Overrunning alternator pulley improves internal combustion engine

3 min read

An overrunning alternator pulley (OAP) can play an important part in the smooth running of the modern internal combustion (IC) engine. But it often gets overlooked by design engineers despite the positive effects it can have on energy efficiency.

The function of the OAP is to decouple the alternator from rotational irregularities. This function is critical, particularly as rotational irregularities that occur in IC engines, including those with turbochargers, can be significantly higher than indicated to a driver by a tachometer needle. In a vehicle's accessory drive, the alternator is the component with the greatest moment of inertia and highest speed. This means that acceleration and deceleration forces acting on the alternator resulting from rotational irregularities have the greatest effect on the belt. Using an OAP therefore ensures that only the accelerating proportion of the crankshaft forces that are transferred to the belt drive are used to drive the alternator. The advantage of the OAP with a one way clutch is reduction in the force level in the belt drive. This 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, the engine runs more smoothly. The OAP also contributes towards the reduction of fuel consumption and therefore CO2 emissions. In urban traffic conditions, where a large proportion of time is spent idling and accelerating, the belt drive with the OAP is subject to significantly lower loads than a belt drive without an overrunning pulley. Use of the OAP also enables more cost effective design of other components in the overall belt drive system. Volker Ploetz, senior manager of transmission components at Schaeffler Automotive, says: "The overrunning alternator pulley works in a similar way to the freewheeling of a bicycle pedal, whereby only the acceleration proportion of the crankshafts 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 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 is being so successfully integrated into both diesel and petrol engines. The component was regarded early on as key for eliminating the rotational irregularities from the belt drive of the new generation of high torque, direct-injection diesel engines. It is also now included in an extensive list of measures for optimising the fuel consumption of internal combustion engines. Schaeffler recently showcased the CO2ncept-10% vehicle to presents a range of optimisation options for internal combustion engines that can be implemented in a short space of time to yield significant fuel saving. Here, the OAP contributed to a reduction of almost 1% in fuel consumption. Dr. Robert Plank, vice president of corporate engineering at Schaeffler, says: "CO2ncept-10% is impressive proof of additional potential for optimisation in a system close to volume production standards. CO2ncept-10% is the sum of the individual components." A comprehensive modular OAP system has been developed that offers many different custom designs for both diesel and petrol engines in passenger cars, commercial vehicles and motorcycles. These also include an OAP manufactured partly to aircraft specifications, which is used in the Schaeffler Audi A4 DTM competing in the German touring car championship. The Schaeffler Group currently manufactures more than 400 different types of belt pulley with OAP and it hopes that its positive effect to engine design gets fair consideration by engineers in the future. Structural design The overrunning alternator pulley consists of a pulley with an outside diameter suitable for using a poly-V belt, a one-way clutch unit with bearing supports, an inner ring and two seals. Following installation, an end cover is snapped onto the front of the alternator shaft to protect it from ambient media such as penetration of contamination like salt water deposits. A modular system has been developed for overrunning alternator pulleys to provide an economical and flexible solution for customer enquiries. The one-way clutch unit and the seal components are standard parts. Unlike spring decouplers, an overrunning alternator pulley does not have its own resonant frequency and does not have to be adjusted to varying alternator sizes. Decoupling function The function of the overrunning alternator pulley is to decouple the generator from the rotational irregularity of the crankshaft of an internal combustion engine. The large mass of the generator cannot follow the high irregularities of the crankshaft at engine start and drive resonances. Therefore it comes to differences of rotation angle speeds of crankshaft to alternator. Contrary to a rigid wheel the overrunning alternator pulley opens in overhauling direction. The alternator mass and their influences are decoupled from the accessory drive. In addition, the overrunning alternator pulley decouples the alternator's inertial torque during a significant engine speed deceleration such as changing gears.