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Low NOx combustion is possible by PREDIC (PREmixed lean DIesel Combustion) in which fuel is injected at a very early stage of the compression stroke and the combustion starts at near the top dead center by self-ignition. To simplify the phenomenon of the PREDIC process, the test engine was operated with gaseous fuels added to intake air to realize combustion of a perfectly homogeneous mixture. The rich limit was observed around λ=2.0∼2.4. This limit was determined by considering the increase in NOx, and the steep pressure rise. During high load operations is not only the ignition timing but also the combustion rate should be controlled. By comparing the homogeneous charge and direct injection case, the mixture heterogeneity could be found to have an influence on the ignition timing and combustion rate, the engine speed and injection timing also had an influenced on these.

Previous research in our laboratory has shown that NOx emissions can be sharply reduced by PREDIC (PRE-mixed lean DIesel Combustion), in which fuel is injected very early in the compression process. However some points of concern remained unsolved, such as a large increase in THC and CO, higher fuel consumption, and an operating region narrowly limited to partial loads, compared to conventional diesel operation. In this paper, the causes of PREDIC's problem areas were analyzed through engine performance tests and combustion observation with a single cylinder engine, through fuel spray observation with a high-pressure vessel, and through numerical modeling. Subsequently, measurable improvements were achieved on the basis of these analyses. As a result, the ignition and combustion processes were clarified in terms of PREDIC fuel-air mixture formation. Thus, THC and CO emissions could be decreased by adopting a pintle type injection nozzle, or a reduced top-land-crevice piston.

A new diesel combustion concept termed MULDIC (MUL-tiple stage DIesel Combustion), which can reduce NOx emissions at high load conditions, was studied by means of engine tests, combustion observation, and numerical simulation. In MULDIC, the first stage combustion corresponds to premixed lean combustion, and the second stage combustion corresponds to diffusion combustion under high temperature and low oxygen conditions. The engine tests showed that simultaneous reduction of NOx and smoke could be obtained with MULDIC operation, even at an excess air ratio of 1.4. Fuel consumption was higher compared to conventional operation because of premature ignition of the first stage combustion and extremely late second stage injection. However, optimization of the first stage combustion increased the degree of constant volume combustion, and hence the thermal efficiency was increased.

A study of the mixing mechanism of fuel with surrounding air is necessary in order to clarify the combustion process. In this study, the flow field near non-evaporating diesel spray as well as spray surface were observed and analyzed using a Nd-YAG laser light sheet. A single shot fuel spray was injected into a high pressure vessel and photographed under double-pulse laser illumination. The images of dispersed particles in the vessel were processed and velocity vectors were obtained by the auto-correlation method. Measured results showed temporal variation in the air movement around the spray. Just after the start of injection, air near the nozzle was pushed outward by the spray tip, after which the flow direction reversed. The air velocity ahead of spray tip was very low compared to fuel spray tip velocity. At a stable injection condition, air near the nozzle tip was pulled by the spray movement and flowed uniformly, and the spray-air boundary was smooth.

Previous research in our laboratory has shown that NOx emissions can be sharply reduced by PREDIC (PRE-mixed lean DIesel Combustion), in which fuel is injected very early in the compression process. However some problems still remain, such as higher fuel consumption, a lack of ignition timing control, and a large increase in THC and CO, compared to conventional diesel combustion. Appropriate mixture formation is necessary to solve these problems. In this paper, the influence of mixture formation on PREDIC was investigated. It was found that the pintle type injection nozzle was shown to be suitable for PREDIC, because it produced a comparatively uniform mixture in the combustion chamber and avoided collision of the fuel spray with the cylinder liner. Modeling by the KIVA-II software package was carried out to improve our understanding of the mixture formation process.

This paper presents the results of engine experiments and spray observations on a VCO nozzle. Two types of VCO nozzles having different hole shapes were investigated. One had a straight step hole (the VCO-S) and the other had a tapered step hole (the VCO-T). Both VCO nozzles could greatly reduce HC emissions in comparison to a standard nozzle. The VCO-S nozzle could reduce NOx emissions more than the VCO-T nozzle, and its spray penetration was shorter than that of the VCO-T.

Ammonia is one of the most useful compounds that react with NOx selectively on a catalyst, such as V2O5-TiO2, under oxygen containing exhaust gas. However ammonia cannot be stored because of its toxicity for the small power generator in populated areas or for the diesel vehicles. A new concept for NOx reduction in diesel engine using ammonia is introduced. This system is constructed from the hydrogen generator by fuel reformer, the ammonia synthesizer, SCR catalyst for NOx reduction and the gas injection system of reformed gas into the cylinder. Experimental results show that, the SCR catalyst provides a very high rate of NOx reduction, reformed gas injection into cylinder is very effective for particulate reduction. WHEN CONSIDERING INTERNAL COMBUSTION ENGINES of the 1990's the question of how to harmonize the engine with the natural environments is one of the greatest problems. The internal combustion engine changes a substance into energy via its explosive combustion.

