An Experimental Characterization Of Hcci Di Mixed Mode Operation Utilizing External Mixture Formation In A 2 5 Liter Common Rail Diesel Engine

An Experimental Characterization of HCCI/DI Mixed-mode Operation Utilizing External Mixture Formation in a 2.5 Liter Common Rail Diesel Engine

Abstract: In order to meet future emissions requirements for internal combustion engines, continuing advances in emission reduction techniques are imperative. A method for in-cylinder emissions reduction currently being investigated quite extensively is Homogeneous Charge Compression Ignition (HCCI). An engine operating in pure HCCI combustion is capable of reducing NOx and PM emissions to near zero levels. However, HCCI engines are typically limited to low and mid-load operation due to high cylinder pressure-rise-rates associated with compression ignition of a homogeneous charge. One method of extending this operating range, investigated in this work, is the combination of HCCI combustion with standard CIDI combustion in a mixed-mode operating scheme. This method combines the emissions benefits of HCCI combustion with the high efficiency operation of standard Diesel combustion. Steady-state engine operating conditions are explored in pure HCCI and HCCI/DI mixed-mode operation and compared to baseline operating conditions of standard CIDI in a production 2.5L common rail Diesel engine. In this work, homogeneous mixture preparation is performed utilizing an external atomization device. In preliminary characterization, the effects of HCCI ratio, EGR ratio and DI timing are explored and the advantages of mixed-mode operation is verified over that of standard CIDI Diesel combustion. Following definition of the most important combustion control parameters, a comprehensive sensitivity analysis of EGR ratio, DI timing, engine speed and engine load is also conducted in mixed-mode operation with a focus on the effects of engine out emissions and combustion characteristics. Additionally, the limits of HCCI operation in this particular engine are also explored in order to define a baseline location for the transition to mixed-mode operation. In order to truly ascertain the benefits of mixed-mode combustion, results of mixed-mode operation are contrasted against manufacturer's engine maps for NOx and PM emissions as well as fuel consumption. The results of rough optimization illustrate that significant reduction in NOx emissions are possible with reasonable PM emissions easily eliminated with a current generation DPF. Fuel consumption is also found to be significantly reduced in most mixed-mode cases where up to 15 percent reductions are possible in the operating range considered. This fuel consumption decrease is also found to extend to pure HCCI operation and close to 10 percent reductions are discovered between operating speeds of 1500 and 2500 RPM at slightly more than 2 bar BMEP engine load. To set the stage for further research, basic transient operation is also investigated. With a firm grasp and understanding of steady-state operating conditions, control of the transition between pure HCCI, mixed-mode and pure CIDI combustion schemes can be explored. The basic structure of this control format includes pure HCCI operation at low-loads, mixed-mode operation in mid-loads and standard CIDI operation at high-loads. In this work, HCCI operation is found to be relatively insensitive to engine speed; however, increases in engine speed may affect the load threshold conditions at which these transitions occur.

Control Strategies for HCCI Mixed-Mode Combustion

Delphi Automotive Systems and ORNL established this CRADA to expand the operational range of Homogenous Charge Compression Ignition (HCCI) mixed-mode combustion for gasoline en-gines. ORNL has extensive experience in the analysis, interpretation, and control of dynamic engine phenomena, and Delphi has extensive knowledge and experience in powertrain compo-nents and subsystems. The partnership of these knowledge bases was important to address criti-cal barriers associated with the realistic implementation of HCCI and enabling clean, efficient operation for the next generation of transportation engines. The foundation of this CRADA was established through the analysis of spark-assisted HCCI data from a single-cylinder research engine. This data was used to (1) establish a conceptual kinetic model to better understand and predict the development of combustion instabilities, (2) develop a low-order model framework suitable for real-time controls, and (3) provide guidance in the initial definition of engine valve strategies for achieving HCCI operation. The next phase focused on the development of a new combustion metric for real-time characterization of the combustion process. Rapid feedback on the state of the combustion process is critical to high-speed decision making for predictive control. Simultaneous to the modeling/analysis studies, Delphi was focused on the development of engine hardware and the engine management system. This included custom Delphi hardware and control systems allowing for flexible control of the valvetrain sys-tem to enable HCCI operation. The final phase of this CRADA included the demonstration of conventional and spark assisted HCCI on the multi-cylinder engine as well as the characterization of combustion instabilities, which govern the operational boundaries of this mode of combustion. ORNL and Delphi maintained strong collaboration throughout this project. Meetings were held on a bi-weekly basis with additional reports, presentation, and meetings as necessary to maintain progress. Delphi provided substantial support through modeling, hardware, data exchange, and technical consultation. This CRADA was also successful at establishing important next steps to further expanding the use of an HCCI engine for improved fuel efficiency and emissions. These topics will be address in a follow-on CRADA. The objectives are: (1) Improve fundamental understanding of the development of combustion instabilities with HCCI operation through modeling and experiments; (2) Develop low-order model and feedback combustion metrics which are well suited to real-time predictive controls; and (3) Construct multi-cylinder engine system with advanced Delphi technologies and charac-terize HCCI behavior to better understand limitations and opportunities for expanded high-efficiency operation.

Using Large Eddy Simulations to Study Diesel DI-HCCI Engine Flow Structure, Mixing and Combustion