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As a method for reducing exhaust emissions from diesel engines, we have experimented on a homogeneous charge diesel combustion technique (HCDC) whereby a portion of fuel is supplied into the intake port to form a homogeneous premixture, this is then fed into the cylinder from the intake port before ignition of the diesel fuel, which is injected directly into the cylinder. Our results have indicated possibilities of substantially reducing both NOx and smoke emissions. If diesel fuel is premixed with air, the premixture under-goes excessively early self-ignition, making it difficult to maintain ignition timing near top dead center and hence limiting the engine operating conditions. While an important target in emission reduction is to realize stable low-emission combustion during a high-load operation, the actual operation of diesel engines mostly involves partial-load conditions.

Under heavy load condition, single fuel operation with diesel fuel was studied experimentally for the homogeneous charge diesel combustion (HCDC) method. HCDC engine, in which pre-mixture was formed by fuel injected into an intake manifold and mixed with air beforehand then ignited by small amount of fuel directly injected into a cylinder, can reduce NOx and smoke simultaneously from the diesel engine. In HCDC the higher the premixed fuel ratio was, the lower the emissions were. Accordingly, it was indicated that homogeneous pre-mixture contributed to improvement of exhaust emissions. However, a diesel knocking due to uncontrolled self-ignition may occur under high premixed fuel ratio conditions in the case of operating heavy loads. Thus, the maximum amount of premixed fuel was restricted by these knocking limits.

A homogeneous Charge Compression Ignition Diesel Combustion (HCDC) system has been experimentally studied for it's effect on exhaust purification of diesel engines. In this system, most fuel is injected into the intake manifold to form homogeneous pre-mixture in the combustion chamber beforehand and the pre-mixture is ignited with a small amount of fuel directly injected into the cylinder by a conventional injection system. Because this system performs homogeneous lean-burn, it can realize low emission which cannot be realized by conventional diesel engines without impairing ignition controllability in the operations ranging from idle to full load. In particular, although the operating regions were strictly limited, extremely low Nox emission levels of as low as 10 to 40 ppm were realized with maintaining low smoke emissions, when the ratio of pre-mixed fuel was increased up to approx. 98%.

An experimental and a numerical analysis wereconducted based on the concept of homogeneous charge diesel combustion (HCDC), in which most of the fuel is supplied for pre-mixed homogeneous charge which is compressed in the cylinder and then ignited by small amount of diesel fuel directly injected into a cylinder. At the previous report, It was indicated that simultaneous improvement of NOx and smoke were possible. Especially under a certain condition, NOx was extremely reduced. This report describes the preliminary analysis for the cause of this emission improvement with HCDC method. As result, direct optical observation of the combustion phenomena and numerical analysis using KIVA2 code suggested that low NOx combustion may be caused by lowered combustion temperature and reduced combustion period due to the uniform lean combustion.

It is well known that during engine cold-start, methanol fueled vehicles have a tendency to emit significant amount of unburnt methanol and formaldehyde, which is an oxidant of methanol The emission behavior and reduction methods of these components are studied in this paper The reduction rate of these unburnt components exceeds 99% when the temperature of a catalyst is enough high However during engine cold-start the oxidative reaction can not begin, and it takes several minutes to warm up the catalyst After the temperature of the catalyst reaches to the light-off temperature it rises steeply and high reduction rates of these components are obtained at the same time Therefore, the catalyst temperature must be raised quickly and effectively in order to realize the proper oxidative reduction of unburnt methanol and formaldehyde emissions during engine cold-start Consequently the effectiveness of installing pre-catalysts was examined in this study Some pre-catalysts (200cm3/piece) were placed after the exhaust manifold Results showed that within 10 minutes of initiating the idling experiment after engine cold-start the pre-catalysts were very effective and decreased emissions of the unburnt components by two thirds Moreover pre-catalysts which were electrically pre-heated with an external heater could more drastically decrease the amount of these components under the same experimental conditions However for such electrical heating to be practical it is necessary to reduce the level of heating energy to as low an amount as possible Therefore two power-saving methods were tried One method consisted of installing a glow plug in the upper stream of the pre-catalyst This method was based on an idea that unburnt components coming in contact with the glow plug are activated and easily oxidized and that they then release thermal energy for quick heating The results showed that this method was effective for reduction (more than 40%) of unburnt methanol but was ineffective for reducing formaldehyde since spot heating caused a balancing of formaldehyde formation/decomposition Therefore another method was examined A small-sized electric heated pre-catalyst(50cm3)was installed in order to heat a full section of the exhaust stream of the catalyst The results showed that this method had a great effect in reducing these harmful substances Moreover, it was demonstrated that this method consumes little energy and is more practical as a means of heating