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System analysis of a diesel engine with VGT and EGR

To fulfil emission requirements specified by environment demands, such as Euro 4 and Euro 5, there is a need to utilize engines based on technologies such as Variable Turbine Geometry (VGT) and Exhaust Gas Recirculation (EGR). A model of an engine using VGT and EGR was created by Ph.D student Johan Wahlstr\"om at Link\"oping University. This thesis evaluates Wahlstr\"om's model and shows how it successfully describes the engine and its behaviour. The thesis also confirms theories about the occurrens of non.minimum phase behaviour in different transfer functions, e.g. from VGT signal to the mass flow through the compressor. An interesting phenomenon when applying VGT and EGR is a nonlinearity leading to, for example, that the same pressure in the intake manifold can occur for two different VGT signals. Such phenomenon can cause problems when designing a control system. Furthermore, this nonlinearity also results in a replacement of the nonminimum phase behaviour with an overshoot when a large (above 80\%) VGT control signal is used. This thesis also provides a linearized model, which describes the engine satisfactory. The linearization results in transfer functions with two zeros and three poles, whose locations do not change much when varying engine speed and load (except at high load and low engine speed). This fact will most likely make it possible to utilize just a few different linearizations for all speeds and loads. However, altering VGT and EGR positions greatly affect the transfer functions. Thus, several linearizations are probably needed to cover all operating points. When designing a future control system a good strategy is to utilize a decoupled system since the model has strong cross-connections. Another solution would be to apply multi dimensional control strategy, e.g. LQ-theory.

Thomas Johansson


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