In diesel engines, ignition delay is determined as the period between fuel injection and ignition. The ignition delay affects the initial combustion and the following combustion process. In this study, the gas fuel (propane, methane or hydrogen) mixed with air was induced into the cylinder and the light oil was injected as ignition source. Here, the ignition delay in this kind of engine was focused as a basic study. Even in the dual fuel like this engine, the ignition delay time is predicted by changing the value of the constant, A in Livengood-Wu equation while the others constants is the same in an ordinary diesel engine. Ignition delay becomes long in air/methane or air/propane mixture because the temperature at injection timing of light oil decreases. In air/hydrogen mixture, the ratio of specific heats increases slightly, however, the mole fraction of oxygen decreases due to decrease of the mass of air. Therefore, the ignition delay in air/hydrogen mixture is a little longer than that in just air. When the light oil is injected near at top dead center, slight heat release appears due to the self-ignition of gas fuel and the cycle-to-cycle variations of the ignition delay and the following combustion increase.

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Techniques for low NOx combustion without degrading the specific fuel consumption were developed for the medium speed diesel engine. First concept for low NOx is the reduction of high temperature combustion zone. Owing to this concept, the high-temperature combustion flame zone is reduced by the injection direction near to the piston surface. Second concept for low NOx is the reduction of the flame temperature owing to the quick mixing of the high-temperature combustion zone and the low-temperature air portion. It is necessary to obtain the air motion which has the quick mixing of combustion zone and air portion. The air motion adopted in this study was the intake swirl induced by the intake valve with shroud. The medium speed engine with a bore of 280mm was used in the application of the new techniques for reducing NOx emission. The engine based on the low NOx techniques has the decrease in NOx emissions by 40 % and almost the same specific fuel consumption compared with the base engine.

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From the viewpoint of global environment protection, regulations for prevention of air pollution from ships have been discussed in the International Maritime Organization (IMO) and the regulations about NOx and SOx are expected to enter into force January 1st of 2000.
And global warming is recently such a serious problem that it is necessary to reduce greenhouse gas emissions. CO₂ and N₂O in exhaust gas contribute to global warming.
In addition to the above, IMO will revise the regulations every five years. Then emission will be expected to be applied to other air pollutants including N₂O in the near future.
Under these circumstances, we studied effect of fuel property or SCR system on harmful exhaust emissions such as N₂O by using a 4-stroke diesel engine.
The following are found:
(1) N₂O concentration from Marine Fuel Oil is approximately 10 times as high value as that from Marine Diesel Fuel.
(2) N₂O concentration from a diesel engine is influenced more strongly by sulphur content in fuel than by nitrogen content in fuel.
(3) In using same fuel for a diesel engine, N₂O concentration is influenced by NOx concentration. N₂O concentration increases with an increase in NOx concentration.
(4) In addition to NOx reduction, reduction of N₂O and HC is possible by using this SCR system.

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In recent years, we must make an effort to develop an advanced technique for recycling of waste plastics in order to utilize scrapped plastics as fuel source for diesel engines. It is necessary for us to cope with the increasing calorific value and the growing needs of environment protection. At Kobe University of Mercantile Marine and Nippo Sangyo K.K, we experiment on the dynamic and exhaust gas characteristics from the standpoint of usage for marine diesel engines.
The experimental fuel oil was obtained by mixing of diesel oil, WPD and water. Although the experiment is difficult, we experiment on NOx reduction by addition of water.
There is great hope of the future development in this experiment about the application of WPD.

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The amount of the land transportation is limited due to the traffic congestion and environmental pollution in Japan.
In order to solve this problem, the way of the transportation is to be sifted from trucks to sea way.
To realize the change, high speed vessels with light and high efficient machinery are required. Especially for the main engine, it is required maximum power with minimum weight.
To cope with this requirement, the authors developed ultra high power, high speed diesel engine having the bore 320 mm, which is the largest high speed engine in the world.
Advanced technology including two-stage turbocharging and Miller systems in a field of large four-stroke diesel engine are adopted in order to achieve such ultra high power.
Finally, more than two times as high power as the conventional engine with the same bore could be achieved. (bmep = 2.89 MPa, mean piston speed =12.0 m/s, 700 kW/cyl, 1000 rpm.)

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The new DK series engines are developed as the next generation of engines, with 5 common concepts: environmental protection, high output with smaller engine size, high efficiency, reduction in maintenance and running cost, and increase in reliability and durability.
Other than the in-line 6 and 8 cylinder types, the DK-20 model comes with 3 and 5 cylinders, and the DK 26 model with a 5 cylinder type. Development of various cylinder number types within the same model allows the new DK series to cover a wide range of output with a small engine model lineup.
With the development of 3DK-20, the low output range that had been covered by high revolution engine running on marine diesel oil can now be covered by engine running on heavy diesel oil. Also, when looking at parts around the piston area, a 3 cylinder engine effectively halves the parts number compared to 6 cylinder engines with the same output. This achieves great reduction inrunning cost and maintenance costs.
The follwing will introduce the new DK series engines, with details on design, performance, and benefits.

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