Several methods to reduce ignition delay period were tested in combination with a high pressure injection and effects on combustion improvement were examined. It was found that the reduction of ignition delay does not give so much improvement at the usual injection timing before TDC, but when the injection timing is considerably retarded or when the original ignition delay is relatively long, shortening of the ignition delay is effective to reduce pre-mixed combustion and NOx emission. Further, assuming the combustion system which conforms to the 1983 Japanese regulation as the reference system, it was found that the combination of pilot injection and high injection pressure, simultaneously reduces NOx by approximately 35% and smoke by 60-80% without worsening the fuel economy.

A new model to describe diesel combustion process has been developed. In this model diesel combustion field is divided into two zones, premixing and combustion. Turbulent mixing process is described by the stochastic approach in each zone separately. Comparison of calculations with experimental results showed that this model can predict the entire course of heat release and nitrogen-oxide formation precisely, under wide-spread conditions. Two-dimensional flame temperature distributions in the combustion field by the two color method were compared with simulation results. Both the measured and the calculated flame temperature distributions showed good agreements with each other. In the diesel combustion process, the injected fuel mixes with air entrained inside the spray. The mixture is thus formed, and ignites at several points. Random expansion of flamelets accelerates both mixing and combustion. Following this, fairly moderate diffusion combustion proceeds.

Two dimensional flame temperature distributions in D.I. diesel engine with high pressure fuel injection were measured by the image analysis of high speed photographs based on two color method. Effects of injection pressure and nozzle hole diameter on flame temperature distribution were examined. The flame temperature in the case of high pressure injection is higher than that in low injection pressure. The higher flame temperature in high pressure injection results from the rapid compression of burned gases. The KL value which is an index of soot density in the combustion chamber decreases as injection pressure increases. The higher oxidation rate of soot at the later period of combustion may contribute to a soot reduction in the case of high pressure injection.

To clarify the detailed structure of high pressure fuel spray, 2-D sectional images of non-evaporating fuel sprays in a high pressure vessel were observed by using the laser light sheet of a copper vapor laser. By this system, many sectional and continuous photographs of the same spray were obtained, and were very effective for the detailed observation of the spray inner structure and its developing process. The spray inner structure was very complicated, and its fuel density distribution was very heterogeneous. And for its developing process, the spray advances straight immediately after injected, then meanders, and deforms into a branch-like structure. Advancing downstream, these branches distribute complicatedly and heterogeneously with low density droplets. The heterogeneity is owing to these branches. And, the developing process is divided into four regions. Further, the effects of some parameters on this process were investigated.

It is well known that increasing the fuel injection pressure is effective for improving the diesel engine combustion. While studying the characteristics of the high pressure fuel spray which is injected in a high pressure vessel, the authors found weak shock waves generating around the fuel spray. To investigate the shock waves effect on the fuel spray the authors measured the propagation speed and pressure amplitude as functions of the injection pressure and ambient pressure. The results indicate that shock waves are generated when the fuel injection speed exceeds the ambient sonic speed. Also it was found that the pressure amplitude of shock wave is approximately 10 % of the ambient pressure and the shock waves spread at a sonic speed. The above results make us think that, isn't it possible to use shock waves for combustion improvement.

The relation between the characteristics of a non-evaporating spray and those of a corresponding frame achieved in a rapid compression machine was investigated experimentally. The fuel injection pressure was changed in a range of 55 to 260 MPa and the other injection parameters such as orifice diameter and injection duration were changed systematically. The characteristics of the non-evaporating spray such as the Sauter mean diameter and the mean excess air ratio of the spray were measured by an image analysis technique. The time required for a pressure rise due to combustion was taken as an index to characterize the flame. It was concluded that the mean excess air ratio of a spray is the major factor which controls the burning rate and that the high injection pressure is effective in shortening the combustion duration and reducing soot formation.

This paper reports on research works of ACE towards the most appropriate injection and combustion system for heavy-duty direct injection diesel engines. Selected items for the study are the effect of nozzle hole diameter, injection rate pattern, swirl ratio, and supercharging under high pressure fuel injection. According to those experimental results, the combination of over 150MPa injection pressure with controlled injection rate, smaller nozzle hole diameter, and quiescent combustion systems shows the best performance and emission. The mechanisms of the combustion improvement are discussed from the turbulent mixing viewpoint, including the results of combustion observation.