Published 29 April 2026

Cylinder Bore and Stroke Selection Criteria for Marine Engines

Cylinder bore and stroke selection sets the fundamental power output, efficiency, and propeller compatibility of a marine diesel engine. The choice between short-stroke, long-stroke, and ultra-long-stroke geometries reflects a balance between mean piston speed, brake mean effective pressure, thermal loading, and the propeller efficiency targeted at the design rotational speed. Modern super-long-stroke engines such as the MAN B&W G-series and WinGD X-series push stroke-to-bore ratios above 4.0 to permit propeller speeds below 80 rpm, raising open-water propeller efficiency by several percentage points. This article surveys the engineering trade-offs that govern bore and stroke selection. Visit the home page or browse the calculator catalogue for related propulsion tools.

Contents

Background

Cylinder bore and stroke are the two principal dimensions of a reciprocating engine. The bore is the internal diameter of the cylinder, and the stroke is the linear distance the piston travels between top dead centre and bottom dead centre. Together they fix the swept volume per cylinder, the basis on which an engine’s nominal displacement and power output are calculated. The ratio of stroke to bore, denoted s/d, is one of the most fundamental design parameters in reciprocating-engine engineering. In slow-speed two-stroke marine diesels, s/d ratios have evolved from values near 2.0 in the 1960s to values exceeding 4.6 in current ultra-long-stroke designs.

The driving force behind this evolution is the relationship between engine rotational speed and propeller efficiency. A propeller’s open-water efficiency rises as its diameter increases relative to its rotational speed, all else equal. Reducing the engine’s rated speed allows a larger, more efficient propeller to be fitted in the same hull, but to maintain the same brake power at a lower speed, the engine must produce more torque per cylinder. More torque, in turn, requires either a larger bore (more cross-sectional area to apply gas pressure) or a longer stroke (more leverage at the crank), or both. Engine designers have generally preferred to extend stroke rather than bore, because longer strokes allow lower mean piston speeds and improve scavenging quality in two-stroke engines.

Bore and stroke selection is therefore not made in isolation. It is the result of a system-level optimization that considers propeller diameter, draught restrictions, ship service speed, fuel consumption targets, hull form, and shaft alignment constraints. The output is a cylinder geometry that balances mechanical, thermodynamic, and hydrodynamic considerations. This article describes how each of these inputs shapes the final bore and stroke, with reference to current commercial offerings.

Geometric definitions

Bore

The bore is measured at the cylinder liner’s running surface, typically expressed in millimetres. Modern marine two-stroke bores range from 260 mm in the smallest auxiliary engines to 980 mm in the largest container-ship main engines. The MAN B&W G95ME-C, for instance, has a bore of 950 mm and a stroke of 3460 mm. WinGD’s X92-B carries a 920 mm bore with a 3468 mm stroke. Bore is the most expensive dimension to extend, because it scales the cross-sectional area of the cylinder cover, the diameter of the cylinder liner, the size of the piston crown, and ultimately the entire transverse footprint of the engine.

Stroke

Stroke is measured between the piston’s two extreme positions and is set geometrically by twice the crank throw. Increasing stroke requires lengthening the crankshaft throws, which in turn raises the height of the engine and lengthens the connecting rod proportionally. A stroke increase from 3000 mm to 3500 mm typically adds 600 to 800 mm to the total engine height and shifts the longitudinal centre of mass of the moving parts by a similar amount. Modern ultra-long-stroke engines have strokes approaching 4 metres. The MAN B&W G70ME-C9.5 has a stroke of 3256 mm against a 700 mm bore for an s/d ratio of 4.65.

Swept volume

The swept volume per cylinder, Vs, is given by the standard formula Vs = (pi / 4) * d^2 * s, where d is the bore and s is the stroke. For an engine with N cylinders, the total displacement is N * Vs. A G70ME-C9.5 with bore 700 mm and stroke 3256 mm has a per-cylinder swept volume of 1.253 cubic metres. A typical 8-cylinder configuration of this engine displaces 10.0 cubic metres, an order of magnitude larger than the largest road-vehicle engines.

Stroke-to-bore ratio

The stroke-to-bore ratio s/d is dimensionless and provides a quick characterization of an engine’s geometric class:

Square (s/d ≈ 1.0): typical of high-speed automotive and small marine four-stroke engines
Long-stroke (s/d 1.5 to 2.5): typical of medium-speed marine and stationary engines
Slow-speed two-stroke (s/d 2.5 to 3.5): older container and bulker engines
Super-long-stroke (s/d 3.5 to 4.0): mainstream slow-speed two-stroke engines from the 1990s onward
Ultra-long-stroke (s/d 4.0 to 4.7): current state of the art for large container, tanker, and bulker propulsion

Power and the BMEP-piston speed plane

Engine brake power can be expressed as P_b = BMEP * Vs * N_cyl * n / k, where BMEP is brake mean effective pressure, Vs is swept volume per cylinder, N_cyl is the number of cylinders, n is rotational speed, and k is the strokes-per-cycle constant (1 for two-stroke, 2 for four-stroke). Since Vs depends directly on bore and stroke, the choice of cylinder geometry is inseparable from the choice of mean piston speed and BMEP.

