2.1 Moderate reductions for IMO compliance
For the moderate emission reductions necessary to ensure compliance with the IMO Regulation, primary methods are used for our low speed two-stroke engines. The methods comprise modifications to the fuel injection system and, in some cases, the use of a new fuel valve design. The fuel nozzle designs exert a strong influence on the formation of NOx because of the influence via mixture formation and combustion on the local temperature level and oxygen concentration in the fuel spray area.
The main method, used on all engine types, involves the application of the so-called Low-NOx layout of the fuel nozzles. 10-25% reductions have been achieved in certain engines by using this method, which is normally associated with a modest fuel penalty (1-2%). Furthermore, this method is generally attractive because of the easy implementation. Fig.1 shows a comparison between the NOx emissions (according to the standard measuring method for IMO) of the previously used fuel-optimised nozzles and the Low-NOx nozzles for a number of MC engines. These Low-NOx nozzles are supplied as standard against all new orders since the engines in question will have to comply with the IMO limits - even though the Regulation has not yet been ratified, and that process might take quite some time! However, when ratified, the limits (and associated paperwork, including certificates) have to be complied with for all vessels with keel laying after 1st January 2000.
Table 1 shows results from tests carried out on a 12K90MC engine with two new types of fuel valve. As can be seen, the mini-sac fuel valve provides very low smoke values and also low CO and HC values. The new (patented) slide fuel valve design provided a significant reduction of CO, HC and smoke values as well as a reduction of the NOx emission so as to comply with the IMO limits. The fuel oil consumption with the slide valve is marginally higher at 100% load, but lower at part load.
As can be seen from Fig.2, the new types of fuel valve incorporate a conventional conical spindle seat as well as a slide inside the fuel nozzle. The mini-sac valve has a slide that divides the sac-volume into two part-volumes and reduces the effective sac-volume to about one-third. The mini-sac valve is standard on some engine types and has shown reduced smoke and combustion chamber deposits in service. The goal is, however, to completely eliminate the sac-volume - which has been achieved with the slide-type fuel valve.
The clearance between the slide and the nozzle in the slide-type valve is of the order of 10-15μm, corresponding to normal fuel injection pump clearances, and measurements of the pressure inside the nozzle between two consecutive injections have confirmed that the leakage past the slide is negligible.
