# Ship Engines Explained: Two-Stroke vs Four-Stroke Marine Diesels > Cargo ship engines are slow two-stroke diesels driving the propeller directly; ferries, cruise ships and tugs use geared or diesel-electric four-strokes. Marine Propulsion By [MARS Engineering & Sales Team](/about/) Published Oct 23, 20266 min read A ship engine on a large cargo vessel is usually a slow-speed two-stroke diesel turning at roughly 60–130 rpm and bolted directly to the propeller shaft, while ferries, cruise ships, tugs and offshore vessels mostly use medium- or high-speed four-stroke diesels driving through a gearbox or generators. Two-strokes win on fuel efficiency and simplicity at very large power; four-strokes win on compact size, power density and flexibility. ## Key takeaways - Marine diesels are grouped by speed: low-speed (commonly below about 300 rpm, almost always two-stroke), medium-speed (roughly 300–1,000 rpm) and high-speed (above about 1,000 rpm), both of the latter typically four-stroke. - A two-stroke diesel fires every revolution; a four-stroke fires every second revolution. - Large two-stroke ship engines are crosshead designs with uniflow scavenging and can reach a brake specific fuel consumption around 170 g/kWh, close to 50 percent thermal efficiency. - The 14-cylinder RTA96-C two-stroke is widely cited as the largest reciprocating engine, rated about 80,000 kW and weighing more than 2,300 tonnes. - Cruise ships typically use diesel-electric propulsion: several medium-speed engines drive generators that power electric propulsion motors or pods. - Since 2020, IMO rules under MARPOL Annex VI cap fuel sulfur at 0.50 percent globally and 0.10 percent in emission control areas, pushing ships to low-sulfur fuel, scrubbers or LNG. ## What type of ship engine do most vessels use? Most commercial ships use diesel engines, chosen for thermal efficiency, fuel flexibility and reliability. The size and speed of the engine depends on the vessel type and how power is delivered. | Engine class | Typical speed | Cycle | Typical power range | Where it is used | | --- | --- | --- | --- | --- | | Low-speed | ~60–130 rpm (large units) | Two-stroke, crosshead | Several MW to about 80 MW | Container ships, bulk carriers, tankers | | Medium-speed | ~300–1,000 rpm | Four-stroke, trunk piston | ~0.5–20 MW per engine | Ferries, cruise ships, offshore vessels, large tugs, ship gensets | | High-speed | Above ~1,000 rpm | Four-stroke | Up to a few MW | Workboats, patrol craft, fast ferries, small tugs, auxiliary gensets | Definitions of the speed bands vary between sources and engine makers, so treat the boundaries as approximate. Every large ship also carries auxiliary engines, usually medium- or high-speed four-stroke gensets, to supply electrical power at sea and in port. ## How does a two-stroke diesel engine work? A two-stroke diesel completes intake, compression, power and exhaust in one crankshaft revolution, so each cylinder fires every time the piston reaches the top. Large marine versions use a turbocharger-fed scavenge air system instead of separate intake and exhaust strokes. - **Power stroke** — fuel is injected near top dead center into air compressed to a high temperature, ignites, and drives the piston down. - **Exhaust** — near the bottom of the stroke, a hydraulically operated exhaust valve in the cylinder cover opens and pressure drops. - **Scavenging** — the piston uncovers ports in the cylinder liner, and pressurized air from the turbocharger and scavenge air receiver sweeps upward through the cylinder, pushing exhaust out through the valve. This one-direction flow is called uniflow scavenging. - **Compression** — the ports close, the exhaust valve shuts and the rising piston compresses fresh air for the next cycle. Because they fire every revolution and run slowly, low-speed two-strokes can be coupled straight to a large fixed-pitch propeller with no gearbox. Most are directly reversible: to go astern, the engine is stopped and restarted in the opposite direction. ### Why are big ship engines crosshead designs? In a crosshead engine, the piston rod connects to a crosshead that slides in guides, and the connecting rod links the crosshead to the crankshaft. This keeps side loads off the cylinder liner and separates the cylinders from the crankcase, so cylinder lubricating oil and system oil can be optimized separately. The penalty is height: crosshead engines are much taller than trunk-piston engines of similar power. ## How is a four-stroke marine diesel different? A four-stroke diesel uses separate intake, compression, power and exhaust strokes, so each cylinder fires every second revolution. Valves in the cylinder head control gas exchange, and the piston connects directly to the connecting rod (a trunk-piston design). Four-stroke ship engines run faster, so they produce more power for their size and weight. That makes them the natural choice where engine-room height and length are limited, such as on