The Overrunning Alternator Pulley – an Unsung Hero of the Internal Combustion Engine?
The OAP is the functional interior of the belt pulley mounted on the alternator. The function of the OAP is to decouple the alternator from the rotational irregularities of an internal combustion engine. The task the OAP performs is therefore a critical one, particularly as the rotational irregularities that occur in the latest internal combustion engines – including turbocharged versions – are significantly higher than those indicated to the vehicle’s driver by the tachometer needle.
In a vehicle’s accessory drive, the alternator is the component with the greatest mass moment of inertia and the highest speed. This means that the acceleration and deceleration forces acting on the alternator resulting from the rotational irregularities have the greatest effect on the belt transferring these forces. The OAP therefore ensures that, at many operating points, only the accelerating proportion of the crankshaft forces that are transferred to the belt drive are used to drive the alternator. The advantages of the OAP with a one-way clutch are clear. The reduction in the force level in the belt drive extends the life (typically by a factor of 10) of individual components, while ensuring an increase in the generator speed and a reduction in noise. In addition to smoother engine running, the OAP also contributes towards the reduction of fuel consumption and therefore CO2 emissions.
For example, in Schaeffler’s CO2ncept-10% concept vehicle, the OAP contributed a reduction of almost one per cent in fuel consumption. In urban traffic conditions – where a large proportion of time is spent idling and accelerating – the belt drive with the OAP is subjected to significantly lower loads than a belt drive without an overrunning pulley. Use of the OAP also enables the more cost effective design of other components in the overall belt drive system.
Volker Ploetz, Senior Manager Transmission Components at Schaeffler Group Automotive comments: “The overrunning alternator pulley works in a similar way to the freewheeling of a bicycle pedal, whereby only the acceleration proportion of the crankshaft’s rotational irregularities is used to drive the alternator. The advantages from this include more effective damping of belt oscillations, which means you no longer need idler pulleys or damping pulleys. You also get a reduction of tensioning forces and lengths, as well as improved noise behaviour, reduced belt width and increased system life. The use of alternators with high mass inertia is also possible. In terms of assembly, the overrunning alternator pulley is easy to fit, as no separate fasteners are required.”
This important contribution towards improving energy efficiency also explains why the OAP has been so successful in both diesel and petrol engines. The component – which has been established for around 15 years now – was regarded early on as a key component for eliminating the rotational irregularities from the belt drive of the new generation of high torque, direct-injection diesel engines. The OAP is now also included in the extensive list of measures for optimising the fuel consumption of internal combustion engines.
A comprehensive modular OAP system has been developed that offers many different custom designs for diesel and petrol engines, passenger cars, commercial vehicles and motorcycles. These include an OAP manufactured partly to aircraft specifications, which is used in the Schaeffler Audi A4 DTM.
The Schaeffler Group currently manufactures more than 400 different types of belt pulley with OAP. In fact, during the last 15 years, around 140 million of these unsung heroes have rolled off the production lines at Schaeffler.
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