WÄRTSILÄ WARTSILA 20, Product Manual

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2. Operating Ranges
2.1 Engine operating modes
If the engine is configured for SCR use then it can be operated in two modes; IMO Tier 2 mode
and SCR mode. The mode can be selected by an input signal to the engine automation system.
In SCR mode the exhaust gas temperatures after the turbocharger are actively monitored and
adjusted to stay within the operating temperature window of the SCR.
2.2 Engine operating range
Below nominal speed the load must be limited according to the diagrams in this chapter in
order to maintain engine operating parameters within acceptable limits. Operation in the
shaded area is permitted only temporarily during transients. Minimum speed is indicated in
the diagram, but project specific limitations may apply.
2.2.1 Controllable pitch propellers
An automatic load control system is required to protect the engine from overload. The load
control reduces the propeller pitch automatically, when a pre-programmed load versus speed
curve (“engine limit curve”) is exceeded, overriding the combinator curve if necessary. The
engine load is derived from fuel rack position and actual engine speed (not speed demand).
The propulsion control must also include automatic limitation of the load increase rate.
Maximum loading rates can be found later in this chapter.
The propeller efficiency is highest at design pitch. It is common practice to dimension the
propeller so that the specified ship speed is attained with design pitch, nominal engine speed
and 85% output in the specified loading condition. The power demand from a possible shaft
generator or PTO must be taken into account. The 15% margin is a provision for weather
conditions and fouling of hull and propeller. An additional engine margin can be applied for
most economical operation of the engine, or to have reserve power.
Wärtsilä 20 Product Guide - a13 - 6 September 2016 2-1
2. Operating Ranges Wärtsilä 20 Product Guide