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. Using diesel engines in these vehicles is a cost-effective, easily implemented strategy for reducing fuel consumption and attendant CO₂ emissions. However, the relatively high emission of NO_x and particulate matter emitted by diesels increases the cost and raises environmental barriers that prevent their widespread market penetration.

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 involves carefully coordinated experimental, modeling, and simulation efforts. Detailed measurements of in-cylinder flows, fuel and pollutant spatial distributions, and other thermochemical properties are made in an optical engine facility with geometric and thermodynamic characteristics that allow it to closely match the combustion and engine-out emissions behavior of a traditional, all-metal test engine. These measurements are closely coordinated and compared with the predictions of numerical simulations, performed by partners at the University of Wisconsin. The experimental and numerical efforts are complementary. Detailed measurements of the in-cylinder variables permit the evaluation and refinement of the computer models, and validated model results can be used to interpolate between the limited measurements that are available. These efforts jointly address the principal goals of this project: developing the physical understanding to guide and the modeling tools to refine the design of optimal, clean, high-efficiency diesel combustion systems.

The work is supported financially by the U.S. DOE Office of Vehicle Technology and by General Motors Research and Development; invaluable technical support is also provided by General Motors.

The optical research engine (left) shares all of the attributes of a modern diesel engine: four valves; a central, vertical fuel injector; and a displacement of about 0.5 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 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 (below). Detailed measurements of flow velocity in the optical engine helped clarify how these flow structures develop and how turbulence enhances mixing at the interface.

In a second example of recent results, a two-photon laser-induced fluorescence technique has been developed to measure the in-cylinder spatial distributions of unburned hydrocarbons and CO. These species represent the products of incomplete combustion, and they prevent low-temperature diesel combustion systems from realizing their full CO₂ emission reduction potential. The discrepancy observed between the measured distributions and the simulation results (below) has led to improved reduced kinetic mechanisms employed in the simulations, but additional work is needed to accurately model the fuel–air mixing processes.