Name a common type of main engine used in very large cargo ships that typically operates at low RPM and directly drives the propeller.

The common type of main engine used in very large cargo ships that typically operates at low RPM and directly drives the propeller is a slow-speed two-stroke marine diesel engine. This engine is specifically engineered for the high power and continuous operation demands of massive vessels. The term slow-speed refers to the engine's operational characteristic of turning at a low number of revolutions per minute (RPM), typically ranging from 60 to 120 RPM. This low speed is vital because it enables the engine's crankshaft to directly drive the propeller shaft, meaning there is no gearbox or speed-reducing mechanism between the engine and the propeller. This direct drive arrangement enhances mechanical efficiency and reliability by eliminating complex components and allows the large propeller to operate at its most efficient rotational speed for propulsion. As a two-stroke engine, it completes an entire power cycle—including the intake of fresh air, compression, combustion (power stroke), and exhaust—in just two movements (strokes) of the piston, which corresponds to one complete revolution of the crankshaft. This design provides a power stroke with every crankshaft revolution, delivering extremely high power output for the engine's physical size and weight, which is ideal for ship propulsion. Lastly, it is a diesel engine, which means it is an internal combustion engine that ignites fuel through compression ignition: air is compressed to a very high temperature, and then fuel is injected into this hot, compressed air, causing it to spontaneously combust without the need for a spark plug. This compression ignition process allows for highly efficient combustion, especially when operating on the heavy fuel oils commonly used in marine transport.