Mean piston speed

Mean piston speed is c_m = 2 * s * n, where s is stroke in metres and n is rotational speed in revolutions per second. For a slow-speed two-stroke at 80 rpm and 3500 mm stroke, c_m = 2 * 3.5 * 80 / 60 = 9.33 m/s. Mean piston speed correlates directly with mechanical wear on liner-ring contact pairs, with frictional losses, and with piston ring pack stress. Marine slow-speed engines historically operated below 7 m/s; modern designs have risen to roughly 8.5 m/s at maximum continuous rating. Pushing further is constrained by tribology, lubricant residence time, and cooling-flow capacity in the piston crown.

Brake mean effective pressure

BMEP characterizes the average gas pressure that, acting through one stroke, would produce the same work output as the actual engine cycle. Modern marine two-strokes operate at BMEP values of approximately 21 bar at maximum continuous rating, having risen from 12 to 14 bar in the 1980s. Higher BMEP increases the gas-loading of cylinder components: peak firing pressures rise, the bedplate and tie rods see higher tensile loading, and cylinder cover bolts must be sized for greater clamping force. Beyond about 22 bar BMEP, the structural and thermal trade-offs become prohibitive for current materials.

Power density and the design envelope

The product of mean piston speed and BMEP, c_m * BMEP, has units of power per unit piston area and represents the engine’s areal power density. This quantity is bounded by available materials and cooling technology. In modern marine two-strokes the limit sits near 9 m/s * 21 bar = roughly 19 MW per square metre of total piston area, which is why doubling the bore is approximately equivalent in power output to quadrupling the cylinder count, all else equal.

Why long stroke benefits propellers

Propeller open-water efficiency is governed by the ratio of advance velocity to rotational tip velocity. For a fixed advance velocity (i.e. a fixed ship speed), reducing rotational speed permits a larger diameter and higher pitch, both of which raise efficiency. For a typical post-Panamax bulk carrier, dropping engine speed from 105 rpm to 75 rpm increases open-water propeller efficiency by roughly 4 to 6 percentage points, depending on hull form and load condition.

This is why propulsion-system designers push for ever-lower engine speeds. To maintain rated power at lower rpm requires either more cylinders, higher displacement per cylinder, or higher BMEP. Of these, increasing displacement per cylinder via longer stroke is the most thermodynamically rewarding, because:

Longer stroke increases swept volume without enlarging the combustion-chamber surface-to-volume ratio at top dead centre, preserving thermal efficiency.
Longer stroke gives better scavenging quality in uniflow-scavenged cylinders, because the piston dwell at bottom dead centre (where exhaust valves are open) is proportionally longer.
Longer stroke spreads the pressure-volume curve over a wider angular window, lowering peak gas-acceleration loads on the connecting rod.

The cost is engine height and the consequent constraints on engine-room layout, cargo-hold depth, and crankshaft manufacturing. The largest marine engines today are approximately 18 metres tall, the height of a six-storey building.

Bore selection drivers

While stroke is favoured in slow-speed engines, bore selection also matters and is driven by:

Power per cylinder target

A target output per cylinder, divided by the achievable BMEP and mean piston speed, fixes the required piston area and therefore the bore. For a 6 MW per cylinder target at 21 bar BMEP and 8.5 m/s mean piston speed, the required piston area is about 0.34 square metres, corresponding to a bore near 660 mm.

Structural ratios

Cylinder cover, liner, and tie-rod stresses scale with the square of bore for a given gas pressure. Doubling the bore quadruples the gas force on the cover and the seating face. Bore is therefore extended cautiously and only when proven materials and welding techniques permit.

Combustion-chamber geometry

A larger bore lengthens the flame-travel distance from the central fuel injection location to the cylinder wall. Combined with the relatively low engine speed of slow-speed designs, this can lead to incomplete combustion if injector spray patterns and atomization are not optimized. Modern engines use multiple injectors per cylinder to keep flame-travel distances within acceptable bounds.

Manufacturing constraints

Cylinder liners above approximately 980 mm bore approach the limits of current centrifugal casting and machining capacity. Liners must be cast, normalized, machined, honed, and inspected as a single piece, and very large diameters strain foundry crane and lathe capacity. Forged or fabricated liners are not yet practical at this scale.

