New Requirements on Engine Technology Casting of Cylinder Crankcases - Challenge Cylinder Running Surface
The replacement of the New European Driving Cycle by the WLTP to determine exhaust emissions has further tightened the requirements placed on engine technology.
In order to reliably meet future emission targets, the share of hybrid drives combining an internal combustion engine with an electric motor needs to increase. To this end, the implementation of lightweight design targets remains an integral demand. The cylinder crankcase, as a central component of the combustion engine and part of a hybrid drive system, continues to play an important role in achieving these goals. In order to ensure acceptance of hybrid drives by the end customer, there are not only demanding technical requirements to be handled, there is also the need to significantly reduce the costs in production technology.
From Metal Application to Mechanical Processing
All casting processes currently used in series production, such as vacuum-assisted die casting, Rotacast gravity die casting 1 and CPS (Core Package System) sand casting 2, contribute significantly to achieving this goal by providing targeted technological developments in all steps of the production chain - from metal application to mechanical processing. The cylinder bore as a friction partner for the piston ring and piston is part of the combustion chamber and has a strong influence on the emission behavior. Minimum space between bores, realized by thermally joined liners or thermal sprayed bores, are competing with cast-in liners.
Die Cast Process Improvements
In the die casting process, for example, significant improvements were made resulting in significantly higher fatigue strength values. These were achieved through consistent process development in the areas of material use, melting technology and melt cleaning (Fig. 1 in the picture gallery).
A further focus of the improvement measures was focusing on running surface design. For the most cost-effective solution in die casting with cast liners made of GJL, it is necessary to achieve a reproducible mechanical or metallic bond with the aluminum casting.
Due to the complex geometry of the crankcase, however, this poses a challenge on the casting process. By carefully adjusting the process parameters, faultless casting can be achieved without cracking or insufficient form closure (Fig. 2 in the picture gallery).
The development of the thermally sprayed bore surfaces is characterized by two essential process steps: the pre-machining of the aluminum cylinder surface to adjust the necessary bond with the spray coating and the spraying process itself (Fig. 3 in the picture gallery).
The following variants are widespread in the pre-machining of aluminum cylinder surfaces:
- mechanically activated internal cylinder surfaces using high-pressure water jets,
- mechanically machined internal cylinder wall with defined undercuts or
- mechanically machined internal cylinder surface, without the use of defined cutting edges.
Coating adhesion is an important parameter for evaluating different roughening processes. A benchmark test showed that the required minimum adhesive tensile strength was achieved by all process variants.
1) S. Dengler, H. Meishner: The New Audi 2.5l TFSI Five Cylinder Engine for the New Audi TT-RS - A Lightweight Design High Performance Engine, 9th Edition VDI Symposium "Casting Technology in Engine Construction 2017"
2) M. Speicher et al: Aluminum engine blocks of the latest generation in the CPS process, 9th VDI symposium "Casting technology in engine construction 2017"