Auxiliary engine

An auxiliary engine is a medium-speed or high-speed four-stroke diesel driving an alternator to supply the ship’s electrical load, distinct from the main propulsion engine. Two to four sets typically feed the main switchboard, sized so that one unit can be off-load for maintenance while the rest carry the sea or port demand. Auxiliary engines run on heavy fuel oil or marine gas oil and must meet MARPOL Annex VI NOx limits. Common families include the Wartsila 32, HiMSEN H21/32, Daihatsu DK-20, Caterpillar 3500, and Yanmar 6EY26W. On dual-fuel ships such as LNG carriers, four-stroke dual-fuel sets like the Wartsila 34DF serve this role.