Generally, soot emission is increased in smaller bore-size diesel engine than larger one because spray-impingement on cavity wall is more significant, while keeping at constant specific output power. The objective of study is to clarify what constraints are necessary for engine/nozzle specifications and injection conditions to achieve the same combustion characteristics (such as heat release rate and emissions) in diesel engines with different bore sizes. In the first report, ‘Geometrical similarity’ was applied to the two different bore-size engines, which have similarity shape of piston cavity. Then, smoke of smaller engine is larger. This is because air entrainment decreases due to shrinking of spray angle. A new spray design method has been proposed to suppress soot emission called as ‘Spray characteristics similarity’. The smaller size of nozzle diameter and larger number of nozzle holes are required for keeping the same spray characteristics (such as specific air-entrainment and penetration) when the bore size decreases. In this report, another approach has been proposed while keeping the same nozzle diameter and number of nozzle holes. When similarity ratio is defined as the ratio of bore diameter, this new method called ‘Geometric similarity of fuel spray’ are expressed as functions of spray penetration, spray angle and air excess ratio. This method induces that fuel spray angle needs to be the same regardless of engine sizes. To achieve this condition, newly designed step nozzle has been installed, which makes fuel spray angle bigger than that of conventional straight one. In the case of ‘Geometric similarity of fuel spray’, soot is as same as that of longer type with keeping the same spray angle compared with the conventional straight nozzle. Consequently, it has been confirmed experimentally that theoretical solutions based on ‘Geometric similarity of fuel spray’ can give almost constant soot emission in different sized engines.