4.3 Smokeless low-speed engines
The Sulzer RT-flex concept for low-speed engines is a fully camshaft-free engine, where both the injection and the actuation of the exhaust valve take place according to the common rail philosophy.
Due to the lower frequency of the injections it is possible to use a normal common rail system, which is fed by a pressurizing unit. The low-speed engine has three nozzles per cylinder, and the injection of each nozzle is controlled by its own rail valve.
Furthermore the injected amount to each injector is actively measured, and the injection is cut off when the fight amount is injected. At low load it is possible to run one nozzle at a time in a rotational pattern, which as a consequence will allow lower minimum engine speed and further contribute to no visible smoke. The engine concept is seen in Figure 14, and the test results in Figure 15.
5. NOx TOWARDS STILL LOWER VALUES
The NOx values of diesel engines have developed positively up to now (Figure 16). However, the author's company is committed to continue the development towards still lower NOx values. The target set is 3 g/kWh, and indeed there seem to be ways to reach that value. It is a well-known fact that the presence of water in the combustion space has an influence on the NOx formation. It is to some degree a positive surprise, though, that humidity seems to be capable of taking the emission values to really low. The method which will be pursued by the author's company is steam injection (Figure 17). The steam will be produced from waste heat from the engine itself. In addition to reducing the NOx emissions to a level of 3 g/kWh, the steam injection will contribute to improved efficiency of the engine itself. Depending on engine size, the degree of waste heat recovery and optimization priorities, the efficiency improvement can be from a couple of per cent to 8% in the most positive simulation calculations. The first test with this system will be run this year, but it must be underlined that commercially this technology will be available in four to five years from now.
Another system which will be developed for low-speed engines is the Water Cooled Rest Gas system (Figure 18). Exhaust gas recirculation is known to have a good impact on NOx formation in a diesel Cylinder. However, when running on heavy fuel, external cooling of the gas causes a lot of problems with the system itself. Therefore, a better way is to leave some gas in the cylinder up in the hottest part of the cylinder space and cool that gas with a water spray. As both the rest gas and the water have a positive impact on NOx formation, single-digit NOx values can be reached with this method. It is applicable mainly on low-speed two-stroke engines.
Figure 14. The Sulzer RT-flex comprises common rail injection.
