MES (Mitsui Engineering and Shipbuilding Co., Ltd.) and Kyushu University have been theoretically and experimentally investigating effects of some measures to clear the IMO NO(x) regulations for marine diesel engines, MARPOL Tier 2 starting from 2011 and Tier 3 from 2016. Formation of NO(x), a product from thermal dissociation of combustion air, is strongly influenced by the maximum combustion temperature (flame temperature). For example, lowering the flame temperature of 200K (for example, from 2400K to 2200K) realizes a NO(x) reduction to one tenth. On the other hand the higher the flame temperature, the better combustion results and the higher the maximum temperature in the thermodynamic cycle, the higher the thermal efficiency. For that reason, NO(x) and specific fuel consumption (SFC, directly linked to CO2 emission) are in a 'trade-off' relation. The authors introduce some unique 'trade-off minimum' measures, with which NO(x) can be reduced keeping the sacrifice of SFC to a minimum. Effects of the following measures have been verified with visualization of spray combustion using a specially designed visual test engine and a constant volume combustion chamber (CVCC) simulating the combustion chamber of a marine diesel engine: 1. Water utilization, the measures to cool the combustion flame and restrain NO(x) formation using water 1-1. FWE, fuel water emulsion 1-2. DWI, direct water injection (DWI is defined as the method of water injection into the cylinder from other injection holes than the fuel injection holes.) Applying the water technologies, following results have been derived: For FWE, 30% NO(x) reduction by adding 30% water (100%fuel + 30%water) without any increase of SFC has been confirmed from running tests using a medium-speed engine. Improvement of spray combustion applying FWE that compensates the bad effect of water has been clearly visualized by the visual test engine. Further MES has achieved a long-time record of FWE application to a Mitsui MAN B&W 2-stroke engine for electric power generation in Guam Island. Regarding DWI, a clear drop in flame temperature has been found in the visual data. A 75% NO(x) reduction with less than 3% increase of SFC has been achieved at low load (25% load) as best record using a 2-stroke test engine (400 mm bore). 2. Miller cycle technique, a method to lower the maximum combustion temperature by lowering the temperature at the beginning of combustion (at the compression end). As a fundamental research work, the Miller cycle effect has been examined using the CVCC (constant volume combustion chamber). And 20% NO(x) reduction has been confirmed by lowering the air temperature at combustion start by 50K. This result will encourage the engine designers to apply the Miller cycle technique. 3. Application of EFI (Electronically controlled fuel injection system) to achieve a 'trade-off minimum' Drastic improvement of spray combustion by combining smaller injection holes and higher injection pressure (raised to 150 MPa) has been visualized. Utilizing the visual data and CFD spray combustion simulation, the 'trade-off minimum' measures are being investigated. As an example, 'rate-shaping', controlling of fuel injection pressure at the beginning of injection applying EFI is introduced. A drop in flame temperature that leads to NO(x) reduction, with smaller deterioration of combustion by rate-shaping has been confirmed, compared to the normal way like injection timing retard, by analyzing the high-speed photos taken from the visual test engine.