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ÖgeReduction of engine torsional vibrations via hydrodynamic dampers(Graduate School, 2022-02-10) Aslan, Yavuz ; Akalın, Özgen ; 503191710 ; AutomotiveWithin the scope of this master's thesis, the vibration damper used to dampen the torsional vibrations of internal combustion engines has been examined. In this manner, the hydrodynamic damper, which is a special type of vibration damper, is investigated. The calculation methods of the stiffness coefficient and damping coefficient, which are the two main characteristics of the hydrodynamic damper, have been studied. CATIA and Hypermesh software programs are used in order to perform the needed 3D model creation and analysis. Values obtained from the calculation by analytical and numerical methods are compared with test results performed on measurement systems. After the determination of these characteristic parameters, a crank train model is built over the AVL EXCITE program to examine the effect of the hydrodynamic damper in an 8-cylinder diesel engine. This established model is considered as two different sub-models, with and without vibration damper, and the differences between both cases are determined. Angular displacement and stress values in different sections of the crankshaft are used as the output of the analysis. Thus, the effect of the hydrodynamic damper used on the crank train and the engine has been clearly demonstrated. Currently, efficiency of the engine is increased by increasing power density and reducing engine volumes in internal combustion engines. On the other hand, this trend increases the loads on the components and affect the strength limits. In an 8-cylinder diesel engine with a high power density, the vibrations observed with increasing cylinder pressures and loads also increase. These vibrations seen in the crank mechanism directly affect the operation of the engine and the lifetime of the parts. For this reason, vibration dampers are used for the damping of these vibrations and for the safe operation of the engine by reducing their amplitudes. In this thesis, a hydrodynamic vibration damper coupled to the free end of the crankshaft is used. The hydrodynamic damper is a vibration damper that consists of leaf springs arranged in an inner part and that creates damping by providing oil flow between these spring packages. Firstly, the stiffness coefficient of this damper is determined. For this purpose, finite element analysis is applied to the damper, which is 3D modeled, through the Hypermesh program. As a result of the analysis, the angular displacement values against the acting torque are obtained and the related stiffness coefficient is determined. Also, a test system is designed to measure the stiffness coefficient. The stiffness coefficient value is obtained by the measurement made on this test system. The stiffness coefficient calculated by the numerical method is compared with the stiffness coefficient measured from the test system. As a result of the comparison, it is seen that both values coincided with each other within a certain margin of error. Analytical calculations have also been made to determine the damping coefficient, which is another specific feature of the hydrodynamic damper. The passage of the oil in the damper between the chambers next to each other creates a damping effect. In this context, the damping coefficient is tried to be calculated with two different methods. The first of these methods is the control oriented transient method, which is used in a similar study before. With this method, the damping coefficient is calculated over different coefficients based on the oil flow. However, since the geometry is very small and there are many parameters affecting the flow, the damping coefficient calculated with this method is found to be quite different from the damper coefficient in the catalog information from the manufacturer. The second method is to reduce the damper to an equivalent dashpot system. With this reduction, the damping coefficient is calculated from the dashpot. With this method, the damping coefficient differs from the value in the catalog, depending on the assumptions and reductions used. In addition to the calculations, a test system is designed for a static damping coefficient and it is measured on this system. The damping coefficient is obtained from the measurement result using the logarithmic decrement method. Since the engine operating conditions cannot be reflected in the test system and the damper is tested statically, not dynamically, the value obtained from the measurement does not represent the dynamic damping coefficient. For this reason, the values obtained from the calculations and the values in the catalog have not been compared. Finally, after determining the characteristics, a crank train model is created using the AVL EXCITE program to examine the effect of the hydrodynamic damper on the engine and to reveal the damping effect of the hidrodynamic damper. In this model, the crankshaft, connecting rod, piston and flywheel are modeled under the forces after combustion pressure and inertia forces. In order to see the effect of the damper on the system, two models, with and without the damper, are created. From these two models, the angular