Sulzer Marine Diesel Engines: History 1898 to WinGD

Sulzer Brothers (Gebrüder Sulzer AG, Winterthur, Switzerland) was one of the two companies that created slow-speed marine diesel propulsion as it exists today. From a licence agreement with Rudolf Diesel in 1893 through the 1898 first engine, the 1912 Monte Penedo sea-going installation, and the RD, RND, RTA, and RT-flex engine families, Sulzer drove most of the engineering progress in large two-stroke crosshead diesel engines. The company’s marine division was sold to Wartsila in 1997 and reorganised as WinGD (Winterthur Gas & Diesel) in 2015, with full ownership passing to CSSC (China State Shipbuilding Corporation) in 2016. The direct technological line from that 1898 first engine to the WinGD X-DF and X92 engines in service today is one of the longer continuous engineering genealogies in modern industry.

For a deeper treatment of the scavenging architecture debate, see Loop Scavenging Versus Uniflow Scavenging in Two-Stroke Engines. The common-rail injection system that Sulzer introduced in 2001 is covered in Common Rail Fuel Injection on Two-Stroke Engines. See the system-main-engine-slow-speed-2-stroke calculator for reference performance data on slow-speed two-stroke engines.

Founding and the Winterthur industrial base (1834 to 1892)

Johann Jakob Sulzer-Neuffert founded the firm in Winterthur, in the Swiss canton of Zurich, in 1834. It began as a foundry, producing cast-iron goods for the local textile industry. His sons Johann Jakob Sulzer-Hirzel and Salomon Sulzer-Sulzer expanded the business through mid-century into pumps, steam engines, and central-heating boilers. By the 1880s Sulzer was exporting machinery across Europe and had established a pattern of vertical integration that would later sustain decades of marine diesel development: the Winterthur site could design, cast, machine, assemble, and test major powerplants without depending on external suppliers for any primary component.

This industrial depth was not typical. Most European marine engine builders of the late 19th century were either shipyard-integrated (Burmeister & Wain in Copenhagen, Blohm & Voss in Hamburg) or small machine shops dependent on steel mills and forges elsewhere. Sulzer’s Winterthur works had its own foundry capacity and a test-engine hall that could run large multi-cylinder prototypes continuously. When Rudolf Diesel began looking for a manufacturer capable of proving his engine concept in 1892 and 1893, Sulzer was one of a short list of firms with the machining precision and test-bed capacity the work required.

The Diesel connection: 1893 to 1898

The licence agreement with Rudolf Diesel

Rudolf Diesel signed a manufacturing licence agreement with Sulzer Brothers in 1893, the same year he signed the foundational agreement with MAN in Augsburg. The MAN agreement is the more frequently cited, because MAN built the first running Diesel engine (completed February 1897 in Augsburg). But Sulzer’s 1893 commitment was made in parallel and with comparable engineering seriousness. Diesel himself visited the Winterthur works during the development period, and the collaboration shaped Sulzer’s engine-design philosophy from the outset.

The 1893 licence gave Sulzer rights to manufacture diesel engines commercially. In return, Sulzer committed engineering resources to solving the practical problems Diesel had not resolved in theory: fuel injection reliability at the pressures required for air-blast injection (approximately 60 to 70 bar for the original air-blast system), cylinder head reliability under repeated thermal loading, and the mechanical reversibility needed for marine use.

First Sulzer diesel engine: 10 June 1898

The first Sulzer diesel engine was started on 10 June 1898 in the Winterthur test hall. It was a single-cylinder, four-stroke engine: 260 mm bore, approximately 14.7 kW (20 hp) at 160 rpm. This was a development unit, not a production engine, designed to verify Diesel’s thermodynamic cycle and the Sulzer team’s approach to injection and combustion chamber geometry. The June 1898 date is Sulzer’s documented first running engine; MAN’s first Diesel engine had already run 15 months earlier in February 1897.

Sulzer moved quickly from that 1898 laboratory unit to a programme of multi-cylinder development engines through 1899 and 1900. By 1903 the company was manufacturing diesel engines in commercial series for stationary and industrial customers. Marine adaptation required additional engineering: a marine diesel must be reversible (able to run in both rotation directions for astern manoeuvring) and it must restart reliably from a cold or warm stop within seconds. Neither requirement applied to a stationary pump or generator engine.

First reversing two-stroke marine engine: 1905

Sulzer’s first reversing two-stroke marine diesel ran in 1905. Two-stroke operation (one power stroke per revolution rather than one per two revolutions) appealed for marine use because of higher power density at low rpm, matching propeller torque-speed characteristics directly without a gearbox. Reversibility on the 1905 engine was achieved by a mechanical camshaft shift: the camshaft could be axially displaced to engage a second set of cams with opposite angular timing, reversing the firing sequence and rotation direction.

This 1905 engine was a demonstration unit rather than a commercial product. The practical marine engineering challenges of two-stroke operation, particularly the gas exchange (scavenging) problem at low loads and the sealing of transfer ports under the piston, occupied Sulzer engineers for another seven years before the technology reached sea-going service.

