MAHLE's designs, materials and highly sophisticated analysis tools result in special pin bore system considerations necessary for most efficient and highest value light vehicle piston layouts.
Current light vehicle pistons are exposed to ever increasing loads. Rising engine specific power outputs combined with reduced weight requirements trigger these loads. The corresponding higher temperatures, increased peak cylinder pressures and minimizing of component dimensions quite often occur under marginal engine lubrication conditions. At the same time, great emphasis is put on NVH performance of the entire Power Cell System.
When focusing on pistons, the actual pin/pin bore configuration needs to achieve the best performance with respect to: The piston pin bore and pin layouts go hand-in-hand. This is strongly influenced by the actual piston pin bore temperatures and the piston material. Actual stresses, depending on cylinder gas pressures and engine speeds, are influenced by pin ovalization, bending and the hydrodynamic oil firm pressure distribution in the pin bore. MAHLE computer analysis and test data show that actual pin bore strength can be increased by applying different features to a cylindrical pin bore.
For a typical automotive casting and pin layout, most pronounced results are achieved with straight pin bore side relief's. As equally effective, these results can be accomplished by introducing pin bore ovalities of certain magnitudes. In comparison, the effects of a profiled pin bore are marginal. Pin bore strength increases by combining different pin bore features that are not equal to the sum of their individual improvements. Those stress-reducing pin bore features often have a very positive influence on pin bore lubrication conditions. Depending on the overall engine system, other piston design features possibly also improve the pin bore scuff resistance and wear behavior. Engines respond differently. In the past, pin bore scuff issues were frequently addressed by introducing circumferential oil grooves in the pin bore. However, adding such features at a later time during the engine development certainly requires durability testing to ensure that there are no negatives effects on the pin bore's long term wear behavior. The introduction of pin bore - oil ring groove cross drillings represents a high cost impact in manufacturing. Regardless of the actual piston pin bore layout, piston surface treatments can have an effect on pin scuffing. In general, it appears that phosphate coated pin bores are at least as scuff resistant as tin plated pin bores. Proper pin lubrication during assembly has a very positive influence on pin scuff behavior. The MAHLE high volume PCS assembly process incorporates appropriate pin/pin bore lubrication. Further, pin bore strength and pin bore lubrication features require investigation in regard to their potential effects on piston crown and lower pin boss stresses. In particular, cross drillings applied to high revving VVT engines require careful evaluation of the lower pin boss layout. Challenging crankshaft counter weight to piston clearance situations or tight piston weight restrictions require FEA and corresponding engine test program-specific MAHLE lifetime analysis. Piston pin related NVH behavior is highly dependent on the overall engine system. In general, the advantages of a floating pin vs. a fixed pin configuration are known. A floating pin configuration allows the pin to rotate during engine operation. This eliminates the plastic pin bending "banana shape" as compared to a fixed pin. Pin to pin bore installation clearances can be reduced approximately 50 percent and designs can be set up for higher specific pin bore surface pressures at peak cylinder gas pressure. Reduced pin clearances always have a positive effect. Once the lower limits are established (example scuff issues), upper installation clearances need to be minimized. In case of MAHLE piston and pin system supply, reduced pin installation clearance variations are achieved, for example, with a select pin fit system. The actual pin movement in the pin bore during engine operation is also one of the factors determining noise behavior. In most cases where pin bore stress-relieving features need to be applied, MAHLE experience is that oval pin bores show NVH advantages over straight side relief's. As far as piston dimensional stability is concerned, the MAHLE 142 high-performance piston alloy shows advantages vs. standard alloys, not only in regard to skirt collapse, but also in regard to pin bore deformation. In summary, MAHLE's designs, materials and highly sophisticated analysis tools in combination with our optimized manufacturing and assembly processes result in special pin bore system considerations necessary for most efficient and highest value light vehicle piston layouts. |