Characterizing Emissions Reduction Technologies By Using In-Cylinder Endoscope Imaging

Diesel engines have the potential to reduce the demand for foreign oil significantly because of their high efficiency, relatively low cost, and reliable operation. However, diesels have been plagued by high emissions of soot and nitrogen oxides (NOx). Current research efforts are aimed at solving this emissions challenge. Possible routes to a solution include low-temperature (<1800K peak) combustion to avoid formation of NOx, alternative fuels such as biodiesel blends, and aftertreatments such as NOx catalysts.

Argonne uses highly instrumented production engines to assess performance, including a commercial 1999 Mercedes 1.7L 4-cylinder turbo direct-injection engine with an internal exhaust gas recirculation (EGR) system. This engine is instrumented for numerous diagnostic measurements including gaseous and particulate matter emissions, thermophoretic sampling, particulate characterization, and the use of a Visioscope for endoscopic imaging of in-cylinder combustion kinetics.

This engine was recently used to compare emissions resulting from the use of an HF-333 diesel fuel (D2) with EGR to control NOx reaction kinetics and those from a new SunDiesel fuel derived from wood chips (made by the Choren Company in Germany using a process similar to Fischer-Tropsch). The SunDiesel fuel eliminates aromatics, which are soot precursors, and has a lower viscosity, which changes the fuel injection characteristics. However, it maintains a carbon/hydrogen ratio consistent with D2 fuel. Various injection timings were tested over the full range of engine speed and load. Results showed that SunDiesel fuel significantly reduced particulate matter, NOx, CO, and brake-specific fuel consumption relative to the D2 fuel.

The Visioscope, a commercial device made by the AVL Company, allowed direct observation of fuel injection and combustion kinetics in an engine cylinder through quartz window probes drilled, welded, and epoxyed into the cylinder head. The Visioscope results showed significant combustion differences between the D2 and SunDiesel fuels. The higher volatility of the SunDiesel resulted in a shorter time for mixing with air, which usually results in more soot formation, yet the engine emission of soot decreased. The Visioscope revealed that although soot formation was indeed higher during SunDiesel combustion, oxidation of the soot was more rapid because of the lower sulfur and aromatic content, and the chemical reactions dominated the fluid flow mechanisms. In the case of the D2 fuel, reduction of NOx by EGR resulted in higher soot formation. However, by adjusting fuel injection timing to increase the duration of combustion, more soot can be oxidized to keep emission levels down.

Future tests will explore the effect of diesel engine fuel injection parameters on emissions in more detail, and this type of advanced instrumentation will also be used to study the performance of other engines and other fuels including natural gas and hydrogen.

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Contact Steve Ciatti

August 3, 2005