First sea-going installations: 1910 to 1925

Context: the race for the first diesel ship

From approximately 1905 to 1912, several European marine engine builders competed to put the first practical diesel-powered ocean-going ship into service. The candidates included MAN (Germany), Burmeister & Wain (Denmark), Werkspoor (Netherlands), and Sulzer (Switzerland). The engineering barriers were not trivial: a sea-going engine needed to produce 500 to 2,000 kW continuously for weeks at a time, start reliably under all sea conditions, and manoeuvre in restricted waters without crew error or mechanical failure.

Werkspoor won the “first diesel ship” distinction with the MV Vulcanus, a tanker that entered service in 1910 fitted with Werkspoor four-stroke trunk-piston engines. Burmeister & Wain followed with the MS Selandia in 1912, a 7,400-tonne passenger/cargo ship with 2 × 920 kW four-stroke B&W engines. Selandia is broadly regarded as the first ocean-going motor ship of consequence, combining passenger carriage with regular cargo service on the Copenhagen-Bangkok route.

MV Monte Penedo: Sulzer’s 1912 milestone

Sulzer’s answer to Selandia came in the same year. The German cargo vessel MV Monte Penedo entered service in 1912 fitted with two Sulzer 4S47 valveless crosshead two-stroke engines producing a combined approximately 1,250 kW. Monte Penedo is documented as one of the first ocean-going vessels propelled by slow-speed two-stroke engines, making it Sulzer’s primary claim to priority in a technology that would eventually dominate marine propulsion.

The 4S47 engine (4 cylinders, S-type, 470 mm bore) used a design Sulzer called “valveless”: both scavenge air entry and exhaust gas exit were controlled by ports cut into the cylinder liner rather than by any valve in the cylinder head. The piston uncovered the exhaust ports first (just before bottom dead centre), then the scavenge ports, allowing fresh air to sweep exhaust gases upward and out. This arrangement, which Sulzer would refine into loop scavenging over the following decades, eliminated the mechanically complex valve-in-head configurations that early competitors used and simplified maintenance aboard ship.

The two-stroke configuration also meant Monte Penedo’s engines fired on every revolution, giving a smoother torque curve than contemporary four-stroke installations at the same cylinder count.

Loop scavenging: the engineering choice that defined Sulzer for 50 years

The scavenging arrangement on Monte Penedo and its successors was not yet systematically classified as “loop” scavenging, but the basic geometry was established: scavenge ports near the bottom of the cylinder liner on one side, exhaust ports near the bottom on the other (or just above the scavenge ports), with incoming air directed upward across the cylinder to form an inverted-U loop before the piston closes both port sets on the upstroke.

This is architecturally distinct from the uniflow arrangement that Burmeister & Wain developed in parallel, where scavenge air enters through ports at the bottom of the liner and exhaust gases exit through a valve in the top of the cylinder cover. Uniflow’s one-directional gas flow is thermodynamically cleaner and enables very long strokes, but it requires a mechanically actuated exhaust valve in the cylinder head with its own cam, hydraulic actuation, and cooling system.

Sulzer chose loop scavenging partly for mechanical simplicity and partly because the Winterthur team believed the loop geometry could be optimised to match uniflow efficiency at moderate stroke-to-bore ratios. For engines with stroke-to-bore below about 2.5, that judgement was defensible. It would become contested by the 1970s.

For a systematic technical comparison of the two approaches, see Loop Scavenging Versus Uniflow Scavenging in Two-Stroke Engines. The uniflow scavenging wiki article covers the gas-exchange physics in more detail.

Interwar development: 1920 to 1945

Double-acting two-stroke: 1923

In 1923 Sulzer introduced its first double-acting two-stroke engines, a configuration where combustion occurs on both faces of the piston: conventional combustion above the piston crown and additional combustion below the piston skirt in a lower combustion chamber. Double-acting operation approximately doubles power output per cylinder for a given bore and stroke, which was the primary motivation in an era when individual cylinders topped out at around 300 to 400 kW.

The engineering penalties were severe. Double-acting required piston-rod seals at both the upper and lower stuffing boxes, two complete fuel-injection systems per cylinder, and cylinder-cover arrangements that could sustain gas pressure loads from below. Maintenance complexity was substantially higher than single-acting equivalents, and the lower combustion chamber was exposed to the stuffing-box seal as a potential failure point for crankcase oil contamination. Sulzer built double-acting engines through the 1930s before abandoning the configuration; MAN also explored double-acting designs in the same period and similarly abandoned them. The record for per-cylinder output was eventually beaten more reliably by bore increases and turbocharging.

Turbocharging and the RD-series precursors

Sulzer engineers were active in supercharging and turbocharging research through the 1930s and 1940s. The company adopted Brown Boveri (BBC, later ABB) turbochargers in the postwar period, as did most European marine diesel builders. Turbocharging changed the specific output economics: a turbocharged two-stroke engine at 1950s technology could produce roughly 50% more power from the same cylinder volume as a naturally aspirated equivalent, at the cost of turbocharger maintenance and slightly more complex air-system management.

