Standing 13.5 meters (44 feet) tall and stretching 26.59 meters (87.2 feet) in length, a machine exists that rivals the size of an apartment building. It weighs over 2,300 tonnes (2,535 short tons), a mass equivalent to hundreds of elephants.
This is not a stationary power plant on land, but the Wärtsilä RT-flex96C, a turbocharged two-stroke diesel engine designed to propel the largest container ships across the oceans. When the 14-cylinder version entered commercial service in September 2006 aboard the Emma Mærsk, it claimed the title of the largest reciprocating engine in the world.
Engineering by the Numbers
The physical specifications of the RT-flex96C present a scale of engineering that is difficult to visualize. The Finnish manufacturer, Wärtsilä, designed the engine with a cylinder bore of 960 millimeters (38 inches) and a stroke length of 2,500 millimeters (8.2 feet). This creates a displacement of 1,828.7 liters (111,590 cubic inches) for a single cylinder. With 14 cylinders firing, the total power output reaches 80.08 megawatts (107,390 horsepower).
To handle this immense force, the internal components possess staggering dimensions. The crankshaft alone weighs 300 tonnes (330 short tons). Each piston, which stands 6 meters (20 feet) high, weighs 5.5 tonnes (6 short tons). Despite this mass, the engine operates at a relatively low speed, running between 15 and 102 revolutions per minute (rpm). When operating at full load, the engine consumes approximately 160 grams (5.6 ounces) of heavy fuel oil per cylinder per cycle. Consequently, the entire engine consumes up to 250 tonnes (275 short tons) of fuel every single day.
Eliminating the Camshaft
The RT-flex96C distinguishes itself from its predecessor, the RTA96C, through the integration of common rail technology. Wärtsilä engineers removed the traditional mechanical systems found in older marine engines, such as the camshaft, chain gear, fuel pumps, and hydraulic actuators. Instead, the engine utilizes electronically controlled hydraulics to operate the exhaust valves.
This uniflow-scavenged design allows for precise control over fuel injection and valve timing. The result is an engine that delivers maximum performance even at lower rpm, while simultaneously reducing fuel consumption and harmful emissions. As the piston descends, it compresses combustion air for adjacent cylinders and cushions the piston as it approaches bottom dead center, relieving load from the bearings.
A Unique Lubrication System
Due to the sheer size of the moving parts, the engine utilizes a crosshead bearing design. This ensures that the piston rods remain vertical, creating a tight seal under the pistons. Because of this isolation, the lubrication system is split into two distinct sections. The crankcase uses one type of lubricant, while the cylinders require a different formulation.
The cylinder lubrication involves the continuous, timed injection of a consumable oil. This specific lubricant protects the cylinder liners from wear and neutralizes the acids formed during the combustion of high-sulfur heavy fuel oils. This precise engineering keeps diametral cylinder liner wear down to approximately 30 micrometers (0.0012 inches) per 1,000 hours of operation. Over 300 of these massive engines were either in service or on order, powering the global trade network.
Imagine a beast so colossal it rivals an apartment building, weighing 2,300 tonnes (2,535 tons).
Standing 13.5m (44ft) tall, the Wärtsilä RT-flex96C isn’t a building, but the world’s largest reciprocating engine, powering the giants of the sea since 2006…🧵👇 pic.twitter.com/oowPEJkcoI
— Fascinating True Stories (@FascinatingTrue) November 25, 2025