displacement and the resulting stress values in different parts of the crankshaft are investigated. The results from the model with the damper obviously stated that the hydrodynamic damper effectively reduces the torsional vibrations in the engine compared to the model without the damper. It can be said that the lifetime of the components and therefore the engine is extended since the stress values on the parts are reduced at the same time. It is claimed that adding torsional vibration damper into the crank train has no negative effect on the engine in terms of torsional vibrations. According to the results of the torsional vibration analysis, almost every part of the crank train elements has minimized amplitude level of torsional vibration and angular displacements with hydrodynamic damper. Additionally, 7 different damper case studies are revealed in order to determine the possible optimized stiffness and damping coefficients of the current hydrodynamic damper. In order to compare the different cases, the existing stiffness and damping coefficients of the current damper are changed by 25%. Updated stiffness and damping coefficients of the damper are applied into the torsional vibration analysis and corresponding results are obtained, respectively. These results are compared with the results of the current hydrodynamic damper. In some cases, the angular displacements and the shear stresses are increased due to the change in the stiffness and damping coefficients. On the other hand, some cases have improved results due to the higher stiffness and damping coefficients compared with the current hydrodynamic damper. These improved cases are investigated in order to determine the possible optimized version of the existing damper. It is not hundred percent possible to claimed that the revised damper has better results in the all speed intervals, but it can be said that the updated dampers with high stiffness and damping coefficients have better torsional vibration results in the most of the engine operating speeds. Regarding this aspect, Case 5 and Case 7 have better results than the other cases and the current hydrodynamic damper. Moreover, Case 5 has less angular displacements and shear stresses in the most speed ranges than the Case 7. Although Case 7 has better results in some speeds than the Case 5, there are some speed intervals that the Case 7 has higher angular displacements and shear stresses than the current hydrodynamic damper. Also, the reduction of the results in Case 7 is less than the increase coming from the Case 7 for specific speeds. Thus, the hydrodynamic damper in Case 5 has better results than the Case 7. It can be concluded that the revised hydrodynamic damper in Case 5 can be the optimized version of the current hydrodynamic damper in terms of torsional vibrations. Lastly, the needed structural changes in order to obtaine the stiffness and damping coefficients of the hydrodynamic damper in Case 5 are determined by using the formulations derived in previous sections.
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ÖgeRide quality and handling characteristics of an off-road vehicle with active anti-roll bar suspension(Graduate School, 2024-01-17) Canpolat, Berkay ; Akalın, Özgen ; 503201707 ; AutomotiveA vehicle's suspension system isolates the car from road vibrations, resulting in a comfortable ride. In addition, it provides sufficient handling in the vehicle and helps the vehicle to drive stably and reliably. The anti-roll bar system contributes significantly to safety by reducing body roll during cornering. However, when used passively, this system negatively affects the vehicle's ride quality. Active anti-roll bar systems can adjust stiffness according to the driver's input or road conditions. Most current studies have focused on this feature to improve the stability and handling of the vehicle. However, ride quality is essential for many reasons in military vehicles. It is possible to observe effects such as loss of attention and discomfort and the emergence of health problems when the driver is exposed to poor road conditions. In this study, it is desired to examine the effect of the anti-roll bar on the system. According to the results, the goal is to develop a controller for a vehicle with better ride quality. In this context, analyses were made on roads with the same waviness and roughness values to compare each result clearly. In addition, analyses were made on roads with different phase angles to observe the vehicle's response to various conditions. It would need to be important to check essential parameters such as road holding and stability by increasing the ride quality while performing analyses on the off-road vehicle. As a result, analyses of these suspension systems, which are essential for vehicle dynamics such as cornering and lane change, should also be made. The aim is to create a system that will not adversely affect road holding and stability while contributing to the improvement of ride quality. Four different suspension systems were installed in the analyses to examine the ride quality with different anti-roll bar thicknesses. One of them is a thicker anti-roll bar. A front suspension produces higher torsional stiffness and load transfer in this case. The second is the anti-roll