SLIDE VALVES

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The fuel injector slide valve has been used by the industry & regulators as a nice bit of the green engineering for the advances it fetch in the NOx reduction. It is a key component in the retrofit kit that makes 20-year-old engines fit for the IMO Tier I emissions compliance. The slide valve was not developed to lower NOx, but to enhance engine efficiency & cylinder condition.

Launched by the MAN B&W in the year 2002 for its MC series slow-speed marine diesel engines, the slide valve upgraded on earlier nozzle designs by removing a minute channel – called the sac volume – that ran from the injection valve seat to the fuel inlet hole in to the combustion chamber. The channel holds a small amount of oil after the injection, which would drip into the chamber & burn incompletely. With big ships utilizing more than 200 tons of the fuel oil per day, those tiny drops added up to quite a lot of the wasted fuel. In the slide valve arrangement, the valve sits on the injection holes, reducing the sac volume to almost zero & enhancing fuel efficiency. It also cuts the amount of the unburned hydrocarbons, & particulate emissions.

Cross sections of fuel-valve nozzle tips.

As with the earlier design, slide valves could be regulated to favor fuel efficiency or NOx reduction. In the 6 years since they were liberated, the latter attribute has become the object of much investigation.

“Using computational fluid dynamics, we shape the result of the changing the number of the holes, the spray angels & spray patterns,” says Kjeld Aabo, Director of Promotion & Customer Support for the MAN B&W. “This minimizes the concentration of the fuel oil at the injection point, resulting in the smaller peak temperatures in the combustion chamber. Changing combustion conditions changes the NOx emissions,” he explains. “The benefit is not associated to the slide valve as such, but to the spray pattern. We then were able to develop a kit that could bring our older in service engines up to the Tier I standards.”

For the “Better fuel efficiency, more complete combustion, less smoke & soot & fewer engine deposits – for these reasons we directly employed the slide valves in all the engines, where it was possible to do so,” says Lars-Robert Pedersen, Director of the Regulatory Affairs for AP Moller-Maersk. “From our viewpoint, all shipowners should apply slide valves instantly, where possible, because it makes perfectly good operational sense. The NOx benefit is a free extra.”