Effect of intake valve closure timing on effective compression ratio and gas exchange process of a modern diesel engine

Rajani S Modiyani, Purdue University

Abstract

Advanced combustion strategies including premixed charge compression ignition (PCCI) and lifted flame combustion are promising strategies for meeting increasingly stringent emissions regulations and improving fuel efficiency in next generation powertrains. In order to promote and implement these strategies closed-loop control of the gas exchange process and combustion is critical. Variable valve actuation (VVA) can play a key role in determining and controlling inputs to the combustion process. Modulation of intake valve closure (IVC) timing dictates the effective compression ratio (ECR) and influences the total amount of charge trapped inside the cylinder, and in doing so allows manipulation of the concentration and temperature history of the reactants prior to and during the combustion process. A validated whole engine model for a 6.7 liter six cylinder, turbocharged diesel engine with cooled exhaust gas recirculation (EGR) was used to investigate the gas exchange process behavior for steady state engine operation. The interaction between the primary actuators of the air handling system of a conventional modern diesel engine; the variable geometry turbocharger (VGT) and EGR valve, was analyzed at two different operating points using the model. The effect of different actuator positions and their coupled interactions was investigated via generation of charge flow versus EGR fraction diagrams from both simulation predictions and experimental data. The observations showed an increase in charge flow when the VGT position was modulated from open to close for a constant EGR valve position. There was also a slight increase in EGR fraction due to increase in pressure at turbine and EGR valve inlet as VGT closed. Similarly, an increase in EGR fraction with EGR valve position modulation was observed along with a slight reduction in charge flow, which was explained by reduction in turbine inlet pressure due to reduced restriction in exhaust path. The effect of intake valve closing (IVC) event timing modulation on effective compression ratio (ECR) and gas exchange process was also investigated. The motivation behind this analysis was to explore the control authority of IVC modulation on the gas exchange process and ECR for implementing advanced combustion strategies including Premixed Charge Compression Ignition (PCCI) and lifted-flame combustion in steady state. A reduction in ECR with IVC modulation was observed. It was also shown that a particular ECR-reduction could be achieved via an early or late IVC timing modulation. The gas exchange process showed a decrease in charge flow with reduction in effective compression ratio. The EGR fraction also reduced accordingly. The trends were confirmed by experimental data from Cummins ISB engine with a VVA system. The electro-hydraulic VVA used is only one of its kind in academia, with control software development and implementation done at Herrick labs, Purdue University. The engine data with VVA for six cylinders showed good agreement with simulation results. The work described in this thesis established steady state control authority of IVC timing, EGR and VGT positions on ECR. It also establishes the effect of ECR modulation on gas exchange process of a modern diesel engine with data validation from a multi-cylinder engine with VVA capability. A method to estimate temperature at top dead center is also described and validated against simulation results. Candidate control structures for both steady state and transient operation are also suggested in this thesis. The work described in this thesis is a first step towards demonstrating PCCI on a multi-cylinder engine with VVA capability for steady state operation, and lays the ground work for model investigation and control development for transient engine operation for promotion of advanced combustion modes.

Degree

M.S.E.

Advisors

Shaver, Purdue University.

Subject Area

Mechanical engineering

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