bar, which is actively used in the vehicle. The third one is a lower-thickness anti-roll bar. Thus, the results in the case where the effect of the anti-roll bar is reduced wanted to be examined. Finally, the anti-roll bar has been removed from the suspension system. The anti-roll bar model used does not have a complex structure. For this reason, the calculation method created by the SAE (Society of Automotive Engineers) was used while calculating the torsional stiffness. There are standards and methods such as ISO 2631, BS 6841, absorbed power, and VDI 2057 to evaluate ride quality. Within the scope of this study, the absorbed power method, frequency-weighted RMS, and vibration dose value (VDV) have been used to evaluate ride quality. The absorbed power is expressed in Watts to represent how much power the human body absorbs to measure the level of discomfort. In this method, which shows a number mathematically, high values indicate an increase in discomfort, while the limit value is determined as 6 Watts. In calculating the comfort level, getting data from the driver or passenger locations is more accurate. Therefore, acceleration data at the driver's location were taken for calculations. In order to extract acceleration values, first of all, analyses should be made in the multibody dynamics program. Then, with the acceleration values obtained from here, the absorbed power value can be transformed with the transfer functions specified by TARADCOM. Graphical programming and numeric computing environments were used simultaneously for this transformation, and the transfer functions provided are analyzed. Absorbed power values in all three global axes were obtained, and the results were transferred to tables and graphics. Another assessment criterion was ISO 2631. Examining the frequency-weighted RMS and vibration dose value (VDV) results, comments on ride quality were given. To determine these values, the signal processing application examined the acceleration data in all three axes. While analyzing the ride quality, the vehicle travels 300 meters to make the assessment correctly. In addition, the speed values between 12.87 km/h and 32.19 km/h were examined to observe different speed conditions. In order to examine different phase conditions, analyses were made on the paths at 0, 45, 90, and 180-degree phase angles, and their effects were examined. Anti-roll bars with different torsional stiffness values have been investigated for their impact on ride quality. According to the results obtained, the change in thickness did not have much effect on ride quality at a 0-degree phase angle due to the low effect of the anti-roll bar on the system. It also allows the vehicle to remain more stable at this phase angle. With the increase of the phase angle, significant increases were observed in the absorbed power values of the lateral axis. It has been determined that this difference widens even more, especially in the 180-degree phase angle. The presence of an anti-roll bar significantly increases the discomfort in the lateral axis. In addition, it was determined that the highest absorbed power values were on the vertical axis. It has been observed that the anti-roll bar has a negative effect on the vertical axis at the 180-degree phase angle. As a result, it has been deduced that the anti-roll bar has negatively affected ride quality, especially with the increase in the phase angle. According to the results, the lowest absorbed power values were achieved in the vehicle's longitudinal axis (x-axis). When frequency-weighted RMS and vibration dose value (VDV) values were also analyzed, it was found that similar results were obtained with the absorbed power method. In vehicle dynamics analyses such as cornering and double-lane change, it has been observed that vehicles with anti-roll bars in the front suspension have lower roll angle values. In addition, vehicles with and without an anti-roll bar were subjected to lane change analyses by the ISO 3888-2 standard. This analysis consists of an obstacle avoidance course and is based on double-lane change. According to ISO 3888-2 results, it has been observed that the vehicle with an anti-roll bar can complete the track at higher speeds. When the results of the model with active anti-roll bars operating according to the specified algorithm are analyzed, it is observed that the ride quality values of the vehicles have improved considerably, especially at high phase angles. In the analyses where the handling is evaluated, it is observed that the vehicles with the active anti-roll bar system produced similar results to the standard vehicle and performed better than the vehicles without an anti-roll bar. As a result, while the ride quality of the vehicle with the active system is better, there is no change in handling characteristics.
