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The present study aims to obtain a strategy for optimizing the combination of injection conditions and combustion chamber geometry to achieve low carbon monoxide (CO), nitrogen oxides (NOx) and smoke emissions with high thermal efficiency at low loads in direct-injection premixed charge compression ignition (DI-PCCI) operation in a diesel engine. To this end, experiments were performed using a naturally-aspirated single-cylinder DI diesel engine equipped with a common-rail injection system and a cooled exhaust gas recirculation (EGR) system under various injection conditions, including injection timing, injection angle and injection quantity, and combustion chamber geometry. The results indicate that CO emission was reduced at injection timings that provide high peak heat release rates. To improve the NOx-CO trade-off relation, the spray angle should be properly selected depending on the combustion chamber geometry.

Formation of nitrogen oxides (NOx) in direct-injection premixed charge compression ignition (DI-PCCI) combustion simulated in a constant volume vessel was investigated using an ignition-combustion model that combines a stochastic mixing model with a reduced chemical reaction scheme. Several improvements were made to the model in order to predict the combustion processes in DI-PCCI. Calculations were carried out for the injection and ambient conditions equivalent to the measurements using the constant volume vessel. Analysis of the calculated results clarified the effects of mixture heterogeneity on NO concentrations and the mechanisms are discussed. The results show that the model successfully represents the experimental tendency for NO concentration when the injection conditions and ambient oxygen mole fraction are varied.

The effects of fuel injection conditions (injection pressure, nozzle orifice diameter and fuel injection quantity) on NOx formation in direct-injection Premixed Charge Compression Ignition (DI-PCCI) combustion were investigated using a constant-volume vessel and a total gas-sampling device. The results show that promotion of fuel-air mixing reduces final NOx mass accompanying a delayed hot flame. In particular, under low oxygen mole fraction conditions, in addition to the hot flame delay, the promotion of fuel-air mixing results in a lower heat release rate. In this case, the final NOx mass is further reduced. For a fixed nozzle orifice diameter, the final NOx mass is reduced with increasing injection pressure. This effect is remarkable for smaller nozzle orifice diameters. Regardless of the oxygen mole fraction, under the low injection fuel quantity condition, enhancement of fuel-air mixing reduces the final NOx mass per released heat.

Utilizing the desirable feature of hydrogen, this study demonstrates the improvement of engine performance and exhaust emissions due to the mixing of hydrogen into natural-gas fuel in a spark-ignition engine at the wide-open throttle (WOT) condition. Both hydrogen and natural-gas fuels were injected into the intake port only in the suction flow, which could make the operation under a wide range of conditions without backfire even at a hydrogen fuel. Based on the measured processes of combustion, the knock characteristics were discussed with special attention to the extremely high burning velocity of hydrogen. At a higher compression ratio, the thermal efficiency in the stoichiometric condition was improved, nevertheless a precise control of ignition timing was required to suppress a hard knock. From the experimental results of engine performance in a variety of parameters, optimal use of hydrogen was exhibited for different engine loads.

The Premixed Charge Compression Ignition (PCCI) natural-gas engine has been investigated extensively as a power source for stationary applications due to its potential for high thermal efficiency and very low NOx emissions. However, methane, which is a major component of natural gas, has a high auto-ignition temperature. Stable ignition of natural gas in PCCI engines can be achieved by high compression ratio, intake air heating, internal EGR and various other techniques. Although each of the above-mentioned methods shows positive effects, to some extent, on engine performance and emissions, the literature indicates that stable operation of the PCCI natural gas engine would require a combination of various techniques, which reveals the need for further investigation. The goal of the present study is to control the PCCI natural gas ignition and combustion by ozone addition into the intake air.

Aim of this study is to clarify the NOx formation mechanism in diesel combustion under high-supercharged condition. Effects of ambient conditions and fuel injection parameters on diesel combustion were investigated using a constant volume chamber. NOx formation process was investigated using a total gas-sampling device. The results indicate that by using the above experimental setup it is possible to realize entirely diffusion combustion like what seen in the highly supercharged condition. Increasing ambient pressure up to 8MPa with high injection pressure shortens the ignition delay and offers a heat release rate proportional to the fuel injection rate with a short combustion duration. Increasing ambient pressure gives a higher NOx formation rate and final NOx concentration. This is due to enhancement in the fuel-air mixing which promotes the heat release.

The combustion process in high-speed direct-injection diesel engines is characterized by random turbulent mixing between turbulent eddies having different fuel concentrations. Nitric oxide and soot are formed in hot eddies and fuel-rich eddies. In the present study, the authors elucidate the diesel combustion process, from the viewpoint of such heterogeneity and turbulent mixing, by analysis of high-speed flame photographs. Based on this study the following points are suggested: jet-like flames are formed just after ignition but soon disintegrate into random turbulent flamelets as each flame quickly expands. In the middle and later stages of combustion, uniform and isotropic turbulent motions prevail over the entire space, gradually decaying with time. Such turbulent motions favor the destruction of fuel concentration heterogeneity. Gas expansion due to combustion enhances such random motions, and the swirl prevents their early decay in every burning stage.

To improve performance and exhaust emissions of a converted dual-fuel natural-gas engine, the effects of basic parameters were experimentally investigated. The results show that diesel fuel operation is favorable at very low loads and that a small amount of pilot fuel with a moderate injection rate is effective for suppressing knock at high loads. As for the charge air throttling, there is an optimal combination of charge amount and equivalence ratio to obtain high thermal efficiency and reduced emissions. An optimal strategy for fueling is demonstrated based on the results. Adequate control of pilot fuel amount, injection timing and throttle opening area gives diesel-equivalent thermal efficiency with very low smoke emission over a wide range of loads.

To develop the reliable and practical method for efficiently collecting diesel particulates and for its regeneration, novel approach with a pellet-packed bed was proposed. This trap has the labyrinths of the flow passage which favors inertial impaction to ceramic pellets, ensuring the required trap efficiency with a longer lifetime prior to clogging as long as the appropriate pellet diameter and trap size are selected. From the experimental results on a single-cylinder test engine, it is shown that the pellet-packed bed has smaller pressure loss during engine operation compared to the monolith filter. Also, the regeneration test was made using a peculiar system by pellet circulation in place of conventional oxidation of particulates. Based on the results, the feasibility of the pellet-packed bed for diesel particulate trap was demonstrated.

In an attempt to grab potential issues with a hydrogen direct injection lean burn engine to have similar power output to a gasoline-fuelled engine, emission characteristics of a hydrogen engine was investigated. It is demonstrated that low NOx emission can be achievable without any catalytic converter. Two major issues, however, have been recognized, that is, combustion instability at low load conditions and too low temperature of exhaust gas to get enough boosting pressure. Hydrogen concentration heterogeneous of the mixture was focused in the CFD and visualization study. Hydrogen jet design of an injector could contribute to improvement of mixing.