Impact of Biodiesel Blends on In-cylinder Soot Temperature and Concentrations in a Small-Bore Optical Diesel Engine 2012-01-1311

Biodiesel is a desirable alternative fuel for the diesel engine due to its low engine-out soot emission tendency. When blended with petroleum-based diesel fuels, soot emissions generally decrease in proportion to the volume fraction of biodiesel in the mixture. This paper presents an experimental investigation of biodiesel impact on in-cylinder soot temperature and concentrations in a single-cylinder, small-bore, optical access, compression ignition engine. While in-cylinder soot measurements have been widely performed with two-color thermometry implemented on digital cameras, their finite dynamic range limits the observation of soot due to its dramatically different radiation intensities. To expand the dynamic range of two-color measurements, this investigation utilizes three cameras. A high-speed CMOS color camera with a wide-band Bayer filter is used to obtain simultaneous measurements of soot temperature and KL factor for high intensity soot clouds within one cycle. Additionally, two intensified CCD cameras with one narrow band pass filter on each lens are simultaneously used to measure low intensity soot clouds.

The three-camera, two-color thermometry technique for determination of soot temperature and concentration is applied in an optical engine operated with ultra-low sulfur diesel (ULSD) and a blend of ULSD and biodiesel. Engine-out soot emissions were measured with a micro soot meter under steady skip-firing conditions. High-speed images show that right after the premixed combustion period, B20 combustion has a higher mean soot temperature which results in higher soot oxidation rates. High intensity soot clouds of B20 are also observed to have less homogeneity than B0 in terms of temperature and KL. The more localized soot sites of B20 combustion are believed to result from the fuel's lower volatility, thus greater local equivalence ratio in comparison to B0 combustion. Soot images taken by intensified CCD cameras showed consistent results with high-speed measurements indicating higher mean soot temperature for B20 than for B0 combustion. Higher soot temperatures in the late-cycle suggest higher oxidation rates for B20. It is notable that the observed normalized soot KL summation of B20 combustion is lower than for B0 combustion. These observations are consistent with lower engine-out soot emission with B20 fuel.

In-cylinder soot measurements made with the high-speed CMOS color camera only, intensified CCD cameras only, and combined three-camera setup are compared by taking soot images at the same crank-angle within the same cycle. In the early stage of combustion, measurements done with the high-speed CMOS camera underestimate the total net soot formation by 28.78% relative to the three-camera setup, and predict a lower mean soot temperature due to the omission of low intensity soot clouds. On the other hand, measurements done with two intensified CCD cameras only will overestimate the soot temperature and underestimate the summation of KL by 71.22% due to omission of high intensity soot clouds that saturate the cameras. The comparison between in-cylinder optical soot measurements with engine-out soot emission showed that the peak of in-cylinder soot concentration is one order of magnitude higher than the engine-out soot emission, while in the late-cycle, measurements done with intensified CCD cameras are at least two orders of magnitude lower than the engine-out soot measurements.