The petrol and diesel engines of modern passenger cars are of all-aluminium design, with cost and weight optimisation using the latest conventional production techniques. The weight-to-power ratio of these engines has increased progressively in recent years, with mass-to-power ratios of around 1.1 kg/kw for 3- and 4-cylinder engines, reflecting the balance between material properties, load distribution and structural use of the engine for a given production boundary condition. This at the same time means that further significant weight reduction is not possible with conventional production techniques.
In order to further investigate the potential for continued engine weight reduction in the context of new technologies, the German Federal Ministry of Economic Affairs and Energy has launched a research project called LeiMot (Lightweight Engine), which involves several heavyweights in the automotive industry, such as FEV Europe GmbH, Volkswagen, RWTH Aachen University and the Fraunhofer research institute. The project aims to reduce the weight of key engine components through metal 3D printing and new metallic materials, and in August 2020 the project is now in the final engine prototype phase, which is expected to reduce the weight of the latest generation of internal combustion engines by 30% by replacing traditional metal parts with components made from fibre composites and 3D printing technology.
Integrated manufacturing of automotive engine components using 3D printing to reduce weight by 21% and improve powertrain efficiency for mature modern products
The 2.0L turbocharged direct injection diesel engine of Volkswagen’s EA288Evo series was targeted for optimisation due to its high mechanical load capacity, and two components in particular were selected: the cylinder head and the crankcase. Both parts were manufactured using a selective laser melting process, rather than die-cast aluminium as in the past. the high design freedom of 3D printing was used not only to reduce weight but also to improve engine performance. During the development of the LeiMot engine concept, the design boundaries of the components have adhered to the entire process of 3D printing, including support structures, print orientation and post-processing.
The researchers began with a functional decomposition of the cylinder head and crankcase. In this way, the researchers were able to analyse each function and could optimise the design according to the given boundary conditions. In addition, the researchers needed to ensure that the LeiMot cylinder head would be inter-compatible with the VW crankcase. At the same time, the researchers had to retain the important interfaces and components of the reference engine, in particular the crank-link mechanism, the air distribution mechanism and the air change components.
Optimising cylinder heads – achieving structural designs that are not possible with conventional processes
A special design method has enabled the researchers to achieve cooling, lubrication and air exchange functions in a material no thicker than 2 mm, with a lattice structure that is significantly less than 2 mm thick. this method allows a wide range of wall thickness parameters to be used depending on the load and without the structural weaknesses associated with conventional manufacturing.
In the case of cylinder heads, engineers need to reinforce certain areas that are subject to high mechanical stresses, as the combustion process mainly generates bending loads, while the whole engine is subject to torsional loads. The optimum ratio of weight reduction to stiffness was a combination of an I-beam and an integrated closed console. In the end, the structurally optimised AlSi10Mg cylinder head manufactured using selective laser melting 3D printing technology weighed 2.3 kg less, a 22% weight reduction compared to the original part.
Integrated manufacturing of automotive engine components using 3D printing reduces weight by 21% and improves powertrain efficiency for a proven modern product
Integrated 3D printing of automotive engine components reduces weight by 21% and improves powertrain efficiency for a mature modern product
The design freedom of 3D printing technology allows researchers to integrate various structures directly into the production process. The researchers have targeted insulating lattice elements with air gaps and low thermal cross-sections to optimise the insulation of the exhaust tract and reduce the amount of exhaust heat entering the cylinder head coolant. Under rated power conditions, the insulation system reduces the heat flow to the cylinder head by 5 per cent. This reduces the warm-up time of the exhaust aftertreatment system and increases the turbine inlet temperature.
Remodelling the crankcase – materials and construction to match performance
The crankcase was also 3D printed and manufactured using AlSi10Mg. Considering the weight and stiffness of the crankcase, it was decided to use a short skirt design with an aluminium base. By reducing the main bearing friction diameter of the bearing housing, it was possible to replace the steel bearing cap with an aluminium plate base. The redesigned crankcase, including the base, has a weight reduction of 5.1 kg compared to the original assembly. By using a topological approach, the researchers have optimised the main flow paths of the assembly and designed cavities and lattice structures for low stress areas such as the outer partition.
For the lightweight design solution, which is close to the limit, the researchers have taken an in-depth look at the material properties during the design process. Due to the special microstructure of the material, there are significant differences between the mechanical properties of parts produced by 3D printing technology and those made by conventional casting processes. The researchers have therefore investigated a number of mechanical properties of 3D printed aluminium alloys at different temperatures using samples and have used the results in their calculations.
Integrated manufacturing of automotive engine components using 3D printing reduces weight by 21% and improves powertrain efficiency for modern, proven products
Integrated manufacturing of automotive engine components using 3D printing to reduce weight by 21% and improve powertrain efficiency for a mature modern product
To further improve the cooling performance of the crankcase and to achieve a uniform distribution of cylinder temperatures, the researchers have used elliptical cooling channels, 2mm wide and 3mm high, between the cylinder sleeves to cool the bores. In addition, the interior of the water jacket is a fully optimised lattice structure. This structure increases the heat transfer area, improves the coolant flow process and increases cylinder stiffness. The improved cooling system of the cylinder liner results in a more uniform deformation of the cylinder liner and corresponding improvements in friction and air leakage.
In addition, by using 3D printing technology, the researchers have designed a new oil passage solution without significant deflection. The oil passages in the cylinder head and crankcase (diameter range 3-8 mm) can be printed directly. The curved channels and gently varying cross-sections result in a reduction in pressure loss in the internal piping system of the cylinder head and crankcase by approximately 22%.
Thanks to the additive manufacturing approach, the concept engine developed in this project will ultimately be equipped with a minimum number of components, minimising material use through minimal component and high functional integration, resulting in a weight reduction of approximately 21% in the main components of the baseline diesel-powered passenger car, while also improving powertrain efficiency, operational performance, thermal management capabilities and improving vehicle noise, vibration and comfort.
The LeiMot research project is a strong demonstration of the design feasibility of the new manufacturing process. In addition, the project is helping researchers to explore new methods for the development of internal combustion engines. 2021 will see five LeiMot prototypes built by FEV and tested through mechanical and thermodynamic tests.
In the short to medium term, components produced in large quantities through 3D printing technology will still find it difficult to compete with conventional manufacturing processes for the mass market. 3D printing technology has been successfully applied to small components in small production processes such as aircraft component manufacturing. In the future, researchers could also use hybrid solutions that combine 3D printing technology with traditional manufacturing processes to further improve manufacturing quality.