duct cores, is an ongoing issue in the practice of iron casting. In most cases, remedial and preventive measures against the occurrence of this defect can be taken by selecting a suitable molding base material or by the targeted use of sand additives. Rough surfaces in the intake and exhaust ducts can be prevented by the grain particle size distribution of the mold material used, or by using a coating that seals the gaps between the pores of the sand grains.
In addition to solving purely technical problems, the charge materials used must be as environmentally friendly as possible. Nowadays, the employees, the local residents around the foundry and also the authorities are very interested in keeping harmful emissions, odor, smoke and condensate to a minimum and in these emissions meeting the requirements of occupational safety during the production process.