Experiment of emissions characteristics for double swirl combustion system in diesel engine

Abstract: Due to the growing importance of future emission restrictions, diesel engines are continuously forced to improve the combustion processes in order to reduce the raw emissions, especially for the emitted particulate matters (PM) and nitrogen oxidants (NOx). For the combustion formation and emission characteristics of diesel, fuel-air mixing plays a controlling role and one of the factors that affect the fuel-air mixing is the design of the combustion chamber. Therefore, a serious of investigations concerned on the geometric shape of combustion chamber have been carried out and some of them have shown well emission characteristics. Double Swirl Combustion System (DSCS) has been reported to have a well performance on fuel conservation and combustion reaction in past literatures, but its performance on emission has not studied yet. It can be hypothesized that fuel spray can collide with the circular ridge, splits into two parts, and then forms double swirls (inner-swirl and outer-swirl), which makes the core of the single spray to meet air directly. Hence, the fuel will be distributed more evenly and the fuel-air mixing and burning progress can be improved for achieving better usage of the air in the whole chamber, It is hypothesized that the generation of soot (main section of PM) can be reduced effectively duo to this progress. To verify the hypothesis, the emission characteristics of diesel engine adopted DS chamber and the original chamber were tested in present paper. The engine tests were based on a 132-mm single-cylinders diesel engine, the corresponding stroke length is 145 mm, connecting rod length is 262 mm, and the maximum engine speed is 2500 r/min. The PM level in the exhaust gas was measured with a MAHA MPM-4 PM analyzer, the readings of which are provided as PM concentration in mg/m3, whose accuracy within ±0.01 mg/m3. The NOx concentration in ppm (parts per million, by vol.) in the exhaust was measured with a HORIBA MEXA-720 NOx analyzer with the accuracy within ±1×10-6. The test was conducted at 1300 r/min (full load), 1600 r/min (25% load, 50% load, 75% load, full load), 1900 r/min (full load) and 2100 r/min (rated speed, full load). The 1300 r/min, 1600 r/min, 1900 r/min correspond to the engine speed A, B, C ruled in the European Stationary Cycle (ESC) test. The weight of each engine speed (A, B, C) in the ESC test are 23, 39, 23, so different loads of engine speed B (1600 /min) were tested in our research to study the emission characteristic of the test engine. The emitted PM, NOx, brake specific fuel consumption and cylinder pressure had been measured. Meanwhile, the rate of heat release and mean cylinder temperature had been analyzed in both original diesel engine and the diesel engine matched with DSCS. The test results (full load) show that the PM emitted by the engine matched with DSCS is significantly lower than the original, with the reduction being higher at high engine speed in the range of 7%~47%. While the emitted NOx of the engine matched with DSCS is higher than the original. The test results (25% load, 50% load, 75% load, full load at 1600 r/min) indicated that the emitted PM and NOx differ slightly at lower load, and the reason can be explained as: the penetration of the fuel spray is short due to the reductive fuel supply and the fuel can hardly touch the ridge of the DS chamber, so the fuel were combusted unrestrained in both DS chamber and the original chamber which result a similar combustion process and emission characteristic. Besides, the penetration would become longer with the increase of fuel supply as the load increased and the fuel spray can collide with the ridge adequately via which the emitted PM can be significantly reduced.