Crank Angle Based Virtual Cylinder Pressure Sensor in Heavy-Duty Engine Application

Abstract: The in-cylinder pressure is an important signal that gives information about the combustion process. To further improve engine performance, this information can be used as a feedback signal in a control system. Usually a pressure sensor is mounted in the cylinder to extract this information. A drawback with pressure sensors is that they are expensive and have issues with aging. This master’s thesis investigates the possibility to create a virtual sensor to estimate in-cylinder pressure based on crank angle degree sensor (CAD-sensor) data and physical models of the heavy-duty engine. Instead of using the standard mounted CAD-sensor an optical high-precision sensor measures the elapsed time between equidistant angles. Based on this signal the instantaneous angular acceleration was estimated. Together with the inertia of the crankshaft, connecting rods and pistons, an estimation of the engine torque was calculated. To be able to extract in-cylinder pressure from the estimated torque, knowledge about how the in-cylinder pressure signal propagates in the drivetrain to accelerate the flywheel needs to be known. Two engine models based on the torque balance on the crankshaft are presented. The fundamental difference between them is how the crankshaft is modeled, rigid body or spring-mass-damper system. The latter captures torsional effects of the crankshaft. Comparisons between the estimated torque from sensor data and the two engine models are presented. It is found that torsional effects of the crankshaft is present at normal engine speeds and has a significant influence on the flywheel torque. A separation of the gas torque contribution from one cylinder is done with CAD-sensor data together with the rigid body engine model. The in-cylinder pressure is then estimated by using the inverse crank-slider function and a Kalman filter estimator. The estimated pressure captures part of the compression and most of the expansion at engine speeds below 1200 RPM. Due to the crank-slider geometry the pressure signal disappears at TDC. The torsional effects perturb the estimated pressure during the gas exchange cycle. Further development must be made if this method is to be used on heavy-duty applications in the future.