Heavy-Duty Low-Temperature Combustion/Diesel Combustion

Diesel engines are the dominant power source for heavy-duty trucks and city buses because they are efficient and reliable. Burning of fossil fuels, however, releases pollution into the environment. By 2010, legislation will require that diesel particulates (smoke) and smog-forming nitrogen oxides (NO_x) be reduced by 90% from 2004 levels.

These drastic emissions reductions pose a difficult challenge for the diesel engine industry. Future diesel engines will likely use aftertreatment devices and unconventional engine operating strategies to achieve these emissions targets. Increased exhaust-gas recirculation, multiple fuel injections, swirling in-cylinder air flows, and very early or very late timing of fuel injection are being explored. These strategies could allow future diesel engines to achieve low-temperature combustion (LTC), a domain in which emissions of both NO_x and particulates can be simultaneously reduced. However, a better understanding of these in-cylinder processes is required to fully realize their ultimate potential.

This laboratory uses advanced laser-based diagnostics to gain a better understanding of these processes and provide engineers in the diesel engine industry with the necessary knowledge base to design cleaner, more fuel-efficient engines. Fuel injection, combustion processes, and emissions formation are studied in an optically accessible, single-cylinder version of the Cummins six-cylinder N-14 highway truck engine. The engine has been upgraded with a state-of-the-art common-rail fuel injector, capable of multiple fuel injections at high (>25000 psi) injection pressure. The engine is coupled to a 75 horsepower dynamometer to maintain engine speed during experiments. The air supply system, consisting of a 150-horsepower compressor, refrigerated-air dryer, and an electrical air heater, provides precise control of intake air properties over a full range of typical diesel engine operating conditions. Conventional research engine instrumentation provides measurements of engine exhaust particulate and NO_x emissions, and cylinder pressure for heat release rate analysis.

While retaining the basic geometry of a production engine, the research engine has been modified extensively for optical access. Windows in the cylinder walls provide access for laser illumination, while the piston-crown window in the extended piston provides imaging access to the combustion bowl. An additional window mounted in the cylinder head provides imaging access to the squish region above the bowl. Pulsed light emission from a high-power Nd:YAG laser is used to for various planar measurements, including in-cylinder liquid fuel penetration via Mie scattering, vapor-phase fuel penetration via Rayleigh scattering, and in-cylinder soot formation via laser-induced incandescence of soot. A second Nd:YAG laser is coupled to an optical parametric oscillator (OPO) to obtain color-tunable light emission for spectroscopic diagnostics, such as fluorescence of OH and NO molecules for measurements of flame structure and pollutant formation. The laboratory is also equipped with a flat-flame, gaseous-fuel burner for optical-diagnostic development.