Physically-based modeling, estimation and control of the gas exchange and combustion processes for diesel engines utilizing variable intake valve actuation

Lyle E Kocher, Purdue University

Abstract

In this work, a low-order five state model of the air handling system for a multi-cylinder variable geometry turbocharged diesel engine, with cooled exhaust gas recirculation and flexible intake valve actuation, is developed and validated against 286 steady state and 62 transient engine operating points. The model utilizes engine speed, engine fueling, EGR valve position, VGT nozzle position and intake valve closing time as inputs to the model. The model outputs include calculation for the engine flows as well as the exhaust temperature exiting the cylinder. The gas exchange model captures the dynamic effects of not only the standard air handling actuators (EGR valve position and VGT position) but also intake valve closing (IVC) timing, exercised over their useful operating ranges, and thus is critical for designing robust controllers. The model's capabilities are enabled through the use of analytical functions to describe the performance of the turbocharger, eliminating the need to use look-up maps; a physically-based control-oriented exhaust gas enthalpy sub-model, and a physically-based volumetric efficiency sub-model. Traditional empirical or regression-based models for volumetric efficiency, while suitable for conventional valve trains, are therefore challenged by flexible valve trains. The added complexity and additional empirical data needed for wide valve timing ranges limit the usefulness of these methods. A simple physically-based volumetric efficiency model was developed to address these challenges. The model captures the major physical processes occurring over the intake stroke, and is applicable to both conventional and flexible intake valve trains. The model inputs include temperature and pressure in both the intake and exhaust manifolds, intake and exhaust valve event timings, engine cylinder bore, stroke, connecting rod lengths, engine speed and effective compression ratio. The model is physically-based, requires no regression tuning parameters, is generalizable to other engine platforms, and has been experimentally validated using an advanced multi-cylinder diesel engine equipped with a fully flexible variable intake valve actuation system. The in-cylinder oxygen fraction serves as a critical control input to these strategies, but is extremely difficult to measure on production engines. Fortunately, estimates or measurements of the oxygen fraction in the intake and exhaust manifold, the in-cylinder charge mass, and residual mass can be utilized to calculate the in-cylinder oxygen fraction. This work outlines such a physically-based, generalizable strategy to estimate the in-cylinder oxygen fraction from only production viable measurements or estimates of exhaust oxygen fraction, fresh air flow, charge flow, fuel flow, turbine flow and EGR flow. The oxygen fraction estimates are compared to laboratory grade measurements available for the intake and exhaust manifolds. The oxygen fraction estimates will be shown to be particularly sensitive to errors in the EGR and turbine flow. To improve the EGR flow estimate, a high-gain observer is implemented to improve the estimate of EGR flow. Furthermore, the in-cylinder oxygen estimation algorithm is developed, and proven, to be robust to turbine flow errors. The model-based observer estimates the oxygen fractions to within 0.5% O 2 and is shown to have exponential convergence with a time constant less than 0.05 seconds, even with turbine flow errors of up to 25%. The observer is applicable to engines utilizing high pressure cooled exhaust gas recirculation, variable geometry turbocharging and flexible intake valve actuation. Advanced combustion modes, such as diesel PCCI, operate near the system stability limits. In PCCI, the combustion event begins without a direct combustion trigger in contrast to traditional spark-ignited gasoline engine and direct-injected diesel engines. The lack of a direct combustion trigger necessitates the usage of model-based controls to provide robust control of the combustion phasing. The nonlinear relationships between the control inputs and the combustion system response often limit the effectiveness of traditional, non-model-based controllers. Accurate knowledge of the system states and inputs is required for implementation of an effective nonlinear controller. The previously described in-cylinder oxygen fraction estimator, physically-based volumetric efficiency model, PCCI combustion timing model and gas exchange model provide the required information. A nonlinear controller is developed and implemented based upon these models to control the engine combustion timing during diesel PCCI operation by targeting desired values of the in-cylinder oxygen concentration, pressure, and temperature during early fuel injection. (Abstract shortened by UMI.)

Degree

Ph.D.

Advisors

Shaver, Purdue University.

Subject Area

Mechanical engineering

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