In 1946 Sulzer introduced turbocharging for normal continuous service on its 6TAD48 engine, marking the transition to turbocharged operation as a standard configuration rather than an experimental one. The 6TAD48 is the identifiable precursor to the RD series that followed a decade later.

Wartime production

Through World War II, Sulzer’s marine diesel output was constrained by Switzerland’s neutral status and raw-material availability, but the Winterthur facility continued operating. Engines went to Swiss-flagged and friendly-nation merchant vessels. The wartime period also produced considerable engineering work on fuel flexibility (Swiss manufacturing had to adapt to whatever fuel grades were available), which contributed to the postwar reliability improvements in the RD series.

The RD series: 1957 to 1968

Launch and architecture

In 1957 Sulzer launched the RD series, a systematically redesigned product line that replaced the various pre-war and early-postwar types and became the company’s mainstream marine engine through the 1960s. RD engines were turbocharged, loop-scavenged, two-stroke crosshead engines with a distinctive feature that set them apart from the B&W MC architecture being developed in parallel: rotary exhaust valves.

Instead of the poppet exhaust valves that B&W used (hydraulically actuated, seated in the cylinder cover, controlled by an individual camshaft follower per cylinder), RD engines used a rotary valve arrangement where a rotating disk or sleeve controlled exhaust timing. Sulzer engineers claimed the rotary valve gave more uniform opening and closing than a poppet valve at the cycle frequencies of large slow-speed engines. The rotary valve was mechanically unusual and required precision manufacture, but it did reduce the dynamic loads on the valve seat and potentially improved valve life compared to early-generation poppet valves.

Bore range for the RD series extended from 440 mm (5RD44) through to engines of 900 mm bore in later variants. Power outputs ranged from a few thousand kilowatts for smaller cylinders to over 30,000 kW for large multi-cylinder configurations, covering the full range from feeder vessels to supertankers.

Pulse turbocharging and constant-pressure conversion

RD engines initially used pulse turbocharging, where exhaust gases are directed to the turbocharger in high-energy pulses timed to individual cylinder exhaust events. Pulse turbocharging produces better turbocharger response at low loads but requires a complex pulse-grouping manifold and introduces some mutual interference between cylinders. Through the 1960s, the industry progressively switched to constant-pressure turbocharging, where all cylinders exhaust into a common high-volume manifold and the turbocharger operates on a steady pressure. Constant-pressure systems give better peak efficiency at full load and simplify the exhaust manifold. Sulzer adopted constant-pressure systems on later RD variants and as standard on the RND.

The “R” naming convention

The “R” prefix in all Sulzer engine designations derives from the German revidiert (revised), an internal classification applied to an early 1950s design revision programme denoted RSD. The R prefix was retained through all subsequent series and continues in WinGD product names (X-series engines were initially also marketed as RT-flex descendants). The naming convention has been more symbolic than systematic since the RTA era; WinGD’s current engines carry it as heritage branding.

Licensee network

Sulzer did not build all its engines in Winterthur. The company licensed its designs to manufacturers in shipbuilding nations, receiving royalties per engine. Principal licensees in the RD era included:

• Mitsui Engineering & Shipbuilding, Japan (active from 1948; Mitsui-built Sulzer engines powered a large fraction of Japanese merchant fleet additions in the 1960s)
• Stork (Netherlands)
• Shipbuilding yards in the UK, including Doxford-adjacent facilities at certain periods

The licensee model allowed Sulzer to capture design royalties from the Japanese shipbuilding boom of the 1960s and 1970s without maintaining large manufacturing capacity abroad, a financially effective strategy when Japanese yards built more merchant tonnage per year than all European yards combined.

The RND series: 1968

Simplification of the RD

In 1968 Sulzer launched the RND series, replacing the RD as the mainstream product. The architectural difference was the exhaust arrangement: RND engines eliminated the rotary exhaust valves of the RD and instead used port-only scavenging, with no separate exhaust valve mechanism. Exhaust ports were cut into the liner in the same zone as the scavenge ports, slightly higher in the cylinder so they closed fractionally later on the upstroke and opened fractionally earlier on the downstroke.

The port-only loop-scavenged arrangement was simpler to manufacture and maintain than the rotary valve RD, and it reduced the number of components subject to thermal fatigue failure. The tradeoff was some reduction in exhaust-timing flexibility, but at the stroke-to-bore ratios typical of 1968-vintage engines (approximately 2.0 to 2.5), port-only scavenging gave acceptable gas-exchange performance.

RND engines spanned bores from approximately 520 mm (5RND52) to 840 mm (12RND84), with power ratings across the full range of merchant vessel propulsion. The series was Sulzer’s primary product through the 1970s, competing directly with the B&W MC series that was also entering service in that decade.

Efficiency drives of the 1970s

The 1973 oil crisis changed marine engine specification practice sharply. Before 1973, marine diesel selection was dominated by capital cost, reliability, and delivery time. After 1973, with heavy fuel oil prices roughly quadrupling, specific fuel oil consumption (SFOC) became a principal selection criterion. Sulzer’s Winterthur design team responded with development work on combustion chamber geometry, injection timing, and turbocharged air-fuel ratio optimisation that improved RND fuel economy through the late 1970s. See the engine SFOC sensitivity calculator for the relationship between air inlet temperature and SFOC that was central to this work.

