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Medium-Speed Four-Stroke Marine Engines

Medium-speed four-stroke marine engines operate at 400-1200 rpm, with bores typically 200-460 mm and per-cylinder powers of 200-1500 kW. They are the dominant choice for cruise ship propulsion (diesel-electric), ferries, offshore support vessels, tugs, and gensets on slow-speed-engine ships. Major manufacturers include Wartsila (32, 46, 50DF), MAN (L21, L23, L27, L32, L40, L48, L51), Caterpillar, and Yanmar. This article surveys the medium-speed engine class, applications, manufacturers, and operating characteristics. Visit the home page or browse the calculator catalogue for related propulsion engineering tools.

Contents

Background

Medium-speed four-stroke marine engines occupy the engineering middle ground between slow-speed two-stroke engines and high-speed engines. They run at intermediate rotational speeds (400-1200 rpm), use trunk piston architecture, and serve a wide range of marine applications where ultra-large size of two-stroke engines is impractical and the lower fuel efficiency of high-speed engines is unacceptable.

Medium-speed engines characterise:

  • Bores: 200-460 mm
  • Strokes: 250-580 mm
  • Stroke-to-bore ratio: typically 1.2-1.4
  • Power per cylinder: 200-1500 kW
  • Total engine power: 1-25 MW
  • Cylinder count: 6-20 (inline 6-9, V 12-20)

This article describes the medium-speed engine class, covers the principal manufacturers and engine families, surveys typical applications, and discusses operating characteristics.

Architectural features

Trunk piston

Trunk piston architecture is universal in medium-speed marine engines. Pistons connect directly to connecting rods via wrist pins; no separate crosshead structure exists.

Inline and V configurations

Configurations:

  • Inline (I-engine): 6-9 cylinders in a single row. Common for 1-7 MW range.
  • V-engine: 12-20 cylinders in two rows at 45° or 60°. Common for 5-25 MW range.
  • W-engine: rare, used for very high-power applications

V configurations provide:

  • Greater power density (more cylinders in less length)
  • Better dynamic balance
  • More compact engine room footprint

Inline configurations:

  • Simpler manufacturing
  • Lower cost per cylinder
  • Easier maintenance access

Speed range

Medium-speed engines run at:

  • Low end: 400-500 rpm. Larger bores (400-460 mm), best fuel efficiency, larger engines.
  • Mid range: 600-900 rpm. Medium bores (250-400 mm), balanced characteristics.
  • High end: 900-1200 rpm. Smaller bores (200-300 mm), higher power density, lower fuel efficiency.

The chosen speed depends on:

  • Propulsion vs auxiliary application
  • Direct drive vs reduction gear
  • Power density vs efficiency trade-off

Speed regulation

Most medium-speed engines have speed regulation requirements:

  • Genset operation: precise frequency control (50 or 60 Hz electrical generation)
  • Direct drive: variable speed for ship operation
  • Diesel-electric: variable speed within efficiency band

Modern electronic governors handle multiple modes flexibly.

Engine families

Wartsila

Wartsila is the largest manufacturer of medium-speed marine engines:

Wartsila 32: bore 320 mm, stroke 400 mm, 720 rpm. Power 4-9 MW. Common in cruise ships, ferries, OSV.

Wartsila 46: bore 460 mm, stroke 580 mm, 514-600 rpm. Power 6-22 MW. Common in cruise ships, large ferries.

Wartsila 31: bore 310 mm, stroke 430 mm, 750 rpm. Power 4-11 MW. Highest-efficiency medium-speed engine (170 g/kWh shop test).

Wartsila 50DF: bore 500 mm, stroke 580 mm, 514 rpm. Dual-fuel LNG. Power 5-17 MW.

MAN Energy Solutions

MAN’s four-stroke family:

MAN L21: bore 210 mm, stroke 320 mm, 1000 rpm. Power 0.6-2.0 MW.

MAN L27: bore 270 mm, stroke 380 mm, 1000 rpm. Power 1.0-3.6 MW.

MAN L32: bore 320 mm, stroke 440 mm, 720-750 rpm. Power 2-8 MW.

MAN L48: bore 480 mm, stroke 600 mm, 514 rpm. Power 7-22 MW.

MAN VP: V-configuration variants of the inline engines.

Caterpillar Marine

Caterpillar covers smaller medium-speed range:

3500 series: bore 170-200 mm, 1500-1800 rpm. Power 0.5-3 MW.

3600 series: bore 280-320 mm, 750-900 rpm. Power 2-7 MW.

MaK series (acquired): bore 320-460 mm, 514-720 rpm. Power 3-25 MW.

Yanmar Marine

Yanmar offers smaller medium-speed engines:

6N series: bore 165-260 mm. Power 0.5-1.5 MW.

6EY series: bore 220-260 mm. Power 0.6-2.0 MW.

Other manufacturers

Cummins, Mitsubishi, Daihatsu, ABC: serve specific markets with smaller medium-speed engines.

