The MAN L21/31 is a medium-speed, four-stroke trunk-piston marine and stationary engine produced by MAN Energy Solutions, with a 210 millimetre cylinder bore and a 310 millimetre piston stroke. The engine is one of MAN’s principal small-bore medium-speed products, positioned between the smaller L16/24 and the larger L23/30 and L27/38 in the MAN four-stroke range. The L21/31 is offered in L5, L6, L7, L8, and L9 cylinder configurations with rated outputs from approximately 1,000 kilowatts to 2,000 kilowatts, primarily for marine auxiliary genset and small-vessel main propulsion duty.
Cylinder data and outputs
The L21/31 produces approximately 220 kilowatts per cylinder at 900 or 1,000 revolutions per minute, depending on the application and frequency. The engine is rated for continuous marine genset duty at the L9 configuration up to approximately 1,980 kilowatts, with brake mean effective pressure of approximately 24 bar and specific fuel consumption around 187 grams per kilowatt-hour at full load on diesel fuel.
Marine auxiliary genset role
The L21/31 is principally configured as a marine auxiliary genset:
- Pre-assembled genset packages with MAN alternator, base frame, and switchgear interface.
- Continuous duty rating suitable for primary or backup electrical generation on cargo vessels, tankers, and offshore vessels.
- Standardised installation footprint simplifying shipyard new-build and retrofit installation.
The engine is widely deployed on container ships, bulk carriers, tankers, and ro-pax ferries as the principal auxiliary genset, with typically two to four units per ship in mid-size vessels and three to six units in large vessels.
Fuel and emissions
The L21/31 runs on:
- Marine diesel oil and marine gas oil as the principal fuels.
- Heavy fuel oil on the heavy-fuel-capable variant with appropriate fuel treatment.
- Low-sulphur fuel oil for SOx Emission Control Area service.
- Biofuel blends at progressively higher proportions through the 2020s.
IMO Tier II compliance is standard for the diesel L21/31. Tier III compliance is achieved through MAN’s selective catalytic reduction system, an optional package that has become standard on new-build orders for vessels operating in NOx Emission Control Areas.
Applications
The L21/31 is deployed across:
- Cargo vessel auxiliary gensets on container ships, bulk carriers, and tankers.
- Tanker auxiliary gensets including LNG carrier auxiliaries (although the larger 32/44CR is more common at the upper output range on LNG carriers).
- Offshore supply vessel auxiliary power in conjunction with main propulsion engines.
- Small-vessel main propulsion on tugs, fishing vessels, and small ferries.
- Naval auxiliary gensets on frigates, corvettes, and naval support ships.
- Stationary distributed generation in 1 to 20 megawatt power plants.
Position in the MAN four-stroke range
The L21/31 sits in the MAN four-stroke product portfolio:
- Below: L16/24 small-bore genset engine.
- Above: L23/30, L27/38, L28/32H medium-bore engines for slightly higher outputs.
- Larger: L32/44CR, L35/44DF, L48/60 for main propulsion and large genset applications.
This positioning allows MAN to offer a continuous product range from approximately 1 megawatt to over 20 megawatts in medium-speed four-stroke configurations, with the L21/31 occupying the high-volume small-genset segment.
Production and service
The L21/31 is built at MAN Energy Solutions’ Augsburg works and at licensee facilities in Korea (Doosan Engine, now Doosan Infracore Marine), Japan (selected partners), and China. Service is supported through MAN’s PrimeServ aftermarket organisation and authorised service partners on every major shipping route.
Engineering significance
The L21/31 is significant within the MAN four-stroke range as a high-volume small-bore genset engine with a global installed base in the tens of thousands of cylinders. The product line predates the renaming of MAN Diesel & Turbo to MAN Energy Solutions and has been continuously developed for over two decades. The L21/31 is a typical example of the small-bore marine genset segment in which MAN, Wärtsilä, Caterpillar, and Hyundai-HiMSEN compete principally on output rating, efficiency, service network, and engine room footprint.