Control-Oriented Modeling and Operating Range Expansion of Premixed Charge Compression-Ignited Combustion in a Multi-Cylinder Diesel Engine with Flexible Valve Actuation and Variable Fuel Reactivity
This effort presents the need for advanced combustion modes such as PCCI for use in heavy-duty diesel engines, as a means to meet the strict future pollutant emission and fuel efficiency regulations imposed by the federal government. The state of the art in literature is reviewed for PCCI combustion, combustion timing prediction and modeling, and limitations on the PCCI operating range. The author's specific contributions are outlined, including testbed development, development of a control-oriented PCCI combustion timing model, analysis of the limitations on the PCCI operating range and technologies to improve them, and support for the other efforts within the research group, as all will be combined to enable successful closed-loop control of PCCI combustion. The PCCI combustion timing model is a simple, analytical, control-oriented model for prediction of combustion timing during premixed charge compression ignition combustion with early fuel injection. The model includes direct dependence on in-cylinder temperature, in-cylinder pressure, and the total in-cylinder O2 mass fraction, including the contribution of EGR, residual burned gas, and backflow during the valve overlap period. The model is extensively validated against experimental PCCI data from a multi-cylinder diesel engine utilizing high pressure EGR, variable geometry turbocharging, and flexible intake valve actuation, which allows control over the engine's effective compression ratio. The results show that across a wide range of input conditions the model predicts the start of combustion within ±2°CA of the experimental values for all but three of the 180 data points (98%+ accuracy), with an RMS error of 0.86°CA. The experimental SOC ranges from as early as -19.3°CA to as late as +0.6°CA by heavily exercising the control actuators, specifically the commanded start of injection timing SOIecm, the IVC timing, and the engine's air-handling system (EGR valve position and VGT position). Analysis is performed to isolate the effects and control authority of each actuator on the ignition delay and start of combustion timing. During these actuator sweeps, the model captures complex relationships and predicts the start of combustion within ±2°CA of the experimental values. This PCCI combustion timing model can be coupled with a gas exchange model for control algorithm design and analysis. The PCCI operating range limitation analysis uncovers the limits that constrain effective PCCI combustion to a limited region on the speed-load map and what can be done to expand that region. A stock Cummins 6.7L multicylinder diesel engine with the added capability of variable valve actuation is used to experimentally investigate the limitations on widespread operation of PCCI combustion with considerations for NOx, PM, UHC, and CO emissions, and fuel efficiency, equivalence ratio, combustion noise, and combustion stability. It is discovered that limits on equivalence ratio, brake-specific PM emissions, combustion noise, EGR cooler boiling, and a pumping/efficiency penalty associated with high EGR rates constrain the range of effective PCCI combustion. Characterization of the pumping penalty limit reveals that variable valve actuation, specifically early IVC timings, can reduce the effective compression ratio of the engine and promote effective PCCI combustion while taking the burden off of the turbomachinery to drive EGR. This reduces the pumping penalty and gives recovery of efficiency and torque, extending the effective PCCI range to higher speeds. The use of a less reactive fuel also improves the range of PCCI combustion by realizing even more pumping improvement through EGR reduction and by enabling a late-injection PCCI combustion that has simultaneous reduction of NOx, PM, and combustion noise. Both IVC modulation and reduced fuel reactivity significantly expands the operating range for effective PCCI to higher engine speeds and loads. The high speed limit is increased from 2000 to 2400 RPM and the high load limit is increased to a maximum of 6.4 bar BMEP. Overall, the effective PCCI operating space is expanded from 10.1 to 30.5% of the engine's total speed-load map. Lastly, conclusions are summarized and future work is proposed to perform a more detailed experimental analysis with multiple injection strategies and additional testing with 100% gasoline.
Shaver, Purdue University.
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