Automotive Low-Temperature Diesel Combustion

Reducing greenhouse gas emissions and U.S. reliance on foreign oil imports are strong drivers for increasing the fuel efficiency of passenger cars and light-duty trucks. While the increased use of diesel engines in these vehicles is likely the fastest route toward achieving this goal, relatively high emissions of NOx and particulate matter impede their widespread adoption.

Consequently, research into automotive-class diesel engines is focused on low-temperature combustion techniques that can reduce emissions in-cylinder, without sacrificing engine efficiency and fuel consumption.

The CRF's automotive low-temperature diesel combustion project encompasses three parallel research efforts in a closely coordinated program:

• Detailed flow and thermochemical property measurements are made in an optically accessible laboratory test engine at Sandia
• Emissions, performance, and fuel consumption measurements are made in a traditional single-cylinder test engine at Wayne State University
• Computer simulations are performed and compared to the data obtained in both the optical and traditional test engines by the University of Wisconsin

Natural synergies emerge among these three efforts. For example, detailed measurements of the flow variables permit the evaluation and refinement of the computer models, while the model results can be used to clarify the flow physics-a process that is difficult if only limited measurements are employed. Similarly, traditional test engine measurements serve to identify interesting operating parameter trade-offs that bear further investigation-either numerically or experimentally-in the optical engine.

The optical research engine (above right) shares all of the attributes of a modern diesel engine: 4 valves; a central, vertical fuel injector; and a displacement of about 0.4 liters per cylinder. The geometry of a typical production engine reentrant bowl is reproduced in the quartz piston of the optical engine. Maintaining this geometry is crucial if realistic engine flows are to be preserved.

In recent research, numerical simulations and experiments have been combined to provide new understanding of the role of bulk flow structures on engine combustion and emissions processes in-cylinder. Numerical simulations conducted in collaboration with the University of Wisconsin have identified the role of bulk flow structures in transporting air (O₂) and partially burned fuel (CO) to a common interface as a critical aspect of low-temperature diesel combustion systems (left). Detailed measurements of flow velocity in the optical engine have helped clarify the development of these flow structures and the generation of turbulence to enhance mixing at the interface.