The 1970s also brought the first systematic application of waste heat recovery to marine diesel auxiliaries. Exhaust gas boilers had been standard for decades, but the SFOC pressure drove interest in more elaborate schemes: exhaust-gas turbines on the main exhaust, power-turbine stages between the turbocharger and the exhaust boiler, and combined Rankine cycles on larger vessels.

Why B&W chose uniflow and Sulzer chose loop: the scavenging debate

By the mid-1970s the two dominant slow-speed marine diesel builders had different scavenging philosophies and both were producing reliable, commercially successful engines. Burmeister & Wain’s uniflow (MC series) engines used a central exhaust valve in the cylinder cover, hydraulically actuated from the camshaft. Sulzer’s loop-scavenged RND used only liner ports. The technical argument for each approach was real, not merely tribal:

Sulzer’s case for loop scavenging: No cylinder-head moving parts meant fewer failure modes in the combustion gas path. Loop-scavenged cylinders had simpler, cheaper cylinder covers. Port-only scavenging needed no valve actuation system (no rocker, no hydraulic follower, no valve cooling water circuit in the cover). Maintenance was simpler and the scavenging geometry could be optimised by liner-port profiling.

B&W’s case for uniflow: Air and gas moved in one direction (up from scavenge ports to exhaust valve), avoiding the flow reversal that wastes kinetic energy in loop scavenging. The exhaust valve timing could be set independently of stroke length. As stroke-to-bore ratio increased, the exhaust-port window in a loop-scavenged liner became proportionally smaller relative to the liner height, reducing gas-exchange efficiency. Uniflow had no such geometry constraint.

The debate had a practical resolution: at stroke-to-bore ratios below about 2.5, loop scavenging was competitive. Above 2.5, uniflow had a measurable efficiency advantage. The trend in marine propulsion through the 1970s was toward longer strokes to spin larger, slower, more efficient propellers. By the late 1970s the leading container ship and tanker engines were being specified at stroke-to-bore ratios of 2.8 to 3.2 or higher, and Sulzer engineers could see the loop-scavenging architecture approaching its practical limits.

The RTA series: the uniflow pivot of 1983

The decision

In 1983 Sulzer launched the RTA series with uniflow scavenging, abandoning loop scavenging after 50 years. This was a significant internal commitment: the Winterthur team that had defended loop scavenging against B&W for decades now redesigned the engine architecture around the same gas-exchange approach their principal competitor had used since the 1950s. The engineering arguments are described in the previous section; the commercial pressure was that very-long-stroke engines (stroke-to-bore ratios of 3.0 and above) were what shipowners and operators needed to meet propulsion efficiency targets after the 1973 oil crisis, and loop scavenging could not deliver that.

RTA architecture used a single hydraulically actuated central exhaust valve in the cylinder cover, similar to B&W’s MC architecture. The camshaft drove a hydraulic pump per cylinder that actuated the valve. Fuel injection was mechanically timed by individual fuel cams on the same camshaft. This architecture, while substantially similar to B&W’s, gave Sulzer’s engineers the valve-timing independence and long-stroke gas-exchange performance the RND could not match.

RTA specifications and range

The initial RTA range extended from 380 mm bore (5RTA38) to 960 mm bore (12RTA96). A 12RTA96C in full configuration (96 cm bore, 12 cylinders, long stroke) produced approximately 65,880 kW: at its 1996 introduction the largest output then available from a slow-speed engine in commercial service.

Stroke-to-bore ratios in the RTA series ran from approximately 2.8 (short-stroke variants) to 3.5 and beyond in the later long-stroke “L” configurations. The long-stroke variants also delivered lower rpm, allowing larger propeller diameters and improved propulsive efficiency across a wider speed range. Tankers, bulk carriers, and eventually container ships were specified with RTA engines through the 1980s and into the 1990s.

Mean piston speed in large RTA engines ran at approximately 6.5 to 7.5 m/s at rated rpm. The engine mean piston speed calculator calculates this for any bore/stroke/rpm combination. BMEP in fully rated configurations was approximately 18 to 20 bar. See the engine BMEP calculator for reference.

RTA variants through the 1980s and 1990s

Sulzer extended the RTA series with several sub-variants through its production life:

• RTA-T: turbo-compound variants adding a power-recovery turbine between the main turbocharger and the exhaust boiler, recovering additional shaft power from exhaust energy. The power turbine drove the main shaft through a gearing arrangement, with reported SFOC improvements of approximately 3 to 5 g/kWh at full load compared to the standard RTA.
• RTA-2T: refined second-generation long-stroke versions with improved combustion geometry and updated fuel injection cams, introduced from the late 1980s.
• RTA96C: the 96 cm bore, “C” (compact/improved) variant that preceded the RT-flex96C and ran in mechanical-cam form on several vessels before the common-rail variant superseded it.

