Yongjin Jung

Spray and combustion of waste cooking oil biodiesel in a compression-ignition engine

An experimental study was carried out to compare the spray and combustion characteristics of waste cooking oil biodiesel with those of conventional diesel in a common-rail direct-injection compression-ignition engine. The indicated mean effective pressure of approximately 0.7 MPa was tested under an engine speed of 800 r/min. The fuels were injected at an injection timing of −5 crank angle degree after top dead center with injection pressures of 80 and 160 MPa. The experiments were performed to investigate the combustion and emission characteristics in a metal engine, while spray and combustion flame characteristics in an optical engine. The morphological and compositional features of particulate matter were also analyzed by using transmission electron microscopy and thermogravimetric analysis. The direct imaging of the spray indicated a longer injection delay for waste cooking oil biodiesel. However, it exhibited longer liquid penetration length and narrower spray angle than diesel as the spray developed. Waste cooking oil biodiesel combustion showed a longer ignition delay with a slightly lower peak of in-cylinder pressure and heat release rate than diesel due to the deteriorated atomization and evaporation of fuel spray. In terms of emission, waste cooking oil biodiesel showed benefits in reduction of carbon monoxide, unburned hydrocarbon and particulate matter emissions. The particulate matter from waste cooking oil biodiesel was composed of smaller primary particles and more volatile organic fraction than diesel soot. The combustion flame imaging verified the retarded combustion phase of waste cooking oil biodiesel. Waste cooking oil biodiesel combustion showed lower flame luminosity accompanied by shorter visible flame duration, at both injection pressures. The flame spatial fluctuation and flame nonhomogeneity were also lower for waste cooking oil biodiesel.

Effect of injector configurations on combustion and emissions in a gasoline direct-injection compression ignition engine under low-load conditions

The effect of injector configurations on combustion and emissions is investigated at low-load conditions in a gasoline direct-injection compression ignition engine. A total of five injectors with higher number of nozzle holes (10, 14) and narrow injection angles (70°, 100°) were tested while comparing with the baseline injector with eight holes and injection angle of 146°. Diesel combustion was also performed at similar operating conditions for the comparison with gasoline combustion. First, at the idle operation (engine speed = 800 r/min, engine load = 10%), combustion stability was very low with baseline injector due to the excessive mixing process. The narrow injection angle was helpful to enhance combustion stability, attributed to suppressing the formation of over-lean mixture by the fuel stratification within narrow area. However, higher number of nozzle holes were unfavorable with frequent misfires by excessive fuel/air mixing. Second, at the low-load operation (engine speed = 1200 r/min, engine load = 25%), combustion and emissions were investigated according to the injection timing. For all injectors, partially premixed compression ignition combustion was realized with early injection which exhibited low nitrogen oxide emissions resulted from lean premixed combustion. However, high hydrocarbon and carbon monoxide emissions were produced with wide injection angle due to the over-mixing and trapped fuel in squish and crevice volume. Hydrocarbon and carbon monoxide emissions were significantly reduced with narrow injection angle, attributed to the fuel stratification inside piston bowl. On the other hand, hydrocarbon and smoke emissions were aggravated in diesel combustion with narrow injection angle, attributed to fuel-rich mixture resulted from the significant fuel impingement on piston bowl. Fuel impingement occurred for both gasoline and diesel; however, evaporation process was much slower for diesel fuel due to the lower volatility. Significant pool fire and rich combustion were observed for diesel combustion, while blue chemiluminescence was dominated in gasoline combustion which meant well-premixed combustion.

Premixed compression ignition combustion with various injector configurations in a heavy duty diesel engine

Premixed compression ignition combustion was implemented using early injection timing and exhaust gas recirculation in a direct injection single cylinder diesel engine and was evaluated with respect to the injector configurations. A baseline injector with an injection angle of 146° and eight nozzle holes showed premixed compression ignition combustion at the injection timing of 40° crank angle before top dead centre among three distinct combustion regimes. The burn duration in premixed compression ignition combustion was shortest among the regimes. Premixed compression ignition combustion at an injection timing of 40° crank angle before top dead centre was achieved at an exhaust gas recirculation rate ranging from 0% to approximately 40%. Two different injector configurations were applied to investigate the effect of injection angle and the number of nozzle holes on premixed compression ignition combustion: one had an injection angle of 70° and eight nozzle holes and the other had an injection angle of 70° and 14 nozzle holes. These two injectors could implement premixed compression ignition combustion as well as the baseline injector under the injection timing and exhaust gas recirculation conditions. In case of both injectors with an injection angle of 70°, the indicated mean effective pressures for 8 and 14 nozzle holes increased by 26% and 11%, respectively, because of the increased fuel participating in combustion and the reduced negative work in premixed compression ignition combustion. On the other hand, the injector with an injection angle of 70° and 14 nozzle holes showed the lowest levels of hydrocarbon, carbon monoxide and smoke emissions, which decreased by 82%, 92% and 81%, respectively, in premixed compression ignition combustion. However, the nitrogen oxides emission for the injectors with eight and 14 holes increased by 82% and 68%, respectively, in premixed compression ignition combustion. Natural luminosity from an in-cylinder visualization reveals pool fire of fuel films on the base of the piston bowl when both injectors had a narrow injection angle. For the injector with an injection angle of 70° and eight nozzle holes, a more vigorous pool fire at an exhaust gas recirculation rate of 0% is attributed to a larger amount of fuel film. At an exhaust gas recirculation rate of 40%, however, the portion of unburned fuel films increased the hydrocarbon and carbon monoxide emissions, and the rest of the diffusive pool fire can increase smoke emission surviving under a lack of oxygen. On the other hand, for the injector with an injection angle of 70° and 14 nozzle holes, the hydrocarbon and carbon monoxide emissions were maintained at lower levels due to less formation of fuel film and better air utilization.