Heavy-Duty Low-Temperature and Diesel Combustion

Diesel engines are the dominant power source for heavy-duty applications such as trucking, construction, and railroad transportation because they are the most fuel-efficient and reliable engine today. The majority of diesel engines use petroleum-based diesel fuel, which contributes to U.S. dependence on petroleum and CO2 emissions. To help reduce U.S. petroleum dependence and CO2 emissions, new federal requirements for the fuel efficiency of heavy-duty vehicles have been established. Moreover, although efficient, diesel combustion can create undesirable toxic air pollutants including particulate matter (PM), smog-forming nitrogen oxides (NOx) and unburned hydrocarbons, and carbon monoxide (CO). To address these challenges, strict federal requirements for diesel engine pollutant emissions have been established. For example, PM and NOx emissions from diesel engines produced after 2010 must be reduced by 90% from 2004 levels.

These requirements calling for dramatic and simultaneous increases in fuel efficiency and reductions in emissions pose a substantial challenge for the diesel engine industry. To achieve these targets simultaneously, new diesel engines will likely require unconventional engine combustion and operating strategies that have the potential to enable more fuel efficiency, while minimizing the need for the very costly emissions aftertreatment devices diesel engines currently require. Increased exhaust-gas recirculation, swirling in-cylinder air flows, multiple fuel injection events, very early or very late timing of fuel injection, and dual fueling with both gasoline and diesel fuels are among the approaches being explored to further optimize efficiency and reduce engine-out emissions. These approaches could allow advanced diesel combustion or low-temperature combustion strategies with potential for enabling both increased fuel economy and reduced NOx and PM emissions. However, fully realizing these combustion strategies’ ultimate potential requires a better understanding of in-cylinder processes.

This laboratory uses laser-based and imaging diagnostics to gain a better understanding of these processes and to provide engineers in the diesel engine industry with the necessary science base to design cleaner, more fuel-efficient engines.

Fuel injection, combustion processes, and emissions formation are studied in a research engine. The research engine retains the basic geometry of a Cummins six-cylinder N-14 highway truck engine, but the single-cylinder version has been modified extensively for research. In addition to a standard common-rail diesel fuel injector in the cylinder head, the engine has an additional gasoline direct-injection fuel injector mounted in the cylinder wall. The additional fuel injector allows the formation of a well-mixed charge for combustion research and for certain optical diagnostics. Two injectors also allow operation with two different fuels, which can have emissions and efficiency advantages.

To facilitate optical diagnostics, the research engine has several windows. First, instead of the standard 150 mm (6 in.) tall all-metal piston, the research engine has a 650 mm (26 in.) tall extended piston with a 100 mm (4 in.) diameter crown window to allow imaging access to the piston bowl. An additional 50 mm (2 in.) diameter window mounted in the cylinder head provides imaging access to the squish region above the bowl. Smaller 30 mm by 50 mm (1.2 in. by 2 in.) rectangular windows in the cylinder walls provide access for laser illumination. The engine also has a drop-down cylinder liner that allows manual access to the combustion chamber so the windows can be quickly cleaned between engine runs.

Pulsed light from a high-power Nd:YAG laser is used for various planar measurements, including

  • in-cylinder liquid fuel penetration via Mie scattering,
  • vapor-phase fuel concentration via Rayleigh scattering or fuel-tracer fluorescence,
  • in-cylinder soot formation via laser-induced incandescence of soot, and
  • laser-induced fluorescence of combustion-generated formaldehyde and polycyclic aromatic hydrocarbons.

A second Nd:YAG laser is coupled to an optical parametric oscillator to obtain color-tunable light emission for spectroscopic diagnostics such as fluorescence of OH and NO molecules, which is used to measure flame structure and pollutant formation. The laboratory is also equipped with a flat-flame, gaseous-fuel burner for optical-diagnostic development.