Engine layout diagram and specified MCR
The L1 to L4 layout diagram, the constant mean effective pressure and constant speed boundaries, how the specified MCR is chosen, and what derating moves within it.
The engine layout diagram is the quadrilateral, bounded by the corner points conventionally labelled L1 to L4, within which a two-stroke main engine’s specified maximum continuous rating may be selected. Its edges are physical limits: the L1 to L3 and L2 to L4 lines are constant mean effective pressure boundaries, and the L1 to L2 and L3 to L4 lines are constant engine speed boundaries.
Selecting a specified MCR is the single decision that fixes the engine’s fuel consumption, its turbocharger match, its certified NOx operating point and the propeller it must drive. Moving the selection from the full-rated mean effective pressure line onto the maximum derated line is worth about 4 g/kWh of specific fuel consumption, which is the mechanism behind engine derating for a permanently reduced service speed.
The full article will cover how the propeller design point plus sea and engine margins produce a candidate rating, how the layout diagram constrains it, what changes physically when a rating is derated, and why a derated engine cannot afterwards be run above its new rating. See slow steaming , engine derating for slow steaming and sea margin and engine margin .