The RTA architecture remained mechanically timed throughout its life: fuel injection timing was set by the profile of fixed cams on the camshaft. Changing injection timing for different loads required either accepting a fixed-timing penalty or fitting variable-timing mechanisms that added mechanical complexity. Electronic control of injection timing was the next step.

Cylinder lubrication systems

One technical area where Sulzer invested independently of scavenging architecture was cylinder lubrication. The Winterthur team developed controlled-feed lubrication systems that metered cylinder oil directly to the liner ports at a rate proportional to engine load. The cylinder lubrication systems for two-stroke engines article covers the general technology; Sulzer’s contribution was systematic quantification of the oil-feed-per-stroke relationship with liner wear rate, documented across fleet operating data from the RND and early RTA period. This data supported later reductions in cylinder oil feed rates from approximately 1.2 g/kWh to below 0.7 g/kWh without liner wear deterioration.

Corporate restructuring: 1990 to 1997

New Sulzer Diesel Ltd (1990)

The marine diesel business was capital-intensive in ways that Sulzer Brothers’ other divisions (pumps, surface coatings, compressors, heating equipment) were not. Marine engine R&D cycles ran five to ten years from concept to commercial engine. The 1973 oil crisis and its aftermath had spurred a major redesign investment (the RTA series), and the electronics and software capability needed for the next generation of electronically controlled engines required sustained capital commitment that the parent company’s diversified structure did not easily accommodate.

In November 1990 Sulzer separated its diesel engine business as a distinct subsidiary: New Sulzer Diesel Ltd. (NSD), headquartered in Winterthur. NSD retained the full Winterthur engineering staff, the test-bed facilities, the licensee network, and the RTA product programme. It operated as an independent company even while remaining majority-owned by Sulzer Brothers.

The 1990 separation also reflected commercial pressures from Asian shipbuilding. Japanese yards, which had been Sulzer licensees since the 1948 Mitsui agreement, were now choosing between Sulzer and B&W (now MAN B&W after the 1980 merger) on commercial terms rather than on long-standing relationships. Korean yards, which were growing rapidly in the late 1980s, had less historical alignment with either builder and were price-sensitive. NSD needed the flexibility to negotiate licensee agreements and pricing independently of the Sulzer parent’s broader industrial strategy.

Wartsila enters the picture

Wartsila of Finland was, by the early 1990s, one of the largest European marine equipment manufacturers, with a strong medium-speed diesel engine business (particularly the Wartsila 46 and 32 families) and ship design capabilities. Wartsila did not build slow-speed two-stroke crosshead engines; its diesel engines were four-stroke medium-speed engines for smaller vessels and offshore applications. Acquiring a slow-speed engine business would give Wartsila coverage across the full marine diesel propulsion spectrum.

In parallel, Diesel Ricerche of Italy (a smaller Italian diesel research and engineering firm) was in discussion with both NSD and Wartsila during the early 1990s.

The 1997 merger: Wartsila NSD

In April 1997, New Sulzer Diesel, Wartsila Diesel Oy (Finland), and Diesel Ricerche (Italy) merged to form Wartsila NSD Corporation. The transaction gave Wartsila full ownership of the slow-speed two-stroke product line and the Winterthur engineering operation. Sulzer Brothers received the merger consideration and exited the marine engine business after 99 years of continuous involvement.

The terms of the transaction were not publicly disclosed in detail. Sulzer Brothers AG continued operating its remaining businesses (pumps, surface coatings, turbomachinery) and remains an active Swiss industrial company today, with no operational connection to marine engines.

The 1997 merger is the clean break in the ownership chain. After April 1997:

• Engine names retained “RTA” and later “Sulzer” in marketing references for continuity with the installed base, but the legal and operational entity was Wartsila NSD.
• The Winterthur facility operated as Wartsila Switzerland (the Swiss subsidiary of Wartsila NSD Corporation, later Wartsila Corporation).
• RTA engines continued in production, and the RT-flex development programme, already under way internally, continued under Wartsila Switzerland management.

For the post-1997 corporate history, the WinGD corporate history article covers the Wartsila period and the 2015 WinGD creation in full.

RT-flex: common rail on a slow-speed engine (2001 to 2012)

The technical challenge

Mechanically timed injection on the RTA series meant injection timing, duration, and pressure were all functions of camshaft geometry. A fuel cam ground to inject at 15 degrees before top dead centre did exactly that at every load, every ambient temperature, every cylinder. To change timing required a different cam or a mechanical timing adjustment; neither was practical in service without a shipyard visit.

Electronic common-rail injection, already standard on automotive diesel engines by the mid-1990s and introduced by Robert Bosch for medium-speed marine engines, requires injection pressures above 800 to 1,000 bar in a common accumulator rail shared across all injectors, with solenoid-controlled injector valves that open and close on electronic command. Injection timing, duration, and rail pressure are software parameters. But applying common-rail to a slow-speed crosshead engine presented problems that automotive and medium-speed engineers had not faced:

• Bore sizes of 350 to 960 mm meant fuel flow rates per injection event orders of magnitude larger than automotive injectors.
• Cycle frequencies of 80 to 110 rpm meant injection events were physically slower, requiring larger valve opening durations.
• The injectors needed to survive in a combustion environment of approximately 180 bar cylinder pressure at firing, with thermal cycling at every revolution.
• Rail pressure had to be sustained at approximately 1,000 bar continuously, requiring high-pressure pumps capable of driving the accumulated rail against the back-pressure of individual injections.

