During the past few decades the exhaust emissions of diesel engines have significantly decreased due to efficient emissions regulation. Compared to the situation in the 1990s, the nitrogen oxide (NOx) and particulate matter (PM) emissions, the main challenges for diesel engines, are now reduced 80-95 % in many industrialized countries. To meet the demanding requirements, engine technologies have been updated and improved step by step. These improvements have also kept Specific Fuel Consumption (SFC) figures at a low level or they have even improved. The latter issue is of great significance for consumers (cost) and also for the environment (CO2).Nowadays many diesel engine fuel injection strategies rely on the use of exhaust after-treatment systems. Efficient and clean combustion is obtained by utilizing high injection pressure and advanced injection timing. The resulting high engine-out NOx is reduced to the target level by using an efficient NOx after-treatment system, possibly supported by an EGR system. In several applications no particulate filter is necessarily yet required for a nonroad engine to meet its emission limits. However, according to some studies, this kind of emissions control without particle filtration may significantly increase the amount of very small particles, nanoparticles, in diesel exhaust. The mentioned problem can be considered as a relevant topic in emerging countries where no after-treatment systems are needed for fulfilling the local emission requirements. The total mass of nanoparticles is typically small compared to larger particles and the total PM mass. However, the high amount of the smallest nanoparticles, frequently called as nucleation mode particles, might create other problems, e.g. for human health.Four of the main objectives of this study are to find out if there is a notable increase in the amount of exhaust nanoparticles when no after-treatment systems are used, and how fuel injection parameters affect the size distribution. Third, the study attempts to figure out if it is possible to diminish the potential problem by right parameters without impairing the engine performance and other basic emissions. Additionally, the results of this study could be used as basic information for the adjustments of the engines which will be developed and homologated in emerging countries.In this study the effects of different injection parameters on PM number emissions of a heavy duty diesel engine were studied at several engine loads and speeds. First, the main engine operating parameters, injection pressure and timing, were investigated. Then, the auxiliary injections were activated. In addition to gaseous emissions and PM mass measurements, the particle size distributions and particle volatility were studied. A high injection pressure and early injection timing seemed to be advantageous for PM and SFC but increased the amount of the nucleation mode particles. However, the current results indicated that auxiliary injections have only a minor effect on nucleation mode particles. In general, using high fuel injection pressures in diesel engines seems to decrease the soot emissions to a very low level but to increase the role of nucleation mode particles.