FUEL INJECTOR

The fuel valve design was changed a few years ago from a conventional type to the mini-sac type. The aim was to reduce the sac volume in the fuel nozzle and curb dripping, thus improving combustion; introducing the mini-sac valves reduced the sac volume to approximately one third of the original (see the chapter on Fuel Injection). To  improve combustion even further, however, a new slide-type valve was introduced for all large bore engines, completely eliminating the sac volume (see the chapter on Exhaust Emissions and Control). A significant improvement in combustion is accompanied by reduced NOx, smoke and particulate emissions. The reduced particulates also improve the cylinder condition, and the wear rates of cylinder liner, piston rings and ring grooves are also generally lower with slide-type fuel valves. Slide-type valves were introduced on the K98MC engine from the beginning, their positive effect confirmed on the testbed. With fuel nozzles optimized with respect to heat distribution and specific fuel consumption, the NOx emission values associated with this engine are described as very satisfactory.

The increased mean effective pressure ratings of modern engines require increased flow areas throughout the fuel valve which, in turn, leads to increased sac volumes in the fuel nozzle itself and a higher risk of after-dripping. Consequently, more fuel from the sac volume may enter the combustion chamber and contribute to the emission of smoke and unburned hydrocarbons as well as to increased deposits in the combustion chamber. The relatively large sac volume in a standard design fuel nozzle thus has a negative influence on the formation of soot particles and hydrocarbons. The so-called mini-sacfuel valve introduced by MAN B&W Diesel incorporates a conventional conical spindle seat as well as a slide inside the fuel nozzle. The mini-sac leaves the flow conditions in the vicinity of the nozzle holes similar to the flow conditions in the conventional fuel nozzle. But its much reduced sac volume—only about 15 per cent that of the conventional fuel valve—has demonstrated a positive influence on the cleanliness of the combustion chamber and exhaust gas outlet ducts. Such valves also reduce the formation of NOx during combustion. A new type of fuel valve—essentially eliminating the sac volume— was subsequently developed and introduced by MAN B&W Diesel as standard to its larger low speed engines (Figure 3.9). The main advantages of this slide-type fuel valve are reduced emissions of NOx, CO, smoke and unburned hydrocarbons as well as significantly fewer deposits inside the engine. A positive effect on the cylinder condition in general is reported. Applying slide fuel valves to a 12K90MC containership engine yielded a 40 per cent reduction in smoke (BSN10) compared with the minisac valved engine, while hydrocarbons and CO were  reduced by 33 per cent and 42 per cent, respectively, albeit from a low level. NOx was reduced by 14 per cent, while the fuel consumption remained virtually unchanged and with a slight reduction at part load.