Sulzer’s Winterthur team (by 1997 operating as Wartsila Switzerland) developed solutions to each of these problems through the mid-1990s, with development work tracked under internal project numbers before the RT-flex designation was finalised.

First commercial RT-flex engine: September 2001

The first commercial RT-flex engine entered service in September 2001: a 6RT-flex58T-B installed on the bulk carrier MV Gypsum Centennial. The 58T bore (580 mm) was a mid-range cylinder size chosen for the first commercial installation to limit the consequences if any development problem required corrective action. At 6 cylinders, the engine produced approximately 13,560 kW at its maximum continuous rating.

The RT-flex differed from the RTA not in basic architecture (both were uniflow-scavenged, crosshead, two-stroke) but in the injection and exhaust-valve actuation systems:

• Common rail fuel injection: a high-pressure fuel accumulator rail at approximately 1,000 bar, maintained by variable-delivery high-pressure pumps, feeding electronically controlled injectors per cylinder. Injection timing, number of injections per cycle (multi-pulse capable), and injection duration were all software-controlled.
• Electronically controlled exhaust valve actuation: hydraulic actuation of the exhaust valve timing was also software-controlled, allowing exhaust valve timing to be adjusted for load and ambient conditions independently of any mechanical cam.
• Cylinder-by-cylinder balancing: power output per cylinder could be equalised by adjusting injection quantity per cylinder in software, correcting for manufacturing variation or wear-related differences in compression ratio.

The RT-flex also allowed full-load flexibility down to approximately 10% of MCR without combustion instability, which was significant for slow-steaming operation. An RTA engine at 10% load would have severe combustion instability because the mechanical cam timing was optimised for high load. The RT-flex could retard injection electronically to stabilise combustion at very low loads.

For the technical history of common-rail injection on large two-stroke engines, including the development sequence and comparison with the MAN B&W ME system, see Common Rail Fuel Injection on Two-Stroke Engines and Marine Engine Common Rail Technology.

RT-flex96C: world’s most powerful diesel engine (2006)

The RT-flex96C was launched in 2006 as the top of the RT-flex bore range. With 960 mm bore, the 14-cylinder version of the RT-flex96C produced 80,080 kW at 102 rpm, making it at that time the most powerful diesel engine ever built for any application. It was the main engine for the Emma Maersk class container ships (8 vessels, 2006 to 2008), each carrying approximately 14,770 TEU.

The 80 MW rating was achieved through a combination of the common-rail system’s injection flexibility (allowing higher peak pressures than fixed-cam injection), the long stroke enabling high BMEP without excessive piston speed, and extensive development of turbocharger matching across the full load range. The 14RT-flex96C-B configuration consumed approximately 14,800 tonnes of fuel per year at design conditions, making SFOC (in grams per kilowatt-hour) the dominant operating cost after crew wages.

RT-flex product range

Wartsila Switzerland marketed the RT-flex in bore sizes from 35 cm (RT-flex35, for smaller vessels) through 58, 68, 82, and 96 cm. The full RT-flex product line in production included:

Exhaust valve actuation and the RT-flex system

One structural feature of the RT-flex worth noting is the exhaust valve actuation architecture. On the RTA, exhaust valve actuation was hydraulic but cam-driven: the camshaft follower drove a hydraulic piston that opened the valve. On the RT-flex, the camshaft’s fuel-injection function was eliminated, but a simpler actuation camshaft for the exhaust valve was retained on some early variants. Later RT-flex variants moved to fully camshaft-free operation with electronically controlled exhaust valve actuation (electrohydraulic), removing the mechanical camshaft entirely. The exhaust valve actuation in two-stroke engines article covers the actuation systems in technical detail.

Ownership transfer to WinGD: 2015 to 2016

The Wartsila Corporation period (1997 to 2015)

From 1997 to 2015, the Winterthur operation ran as Wartsila Switzerland, the slow-speed engine division of Wartsila Corporation. The division’s products were marketed as “Wartsila RT-flex” and “Wartsila RTA” engines, with “Sulzer” appearing in historical references and in legacy parts catalogues. The Winterthur design team was largely continuous with the pre-1997 Sulzer staff; several engineers who had worked on the RTA series in the 1980s remained active into the 2000s and participated in the RT-flex development.

Through this period, the slow-speed two-stroke engine business was financially and technically distinct from Wartsila’s medium-speed engine and services businesses. Slow-speed engines sell in volumes of a few hundred per year globally (constrained by global newbuilding), have high design-engineering content, and depend on licensee relationships with Asian yards for most manufacturing. Medium-speed engines sell in thousands per year across industrial, offshore, and smaller marine applications, and have a very different aftermarket services profile. The two businesses did not share much commercially beyond corporate overhead.

