Torsional vibration modelling of a diesel powertrain system and rattle investigation

During the early developments of vehicles, steam powered engines and gasoline engines were widely used. Researchers had studied on many types of propulsion systems in order to increasing power and torque demands. In 1890's Dr. Rudolph Diesel developed a slow burning, compression ignition, internal combustion engine as called by Rudolph Diesel. By that time, steam engine was the predominant power source for large industries. Development of Diesel's invention, needed more time and work to become a commercial success. Many engineers and developers joined in the work to improve the market viability of the idea created by Rudolf Diesel. At the same time, researchers studied on making the diesel engines more efficient for propulsion systems. In 1920's turbocharger technology came along and this increased the efficiency of internal combustion engines. Since then, many regulations came along for lower emission levels which initiated a new era as called downsizing. Diesel and gasoline technologies have crucial impact on customer satisfaction, due to the fact that these technologies directly affect the performance, fuel efficiency and noise levels of engines. There are two types of engines are being used; gasoline engines and diesel engines. Each engine has different combustion characteristics which leads to different dynamic behavior. Compared to gasoline engines, diesel engines has noisier performance since the combustion pressures are significantly higher, leading to higher levels of combustion noise and overall dynamic excitations. Noise and vibration is an area that automotive manufacturers are investigating to satisfy the increasing customer demands. In recent years, significant improvements in terms of downsizing, lower emission levels, together with the increasing use of lighter materials in powertrain components have led to the additional Noise, Vibration and Harshness (NVH) concerns, establishing them as indicators of vehicle quality and customer satisfaction. Noise radiated from powertrains is known to contribute significantly to the noise experienced inside and outside of vehicles. Gear rattle is now recognised by vehicle manufacturers as one of the most important target for NVH improvement, following the continuous developments of downsized engines. Engine torque fluctuations due to combustion and the inertial forces, together with the dynamic response of the complete system result in torsional vibrations of the gearbox components, such as the idler gears and layshafts. Current vehicle design trends towards lighter flywheels and lower idling speeds increase the possibility of gear rattles as a major noise source. Gear rattle is the result of repetitive impacts between teeth surfaces in the presence of backlash at the meshing gear teeth, under unloaded conditions. Transmission of vibration from the gear shafts through bearings to the gearbox housing is the principal mechanism, radiating noise to the environment. Complex interactions between the powertrain system and industry's need for timely solutions have resulted in costly methods. Most automotive manufacturers are investigating the overall rattle performance during the late design phase by conventional testing methods. This leads to excessive test costs due to the need for protoype of powertrain system, anechoic dyno room and complicated data acquisition systems. Even though there are some studies for the prediction of the rattle nosie by numerical modelling methods, physical test correlation and need for rattle evaluation criteria is usually lacking. The aim of this study to investigate the rattle characteristics of a heavy duty diesel powertrain system using CAE methods with sufficient physical test correlation. Two different CAE models are created for the investigation of rattle noise. Natural frequencies and mode shapes are determined and impact-impulse parameter is used for rattle prediction. In accordance with the literature studies, CAE models are created by considering component's rotational degrees of freedom. CAE results are compared with the test data in order to determine the level of correlation. As a last step, some optimization studies are carried out to improve the rattle performance of powertrain system. The work carried out in this thesis is outlines as follows. In the first chapter, general information about the diesel powertrain system, components and working principle are given. Previous studies in the literature are summarised to understand the existing CAE studies for predicting transmission noise. It was decided that, a model which only takes into account of rotational degrees of freedom of the components –as called torsional vibration model- is sufficient for prediction of the rattle noise. In the second chapter, theoretical background of the subject of thesis is presented. Basis of the vibration theory and eigen value problem are given briefly. Equations of motion of a single gear pair are given for understanding the dynamic interaction between gears. Then rattle mechanism is described for beter understanding of the rattle phonomena and impact-impulse response. It is shown that, overall system stiffness and inertia, drag torque, backlash and torsional vibrations are the keywords of the rattle phonomena. Lastly, modelling and solver alghoritm of AVL Excite PowerUnit and modal reduction theory are also described briefly in this chapter. In the third chapter, two CAE models and modelling assumptions are explained. First, a mathematical model is built for understanding the natural frequecies and mode shapes for the entire powertrain system under unloaded condition. AVL Excite Designer software is used for this part of the study. After, a numerical model is built for being able to include the non-linear effects such as backlash and non-linear clutch spring. For this step, AVL Excite PowerUnit is used. With the numerical model results, torsional vibration characteristic of transmission shafts are investigated. Critical gear condition is selected by using the impact-impulse parameter embedded in the software. A new methodology is offered in order to develop a method for predicting the transmission rattle in early phases of a design. Under unloaded conditions of a transmission, impact forces that are acting on the layshaft due to the rattle characteristic, draws an anology with the operational modal analysis. By comparing the layshaft dynamic response under impact forces and without the impact forces a methodology is offered by predicting the rattle by making simple torsional vibration mesaurement. In the fourth chapter, CAE and physical test correlation is investigated. Engine system –without the transmission system- is used for physical testing since the transmission under investigation is still in design phase. Torsional vibrations are measured from crankshaft position sensor mounted on the flywheel ring gear. CAE results are compared with the physical test resuls for various engine orders in terms of angular displacement. Good correlation level is obtained, which indicates that CAE model is sufficiently accurate enough for rattle predictions. As a last step, optimization studies are carried out so as to reduce rattle noise. Backlash, drag torque and flywheel inertia are selected as optimization parameters and results are compared with the base design results. For rattle noise comparisions, impact impulse parameter is used. Results of the optimization studies are evaluated by considering their noise&vibration, performance and durability effect. Lastly, some concluding remarks are presented and some possible future studes are suggested.