For developers, reducing friction in internal combustion engines is now a high priority. By replacing plain bearings in the engine with rolling bearings, significant advantages can be gained, says Schaeffler — including reductions in CO2 emissions
In the push to develop more efficient vehicle drive systems, minimising friction in drive train components is critical. Re¬placing sliding bearings with roll¬ing bearings, for example, can provide significant improvements in energy efficiency.
Despite these potential benefits, rolling bearings, which were once widespread in engine designs, have been largely superseded by plain bearings. However, recent developments in rolling bearings suggest that it is perhaps now time for engine developers to re-evaluate their choice of bearings.
Due to its sheer size and the distribution of friction, the most obvious starting point on an engine is the crankshaft, with its main and conrod bearings. The number of bearings on the latest engine camshafts makes it particularly interesting to consider the potential for reducing friction there. Also, as the use of balancer shafts in passenger cars is becoming more widespread — in order to improve a vehicle’s noise, vibration and harshness (NVH) rating – the rolling bearing concept and its friction-reduction potential must be evaluated under these specific load and operating conditions.
Rolling bearings for the crankshaft
In volume car production, substituting crankshaft plain bearings (main bearings plus crank pin bearings) with rolling bearings is currently not well established. Previous attempts at this have been rejected for various reasons. However, given the current debate on vehicle CO2 emissions, efforts have now resumed in this area.
Besides lower friction levels, roller-mounted crankshafts are expected to reduce the need for pressurised oil flow. In addition, because of their better dry running characteristics and lower lubricating oil requirements, rolling bearing crankshafts are more suited to an increased number of engine start-stop operations.
In the case of an unsplit crankshaft with integrated rolling bearings, the shaft material must not only satisfy the usual requirements for the crankshaft but must also be suitably prepared as a rolling bearing raceway. The bearing cages and outer races need to be split. The steel for such outer races must not only provide high rolling contact fatigue and durability, but must also be easy to machine and forge, and low cost.
As well as the bearing life calculation, another key factor in bearing design is the identification of suitable clearances. Adequate minimum clearance must be guaranteed in all operating condi¬tions, while the clearance also has a dramatic effect on the engine acoustics. Analytical tools are now available that enable adequate rolling bearing models to be integrated into an overall dynamic system simulation. Practical experience also shows that when it comes to acoustics, an engine with a roller-mounted crankshaft is now a very feasible solution.
Friction reduction potential
In many respects, the conrod bearings have to satisfy far more stringent technical requirements than the main bearings. Installation space is severely restricted in both radial and axial directions, and extreme peak loads and additional centrifugal forces occur. The increased wear can be offset by using specific cage designs and coatings — for example, copper/silver coatings and more recently, PVD (Physical Vapour Deposition) coating systems (Triondur).
Friction calculations on individual bearings based on catalogue formulae cannot simply be transferred to this type of application, since the geometrical influences on the bearing, including dynamic tilting, elastic deformation and high dynamic loads are not taken into account in these formulae.
This uncertainty can be resolved by using simulation tools. The friction models used in simulation software are suitable for solid body and mixed friction, as well as elasto-hydrodynamics. The durability of the bearing can be predicted more accurately by integrating the elastic bearing environment. For this reason, rolling bearing manufacturers have invested heavily in developing specific, proprietary software solutions. Schaeffler, for example, uses its in-house developed software — Bearinx for static calculations, Caba for rolling bearing dynamics and Telos for surface roughness in rolling contacts.
Camshaft rolling bearings
In most modern engines, the number of camshaft bearing positions is relatively high and so the potential to reduce friction is considerable. However, compared to the crankshaft, the loads are moderate and so the expected frictional heat relative to the heat transfer capability of the surrounding components (the camshaft and cylinder head) is quite low. This means that when rolling bearings are used, compressed oil does not need to be supplied to the bearings. Indeed, most of the oil acts as a coolant rather than a lubricant.
As far as frictional losses in the camshaft bearings are concerned, both software analysis and torque tests on the cylinder heads demonstrate that, in the case of the plain bearings in the lower speed range, the bearings operate in the mixed friction range. In contrast, at mid-to-high speeds, a separation of the surfaces by a hydrodynamic oil film occurs, which significantly reduces friction.
Optimisation of balancer shaft systems
While the need for improvements in fuel consumption is now clearly a hot topic for consumers, minimising NVH levels is also a given. Balancing the free second order mass forces of four-cylinder engines requires two counter-rotating balancer shafts that rotate at twice the crankshaft speed, exerting considerable forces on the cylinder crankcase that quadruple with speed. Due to the number of bearings required as well as the high forces and high speeds involved, the use of rolling bearings to reduce friction appears to be an obvious solution here.
Initial designs of such a rolling bearing arrangement would be based on conventional methods. However, subsequently, the effects of shaft deformations and elasticity of the housing on fatigue durability need to be considered using additional analyses. Other effects on bearing life include oil quality, in terms of its viscosity, contamination and additives, as well as the non-transient peak temperatures that occur during operation. Ultimately, the high peak speeds of balancer shafts in four-cylinder engines often require special bearing and/or cage designs.
For cost reasons, the testing of the bearing design is often initially conducted on a separate test rig, which, for practical reasons, uses four balancer shafts driven in a specific synchronised sequence, in order to compensate for internal inertia forces. The speed capability of the bearing design, the temperature conditions and the bearing life under various operating conditions are analysed in detail on the test rig. Plain bearing variants can also be directly com¬pared with rolling bearings in terms of required torques, enabling friction reduction potential to be determined.
The reduced friction provided by rolling bearings compared to plain bearings is typically around 50 per cent or higher, depending on design. This is remarkably consistent over test temperatures of between 30 and 120degC. Expressed as absolute values, this is more than 1.5kW at an operating temperature of around 100degC in the upper speed range.