By the early 2010s, Wartsila was reconsidering whether the capital-intensive slow-speed design business was strategically core. The Chinese shipbuilding industry had grown to account for over 40% of global newbuilding tonnage by 2012 to 2013, and CSSC (China State Shipbuilding Corporation) was the group behind several of the largest Chinese yards.

WinGD formation: January 2015

On 19 January 2015, Wartsila and CSSC announced the formation of WinGD (Winterthur Gas & Diesel Ltd.) as a joint venture to take over the slow-speed two-stroke engine business. At formation, CSSC held 70% and Wartsila held 30% of the joint venture. WinGD was incorporated in Switzerland, headquartered at Winterthur, and staffed by the former Wartsila Switzerland engineering team.

The arrangement gave CSSC access to the design rights, product portfolio, and licensee relationships of the world’s largest slow-speed engine design organisation. It gave Wartsila an orderly exit from a capital-intensive business it considered non-core, with a retained 30% stake that could be monetised later.

Full CSSC ownership: June 2016

On 29 June 2016, Wartsila announced it would divest its remaining 30% stake in WinGD to CSSC. The transaction resulted in Wartsila taking approximately EUR 21 million in write-downs in second-quarter 2016 results, implying the book value of the 30% stake exceeded the disposal proceeds. CSSC has owned WinGD 100% since late 2016, with WinGD operating as an independent Swiss company within the CSSC group.

The Winterthur engineering centre remained in place. WinGD continued to maintain design authority over the engine designs, support the global licensee network, and develop new engine types. The engineers in Winterthur remained the technical reference point for engines bearing the Sulzer/RT-flex/X-series genealogy worldwide.

WinGD’s X-series: the post-Sulzer lineage

Branding transition

From 2015, WinGD progressively rebranded its engine portfolio from “RT-flex” to the X-series. The X designation replaced RT-flex as the commercial name, with bore sizes indicated by two digits: X35, X40, X52, X62, X72, X82, X92. An X62 is a 620 mm bore engine; an X92 is 920 mm bore. The X-series engines are architecturally continuous with the RT-flex: uniflow scavenging, common-rail injection, electronic exhaust valve actuation, camshaft-free.

X-DF: dual-fuel operation

The X-DF (X-series Dual Fuel) engines use the same base architecture but add a low-pressure gas-admission system for operation on LNG. Unlike MAN B&W’s ME-GI approach (which injects gas at high pressure, approximately 300 bar, in Diesel cycle combustion), the X-DF uses low-pressure gas admission at approximately 16 bar into the scavenge air stream, with Otto-cycle combustion. The tradeoff is that Otto-cycle gas combustion has a risk of methane slip (uncombusted methane passing into the exhaust), whereas high-pressure Diesel-cycle combustion does not. WinGD addressed methane slip through the iCER (Intelligent Control by Exhaust gas Recycling) system, introduced on the X-DF2.0.

The first commercial X-DF engine entered service in 2016 on the vessel SK Audace, an LNG-fuelled container ship. By 2024 WinGD had delivered over 200 X-DF engines across the container ship, tanker, and LNG carrier segments. The WinGD X-DF dual-fuel architecture article covers the X-DF technical architecture in full.

Ammonia and methanol variants

WinGD has announced the X-DF-A (ammonia-fuelled) and X-DF-M (methanol-fuelled) variants of the X-series as responses to IMO decarbonisation requirements. The X-DF-M had first full-load running reported in December 2024. X-DF-A deliveries were scheduled for 2025 to 2026 on EXMAR vessels. These developments continue the Sulzer-lineage engine’s adaptation to alternative fuels that began with the 1972 gas experiment on an LNG carrier, though the current generation represents a substantially different engineering proposition from that early isolated test.

Ownership chain: summary table

Engine series chronology

Why Sulzer stayed with loop scavenging from 1912 to 1983

The 70-year persistence of loop scavenging at Sulzer, alongside B&W’s uniflow approach, is not adequately explained by conservatism alone. Several factors sustained the choice:

No cylinder-cover moving parts. A loop-scavenged engine with port-only gas exchange has no valves, seats, guides, or actuators in the cylinder cover. The cylinder cover is a static pressure boundary. On a uniflow engine, the exhaust valve is a machined component operating at approximately 400 to 600 degrees Celsius, subject to corrosive attack from heavy fuel combustion products, and requiring replacement on a planned maintenance interval of approximately 6,000 hours. The Sulzer team argued, not without evidence, that eliminating the exhaust valve reduced maintenance burden and failure risk per cylinder in service.

Port profiling flexibility. Loop scavenge port geometry could be adjusted through liner design without affecting anything external to the cylinder. Changing the angle, height, or number of ports to optimise gas exchange for a new stroke or bore combination required only a revised liner, not a revision to the cylinder cover actuation system. This gave the Winterthur team more degrees of freedom in port-geometry optimisation across a wide bore range.

