New Methods Produce Low-Temperature, Soot-Free Diesel Combustion

Researchers at Sandia National Laboratories' Combustion Research Facility (CRF) have discovered methods for producing non-sooting and low-flame-temperature diesel-like combustion. The research is motivated by the need to minimize nitrogen oxide (NO_x) and particulate matter (PM) emissions from high-efficiency diesel engines and is being conducted in collaboration with automotive and heavy-duty diesel manufacturers, with funding from the Department of Energy's Vehicle Technologies Office.

Sandia researchers Lyle Pickett and Dennis Siebers conducted experiments at the CRF using reacting diesel fuel jets under well-controlled ambient and fuel injector conditions. They sought to determine what factors affect soot formation during mixing-controlled diesel combustion with flame temperatures less than approximately 2000 K — too low for significant NO_x formation. In the course of the work, Pickett and Siebers identified three methods for producing fuel jets that did not form soot in any region of the reacting jet and that simultaneously had a low flame temperature. The operating conditions and regions of heat release in the fuel jet (OH chemiluminescence images) for the three methods are shown in Figure 1. Simultaneously acquired planar laser-induced incandescence images (not shown) confirmed that the fuel jets were soot-free.

Figure 1. OH chemiluminescence images of three methods for low-flame-temperature, soot-free diesel combustion. Images are time-averaged during mixing-controlled heat release phase of combustion. The fuel injector is at the left edge of the image. Ambient temperature, density, oxygen concentration, and injector nozzle size are listed at the right. The injector orifice pressure drop was 1400 bar.

The first method (1) uses a small nozzle coupled with a low ambient oxygen concentration. The small nozzle induces fast fuel-air mixing prior to the flame lift-off length to produce equivalence ratios less than two in the fuel jet prior to combustion, thereby avoiding soot-forming rich fuel-air mixtures. The ambient gas oxygen concentration of 10% keeps stoichiometric flame temperatures below 2000 K, as indicated by the solid portion of the adiabatic flame temperature curve. The second method (2) uses a small nozzle coupled with a cooler ambient temperature to extend the lift-off length and induce more fuel-air mixing prior to the lift-off length, resulting in an equivalence ratio of approximately 0.6 prior to combustion. The lean mixture avoids both soot formation and a high-temperature stoichiometric diffusion flame. The third method (3) relies on the use of very high EGR (8% ambient oxygen) resulting in mixtures in the fuel jet that are rich, but with peak adiabatic flame temperatures that are too cool for soot inception at diesel timescales.

A schematic illustrating how each of these methods avoids soot formation and produces low flame temperatures is shown in Figure 2. The figure is an equivalence ratio-versus-temperature plot with contours indicating the locations where soot and NO_x formation occur for a diesel-like fuel. Curves (dashed lines) overlay the plot, showing the predicted adiabatic flame temperatures for fuel-air mixtures at the given equivalence ratios for four ambient oxygen concentrations. The reduced oxygen concentrations simulate the effect of various levels of exhaust-gas recirculation (EGR) in an engine.

Figure 2. Schematic of equivalence ratio versus adiabatic mixture flame temperature.

The lack of soot formation and low-flame temperatures realized in the reacting fuel jets suggests that these diesel combustion methods offer the potential for a simultaneous soot and NO_x reduction in an engine, while maintaining a mixing-controlled heat release rate. Although much more research is needed to determine if these non-sooting, low-temperature mixing-controlled combustion strategies can be employed in engines, the results are providing guidance to engine designers on future directions for lowering soot and NO_x emissions.

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