What are the main differences between a two-stroke crosshead marine diesel and a four-stroke engine, in terms of scavenging, maintenance, and firing cycle?

• A. Two-stroke uses scavenging air to push out exhaust with continuous power strokes; four-stroke has distinct intake/compression/exhaust/fuel events; maintenance intervals and layouts differ, with crosshead designs separating piston/connecting rod motion from the main crankcase.
• B. Two-stroke relies on separate crankcase lubrication; four-stroke uses cam-driven poppet valves; scavenging is identical.
• C. Two-stroke uses electric motor scavenging; four-stroke uses turbocharging only; firing cycle is identical.
• D. Two-stroke crosshead has three crankshafts; four-stroke has two.

Answer: (C)

The main idea here is how charging and clearing the cylinder, how often combustion happens, and how the engine is built around the piston and crank mechanism differ between a two-stroke crosshead and a four-stroke.

In a two-stroke crosshead engine, scavenging is achieved by forcing fresh air into the cylinder to push out the burnt gases; this air-scavenging process occurs within the same cycle and there isn’t a separate, distinct intake and exhaust stroke controlled by valves. As a result, the cylinder can produce a power pulse with every crankshaft revolution, giving higher power density.

In a four-stroke engine, air is drawn in during a separate intake stroke, is compressed, ignites during the power stroke, and is expelled during a separate exhaust stroke, all controlled by cam-driven valves. The firing cycle thus occurs only on every other revolution, not every revolution.

The mechanical layout also differs: a two-stroke crosshead uses a crosshead to separate the piston/connecting rod motion from the main crankcase, which changes lubrication and maintenance needs compared with a four-stroke engine that uses a conventional crankcase and a cam-driven valve train.