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Diesel combustion and emissions with four kinds of oxygenated agents as main fuels were investigated. Significant improvements in smoke, particulate matter, NOx, THC, and thermal efficiency were simultaneously realized with the oxygenates, and engine noise was also remarkably reduced for the oxygenates with higher ignitability. The improvements in the exhaust emissions and the thermal efficiency depended almost entirely on the oxygen content in the fuels regardless of the oxygenate to diesel fuel blend ratios and type of oxygenate. The unburned THC emission and odor intensity under starting condition with an oxygenate were also much lower than with conventional diesel fuel.

Synthetic fuels (e-fuels) synthesized from H2 and CO by renewable electricity are expected as the next- generation diesel fuels and two types of e-fuels have received extensive attention: Fischer-Tropsch (FT) fuel and Oxymethylene dimethyl ether (OME). In this study the effects of OME blending ratios with 0 to 50 vol.% in FT fuels on combustion, emissions and spray characteristics in diesel engines are investigated. The results suggest that the OME blends to FT fuels suppressed the deterioration in combustion efficiency under low intake oxygen concentration conditions. The smoke emissions of FT fuels and OME blended fuels were both lower than those of diesel fuel and decreased with the increase in the OME blend ratio, and the soot-NOx trade-off relation in diesel engines can be improved.

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.

Diesel soot suppression effects of catalytic fuel additives for a range of fuels with different properties were investigated with calcium naphthenate. A single cylinder DI diesel engine and a thermobalance were used to determine the soot reduction and its mechanism for seven kinds of fuels. Experimental results showed that the catalytic effect of the fuel additive was different for the different fuels, and could be described by a parameter considering cetane number and kinematic viscosity. The fuel additives reduced soot more effectively for fuels with higher cetane number and lower kinematic viscosity. This result was explained by soot oxidation characteristics for the different fuels. Oxidation of soot with the metallic additive proceeds in two stages: stage I, a very rapid oxidation stage; and stage II, a following slow or ordinary oxidation stage.

Thermodynamic characteristics of premixed compression ignition combustions were clarified quantitatively by heat balance estimation. Heat balance was calculated from temperature, mole fractions of intake and exhaust gases, mass and properties of fuels. Heat balance estimation was conducted for three types of combustion; a conventional diesel combustion, a homogeneous charge compression ignition (HCCI) combustion; fuel is provided and mixed with air in an intake pipe in this case, and an extremely early injection type PREmixed lean DIesel Combustion (PREDIC). The results show that EGR should be applied for premixed compression ignition combustion to complete combustion at lower load conditions and to control ignition timing at higher load conditions. With an application of EGR, both HCCI and PREDIC showed low heat loss characteristics at lower load conditions up to 1/2 load.

Time series analysis of diesel exhaust gas emissions under transient operation was carried out using a uniquely developed gas sampling system to efficiently collect all exhaust gas throughout transient cycles. The effects of fuel properties and other engine operation parameters on the exhaust emissions under transient runs when fuel amounts abruptly increase were analyzed. The results showed that THC increased abruptly to 2 or 6 times the final steady-state concentration immediately after the start of acceleration and then decreased to the steady-state values after 70∼200 cycles. At acceleration, NOx increased abruptly to about 80 % of the final NOx concentration, and then increased gradually to reach the final values after 60∼500 cycles. The behaviors of THC and NOx during transient operation can be described by exponential functions of the elapsed cycle numbers and the final emission concentrations.

Time resolved changes in unburned hydrocarbon emissions and their components were investigated in a DI diesel engine with a specially developed gas sampling system and gas chromatography. The tested transient operations include starting and increasing loads. At start-up with high equivalence ratios the total hydrocarbon (THC) at first increased, and after a maximum gradually decreased to reach a steady state value. Reducing the equivalence ratio of the high fueling at start-up and shortening the high fueling duration are effective to reduce THC emissions as long as sufficient startability is maintained. Lower hydrocarbons, mainly C1-C8, were the dominant components of the THC and mainly determined the THC behavior in the transient operations while the proportion of hydrocarbon (HC) components did not significantly change. The unregulated toxic substances, 1,3 butadiene and benzene were detected in small quantities.

