Background
The four-stroke diesel engine cycle, named for its four distinct piston strokes per power cycle, was developed by Rudolf Diesel in the 1890s. Four-stroke engines have dominated automotive and locomotive applications since their inception. In marine applications, four-stroke engines occupy a complementary niche to slow-speed two-stroke engines, serving:
- Smaller ships: where slow-speed two-stroke engines are oversized
- Ships with multiple engines: providing redundancy and power flexibility
- Diesel-electric propulsion: where multiple gensets feed an electrical bus
- Auxiliary power: gensets on ships with main two-stroke engines
- High-speed applications: ferries, fast craft, naval vessels
Four-stroke engines cover a wide power range:
- High-speed (above 1500 rpm, up to ~3500 rpm): fast ferries, naval craft, recreational craft
- Medium-speed (400-1200 rpm): cruise ships, OSVs, tugs, ferries
- Low-speed four-stroke (below 400 rpm): rare, some specialised applications
This article covers the four-stroke cycle, engine architecture and configuration variants, the principal manufacturers and engine families, and the design considerations that distinguish four-stroke from two-stroke marine engines.
The four-stroke cycle
A four-stroke engine completes one combustion-power-exhaust cycle every two crankshaft revolutions. The four strokes:
1. Intake stroke
The piston descends from TDC to BDC. The intake valve opens; air (turbocharged in marine engines) enters the cylinder. The exhaust valve is closed. Cylinder volume increases, drawing in fresh charge.
2. Compression stroke
The piston ascends from BDC to TDC. Both intake and exhaust valves are closed. Cylinder volume decreases; pressure and temperature rise. Near TDC, fuel is injected.
3. Power stroke (combustion and expansion)
Fuel ignites and combusts; pressure peaks shortly after TDC. The piston descends from TDC to BDC under gas pressure, doing useful work on the crankshaft. The exhaust valve opens near BDC, ending the expansion phase.
4. Exhaust stroke
The piston ascends from BDC to TDC. The exhaust valve is open; combustion products are pushed out. Near TDC, the intake valve opens (some valve overlap may occur). The exhaust valve closes shortly after TDC.
The cycle then repeats. Total: 720°CA per cycle, two revolutions of the crankshaft.
Architecture
Trunk piston design
Most four-stroke marine engines use trunk piston architecture (in contrast to the crosshead architecture of slow-speed two-strokes). In a trunk piston:
- The piston extends below the combustion chamber to the connecting rod connection point
- A wrist pin connects the piston directly to the connecting rod’s small end
- No separate crosshead structure between piston and connecting rod
- The piston itself receives the side-thrust load from the connecting rod’s angle
Trunk piston engines are more compact than crosshead designs, with lower height per cylinder, but at the cost of:
- Cylinder oil contamination (system oil can contact the cylinder)
- Higher piston friction (side thrust against cylinder wall)
- Lower efficiency than equivalent crosshead designs
Modern four-stroke trunk piston engines achieve 175-185 g/kWh SFOC at full load, compared to 160-175 g/kWh for slow-speed two-stroke engines.
Cylinder configuration
Four-stroke marine engines come in various cylinder configurations:
- Inline (I): cylinders in a single row. Common for 6-9 cylinders, 1-5 MW range.
- V-shaped (V): cylinders in two rows at 45° or 60° angle. Common for 8-20 cylinders, 5-25 MW range.
- W-shaped: rare but exists for very high-power applications
V configuration provides:
- Greater power per unit length
- Better dynamic balance
- More compact engine room footprint
Inline configuration provides:
- Simpler manufacturing
- Easier maintenance access
- Lower cost per cylinder
Engine speed and bore
Four-stroke marine engines vary widely:
| Class | Speed | Bore | Application |
|---|---|---|---|
| High-speed | 1500-3500 rpm | 100-200 mm | Fast craft, leisure |
| Medium-speed | 400-1200 rpm | 200-500 mm | Cruise, OSV, tug, genset |
| Low medium-speed | 250-400 rpm | 400-650 mm | Some specialty applications |
Power per cylinder ranges from a few hundred kW (high-speed) to 1500-2000 kW (medium-speed at full size).
Valve actuation
Four-stroke engines have separate intake and exhaust valves operating once per cycle:
Camshaft
A camshaft, geared from the crankshaft at half engine speed, drives valve operation. Cams have specific lift profiles for intake and exhaust events.
Pushrods or overhead camshaft
Two principal valve train configurations:
- Pushrod (cam-in-block): camshaft in the engine block, pushrods transfer motion to rocker arms above the cylinders, rockers actuate valves
- Overhead camshaft (OHC): camshaft mounted above the cylinder head, directly actuating valves through followers
Modern marine engines mostly use OHC configurations for better high-speed valve dynamics.
Variable valve timing
Some advanced four-stroke marine engines include variable valve timing, electronically adjusting intake and exhaust valve events:
- Adjusting valve overlap for emissions and efficiency
- Optimising for different operating regimes
- Compensating for ambient conditions
Variable timing is more common on automotive applications but appearing in marine.
Multiple valves per cylinder
Modern engines often have 4 valves per cylinder (2 intake + 2 exhaust), providing:
- Better breathing
- Higher power density
- Better cylinder filling
Older designs use 2 valves per cylinder.
Applications
Cruise ships
Modern cruise ships typically use diesel-electric propulsion: 4-6 medium-speed four-stroke engines (typically Wartsila or MAN) generating electricity, with electric motors driving propellers. Total installed power: 50-100 MW.
