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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.

This research investigates the influences of the injection timing, injection pressure, and compression ratio on the combustion and exhaust emissions in a single cylinder 1.0 L DI diesel engine operating with ultra-high EGR. Longer ignition delays due to either advancing or retarding the injection timing reduced the smoke emissions, but advancing the injection timing has the advantages of maintaining the thermal efficiency and preventing misfiring. Smokeless combustion is realized with an intake oxygen content of only 9-10% regardless of the injection pressure. Reduction in the compression ratio is effective to reduce the in-cylinder temperature and increase the ignition delay as well as to expand the smokeless combustion range in terms of EGR and IMEP. However, the thermal efficiency deteriorates with excessively low compression ratios.

As a research of low temperature regeneration of DPF, combustion mechanism of diesel particulate matter (PM) trapped in DPF was investigated. For the assumption of PM combustion mechanism, the relationship between PM combustion characteristics and the physical properties of PM particles was investigated by using thermal and spectroscopic analysis methods. Experimental PM samples were produced under typical engine operating conditions using three representative fuels, two commercial diesel fuels containing aromatics (JIS-2 and Class 1) and a paraffin fuel that was prepared in a gas-to-liquid (GTL) process and did not contain any aromatics. Based upon these characteristics and combustion test results of the PM samples, a mechanism of the PM combustion was assumed. And the crystallinity of PM particles and existence of some surface functional groups containing oxygen are thought to be the important factors to lower the temperature of PM combustion., independent of the fuel type.

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.

To clarify the microcrystal structure of soot particulate in the combustion chamber, we examined sampling methods which freeze the reaction of sample specimens from the combustion chamber and collected the soot particulates on microgrids. We investigated the microcrystal structure with a high resolution transmission electron microscope. The results were: the particle size distribution and the microcrystal structure of the soot particulates is little different for the cooled freezing method and room temperature sampling. The typical layer plane structure which characterizes graphite carbon is not observed in the exhaust of diesel engines, but some particulates display a somewhat similar layer plane structure. The structure of soot particulate is a turbostratic structure as the electron diffraction patterns show polycrystals. The soot particulates in the combustion chamber is similar to exhaust soot particulates.

Soot emission from diesel engines generally increases with shorter ignition lags. However, the detailed process and mechanism of this phenomenon has not been well understood. This investigation attempts to observe and analyze the in-chamber soot formation process at various ignition lags by high-speed photography of the direct flame images and laser shadowgraphs as well as the laser light extinction. In the experiment, the separation of soot concentration from the soot-fuel mixture concentration was established by subtracting the laser light extinction intensity through a non-firing chamber from that through a firing chamber. It was found that the soot concentration in the swirl chamber reached a maximum value immediately after the start of combustion, and then decreased rapidly. With shorter ignition lags, the maximum and final soot concentrations in the chamber increased.

In a DI diesel engine, THC emissions increase significantly with lower compression ratios, a low coolant temperature, or during the transient state. During the transient after a load increase, THC emissions are increased significantly to very high concentrations from just after the start of the load increase until around the 10th cycle, then rapidly decreased until the 20th cycle, before gradually decreasing to a steady state value after 1000 cycles. In the fully-warmed steady state operation with a compression ratio of 16 and diesel fuel, THC is reasonably low, but THC increases with lower coolant temperatures or during the transient period just after increasing the load. This THC increase is due to the formation of over-lean mixture with the longer ignition delay and also due to the fuel adhering to the combustion chamber walls. A low distillation temperature fuel such as normal heptane can eliminate the THC increase.

The thermal cracking and polyaromatic hydrocarbon (PAH) formation processes of dimethyl ether (DME), ethanol, and ethane were investigated with chemical kinetics to determine the soot formation mechanism of oxygenated fuels. The modeling analyzed three processes, an isothermal constant pressure condition, a temperature rising condition under a constant pressure, and an unsteady condition approximating diesel combustion. With the same mole number of oxygen atoms, the DME rich mixtures form much carbon monoxide and methane and very little non-methane HC and PAH, in comparison with ethanol or ethane mixtures. This suggests that the existence of the C-C bond promotes the formation of PAH and soot.

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.

The blending of dimethyl carbonate (DMC), which contains 53% of oxygen, in diesel fuel is very effective to suppress the formation of exhaust particulates, however, the mechanism of the suppression has not been made clear. In this study, the comparison on the performance of gas oil and DMC mixture was achieved. The effect of the oxygen in DMC molecule has to suppress the formation of particulates was monitored by way of using thermal cracking analyzer under various conditions.

Since there is a trade-off relationship between NOx and particulates in exhaust gas emitted from a diesel engine, simultaneous reduction of the amounts of NOx and particulates in a combustion chamber is difficult. However, the amount of particulates produced in the combustion process could be reduced in a state of almost complete combustion, and the amount of NOx produced during the combustion process could be reduced by the use of a catalyst and reducing agent in the exhaust process. It has been demonstrated that the use of ethanol as a reducing agent on a silver-base catalyst in the presence of oxygen is an effective means for reducing NOx, although the mechanism of the reduction has not been elucidated. Therefore, in the present study, an NOx-reduction apparatus was conducted, and model experiments on NOx reduction were carried out in an atmosphere simulating exhaust gas emitted from a diesel engine and at the same catalyst temperature as that in a combustion chamber.