Diesel combustion and exhaust gas emissions under transient operation (when fuel amounts abruptly increased) were investigated under a wide range of operating conditions with a newly developed gas sampling system. The relation between gas emissions and piston wall temperatures was also investigated. The results indicated that after the start of acceleration NOx, THC and smoke showed transient behaviors before reaching the steady state condition. Of the three gases, THC was most affected by piston wall temperature; its concentration decreased as the wall temperature increased throughout the acceleration except immediately after the start of acceleration. The number of cycles, at which gas concentrations reach the steady-state value after the start of acceleration, were about 1.2 times the cycle constant of the piston wall temperature for THC, and 2.3 times for smoke.

Significant improvements in exhaust emissions and engine performance in an ordinary DI diesel engine were realized with highly oxygenated fuels. The smoke emissions decreased sharply and linearly with increases in oxygen content and entirely disappeared at an oxygen content of 38 wt-% even at stoichiometric conditions. The NOx, THC, and CO were almost all removed with a three-way catalyst under stoichiometric diesel combustion at both the higher and lower BMEP with the combination of EGR and a three-way catalyst. The engine output for the highly oxygenated fuels was significantly higher than that with the conventional diesel fuel due to the higher air utilization.

Ultra low emissions and high performance combustion was achieved with a combination of high EGR, a three-way catalyst, and a highly oxygenated liquid fuel, neat dimethoxy methane (DMM), in an ordinary DI diesel engine. The smokeless nature of neat DMM effectively allowed stoichiometric diesel combustion by controlling BMEP with EGR. NOx, THC, and CO emissions were reduced with a three-way catalyst. At lower BMEP with excess air, the EGR effectively reduced NOx. High-speed video in a bottom view type engine revealed that luminous flame decreased with increased fuel oxygen content and almost disappeared with DMM.

The volatile organic compounds (VOC) from diesel engines, including formaldehyde and benzene, are concerned and remain as unregulated harmful substances. The substances are positively correlated with THC emissions, but the VOC and aldehyde compounds at light load or idling conditions are more significant than THC. When coolant temperatures are low at light loads, there are notable increases in formaldehyde and acetaldehyde, and with lower coolant temperatures the increase in aldehydes is more significant than the increase in THC. When using ultra high EGR so that the intake oxygen content decreases below 10%, formaldehyde, acetaldehyde, benzene, and 1,3-butadiene increase significantly while smokeless and ultra low Nox combustion is possible.

Diesel-like combustion of an emulsified blend of water and diesel fuel in a constant volume chamber vessel was visualized with high speed color video, further analyzing with a 2-D two color method and shadowgraph images. When the temperature at the fuel injection is 900 K, here while the combustion with unblended diesel fuel in the vessel is similar to ordinary diesel combustion with diffusive combustion, combustion with the emulsified fuel is similar to premixed diesel combustion with a large premixed combustion and very little diffusive combustion. With the emulsified fuel the flame luminosity and temperature are lower, the luminous flame and high temperature regions are smaller, and the duration of the luminous flame is shorter than with diesel fuel. This is due to promotion of premixing with increases in the ignition delay and decreases in the combustion temperature with the water vaporization.

Low NOx and soot free premixed diesel combustion can be realized by increasing ignition delays in low oxygen atmospheres, as well as the combustion here also depends on fuel ignitability. In this report single intermittent spray combustion with primary reference fuels and a normal heptane-toluene blend fuel under several oxygen concentrations in a constant volume combustion vessel was analyzed with high-speed color video and pressure data. Temperature and KL factor distributions are displayed with a 2-D two-color method. The results show that premixing is promoted with a decrease in oxygen concentration, and the local high temperature regions, above 2200 K, as well as the duration of their appearance decreases with the oxygen concentration. With normal heptane, mild premixed diesel combustion can be realized at 15 vol% oxygen and there is little luminous flame.