Technical Information
– Optimization focus: camshafts –
Assembled camshafts with rolling bearing support lower CO2 emissions
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Stuttgart, Germany, September 2010—Despite a number of advances already achieved, commercial vehicle engines offer still more opportunities for optimizing efficiency. Alongside innovations in the power cell unit, MAHLE is focusing its efforts on the valve train. Lightweight assembled camshafts fitted with low-friction rolling bearings offer still significant CO2 savings potential.
Assembled camshafts for commercial vehicles
For decades, MAHLE has been a large-scale supplier of assembled camshafts for passenger cars, including its variable CamInCam® since 2007. MAHLE has now added assembled camshafts for commercial vehicles to its product portfolio. There are currently three tendencies in commercial vehicles: cast camshafts are generally used in light-duty drives and steel camshafts for higher loads. The growing distribution of commercial vehicle engines with overhead cam (OHC) valve trains has fostered the use of assembled camshafts.
MAHLE's production method for assembled camshafts offers optimal prerequisites for this application. Using a thermal shrink-fit method, parts such as cams, pulse-generator wheels, and driving and output elements are joined to precision steel pipes having high degree of torsional stiffness and flexural strength. The purely elastic transverse press fit guarantees a consistently tight interference fit of the joined components over the entire service life—all of the two million kilometers required in commercial vehicle engines. By contrast, assembled camshafts produced by means of plastic deformation in the assembled press fit exhibit an aging effect and are therefore less suitable for an extended service life.
At the same time, assembled camshafts—especially those with lobes made of inductively hardened roller bearing steel—are built to withstand high mechanical loads and offer a great deal of freedom in terms of the materials and design used for the driving elements and output elements. Additional positive traits include high possible contact stresses between the cam and cam follower as well as weight advantages. Assembled camshafts provide a significant weight advantage. Compared to all-steel camshafts, they are approximately 50% lighter!
MAHLE has already gleaned experience with assembled commercial vehicle camshafts in a number of customer projects and has manufactured these under near-series conditions. Endurance testing has since been conducted as well. The results are convincing: material damage resulting from rolling fatigue, for example, is nonexistent. With the experience we have gained to date in terms of design, manufacturing process, and testing, assembled camshafts for commercial vehicles are an attractive addition to MAHLE's product portfolio. They are an ideal way to meet the growing requirements for longer service life and minimal weight.
LFC technology with rolling bearing support
Another way to reduce frictional loss in the valve train is to use rolling bearings as the main camshaft bearings. Along with reducing the mechanical bearing friction, this solution provides the added advantage of eliminating the need to supply pressure oil to the camshaft's plain bearing support, since the oil mist in the cylinder head is sufficient to lubricate the rolling bearings.
MAHLE's Low Friction Camshaft (LFC) technology with rolling bearing support is based on the standard MAHLE process for assembled camshafts, making it efficient and inexpensive to produce. A special needle bearing with minimal space requirements is used as the rolling bearing. The inner bearing ring is eliminated in this configuration, and the needles run directly on the locally hardened camshaft tubes. The tolerance requirement for the bearing seat in the cylinder head remains unchanged. A special bearing cage positions the bearing during assembly in the cylinder head. It features additional grooves for supplying the needle bearings with splash oil from the cylinder head. For the joining process, MAHLE uses exclusively fully machined lobes to avoid the need for subsequent grinding, thereby avoiding contamination of the bearings.
Because the amount of frictional loss that can be achieved depends on the number of bearing points, this concept shows the greatest potential for DOHC applications. In the valve train systems tested by MAHLE to date, a reduction in frictional loss of over 40% was measured in the low-end rpm range. This is also extremely advantageous at low oil temperatures—during cold-starting, for instance.
Along with reduced mechanical loss, another winning aspect of camshafts fitted with roller bearings is a lower oil requirement in the cylinder head. Because this design does away with hydrodynamic plain bearings, it requires significantly less oil in the valve train. Consequently, the delivery rate of the oil pump can be reduced, resulting in a savings potential similar in scale to the savings that can be achieved by minimizing mechanical losses through the use of a camshaft with roller bearings.
In a direct comparison of the oil delivery rate in an otherwise identical engine, the roller bearing-fitted camshaft had a clear advantage over a plain bearing shaft with the same bearing diameter. The roller bearing camshaft required around
40% less oil delivered to the cylinder head. The use of LFC camshafts in combination with further reduction of the oil delivery rate results in fuel consumption optimization by approximately 1–2%.
MAHLE has analyzed the acoustic performance of a roller bearing camshaft using structure-borne and airborne-sound measurements taken on an acoustic test bench with a driven cylinder head. In addition to an only slightly higher overall noise level, the analyses revealed a shift to lower frequencies, which are subjectively perceived to be more pleasant than high frequencies. Hence engines fitted with the LFC camshaft are not expected to exhibit problematic noise behavior.
The MAHLE Group is one of the 30 largest companies in the automotive supply industry worldwide. With its two business units Engine Systems and Components and Filtration and Engine Peripherals, MAHLE ranks among the top three systems suppliers worldwide for piston systems, cylinder components, as well as valve train, air management, and liquid management systems. The newly formed Industry business unit bundles the MAHLE Group's industrial activities. These include the areas of large engines, industrial filtration, as well as cooling and air-conditioning systems.
In 2009, the MAHLE Group generated sales of approximately EUR 3.9 billion; around 43,000 employees work at over 100 production plants and eight research and development centers.
Press contact
MAHLE GmbH
Birgit Albrecht
Corporate Communications/Public Relations
Pragstrasse 26–46
70376 Stuttgart
Germany
Phone: +49 (0) 711/501-12506
Fax: +49 (0) 711/501-13700
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