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Typical DI diesel engines operate with fuel injection taking place within a range of about 30 crank angle degrees before top dead center, at the end of the compression stroke. When injection takes place far earlier, at the beginning of the compression stroke, another form of combustion occurs, which we termed PREmixed lean Diesel Combustion, or PREDIC. With PREDIC operation, self-ignition occurs near top dead center and NOx emissions are drastically lower. When ignition occurs, the fuel-air mixture is thought to be nearly homogeneous, with only slight heterogeneity. Appropriate fuel spray formation is very important for successful PREDIC operation. Using a single-zone NOx formation model, calculations showed that the mean excess air ratio in the PREDIC combustion zone was 1.87, which resulted in very low (20 ppm) NOx emissions. Conventional combustion at the same conditions resulted in a mean combustion zone excess air ratio of 0.88.

The effects of fuel injection velocity and ambient gas pressure on the spray formation and atomization process for a non-evaporating diesel spray were observed and analyzed with greatly magnified photographs illuminated by a pulsed ruby laser light sheet. Individual fuel droplets were distinguishable at the peripheral regions of the spray in these photographs. The spray width became narrower with an increase in injection velocity, and the spray spread out further with increase in ambient gas pressure. The branch-like structure in the spray originated from local high and low fuel particle number density regions and the difference in number density between these two regions increased with higher injection velocity. The ruby laser was double-pulsed to enable fuel particle velocity vectors to be characterized at the peripheral regions of the fuel spray. The vorticity scale was smaller and vorticity magnitude grew higher with increase of injection velocity.

A conventional single cylinder direct injection diesel engine was fitted with three fuel injectors: one mounted vertically on the center, and the others mounted diagonally from the side direction. With this system, it was possible to control the fuel injection timing and injection quantity of each injector independently. It was also possible to independently control the fuel injection pressure of the center and side injectors. Using this system, it was possible to control the spatial and temporal distributions of the fuel injected into the combustion chamber, which are impossible to obtain with conventional injection equipment. In this study, an improvement in particulates and specific fuel consumption was obtained, while maintaining low NOx, by injecting a small amount of fuel from the two side injectors after the main fuel injection from the center injector.

Two ways to attain low emission diesel combustion, which are capable of meeting future regulations, are the so-called two-stage “rich and lean” combustion and ‘lean” diesel combustion. To actually achieve these types of combustion, homogeneous lean air-fuel mixture formation is very important In this study, two methods of producing a desirable air-fuel mixture axe investigated experimentally by observing fuel sprays from several unique injection nozzles in a high-pressure vessel. One was a slit shaped hole nozzle, which might result in increased air entrainment into the spray because of the larger surface area. The other was impinging flow nozzle, which generated a more homogeneous mixture by its high turbulence. It was observed that with the slit shaped hole nozzle, the cross-sectional shape of the spray was unexpectedly circular, which was attributed to a greater dispersion of the spray perpendicular to the lengthwise slit axis.

The effects of a multi-hole nozzle with throttle construction (NTC) on combustion and emissions were investigated at high pressure fuel injection conditions. The throttle area was larger than the total injector hole area, therefore its fuel flow quantity was about the same as the standard nozzle under steady flow conditions. But the initial fuel injection rate was lower under unsteady flow conditions and smoke emissions were improved with the NTC. It is postulated that these effects were due to fuel flow turbulence inside the nozzle during the time of needle valve lift.

A heavy-duty, naturally aspirated diesel engine was converted into a turbocharged, aftercooled, compressed natural gas engine. Engine test results show that excess air ratio and ignition timing strongly affect NOx and THC emissions. Leaning the air-fuel mixture reduces NOx emission, but it increases THC emission and combustion becomes unstable above a certain excess air ratio. Retarding the ignition timing reduces both the NOx and THC emissions. Dual-plug ignition improves brake thermal efficiency. The NOx emission level can be reduced to meet the Japanese long-term emission regulation limit for heavy-duty gasoline engines with a sufficient safety margin by appropriately selecting the air-fuel ratio and ignition timing so as to keep the THC emission level below the regulation limit without using any after-treatment. The engine full torque characteristics were almost the same as the base engine throughout the engine speed range, while the maximum exhaust gas temperature was lower.

We studied the use of SiC porous material for diesel particulate filters. Crystallities of SiC grow into hexagonal plates during sintering,but their sizes have not been controllable yet. We developed a production process that makes the average pore diameter as desired in limiting the pore size distribution to a narrow range. Diesel particulate filters made of SiC greatly reduce pressure loss even when a large quantity of particulates accumulates as compared with conventional wall-flow type filter of cordierite of the same size. This enables particulate filter to be made smaller. The good thermal conductivity of SiC allows fast regeneration without a temperature spike, but its large coefficient of thermal expansion causes heat cracks during rig tests and engine bench tests. The rig tests showed that the heat cracking problem can be solved if the length and diameter are below a certain threshold for each.

The characteristics of a new diesel particulate filter material made of SiC were studied through engine tests in varying material properties, such as average pore diameter, and wall thickness. Compared to a conventional cordierite filter of the same size, particulate trapping efficiency is almost the same, and the pressure loss and the deterioration of fuel consumption can be reduced to about half with the optimum material properties. If the same pressure loss is allowed, the filter size can be reduced by 30%. Its good thermal conductivity prevents local temperature increases, which doubles the permissible amount of trapped particulates. As heat crack problems occurred in integral-type filters due to the high thermal expansion of SiC, a split-type filter having 49 filter segments with a square section was developed.

Several types of oxidation catalyst material are tested in repeated particulate emission measurements over the US HDD transient test procedure. Particulates are effectively reduced in the initial stage of the measurements. However, particulates tend to increase when repeating the measurements. This is believed to be caused by sulfate adsorption on the catalyst surfaces. Hence, oxidation catalysts are tested after stabilizing surface adsorption. Test results show that an oxidation catalyst which forms more sulfates is not effective in reducing particulates because the sulfate increase offsets the SOF reduction effect. An effective catalyst for particulate reduction is developed by suppressing sulfate formation.