Model based optimal longitudinal vehicle control

Considering the competitive environment in automotive industry, original equipment manufacturers (OEMs) in this industry are in a challenging competition with each other to offer their customers more attractive vehicles. Cost, emissions, fuel economy, noise vibration & harshness (NVH), durability, performance and driveability properties make a product able to distinguish from its competitors' products. Each of these attributes has a major contribution of forming a perception of the customers' choosiness. New technologies as a result of the research and developments activities in electronics resulted with complex electro-mechanical systems in automobiles. With the addition of recent developments in materials and manufacturing processes on top of it, especially in diesel fuelled internal combustion engines (ICE), torque and power delivery had almost doubled with respect to the conventional engines developed not more than two decades ago. Additionally as a result of latest developments at air path and gas exchange systems control, torque build up rate had significantly increased enabling the vehicles to be more agile and reactive to load change request manoeuvres. As a result of all these capability improvements, vehicle response characteristics to high torque and power capacity engines changed extremely altering the necessity of proper and robust driveability calibration requirements. Driveability properties of the vehicles had gained significant importance in terms of customer satisfaction. This dissertation focuses on improving vehicle driveability properties taking advantage of simulation tools and model based control. The overall profit of this thesis is providing improved driveability via using engine torque production and vehicle models and controllers at the same time. Torque transmission from the vehicle's power unit to the road surface via tires is a complex structure which should be handled with extreme care considering the overall driveability performance of the vehicle. An agile throttle response of the vehicle is aimed without error modes like acceleration initial kick, bump, response delay, stumble or shuffle. However considering the nonlinearities resulting from the complex structures at the drivetrain of the vehicle, this requirement becomes significantly challenging. Despite mechanical control at longitudinal motion in conventional vehicles, modern vehicles are equipped with electromechanical systems. Thanks to technological developments in the automotive industry that current capability of the vehicles enables us to develop better platforms for improving driveability characteristics. Modern engine control units (ECUs) have the capability of processing thousands of signals in a less than tens of milliseconds and as a result regulate numerous actuators which results with displacement of the vehicle complying all regulative requirements and customer expectations. Acceleration throttle pedal input signal is recorded by electronic control unit, processed and finally used to control the parameters for the combustion systems. In terms of driveability control, automotive manufacturers' engine control algorithms employ input shaping or simple filtering algorithms. These algorithms use look-up tables and main control strategy is to slew the pedal oriented torque request for the tip-in and tip-out manoeuvres in an open loop control methodology especially in backlash transition region of the driveline. Considering the fact that there is no close loop control and these features become subjective calibration methodologies and outcome becomes strongly dependant on calibrator's capability and performance. Moreover filling look-up tables for all gear, engine speed and pedal position combinations requires significant amount of calibration development time. Taking into consideration all of these obstacles of the current driveability features, the subject of automated torque control for improved driveability is a state of the art research topic both within automotive manufacturers and academic researchers as it can be described as an optimization problem dealing with performance and comfort counter measures. Knowledge of the instantaneous produced torque by the engine is a key item with respect to satisfying above stated attributes in vehicle longitudinal motion control. Currently common approach for combustion management is the usage of look-up table based structures with the drawback of poor conformity of the produced torque. Look-up tables define air and fuel quantity setpoints in order to produce requested indicated torque without feedback of the produced torque. These look-up tables are filled at engine dynamometer test benches at normal ambient conditions. In general fuel and air quantity setpoint maps have the axes of engine speed and indicated torque and requested amount of desired variable is filled to the corresponding point of the look-up table. In real world driving conditions fuel quantity control is robust however especially with turbocharged systems; requested air quantities may deviate from the setpoint values especially when considering transient manoeuvres. This phenomenon is called "turbo/boost lag" and significantly affects the produced torque. The situation is much worse for non-standard conditions, extreme hot and cold and altitude. In the literature most of the proposed vehicle longitudinal motion control related engine torque control algorithms base on the fact that requested torque will be generated immediately from the diesel engine. However as explained above this is not the case in real life applications. Therefore engine characteristic is either not included or covered with a simple filtering algorithm in conventional vehicle longitudinal motion related engine torque control methodologies. Engine brake torque model combined driveability control algorithm proposed in this thesis is differentiated from the previous studies in the literature within this perspective. Proposed "In cylinder pressured based engine brake torque model algorithm" works in harmony with the driveability control structure and improves overall vehicle response characteristics. Within the scope of this study a 4 degree of freedom powertrain model consisting of 4 inertias, 2 set of spring and damper elements with tyre characteristics, is built in MATLAB/Simulink environment. Model validation considering longitudinal vehicle dynamics is performed with employing vehicle level tests using a tip-in followed by a tip-out acceleration pedal signal input load change manoeuvres. Comparison of simulation results and measured vehicle test data shows that proposed model is capable of capturing vehicle acceleration profile revealing unintended error states for the specified input signals. Considering the driveability control perspective, a Model Predictive Control (MPC) algorithm employed to manipulate the pedal map oriented torque demand signal in an automotive powertrain application in order attenuate the powertrain oscillations in longitudinal vehicle motion control. 4 mass model could not be employed at with the MPC algorithm due to very high level of nonlinearity. Therefore two simplified versions of 2 and 3 mass models have been developed. It has been verified that both 2 and 3 mass vehicle models are accurate enough to employ the MPC torque control algorithm. As the aim of this study is to develop a close loop driveability algorithm for real world applications, the 4 mass vehicle model is used as replacement environment for the subjected vehicle in order to employ 2 and 3 mass vehicle model based control algorithm. MPC algorithms via using both models showed good capability, however smoothness of the driving profile with the 2 mass vehicle model is slightly better than the 3 mass model. Moreover to further improve the powertrain oscillations without compromising from overall system response speed, an additional anti-shuffle control element, basically a P controller based on the speed difference of engine and vehicle speeds, has been implemented to the MPC control algorithm. Literature review about the engine torque control for improved driveability show that all the researcher use MPC alone. Proposed MPC with additional P controller is a new contribution to the literature in the subjected area of research.