Applications

Cruise ship propulsion

Modern cruise ships:

  • 4-6 medium-speed engines (typically Wartsila 46 or MAN L48)
  • Diesel-electric propulsion: engines drive generators, electric motors drive propellers
  • Total installed power: 50-100 MW
  • Engines start/stop based on demand

Ferry propulsion

Ferries vary widely:

  • Small ferries (10-30 cars): 1-2 medium-speed engines, 1-3 MW total
  • Medium ferries (50-200 cars): 2-4 engines, 4-10 MW total
  • Large ferries (300+ cars): 4-6 engines, 15-25 MW total

Often diesel-electric for redundancy and manoeuvrability.

Offshore Support Vessels (OSV)

OSV propulsion:

  • 4-6 medium-speed engines (Wartsila 32 or MAN L32)
  • Diesel-electric for dynamic positioning (DP)
  • Total power 5-20 MW
  • Redundancy critical for offshore safety

Tugboats

Tugboats:

  • Conventional tugs: 1-2 medium-speed main engines
  • ASD/Tractor tugs: 2 medium-speed engines driving Z-drives
  • Power 2-10 MW per engine

Some naval ships use medium-speed engines:

  • Patrol vessels and corvettes
  • Auxiliary ships (supply, transport)
  • Combined with gas turbines on warships

Gensets

Gensets on slow-speed-engine ships:

  • 3-4 medium-speed engines per ship
  • Each 1-3 MW
  • Provide auxiliary and emergency power
  • Run continuously at sea

Power generation in fleet

Medium-speed engines have very high installed power across the world fleet:

  • Cruise ships: ~10 GW total
  • Ferries: ~30 GW
  • OSVs: ~20 GW
  • Tugs: ~10 GW
  • Gensets on slow-speed ships: ~50 GW
  • Other applications: ~30 GW

Total medium-speed marine fleet: approximately 150 GW.

Performance characteristics

SFOC

Modern medium-speed engines achieve:

  • Best in class: 170-175 g/kWh (Wartsila 31)
  • Mainstream: 175-185 g/kWh
  • Older designs: 185-200 g/kWh

Lower than slow-speed two-stroke engines (~165 g/kWh) but better than smaller high-speed engines.

Power density

Power per displacement volume:

  • Wartsila 32: ~35 kW/litre
  • Wartsila 31: ~50 kW/litre (improved efficiency)
  • MAN L32: ~30 kW/litre
  • High-speed comparison: 60-100 kW/litre

BMEP

BMEP at MCR:

  • Mainstream: 22-25 bar
  • High-output: 26-31 bar (Wartsila 31)
  • Lower-output: 18-22 bar

Mean piston speed

Mean piston speed at MCR:

  • Most engines: 9-12 m/s
  • High-speed end: up to 14 m/s

Higher than slow-speed two-stroke (~9 m/s) but lower than high-speed engines.

Maintenance

Major intervals

  • Top overhaul: 8,000-16,000 hours (vs 16,000-24,000 for slow-speed)
  • Major overhaul: 24,000-48,000 hours
  • Engine life: 200,000-300,000 hours typical

The shorter intervals reflect the higher rotational speeds and trunk piston architecture.

Periodic service

Common medium-speed maintenance items:

  • Cylinder head removal and inspection
  • Valve grinding/regrinding
  • Piston ring replacement
  • Big end bearing inspection
  • Fuel injection equipment service

Spare parts

Marine engineers carry sets of spares for:

  • Common wear items (rings, valves, gaskets)
  • Critical-path items (injectors, bearings)
  • Special items (e.g., camshaft sections)

Modern developments

Higher BMEP and lower SFOC

Wartsila 31 (2015) achieved 31 bar BMEP and 170 g/kWh SFOC. Subsequent generations push toward 32+ bar BMEP and below 170 g/kWh.

Dual-fuel

Dual-fuel medium-speed engines are well-established:

  • Wartsila 50DF: LNG dual-fuel since 2010s
  • MAN L51 dual-fuel: introduced
  • Wartsila 32DF and 46DF: LNG-capable
  • Methanol DF: emerging in 2020s
  • Ammonia DF: in development

Variable valve timing

Some new designs include variable valve timing for:

  • Better fuel economy across operating envelope
  • Compliance with emission regulations
  • Improved transient response

Common rail injection

Common rail fuel injection has spread from slow-speed to medium-speed engines:

  • More precise fuel control
  • Variable injection timing
  • Better emissions

Electronic engine management

Modern medium-speed engines have engine control systems comparable to slow-speed ME-C:

  • Cylinder balancing
  • Variable timing
  • Diagnostic capability
  • Network integration

See also

References

  • Wartsila. (2023). Wartsila 31 Product Guide. Wartsila Corporation.
  • Wartsila. (2023). Wartsila 46 Engine Reference. Wartsila Corporation.
  • MAN Energy Solutions. (2023). Four-Stroke Marine Engine Programme. MAN Energy Solutions.
  • Caterpillar Marine. (2023). Marine Engine Selection Guide. Caterpillar Inc.
  • Lloyd’s Register. (2022). Marine Engine Selection Guide.