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ÖgeRide quality evaluation methods for off-road vehicles(Graduate School, 2022) Çalkan Tütüncü, Hande ; Akalın, Özgen ; 717834 ; Automotive Engineering ProgrammeAnalyses were made on the FED-Alpha vehicle model using the ADAMS Car program to observe the effect of road characteristics on ride quality. The vehicle was developed by Ricardo Inc. as a fuel-efficient all-terrain military vehicle. The FED-Alpha is a 4x4 wheeled vehicle and the front and rear axles have an independent double-wishbone suspension system. The suspension system has air bellows and titanium helical springs and dampers. The dampers give different damping characteristics at low and high frequencies. In the ride quality evaluation standards, the constraints of analysis or test have been determined. The constraints mentioned were discussed and analysis studies were performed with these constraints. For example, speed deviation, which is one of the absorbed power method constraints, is examined and the effect of this value on ride quality is examined by exceeding the restricted value. Analyses were made by collecting data on three axes at three different vehicle locations. Waviness, phase angle, wavenumber, wavelength and roughness were chosen as the road parameters to examine ride quality effects. Synthetic roads were created by changing the road parameter to be examined. Synthetic roadways were modeled in a format that could be analyzed with ADAMS Car. Absorbed power, vibration dose value, and frequency weighted rms acceleration methods were used for ride quality evaluation in the scope of this thesis. Absorbed power was calculated with the transfer function and the FFT method, and the different results between the two methods were examined. Since the difference in results was negligible, the calculations continued with the transfer function. MATLAB and SIMULINK software were used simultaneously in the ride quality evaluation method calculations. The acceleration data was collected from the driver seat base, passenger seat base, and middle point of the rear passenger seat base. Acceleration data were exported from the ADAMS Car. With the help of MATLAB code, the tab files were imported to SIMULINK and the acceleration data was passed through transfer functions. Obtained results are converted into graphics with the help of MATLAB. Analyses were made between the speed of 8,05 km/h and 24 km/h to determine vehicle 6-Watt speed in the z-axis on synthetic roads. First, the found absorbed power values are plotted in the speed domain. Then, 6-Watt speed was determined by fitting a second-degree polynomial to the plotted graph in the speed domain. Finally, using the fitted second-degree polynomial, 6-Watt speed was calculated. Using the same second-degree polynomial calculation model, the vibration dose value and frequency weighted rms acceleration values corresponding to the 6-watt speed were calculated. The purpose of comparing these values is to determine how other methods evaluate the 6-watt ride quality limit defined in the absorbed power method. A higher correlation rate was detected with frequency weighted rms acceleration and a slightly lower correlation with vibration dose value. Ride quality result graphs were plotted in three axes by analyzing four rms roughness values. As the road rms roughness value increases, all vibration evaluation values in the z-axis increase. This result means that ride quality deteriorated in the z-axis due to roughness. It has been determined that the ride quality decreases depending on the increasing speed. It has been observed that the absorbed power values are most affected by the change of vehicle speed in the z-axis. It has been determined that with the increase of the road rms roughness, the ride quality decreases in the x and y axes. But the change is not continuously increasing depending on the speed. Analyses were made at three different waviness values and ride quality graphs were plotted for each axis. As the waviness increase, the ride quality increase in the z-axis. The ride quality decrease depending on the speed increase. No change was detected in the ride quality in the y-axis depending on the waviness change. The decrease in the waviness led to a decrease in the ride quality in the x-axis. Still, no correlation could be observed in the change depending on the speed in the x-axis. Analyses were made on roads with four different wavenumber bandwidths. It is observed that the ride quality change as the wavenumber bandwidth change in the z-axis. The ride quality increase on wider bandwidth at low wavenumber range. The ride quality increase on narrow bandwidth at high wavenumber range at low speed. The ride quality increase on narrow bandwidth at high wavenumber range at high speed. As the speed increases, the ride quality in the z-axis decrease, but it has been observed that the slope of the speed-related increases differs on the roads with different wavenumber bandwidths. It was determined that one of the parameters affecting the ride quality in the x and y axes is the wavenumber bandwidth, but no correlation could be detected. Analyzes were made on the road with four different phase angles. It has been determined that as the phase angle increases, the ride quality in the z-axis increase. As the speed increases, the ride quality values in the z-axis decrease. As the phase angle increases, the ride quality on the y-axis decrease. It is evaluated that the ride quality decrease is the roll motion caused by the opposite movement of the right and left wheel. It was observed that the phase angle change in the x-axis did not have much effect on the ride quality. Analyzes were made at different speed profiles on the 5,08 