Ferries
Ferries use various configurations:
- Smaller ferries: 2-4 high-speed or medium-speed engines
- Large ferries: 4-6 medium-speed engines
- Often diesel-electric for redundancy
OSVs (Offshore Support Vessels)
OSVs typically use 4-6 medium-speed four-stroke engines in diesel-electric configuration, providing:
- Redundancy for safety in offshore operations
- Power flexibility for varied operations
- Dynamic positioning capability
Tugboats
Tugs use various configurations:
- Conventional tugs: 1-2 medium-speed main engines
- ASD (azimuth stern drive) tugs: 2 medium-speed engines driving Z-drives
- Tractor tugs: similar to ASD with different layout
Naval vessels
Naval ships often use four-stroke main engines plus gas turbines:
- COmbined Diesel-electric And Gas turbine (CODAG) for warships
- Diesel-electric for support vessels
- High-speed four-stroke for fast craft
Gensets on slow-speed-engine ships
Ships with slow-speed two-stroke main engines typically have 3-4 four-stroke gensets for:
- Auxiliary power at sea
- Hotel load in port
- Backup if main engine fails
Manufacturers
Wartsila
Wartsila is the largest medium-speed marine engine manufacturer:
- Wartsila 14, 20, 26, 31, 32, 34DF, 38, 46, 50DF: families across speed and power ranges
- Bore range: 200-460 mm
- Power: 1-25 MW per engine
- Wartsila 31 (2015) achieved best-in-class SFOC
MAN Energy Solutions (four-stroke)
MAN’s four-stroke family complements its slow-speed two-stroke business:
- MAN L21, L23, L27, L32, L40, L48, L51, L58: medium-speed
- MAN VP, VPM: V-configuration
- Bore range: 200-580 mm
- Power: 1-22 MW per engine
Caterpillar Marine
Caterpillar serves smaller and medium marine applications:
- 3500 series, C-series, MaK series: from 100 kW to 25 MW
- Strong in tugs, ferries, fishing, OSV
- Wide service network
MTU (Rolls-Royce Power Systems)
MTU specialises in high-speed four-stroke:
- MTU Series 2000, 4000, 8000, 1163: 200 kW to 9 MW
- Strong in fast craft, naval vessels, leisure
- High power density
Yanmar Marine
Yanmar serves smaller marine applications:
- Wide range of small to medium engines
- Strong in fishing vessels, small ferries, leisure
Cummins Marine
Cummins offers high-speed and medium-speed engines:
- QSK, QSM, B-series: 100-2000 kW
- Common in OSV, fishing, leisure
Comparison with two-stroke
| Feature | Four-stroke marine | Two-stroke slow-speed marine |
|---|---|---|
| Speed | 400-3500 rpm | 60-115 rpm |
| Bore | 100-580 mm | 350-980 mm |
| Power per cylinder | 100-2000 kW | 1000-7000 kW |
| Total power range | 0.1-25 MW | 5-87 MW |
| SFOC (best) | 175 g/kWh | 165 g/kWh |
| Cycles per cylinder | 1 per 2 rev | 1 per 1 rev |
| Architecture | Trunk piston | Crosshead |
| Application | Aux, cruise, ferry, tug, OSV | Container, tanker, bulker |
| Typical cylinder count | 6-20 | 4-14 |
| Reversal | Via gearbox/CPP | Direct or CPP |
| Capex | Lower | Higher |
| Footprint | Smaller | Larger |
| SFOC penalty at part load | Higher | Lower |
| Maintenance interval | Shorter | Longer |
| Lifecycle | 25,000-40,000 hours | 100,000+ hours |
The two engine classes are largely complementary rather than competitive. Each suits specific applications.
Multi-engine configurations
Father-son arrangement
Some ships use a “father-son” configuration: one large medium-speed engine (father) plus one smaller (son). The two share a gearbox driving a single propeller. The smaller engine handles cruising; the larger adds power for higher speed or harbour manoeuvring.
Twin engine
Twin-engine installations have two engines driving two propellers. Common in tugs, fast craft, and some commercial vessels. Provides redundancy and manoeuvrability.
Multi-engine diesel-electric
Larger vessels (cruise ships, OSVs) use 4-6 engines driving a common electrical bus, with electric motors to propellers. Allows engines to start/stop based on load demand, improving efficiency.
Future developments
Alternative fuels
Four-stroke marine engines are leading some alternative fuel transitions:
- LNG dual-fuel: Wartsila DF series, MAN dual-fuel
- Methanol: Wartsila and MAN methanol-capable
- Ammonia: in development
- Hydrogen: pilot programmes
- Battery hybrid: integrated four-stroke + battery systems
Higher efficiency
Modern engines target SFOC below 175 g/kWh through:
- Higher BMEP and Pmax
- Better thermal management
- Improved combustion design
- Variable valve timing
- Common rail injection
Digital integration
Like slow-speed engines, four-stroke marine is integrating:
- Predictive maintenance
- AI-driven control
- Digital twins
- Cloud analytics
Related Calculators
- Four-Stroke Cycle Calculator
- Engine Power Per Cylinder Calculator
- Mean Piston Speed Calculator
- Specific Fuel Oil Consumption Calculator
- Diesel-Electric Power Calculator
See also
- Two-Stroke Marine Diesel Engine Fundamentals
- Crosshead Diesel Engine Architecture Overview
- Engine Power and BMEP Relationships
- MAN B&W ME-C Electronic Control Overview
References
- Heywood, J. B. (2018). Internal Combustion Engine Fundamentals (2nd ed.). McGraw-Hill.
- Woodyard, D. (2009). Pounder’s Marine Diesel Engines and Gas Turbines (9th ed.). Butterworth-Heinemann.
- Wartsila. (2023). Marine Engine Selection Guide. Wartsila Corporation.
- MAN Energy Solutions. (2023). Four-Stroke Marine Engine Programme. MAN Energy Solutions.
- Lloyd’s Register. (2022). Marine Engine Selection and Operation Guide.