Abstract
The paper presents a Variable Valve Timing methodology for turbocharged diesel engines based on a modified theoretical engine modelling and optimum control of the air path and volumetric efficiency of a turbocharged diesel engine. Using camless valvetrain strategy, results in better fuel economy together with improved air intake charge throughout the engine operating map. The system is capable of continuously and independently controlling virtually all standard parameters of valve motion variably. This permits optimization of valve events for any operating condition without compromise. The optimized intake valve profile is determined, to have the best volumetric efficiency and proper operation for each running condition based on the existing model and by using numerical techniques.
We modify a theoretical nonlinear model of a four-cylinder turbocharged (TC) diesel engine. This model captures the interactions between VVT actuation, the turbocharger dynamics, and the cylinder-to-cylinder breathing characteristics. The model treats the cylinder strokes and the manifolds as thermodynamic control volumes by using the filling and emptying method, solving energy and mass conservation equations with submodels for turbine, turbocharger, compressor, manifold, variable valve timing and cylinder breathing dynamics. The model has been implemented in Matlab/Simulink. The model shows good results in the simulation and the validation and it is validated using experimentally measured engine parameters, giving good agreement.
In order to optimize engine performance characteristics an optimal control strategy is applied that varies inlet valve profile parameters to maximize the engine volumetric efficiency under maximum inlet valve lift constraint during different valve timings.
Finally, the numerical engine optimization highlights the fact that camless optimized intake valve profile has the capability to increase the volumetric efficiency both at low and high speed conditions. Following pages show some of the simulation results such as air mass flow to cylinder, manifold pressure, manifold temperature and cylinder pressure of the modelled turbocharger diesel engine. These variables are often required to achieve better power performance and lower fuel consumption.
Keywords: Variable valve timing, Turbocharged diesel engine, Optimal control, Volumetric efficiency, Computer simulation.