The Auxiliary Belt drive system (ABDS) should not be confused with the Timing Belt drive system (Synchronous Belt Drive System -SBDS) of the internal combustion engine. Timing Belt drive takes power from the crankshaft and drives the camshaft/s that in turn drives the inlet and outlet valves of the engine.
On the other hand, an Auxiliary Belt drive also takes power from the Crankshaft and drives various types of key peripheral components (such as the Alternator, Water pump, Power steering pump, AC compressor, air Pump, etc.) via a system of pulleys. In general, today’s modern and advanced engines have an auxiliary belt drive system. ABDS is also known as FEAD (Front end auxiliary drive).
Basics of ABDS System working and its importance-
In the old engines, the belt (V Belt) simply connected the crank with the radiator fan, so that fan runs continuously with the engine even if engine cooling is not required. But over the years, as engines became more complex and advanced, the fans became electrical rather than mechanical. The fan V-belt drive was superseded by the micro-V belt drive those powers one or more additional components, such as the alternator, compressor, pump etc. It’s not uncommon to find more than one V-belt in the same engine depending on the application. For example, many of tractors, gen-sets, etc. engines still use traditional V-belt drive.
Modern high-performance Engines have Micro-V belt (also called Muti V belt, and Ribbed V belt) that is wider and has smaller cross-section compared to V belt. A single long-length belt drives the Alternator, Water pump, Power steering pump, AC compressor, air Pump, etc. As the auxiliary drive belt is naturally long, it is wise to have idler pulleys and/or a tensioner pulley in it to support and provide constant grip on all the pulleys.
In a belt drive system of the engine, the setting of proper belt tension is of great importance. While high tension will definitely cause premature failure of the bearings supporting the pulleys (due to excessive radial load) and also surface wear of the belt itself. Conversly, if the belt is too slack or loose, excessive slip may occur on the pulley that will result in power or efficiency loss and system vibrations. In this context, the ABDS system has key element known as Auto-tensioner or simply the belt tensioner. It has the role of maintaining a suitable amount of tension on the belt, especially on the slack span, during all the load conditions of the engine. The device is characterized by a small stiffness to ensure the required pre-tension in the belt over the lifetime of the engine, disregarding belt elongation, deformation, or wear with the mileage.
Components of ABDS (Tensioner, Idler Pulley, and Micro-V belt)
Auto-Tensioner – It is one of the most critical and important parts of the ABDS, it maintains the constant tension in the belt so that there is minimum or no slippage on the pulley. As the engine crank rotates at various RPMs depending on the condition, the belt tension will vary if there is no auto-tensioner. At low tension, the belt will slip which will lead to power loss (mechanical efficiency loss), and at high tension, belt wear & tear will take place. Auto-tensioner may be spring-loaded or hydraulic. Reduced slip with auto-tensioner can allow the use of lower-ratio pulleys; this reduces the load on the engine due to lightweight pulleys, increasing fuel economy.
Idler Pulley – Idler pulley is generally a flat pulley to support the Micro-V belt. Micro-V belt runs over various Pulleys of Alternator, Water pump, Power steering pump, AC compressor, and air Pump like a serpentine. This belt needs to be supported/guided with a large wrap angle to prevent it from vibrations, slippage, or coming out of pulley grooves. Idler pulleys are used for the same purpose. There may be more than 1 idler pulley in an engine depending on the design.
Micro-V belt- This is the belt running over all the pulleys in the system. Micro-V stands for V-shaped grooves on the belt surface. To accommodate bi-directional flexing (serpentine shape) while remaining strong enough to transfer the total force required by multiple loads, the belt has multi-groove (multi-v, poly-v, or multi-rib) construction. It is more efficient than the older multiple-belt system and may consume less space in the engine compartment. By using a single, wider belt instead of multiple, thinner belts, the belt may be put under increased tension without stretching. Also, the tendency of individual (multiple) V-belts to “flip over” in the pulley groove (at high RPM and/or when the belt stretches) is eliminated by using a single and wide micro-v belt.
Micro Hybrid System in IC Engines (BSG: Belt Starter Generator)
Now that we have learned about the importance of the ABDS system in modern engines, It is worthwhile to note that with certain modifications the normal ABDS system can be utilized to create a micro-hybrid system. In a normal engine, the belt drives the alternator to change the battery. However, in BSG (Belt starter generator) system, the alternator is replaced by a special motor that charges the battery and also provides mechanical power to the system when required. Motor mode is exploited as an electric boost, where the BSG assists the ICE for transient accelerations. Moreover, this mode is also used for the start and stop function, through which the cranking of the ICE is performed without the need of a separate starter electric machine.
When dealing with micro-hybrid systems, the tensioner features need to be extended to the operating conditions inherent to the use of a belt starter generator (BSG). The belt tensioner is placed on one of the slack spans in order to maintain the tension inside a reasonable operating interval. In contrast with the traditional alternator, the BSG can be activated to work also as a motor, thus delivering power to the system. This mode changes the tensioning conditions of the Belt Drive System and leads to a fast exchange between slack and tight spans. Therefore, the traditional tensioner acting only on one span has a null effect on the system because the force associated to its low stiffness is easily overcome by the new tight span. As a result, the BSG-based BDS requires a dedicated tensioning mechanism able to quickly adapt itself to this dual tension behavior.