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The effects of combining premixed, low temperature combustion (LTC) with biodiesel are relatively unknown to this point. This mode allows simultaneously low soot and NOx emissions by using high rates of EGR and increasing ignition delay. This paper compares engine performance and emissions of neat, soy-based methyl ester biodiesel (B100), B20, B50, pure ultra low sulfur diesel (ULSD) and a Swedish, low aromatic diesel in a multi-cylinder diesel engine operating in a late-injection premixed LTC mode. Using heat release analysis, the progression of LTC combustion was explored by comparing fuel mass fraction burned. B100 had a comparatively long ignition delay compared with Swedish diesel when measured by start of ignition (SOI) to 10% fuel mass fraction burned (CA10). Differences were not as apparent when measured by SOI to start of combustion (SOC) even though their cetane numbers are comparable.

Chemiluminescence imaging has been applied to a parametric investigation of diesel autoignition. Time-resolved images of the natural light emission were made in an optically accessible DI diesel engine of the heavy-duty size class using an intensified CCD video camera. Measurements were obtained at a base operating condition, corresponding to a motored TDC temperature and density of 992 K and 16.6 kg/m3, and for TDC temperatures and densities above and below these values. Data were taken with a 42.5 cetane number blend of the diesel reference fuels for all conditions, and measurements were also made with no. 2 diesel fuel (D2) at the base condition. For each condition, temporal sequences of images were acquired from the time of first detectable chemiluminescence up through fully sooting combustion, and the images were analyzed to obtain quantitative measurements of the average emission intensity.

Premixed diesel combustion is intended to supplant conventional combustion in the light to mid load range. This paper demonstrates the operating load limits, limiting criteria, and load-based emissions behavior of a direct-injection, diesel-fueled, premixed combustion mode across a range of test fuels. Testing was conducted on a modern single-cylinder engine fueled with a range of ultra-low sulfur fuels with cetane number ranging from 42 to 53. Operating limits were defined on the basis of emissions, noise, and combustion stability. The emissions behavior and operating limits of the tested premixed combustion mode are independent of fuel cetane number. Combustion stability, along with CO and HC emissions levels, dictate the light load limit. The high load limit is solely dictated by equivalence ratio: high PM, CO, and HC emissions result as overall equivalence ratio approaches stoichiometric.

A program to explore the effects of natural and additive-derived cetane on various aspects of diesel performance and combustion has been carried out. Procedures have been developed to measure diesel engine fuel consumption and power to a high degree of precision. These methods have been used to measure fuel consumption and power in three heavy-duty direct-injection diesel engines. The fuel matrix consisted of three commercial fuels of cetane number (CN) of 40-42, the same fuels raised to CN 48-50 with a cetane improver additive, and three commercial fuels of base CN 47-50. The engines came from three different U.S. manufacturers and were of three different model years and emissions configurations. Both fuel economy and power were found to be significantly higher for the cetane-improved fuels than for the naturally high cetane fuels. These performance advantages derive mainly from the higher volumetric heat content inherent to the cetane-improved fuels.

A combination of optical imaging diagnostics has been applied to the fuel jet of a direct-injection diesel engine to study the ignition and early soot formation processes. Measurements were made in an optically accessible direct-injection diesel engine of the “heavy-duty” size class at a representative medium speed (1200 rpm) operating condition. Two fuels were used, a 42.5 cetane number mixture of the diesel reference fuels and a new low-sooting fuel (needed to reduce optical attenuation at later crank angles) that closely matches both the cetane number and boiling point of the reference fuel mixture. The combustion and soot formation processes are found to be almost identical for both fuels. Ignition and early combustion were studied by imaging the natural chemiluminescence using a calibrated intensified video camera. The early soot development was investigated via luminosity imaging and simultaneous planar imaging of laser-induced incandescence (LII) and elastic scattering.