Stroke selection drivers

Stroke selection is governed by:

Target rotational speed

Set by propeller-efficiency optimization and the gearing arrangement (direct-drive vs reduction gear). For direct-drive container ships at 75 to 95 rpm, strokes of 3000 to 3500 mm are typical. For LNG carriers at 85 rpm, strokes near 3000 mm. For VLCCs at 65 to 75 rpm, strokes of 3500 to 4000 mm.

Mean piston speed limit

Stroke times rotational speed must not exceed the mean piston speed limit (about 8.5 m/s for slow-speed engines). Together with the rotational-speed target, this constrains stroke to specific bands.

Engine height available

Engine height grows roughly linearly with stroke. For container ships and bulk carriers with deep engine rooms, 4-metre strokes are accommodated. For shallow-draught vessels (some tanker designs, river-going ships), stroke may be limited by deckhead clearance.

Crankshaft manufacturability

Crank throws extend half the stroke. Forging crankshafts with throws above 1.7 metres demands very large open-die forging equipment and post-forge machining. Welded-in main bearing journals (semi-built crankshafts) are commonly used at this scale.

Comparison: short, long, and ultra-long-stroke designs

Feature	Short-stroke (1980s)	Long-stroke (1990s)	Ultra-long-stroke (2020s)
Typical s/d	2.0 to 2.5	3.0 to 3.5	4.0 to 4.7
Rated speed	100 to 120 rpm	85 to 105 rpm	60 to 90 rpm
BMEP	12 to 14 bar	17 to 19 bar	19 to 21 bar
Mean piston speed	6.5 to 7.5 m/s	7.5 to 8.0 m/s	8.0 to 8.5 m/s
Open-water propeller efficiency	0.55 to 0.62	0.62 to 0.66	0.66 to 0.71
Engine height	11 to 13 m	13 to 15 m	16 to 19 m

The progression from short to ultra-long-stroke has been continuous over four decades. Each generation has reduced specific fuel oil consumption by roughly 8 to 12 g/kWh by exploiting propeller-efficiency gains, with engine internal efficiency gains contributing a smaller share.

Manufacturer offerings

MAN Energy Solutions B&W

MAN Energy Solutions offers the G-series (G50, G60, G70, G80, G90, G95) where “G” denotes ultra-long-stroke. The “ME-C” suffix indicates electronic control. The G95ME-C10.5, the largest, has a 950 mm bore and 3460 mm stroke for an s/d of 3.64 and outputs 6870 kW per cylinder at 80 rpm. The S-series (S35, S40, S46, S50, S60, S65, S70, S80, S90) is the prior super-long-stroke generation, with s/d values around 4.0.

WinGD

WinGD (formerly Wartsila two-stroke) offers the X-series with bores from 350 to 920 mm. The X92-B has a 920 mm bore and 3468 mm stroke for an s/d of 3.77, producing approximately 6240 kW per cylinder at 80 rpm. The X-DF series adds dual-fuel capability to the same architectural base.

Mitsubishi UE

Mitsubishi’s UEC engines are licensed under various MAN and self-developed designs and serve the smaller container, bulk-carrier, and chemical-tanker market with bores 33 to 60 cm and strokes up to 2.4 m.

Calculator and design tools

Bore and stroke once selected feed directly into the engine performance calculations: brake power, BMEP, mean piston speed, SFOC, and indicated diagrams. The BMEP calculator accepts bore, stroke, cylinder count, rated speed, and rated power to back-calculate operating BMEP. The piston displacement volume calculator returns swept volume given bore and stroke. The engine rating point calculator plots a chosen engine against the design layout diagram.

Operations and maintenance implications

Bore and stroke have lasting consequences for operations and maintenance:

Cylinder liner wear life scales inversely with mean piston speed and with cylinder oil feed rate adequacy. Long-stroke engines, despite higher absolute piston travel, often achieve better wear life through lower piston speed.
Piston ring pack life depends on stroke, BMEP, and ring-belt cooling. Modern ultra-long-stroke engines achieve 24,000 to 30,000 hours between top overhauls.
Cylinder cover and exhaust valve overhaul intervals are largely independent of bore and stroke, being driven by combustion-deposit and corrosion mechanisms tied to fuel quality.
Spare parts costs for very large bore liners, pistons, and covers can dominate maintenance budgets on aged engines, motivating careful spare-parts management.

References

MAN Energy Solutions. (2023). Marine Engine Programme: Two-Stroke Engines. MAN Energy Solutions.
WinGD. (2023). X-Series Engine Selection Guide. Winterthur Gas & Diesel.
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.
Carlton, J. S. (2018). Marine Propellers and Propulsion (4th ed.). Butterworth-Heinemann.