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In order to utilize bio-alcohols as the fuel for diesel engines, combustion characteristics of alcohol blended with gas oil were compared between ethanol and n-butanol in a direct injection diesel engine. In the case of the same cetane number between ethanol and butanol blends, the time-history of combustion, in other words, the ignition delay, the diffusion combustion and the combustion duration, coincided almost completely in both blend fuels. However, the smoke density of the butanol blend was smaller than that of the ethanol blend. This result must be caused by difference in soot formation process between ethanol and butanol blends. Thus, it is difficult to predict the trend of the soot emission in combustion of alcohol blends only by using the existing phenomenological model of the soot formation in the combustion of gas oil.

In order to realize a premixed compression ignition (PCI) engine by utilization of bio-alcohol, combustion characteristics of bio-alcohol blended with gas oil were compared between ethanol and n-butanol in a diesel engine. The effects of the ethanol blend ratio and the butanol blend ratio on ignition delay, premixed combustion, diffusion combustion, fuel consumption and exhaust emissions such as smoke density and NOx were investigated experimentally. It is found that ethanol almost burns out together with combustion of low evaporation temperature composition of gas oil in the premixed combustion period and the heat release in the diffusion combustion is based on mainly high evaporation temperature composition of gas oil, then, soot is formed in the diffusion combustion of gas oil. On the other hand, a part of butanol burns in the diffusion combustion, however, the combustion of butanol in the diffusion stage is not the cause of soot formation.

Homogeneously charged natural gas was burned in a diesel engine assisted by a small amount of gas oil injection. The effects of the equivalence ratio of natural gas, the intake temperature and the EGR rate on ignition of gas oil, the burning rate of natural gas and the knock limit were clarified experimentally. As results, a marked improvement in the trade-off between smoke and NOx was obtained maintaining a high thermal efficiency by a combination between the natural gas charge rate, the EGR rate, the intake preheating and the nozzle orifice size suitable at each engine load.

In order to improve the trade-off between smoke and NOx in a DI diesel engine, the effect of premixed natural gas on combustion and exhaust emission has been investigated experimentally. The effects of the CNG charging rate and the EGR rate on ignition and combustion rate are mainly examined, in addition, the effects of CNG and EGR on NOx reduction was analyzed from the viewpoint of water content in the EGR gas. As a result, it is found that (1) the burning rate of natural gas pre-mixture is larger under the higher temperature condition, thus, the larger burning rate results in shortening the combustion duration and leads to lower fuel consumption, (2) ignition and the burning rate of natural gas are suppressed by EGR, (3) significant NOx reduction is obtained by charging natural gas under the low load condition, and (4) the NOx reduction rate due to EGR is about twice of the one due to the port water injection.

In order to clarify the characteristics of turbulence in diesel combustion, fluctuations of pressure and temperature were measured in a DI diesel engine, and the root mean square value, the auto-correlation coefficient and the power spectrum density of the measured fluctuations were analyzed comparing between the cases with and without pilot injection. The following concluding remarks are obtained. (1) The frequency power spectrum of in-cylinder pressure history is decreased by pilot injection in two frequency ranges from 0.2 to 2.0 kHz and from 2 to 5 kHz. (2) Fluctuation of combustion pressure is dependent on the maximum rate of pressure rise, which is dependent on ignition delay. (3) The maximum RMS of soot temperature fluctuation in the diffusion combustion is dependent on the maximum RMS of pressure fluctuation in the initial combustion. (4) Fluctuation of temperature during diffusion combustion period has the characteristics of isotropic turbulence.

The effect of port water injection on NOx formation was examined theoretically as well as experimentally. In the experiment, water was injected into each suction port of a four cylinder turbocharged DI diesel engine using gasoline injectors. The exhaust NOx was reduced significantly by port water injection, and about 50 % reduction in NOx concentration was attained under various engine operation conditions by injecting the amount of water of 0.03 kg per unit kg of dry air. Comparing the experimental results and the analytical ones based on the two-zone model developed by the authors, it is shown that the NOx reduction rate due to port water injection is dependent on the equivalent absolute humidity of the injected water and is independent on engine load, fuel injection timing and water injection timing.

For reducing the exhaust emissions and improving the ignition characteristics, the effect of pilot injection was investigated experimentally in a turbocharged direct injection diesel engine. The pilot injection quantity was varied by changing the seat diameter of the Dodge plunger installed in the newly developed pilot injector while the separation period between the beginning of pilot injection and that of main injection was fixed at a short interval in the present experiment. The pilot injection effect on combustion was compared with the case of normal injection in two fuel oils with the cetane indexes of 55 and 40 respectively.

The effect of water in the suction air and in the emulsified fuel on NO formation was analyzed experimentally and theoretically from the point of view of changes in specific heat and the entrained air rate of the burned gas in the combustion chamber. The specific heat was calculated accurately through the chemical equilibrium composition analysis considering the absolute humidity of the suction air, the amount of water added in the emulsified fuel as well as the residual gas fraction. In order to estimate the NO formation rate, the burned gas temperature which is dominated by the excess air ratio and the specific heat of the burned gas was calculated by using the two-zone combustion model. The effect of absolute humidity of the suction air on NO formation is so large that a change in absolute humidity of 0.01 kg/kg results in about 20% reduction of NOx. This large reduction is based on only about 1% change in specific heat of the burned zone gas.

Cylinder pressures at several locations and the radiant heat emission were measured simultaneously in order to better understand the combustion behavior in the combustion chamber of a high speed DI diesel engine. The radiant measurements utilized an optical fiber thermometer (OFT) with a light pipe sensor. The exhaust emissions of NOx, smoke and particulates were also measured. The formation of NOx and soot and the soot burn-up processes were analyzed applying an in-cylinder two-zone model. The results were compared with the experiment. It was confirmed that diffusion burning stage started close to the first peak of the heat release rate curve. The OFT output was closely related to the diffusion combustion process. The measured exhaust smoke correlated well with the cumulative diffusion combustion quantity. The dry soot in the particulate emission also correlated well with the Bosch smoke density except for the cases of high sulfur fuel and water-emulsified fuel.

A new method named the modified correlation method with slotting (MCS) has been proposed to analyze the turbulence scale accurately from the random discrete data with non-uniform time intervals, usually obtained by LDV measurements. The reliability of the method was confirmed by the model data. By applying the method, the authors have analyzed the air flow and the turbulence in a combustion chamber of a motored diesel engine under the conditions of three different intensities of suction swirl and also around the fuel spray injected intermittently under atmospheric condition. The main feature of turbulence confirmed were as follows. The integral time scale increased once at the beginning of the compression stroke and then decreased toward the compression top dead center.