Adequate efficiency at moderate stroke-to-bore ratios. For the stroke-to-bore ratios common through the 1960s (approximately 1.8 to 2.5), loop scavenging’s gas-exchange efficiency was within a few percent of uniflow’s, as demonstrated by test-bed measurements that both builder teams published in technical society papers through the 1960s and 1970s.

The argument ended when the market demanded stroke-to-bore ratios above 3.0 for very-large propeller configurations. At that geometry, the scavenge and exhaust port window in a loop-scavenged liner becomes too small relative to cylinder displacement to achieve adequate gas exchange within the available crank-angle window. Sulzer’s 1983 decision to switch to uniflow for the RTA was, in effect, an acknowledgement that the efficiency argument had a geometric limit that the market had reached.

The loop scavenging versus uniflow scavenging article covers the thermodynamics and measured efficiency data in detail. See also cross-scavenging in legacy two-stroke designs for the third scavenging approach that neither Sulzer nor B&W adopted for large crosshead engines.

Notable vessel installations

Sulzer engines, under Sulzer, Wartsila, and WinGD branding, powered a substantial fraction of the world’s large merchant fleet across the 20th and early 21st centuries:

• MV Monte Penedo (1912): 2 × 4S47, the first ocean-going two-stroke Sulzer installation.
• Emma Maersk class (2006 to 2008, 8 ships): 1 × 14RT-flex96C-B, 80,080 kW, largest diesel engines yet built.
• SK Audace (2016): first commercial X-DF engine installation, LNG dual-fuel.
• Thousands of bulk carriers, tankers, and container ships built through Japanese, Korean, and Chinese yards from the 1960s to the present, with Mitsui-built Sulzer/Wartsila/WinGD engines forming a large fraction of that total.

No complete fleet census of Sulzer-licensed engines exists in public documentation, but Wartsila’s own historical references cite over 10,000 slow-speed engines in service or production under the Sulzer/Wartsila/WinGD lineage through the 2010s.

Service and spares for legacy engines

Engines built before 1997 under the Sulzer name are the responsibility of:

• WinGD, for design authority, technical bulletins, and OEM parts still in production.
• Wartsila Marine (which retained the QuantiParts spare-parts business from the pre-WinGD period), for legacy parts on Sulzer RTA and RT-flex engines.
• Specialist aftermarket suppliers for older RD and RND engines, which date from 1957 to the mid-1980s and are reaching the end of their economic service lives.

Engines built between 1997 and 2015 under Wartsila branding (Wartsila RTA, Wartsila RT-flex) are supported by both WinGD and Wartsila Marine, with WinGD holding design authority and Wartsila Marine holding parts inventory. Engines built after 2015 under WinGD branding are exclusively a WinGD responsibility.

Limitations

This article traces the corporate ownership chain and engine technical development from documented primary sources (WinGD history pages, Wartsila press releases, Pounder’s Marine Diesel Engines). Several caveats apply:

Specific performance figures are nominal. Power ratings, SFOC values, and stroke-to-bore ratios given here are as published in engine programme documentation. Actual ship installations were often derated, and fuel consumption in service varies with propeller matching, hull condition, and operating profile. The engine BMEP calculator and SFOC sensitivity calculator allow site-specific calculations.

Licensee production history is incomplete in public records. The full quantity of Sulzer-designed engines built by Mitsui, Stork, and other licensees through the 20th century has not been consolidated into any single public source accessible at the time of writing. Figures like “over 10,000 engines” are Wartsila marketing-document figures, not independently audited production counts.

The 1997 merger terms are not publicly disclosed. The financial terms of the NSD-Wartsila-Diesel Ricerche merger were not publicly reported at the time and have not been disclosed subsequently. The description here follows the structural facts of the merger (entities, timing, outcome) without attempting to characterise valuations.

The double-acting engine programme’s end date is imprecise. Sulzer’s double-acting two-stroke programme, introduced in 1923, wound down in the 1930s, but the precise year of last production is not documented in publicly accessible sources.

WinGD X-DF-A and X-DF-M status is current as of mid-2025. Alternative-fuel engine programmes are subject to ongoing development, regulatory change, and customer order timing. Dates and specifications for ammonia and methanol variants should be verified against current WinGD product documentation before use in vessel specifications.

See also

• WinGD Corporate History
• WinGD X-DF Dual-Fuel Engine Architecture
• MAN B&W ME-C Electronic Control Overview
• Two-Stroke Marine Diesel Engine Fundamentals
• Loop Scavenging Versus Uniflow Scavenging
• Uniflow Scavenging in Two-Stroke Marine Engines
• Common Rail Fuel Injection on Two-Stroke Engines
• Marine Engine Common Rail Technology
• Exhaust Valve Actuation in Two-Stroke Engines
• Cylinder Lubrication Systems for Two-Stroke Engines
• Burmeister & Wain History
• NOHAB and Polar Marine Engines
• Specific Fuel Oil Consumption
• Cross-Scavenging in Legacy Two-Stroke Designs

Related calculators:

• Main Engine Slow-Speed 2-Stroke System
• Engine BMEP Calculator
• Engine Mean Piston Speed
• SFOC Sensitivity to Air Temperature
• Engine Model Decoder