Modeling and experimental characterization of belt drive systems in micro-hybrid vehicles

Belt Drive Systems (BDS) constitute the traditional automotive mechanism used to power the main internal accessories (such as the alternator, water pump and air conditioning pump) taking power from the engine's crankshaft rotational motion. BDS usually work in the severe ambient conditions of the engine compartment and are subject to highly dynamic excitations coming from the crankshaft harmonics. The substitution of the traditional alternator with an electric machine, namely Belt Starter Generator (BSG), is the most promising micro-hybrid technology towards a quick and effective satisfaction of the current regulations of fuel consumption and pollutant emissions reduction. The use of a BSG leads to increased stresses in the already complex front end accessory drive. As a matter of fact, a BSG is an electrical machine able to work both as motor and as generator and defines two distinct functioning modes of the drive, namely motor and alternator modes. The relative alternation of tight and slack spans profoundly changes the functionality of the overall drive and affects its transmissions capability and efficiency, furthermore resulting in NVH (noise vibration harshness) effects that need to be carefully addressed. Traditional automatic tensioners acting on the slack span of the alternator mode application are not capable of facing the irregular stresses of a BSG-based BDS which requires the use of a tensioning device capable of keeping the belt tension inside a safe range and of preventing slippage during all the operating conditions of the drive. With this goal many solutions are currently being investigated, such as the cooperation of two tensioners one for each span, active tensioners, double arm tensioners or hydraulic tensioners. The critical issues due to the involvement of BSG in BDS require a deep study focused on the tension conditions of the belt and its influence on the overall efficiency of the system. The aim of the research described in this thesis is to obtain a defined modelling approach of belt drive systems for micro-hybrid vehicles and to validate it through extensive experimental analysis. To obtain a reliable testing environment, a dedicated full-electric test rig was designed and realized. The test rig presented in this work is capable of assuring the repeatability and accuracy of the measurements leaving aside the uncertainties deriving from the irregularities of the ICE behaviour that usually affect the experimental activities conducted on front engine accessory drives. After providing both the modelling and testing environment as assets for the analysis, several experimental activities are carried out with the goal of assessing the dynamic behaviour of belt drive systems and their efficiency, comparing the performances of different tensioning solutions, understanding the behaviour in static and dynamic conditions of a traditional automatic tensioner and one example of an omega twin arm tensioner, which is the tensioning solution most explored by the manufacturers at present. The ultimate goal of gaining a complete understanding of belt drive systems in the special case of micro-hybrid vehicles is eventually fulfilled by an experimental validation of the static and dynamic models proposed.