Effects of multiple injections strategy on combustion process of mixed fuels with dimethyl carbonate and diesel

Abstract: In order to achieve the target of emission purification within the aspect of combustion optimizing in a cylinder, as well as the partial replacement of fossil fuels, a study on the combustion process of dimethyl carbonate (DMC)-diesel fuel blend was carried off. A fuel blend D10 (10% DMC and 90% diesel by volume) was chosen as the test fuel. The experiments were conducted on a single-cylinder research engine originated from a Daimler Benz OM646 2.2 litre common rail direct injection four-cylinder in-line diesel engine. As for the research engine, the indicated mean effective pressure pmi was adopted as the baseline for assessment of engine performance. A given load at 1900 r/min was chosen as the engine operating mode. According to the first law of thermodynamics, the heat release rate can be presented in real-time by the IndiCom software. The exact position in the crank angle of the center of heat release (COHR, 50% of the total heat release) can be figured out by interpolation. It was a key parameter to describe the combustion process of the engine. Through a fuel injection strategy characterized by an adjustable first pre-injection phase but constant phase intervals between the three injections and adjustable main injection duration but fixed durations of first pre-injection and second pre-injection, the accurate COHR target at a steady working mode can be implemented. Therefore five modes with an even interval of COHR, under the same engine speed and pmi, were inspected. As for D10 fuel, to compensate for its energy density falling, the rail pressure of D10 was 3 MPa higher than that of a diesel. While using D10 fuel, two schemes were considered. One was to keep the injection parameters of fuel system unchanged. The other was to slightly adjust the injection parameters, thus the COHR can be precisely accorded with the original diesel engine. When multiple injection strategy was adjusted to achieve the exact COHR which was delayed in constant step, the features of the combustion process of a diesel engine were analyzed. The ignition for each operating mode occurred between the second pilot injection and the main injection. For every two adjacent modes varying with COHR, the crank angle intervals of the fuel injection timings as well as center of heat release and the location of heat release peak were almost identical. With the increase of COHR, the duration in crank angle taken from injection timing to ignition became shorter, while the duration in the crank angle taken from ignition to the center of the heat release became longer. As compared with the location of a peak pressure in-cylinder, the COHR would be moved backward far away. As for D10 fuel, because the easy vaporization of dimethyl carbonate promoting the mixing of fuel and air and combustion, both crank angle intervals, from the ignition to 10% of heat release and from ignition to 90% of heat release, were shorter, which indicated that the added dimethyl carbonate could help to promote the combustion process. These analyses of combustion features based on COHR will provide a fundamental guidance for the application of dimethyl carbonate/diesel blends, multiple injections regulating the combustion process, and emission control theory.