ferries, offshore supply vessels and tugs. Their higher speed means they usually drive the propeller through a reduction gearbox, often with a controllable-pitch propeller, or drive generators in a diesel-electric plant. | Feature | Low-speed two-stroke | Medium/high-speed four-stroke | | --- | --- | --- | | Firing frequency | Every revolution | Every second revolution | | Connection to propeller | Direct drive, no gearbox | Reduction gearbox or diesel-electric | | Size per kW | Large and heavy | Compact | | Fuel efficiency | Highest of any marine engine | High, slightly lower at equal power | | Gas exchange | Liner ports plus exhaust valve | Intake and exhaust valves | | Redundancy | Usually single main engine | Multiple engines common | | Typical vessels | Deep-sea cargo ships | Ferries, cruise ships, tugs, offshore | ## What engines power cargo, container and cruise ships? Cargo and container ship engines are almost always single low-speed two-strokes because these ships run at steady speed for weeks and fuel is their largest operating cost. Cruise ship engines are usually several medium-speed four-strokes in a diesel-electric arrangement. - **Container ship engine** — a single large two-stroke, often 8–12 cylinders on the biggest vessels, driving one large fixed-pitch propeller. The largest container ships have used engines of the RTA96-C class, which in 14-cylinder form has a 960 mm bore, a 2,500 mm stroke and a rating of about 80,000 kW. - **Bulk carrier and tanker** — similar slow-speed two-strokes, usually smaller in bore and cylinder count. - **Cruise ship engine** — typically four to six medium-speed engines feeding a common electrical network. Electric motors drive shafts or azimuthing pods, and the number of engines running is matched to load for efficiency, while hotel power comes from the same plant. - **Ferries and offshore vessels** — medium- or high-speed four-strokes through gearboxes, sometimes in diesel-electric or hybrid configurations. Smaller ferries and coastal vessels use high-speed engines in the hundreds to low thousands of kW. For example, the [SC33W1000CA2](/products/diesel-engines/sdec-sc33w1000ca2-marine-diesel-engine-680-1100-hp-with-advance-gearbox/) is offered with an integrated gearbox in the 680–1,100 hp range, the type of engine seen on the [passenger ferry propulsion project](/projects/passenger-ferry-sdec-sc33w/) . ## Which fuels do ship engines burn, and what about LNG? Large two-strokes were designed to burn heavy fuel oil, and many still do with exhaust gas scrubbers, while others burn very low sulfur fuel oil or marine gas oil to meet MARPOL Annex VI sulfur limits. LNG-fueled ships use dual-fuel engines that can switch between gas and liquid fuel. Dual-fuel two-strokes come in two main concepts: high-pressure gas injection, which burns gas on the diesel cycle, and low-pressure gas admission, which burns a premixed charge on the Otto cycle with a small pilot of liquid fuel. LNG cuts sulfur oxides and particulates almost entirely and reduces CO2, although unburned methane slip must be managed. Engine makers also offer methanol-capable engines and are developing ammonia-fueled designs as shipping works toward IMO greenhouse gas targets. Nitrogen oxide emissions are regulated through IMO Tier limits. Tier III, which applies in designated NOx emission control areas to ships built after the relevant dates, usually requires selective catalytic reduction or exhaust gas recirculation on diesel operation. ## What does this mean for buyers of smaller marine propulsion engines? For most vessels below deep-sea cargo size, the real choice is between medium-speed and high-speed four-stroke engines, matched to a marine gearbox. Slower, larger engines generally last longer at continuous load; faster engines save weight and space. - Tugs, dredgers and heavy workboats often pair 1,000 kW-class engines, such as the [CW6250ZLC-1N main engine](/products/diesel-engines/weichai-cw6250zlc-1n-marine-main-engine-1058-kw/) or the [V12 12M33C1000-15](/products/diesel-engines/weichai-baudouin-12m33c1000-15-v12-marine-engine-1000-hp/) , with heavy-duty gearboxes. - Fishing boats and coastal workboats typically use 100–400 kW high-speed engines, as discussed in our [marine diesel engine buying guide](/blog/marine-diesel-engine-buying-guide/) . - Auxiliary gensets on any ship follow ISO 8528 ratings, while marine propulsion engine ratings are typically based on ISO 3046 reference conditions with the manufacturer's duty definitions. MARS supplies marine propulsion engines and gearboxes in these classes and can test the engine before dispatch with a written report and video. To compare options for a vessel, [send the vessel and duty details](/contact/) . ## Frequently asked questions What is the largest ship engine in the world? The 14-cylinder RTA96-C two-stroke diesel is widely cited as the largest reciprocating engine, rated about 80,000 kW (around 107,000 hp) and weighing more than 2,300 tonnes. Newer