cm rms roughness road. As a result, it has been determined that even if the speed deviation factor specified in the standard is exceeded and the average speed is maintained and driven for approximately 300 meters, the absorbed power values in the z-axis will not change. However, this result needs to be supported by the results of the analysis to be made in different vehicles. When the ride quality evaluation methods are compared, it has been determined that the absorbed power will give the most sensitive results changing by vehicle speed. It is considered that the absorbed power, vibration dose value and frequency weighted rms acceleration methods will be sufficient for the evaluation of the ride quality in the z-axis. However, it is thought that it is necessary to include the suspension and wheel parameters in the x and y-axis evaluations.
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ÖgeViability of differential braking based steering redundancy for an autonomous vehicle(Graduate School, 2024-01-17) Tokay, Dorukhan ; Akalın, Özgen ; 503201709 ; AutomotiveThe exploration of differential braking as an alternative steering method for autonomous vehicles is a pivotal theme of this research, particularly in the context of steering system failure. Differential braking, which applies varying braking forces to the wheels, has been integral to enhancing vehicle stability and maneuverability in systems like Electronic Stability Control and Differential Braking Assisted Steering. This thesis underscores the importance of redundant systems in autonomous vehicles, given the rapid advancements in autonomous driving technologies and the inherent risks of steering system failures, especially at high speeds. Differential braking emerges as a promising safety net in such critical situations. The historical progression of differential braking is examined, from its initial role in cornering stability to its integration with advanced control strategies that enable sophisticated vehicle control tasks. The technology has evolved significantly, now being combined with active steering systems to improve vehicle handling. The research methodology involves assessing differential braking as an emergency steering system through double lane change maneuvers based on NATO standard. A reference trajectory is generated for the vehicle, and two PID controllers are optimized to minimize trajectory and yaw angle deviations. The optimization process uses a heuristic algorithm inspired by natural swarms, Particle Swarm Optimization, which iteratively updates to find the optimal solution. The algorithm's parameters are carefully chosen to ensure stability and convergence, with the optimization process tailored to reduce computation time. A pseudo-ABS system is introduced to manage wheel slip during intense braking, ensuring optimal braking performance and preventing loss of traction. The system adjusts braking force based on longitudinal slip, with control margins customized to the tire and ground characteristics. A co-simulation approach is employed, integrating a high-fidelity vehicle model with the controller, to evaluate the system. The simulations are conducted at different vehicle speeds, with the optimized controller gains derived from the higher speed. The vehicle model used is well-documented and validated, ensuring the reliability of the simulation results. The findings reveal that at higher speeds, the vehicle can successfully navigate the maneuver within the course boundaries, though with some response lag. The pseudo-ABS system effectively controls longitudinal slip, maintaining tire traction. At lower speeds, the vehicle performs the maneuver with minimal delay, but the gains tuned for higher speeds may be too aggressive, indicating the need for speed-dependent tuning. The impact of front wheel geometry on differential braking performance is also explored, with the scrub radius identified as a critical suspension parameter. Simulations with varying scrub radii show that there is a threshold below which the necessary steering torque for successful maneuvers is not produced. While larger scrub radii improve differential braking effectiveness, they are not typically preferred for road vehicles due to the negative implications for steering effort, tire wear, and road feel. The study concludes that the default scrub radius offers an optimal balance, allowing the vehicle to follow the reference path with minimal steering wheel oscillations. In conclusion, the research confirms the viability of differential braking as a redundant steering system for autonomous vehicles, as demonstrated by successful simulations of obstacle avoidance maneuvers. The simple yet effective PID controllers could be further enhanced by optimizing control parameters for specific vehicle speeds. The significant influence of front wheel geometry, particularly the scrub radius, on the success of differential steering is highlighted, with an optimal radius being necessary to generate sufficient steering torque while minimizing unwanted steering vibrations. The potential of differential steering as a reliable redundant steering system is underscored, with future work aimed at leveraging predictive model-based control systems to refine such systems in autonomous vehicles.