The combustion characteristics in a partially premixed charge compression ignition (PCCI) engine with n-hexane were compared with ordinary diesel fuel to evaluate combustion improvements with lower distillation-temperature fuels. In the PCCI engine, a lean mixture was formed reasonably with early stage injection and the additional fuel was supplied with a second stage fuel injection after ignition. With n-hexane, thermal efficiency improved while simultaneously maintaining low NOx and smokeless combustion. A CFD analysis simulated the mixture formation processes and showed that the uniformity of the mixture with the first stage injection improves with lower distillation-temperature fuels.

Smokeless and ultra low NOx combustion without knocking in a dual-fuel diesel engine with induced LPG as the main fuel was established with a uniquely developed piston cavity divided by a lip in the sidewall. A small quantity of diesel fuel was directly injected at early compression stroke into the lower part of the cavity as an ignition source for this confined area, and this suppressed explosively rapid combustion just after ignition and spark-knock like combustion at later stage. A combination of the divided cavity, EGR, and intake air throttling was effective to simultaneously eliminate knocking, and reduce THC and NOx significantly.

The fundamental parameters related to engine combustion and performances, such as, heating value, theoretical air-fuel ratio, adiabatic flame temperature, carbon dioxide (CO2), and nitric oxide (NO) emissions, specific heat and engine thermal efficiency were investigated with computations for a wide range of oxygenated fuels. The computed results showed that almost all of the above combustion-related parameters are closely related to oxygen content in the fuels regardless of the kinds or chemical structures of oxygenated fuels. An interesting finding was that with the increase in oxygen content in the fuels NO emission decreased linearly, and the engine thermal efficiency was almost unchanged below oxygen content of 30 wt-% but gradually decreased above 30 wt-%.

Most countries consider it is harmful for humans to inhale SPM of fine organic particles and elemental carbon less than 2.5 μ in diameter1,2). It is generally believed that organic matters in SPM are mainly composed of diesel exhaust particulate and soot from residential chimneys or industrial smokestacks3,4). To determine the contribution ratios of several organic substances to SPM, we characterized SPM, diesel exhaust particulate (DEP), powdered summer radial tire, and bitumen, using high performance liquid chromatography, field desorption mass spectrometry and linear theory.

Light naphtha, which exhibits two-stage ignition, was induced from the intake manifold for ignition enhancement and a low ignitability fuel or water, which does not exhibit low temperature oxidation, was directly injected early in the compression stroke for ignition suppression in an HCCI engine. Their quantitative balance was flexibly controlled to optimize ignition timing according to operating condition. Ultra-low NOx and smokeless combustion without knocking or misfiring was realized over a wide operating range. Alcohols inhibit low temperature oxidation more strongly than other oxygenated or unoxygenated hydrocarbons, water, and hydrogen. Chemical kinetic modeling for methanol showed a reduction of OH radical concentration before the onset of low temperature oxidation, and this may be the main mechanism by which alcohols inhibit low temperature oxidation.

The influence of aromatic content in fuels on the soot and NOx emissions from a diesel engine was analyzed under controlled ignition lags with spark-assisted operation. Monocyclic aromatic hydrocarbons and n-hexane mixtures were used as fuels, and the aromatic content was varied from 0 to 75 v-%. The experiments showed that, at the same equivalence ratio and regardless of the molecular structure of the fuel, the soot concentration in the exhaust gas could be described by a linear-combination function with two variables representing the ignition lag and C/H atom-ratio of the fuels. For unchanged ignition lags, the soot emissions increased linearly with increased C/H atom-ratios, which are controlled by the aromatic content. The degree of increase in soot emissions with increasing C/H atom-ratio decreased with decreasing equivalence ratios. The NOx emission increased slightly with increases in the C/H atom-ratio and ignition lag.

Ultra-low NOx and smokeless operation at higher loads up to half of the rated torque is attempted with large ratios of cold EGR. NOx decreases below 6 ppm (0.05 g/(kW·h)) and soot significantly increases when first decreasing the oxygen concentration to 16% with cold EGR, but after peaking at 12-14% oxygen, soot then deceases sharply to essentially zero at 9-10% oxygen while maintaining ultra low NOx and regardless of fuel injection quantity. However, at higher loads, with the oxygen concentration below 9-10%, the air/fuel ratio has to be over-rich to exceed half of rated torque, and thermal efficiency, CO, and THC deteriorate significantly. As EGR rate increases, exhaust gas emissions and thermal efficiency vary with the intake oxygen content rather than with the excess air ratio.

Exhaust Particulate emitted from diesel engines is a serious problem form the point of view of the environment and energy saving. Exhaust particulate is consist of dry soot and SOF (soluble organic fraction). To clarify the formation process of SOF in the combustion chamber of diesel engines, first lower temperature column condensed method was investigated. The gas from combustion chamber was collected to the sampling column using this method, and the cracked as well as the condensation polymerized components were analyzed with gas chromatography. The sampling condition of the low temperature column condensation method are length of condensation column 600mm, cooling temperature 198K, and dilution ratio 5. The diesel fuel injected into the combustion chamber, first cracks into lower boiling point hydrocarbons, this is followed by dehydrogenation and formation of benzene ring compounds through condensation polymerization. This is followed by the formation of PAH.

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.