engine generations from the major licensors have comparable ratings. Why do ships use two-stroke engines? Low-speed two-strokes are the most fuel-efficient engines available at very high power, and they turn slowly enough to drive a large propeller directly without a gearbox. For ships that run at steady speed for long voyages, that saves fuel and complexity. How fast does a ship engine turn? Large two-stroke ship engines typically run at roughly 60–130 rpm at rated power. Medium-speed four-strokes run at roughly 300–1,000 rpm and high-speed engines above about 1,000 rpm. What engine does a cruise ship use? Most modern cruise ships use diesel-electric plants with several medium-speed four-stroke engines driving generators, and electric motors or pods driving the propellers. Some newer cruise ships run on LNG with dual-fuel engines. Are ship engines diesel? Most are. Diesel and dual-fuel engines power the large majority of the world's merchant fleet, with gas turbines, steam turbines and battery-electric systems used only in specific niches. M **MARS Engineering & Sales Team** Written by the engineering and sales team at Shanghai Mars Industrial Co., Ltd., who specify, inspect and ship marine and industrial power equipment from Shanghai every week. ## Share - [Share on LinkedIn](https://www.linkedin.com/sharing/share-offsite/?url=https%3A%2F%2Fwww.marine-equipment-supply.com%2Fblog%2Fship-engines-two-stroke-vs-four-stroke-marine-diesel%2F) - [Share on WhatsApp](https://wa.me/?text=Ship+Engines+Explained%3A+Two-Stroke+vs+Four-Stroke+Marine+Diesels+https%3A%2F%2Fwww.marine-equipment-supply.com%2Fblog%2Fship-engines-two-stroke-vs-four-stroke-marine-diesel%2F) - [Share by email](mailto:?subject=Ship+Engines+Explained%3A+Two-Stroke+vs+Four-Stroke+Marine+Diesels&body=https%3A%2F%2Fwww.marine-equipment-supply.com%2Fblog%2Fship-engines-two-stroke-vs-four-stroke-marine-diesel%2F) **Products mentioned** - [Weichai CW6250ZLC-1N Marine Main Engine (1058 kW)](/products/diesel-engines/weichai-cw6250zlc-1n-marine-main-engine-1058-kw/) - [SDEC SC33W1000CA2 Marine Diesel Engine (680-1100 HP) with Advance Gearbox](/products/diesel-engines/sdec-sc33w1000ca2-marine-diesel-engine-680-1100-hp-with-advance-gearbox/) - [Weichai Baudouin 12M33C1000-15 V12 Marine Engine (1000 HP)](/products/diesel-engines/weichai-baudouin-12m33c1000-15-v12-marine-engine-1000-hp/) - [Advance GWC Series Marine Gearbox — 2000 HP](/products/marine-gearboxes/advance-gwc-series-marine-gearbox-2000-hp/) - [SDEC D683 Marine Diesel Engine (320 HP)](/products/diesel-engines/sdec-d683-marine-diesel-engine-320-hp/) ## Need a quotation? Send your model, application and quantity — we reply within 24 hours on working days. [Get a quote](/contact/#inquiry) ## Products from this guide Weichai · Diesel Engines ### [Weichai CW6250ZLC-1N Marine Main Engine (1058 kW)](/products/diesel-engines/weichai-cw6250zlc-1n-marine-main-engine-1058-kw/) The Weichai CW6250ZLC-1N is a large heavy-duty medium-speed inline 6-cylinder, turbocharged and aftercooled… View details → SDEC · Diesel Engines ### [SDEC SC33W1000CA2 Marine Diesel Engine (680-1100 HP) with Advance Gearbox](/products/diesel-engines/sdec-sc33w1000ca2-marine-diesel-engine-680-1100-hp-with-advance-gearbox/) The SDEC SC33W1000CA2 is a large heavy-duty inline 6-cylinder, twin-turbocharged water-cooled marine diesel… View details → Weichai · Diesel Engines ### [Weichai Baudouin 12M33C1000-15 V12 Marine Engine (1000 HP)](/products/diesel-engines/weichai-baudouin-12m33c1000-15-v12-marine-engine-1000-hp/) The Weichai Baudouin 12M33C1000-15 is a large high-power V12, turbocharged and aftercooled water-cooled… View details → Advance · Marine Gearboxes ### [Advance GWC Series Marine Gearbox — 2000 HP](/products/marine-gearboxes/advance-gwc-series-marine-gearbox-2000-hp/) The Advance GWC Series is a large heavy-duty marine reduction gearbox for high-power ship and boat… View details → ## Keep reading [All articles](/blog/) Marine Propulsion · Oct 23, 2026 ### [How Much Does It Cost to Repower a Boat? Planning Guide](/blog/boat-repower-cost-guide/) The cost to repower a boat is the engine plus rigging, gearbox, shaft, exhaust and labor. Indicative ranges, cost drivers and ways to cut downtime… Read article → Marine Propulsion · Oct 23, 2026 ### [Inboard vs Outboard vs Sterndrive: Boat Propulsion Compared](/blog/inboard-vs-outboard-vs-sterndrive/) Inboard vs outboard comes down to use: outboards win on space and easy service, sterndrives on smooth power, inboards on durability and heavy loads. Read article → Marine Propulsion · Oct 23, 2026 ### [Marine Heat Exchanger vs Keel Cooling: Engine Cooling Systems](/blog/marine-engine-cooling-systems-heat-exchanger/) A marine heat exchanger keeps engine coolant in a closed loop and uses pumped seawater to remove heat; keel coolers do it through the hull. Compare… Read article → URL: https://www.marine-equipment-supply.com/blog/ship-engines-two-stroke-vs-four-stroke-marine-diesel/