Exhaust gas recirculation (EGR) on marine engines
Marine EGR routes scrubbed and cooled exhaust back into the charge air to cut peak combustion temperature and reach NOx Tier III without urea.
Exhaust gas recirculation reaches NOx Tier III by lowering peak combustion temperature rather than by treating the exhaust downstream. A fraction of the exhaust is cooled, cleaned and returned to the scavenge air, raising the specific heat capacity of the charge and displacing oxygen, which suppresses thermal NO formation.
The point that surprises people coming from the sulphur side is that a marine two-stroke EGR system contains its own water-based scrubber inside the recirculation loop, to strip sulphur dioxide and particulate before the gas re-enters the charge air. An EGR ship burning high-sulphur fuel is already operating a scrubber, and the design question is whether its water treatment and bleed handling are shared with the exhaust gas cleaning system on the uptake or kept separate.
The full article will compare EGR with selective catalytic reduction on capital cost, consumables, fuel penalty and space, cover the high and low pressure arrangements, the control strategy on ECA entry, and the retrofit route covered in EGR retrofit on two-stroke engines .