The Wärtsilä RT-flex96C is a two-stroke turbocharged low-speed diesel engine designed by the Finnish manufacturer Wärtsilä. It is currently considered the largest reciprocating engine in the world, designed for large container ships, running on heavy fuel oil. It stands at 13.5 metres (44 ft) high, is 27.3 m (90 ft) long, and weighs over 2300 tonnes in its largest 14-cylinder version — producing 80080 kW.
The 14-cylinder version was put into service in September 2006 aboard the Emma Mærsk. The design is based on the older RTA96C engine, but revolutionary common rail technology has done away with the traditional camshaft, chain gear, fuel pumps and hydraulic actuators. The result is better performance at low revolutions per minute(rpm), lower fuel consumption, and lower harmful emissions.
The engine has crosshead bearings so that the always vertical piston rod allows a tight seal under the piston. One of the reasons that the large two-stroke diesels use this design is so that thelubrication in the combustion area is separated from the crank case oil, which stays clean from combustion products. The upper portion is lubricated by continuous injection of consumable lubricant which is formulated to stand up to high temperatures and high sulfur. Another reason is to reduce sideways forces on the piston, keeping diametral cylinder liner wear in the order of only about 0.03 mm per 1000 hours.
The descending piston is used to compress incoming combustion air for the adjacent cylinders which also serves to cushion the piston as it approaches bottom dead centre (BDC) to remove some load from the bearings.
25 engines are in service with 86 are on order.