With few exceptions in the forthcoming years all civilian sea-going vessels are going to be propelled by large diesel engines. These represent the most efficient thermal engine type having efficiencies ranging from 40% to over 50%. Almost one third of the large sea-going vessels will be equipped with medium-speed four-stroke engines, which deliver up to approx. 30 MW of power output.

Although the critical exhaust components, i.e., nitrogen oxides (NOx), sulfur dioxide (SO₂) and particles, constitute far less than 1% of the diesel exhaust gas, it is estimated that approximately 8 to 10 % of the total global emissions is caused by the approx. 35,000 commercial ships. It is, therefore, worthwhile to reduce pollutant emissions from large marine diesel engines, especially in regions with high traffic.

A goal of the FVV project "Process Management of large diesel engines" was to clearly reduce the nitrogen oxide and smoking emissions using in-motor measures, while preserving fuel consumption characteristics as constant as possible. Using a combination of higher compression, higher injection pressures as well as changes in the valve timing (Miller process) the NOx emissions can be lowered by approx. 25 % with an identical fuel consumption and similar smoke values. It is still desirable, however, to improve the smoke behaviour under partial load operation. Future electronically-steered high-pressure injection systems for heavy fuel oil operation offer here the possibility of multi-step injection, so as to bring the fuel into the cyliner in two or more stages, whereby making the fuel quantity of each single injection in the ideal case freely selectable.

Systematic investigations with an electronically steered single injection pump with solenoid valves showed that pre-injection with a pre-injection fuel quantity of approximately 2-3 % of the total amount during the cycle decreases combustion-generated noise substantially. In particular at low-load operating points with reduced rotation speeds (operation as generator) pre-injection brings clear improvements as regards smoke output and fuel consumption while achieving, at the same time, smaller NOx emissions. An injection cycle with the fuel quantity split in two stages following a 35%/65% scheme can achieve, in certain operating modes, clearly lower nitrogen oxide emissions with the same consumption and similar smoke values as the single injection scheme with high injecting rate and short burn duration.

Future investigations will aim at achieving invisible smoke emissions also in the lower load ranges. Novel injection systems for large diesel engines such as, for example, Common Rail injection systems, which enable a short and intensive post-injection, can make a valuable contribution here.