Evolving a popular medium speed programme

SEMT Pielstick’s PC engines were refined and expanded over 40 years to meet changing market requirements with 400mm to 570mm-bore designs

SEMT Pielstick of France forged a pioneering role in developing and promoting four-stroke engines for propulsion and genset drives from the late 1940s before coming under the umbrella of Germany’s MAN Diesel group.

The histories of MAN and SEMT Pielstick were already closely linked. The latter’s German founder, Gustav Pielstick, began working for MAN in 1911 and stayed with the company for almost 35 years. After World War II he became technical manager of Société des Études de Machines Thermiques (SEMT), which had been created by the French government in 1946.

An initial project was to adapt the MAN 40/46 submarine engine for commercial markets, SEMT subsequently pursuing the development of its own medium speed PC and high speed PA diesel engines. SEMT Pielstick evolved to become a driving force in the design and manufacture of four-stroke marine engines, with volume production undertaken in France and by licensees in key shipbuilding countries, notably Japan.

From 1988 SEMT was jointly run by MTU Friedrichshafen and MAN Diesel, first as a 50:50 joint venture and then, from early 1998, under an arrangement in which MAN held two-thirds and MTU one-third of the shares. In October 2006 MAN Diesel SE of Augsburg acquired full control of SEMT Pielstick which became a brand in the German parent’s portfolio, with the French subsidiary operating as MAN Diesel SA.

Numerous examples remain in service at sea but PA and PC engines are no longer produced for marine applications (although some models are still manufactured for nuclear power station back-up and submarine propulsion). The main focus of production at St-Nazaire is now on the larger versions of MAN Diesel’s successful 48/60 series medium speed engines, whose latest derivative features common rail fuel injection.

The Pielstick marque originated in the early 1950s as part of a family of monobloc multiple-crankshaft engines designated the PC1 series. The first PC1 engine ran in 1951 and initial tests with heavy fuel oil (380 cSt) followed in 1953. The first commercial installation, a power station engine, was commissioned in 1953 and the first PC marine engine ran on heavy fuel in 1959.

Further development resulted in more conventional single-crankshaft designs: the 400mm-bore PC2 series, introduced in the mid-1960s; the 480mm-bore PC3 series, introduced from 1971; and the 570mm-bore PC4 series which was launched in the late 1970s. The PC3 series was phased out but successive modernisation and upgrading programmes for the PC2 and PC4 designs resulted in the PC2-6B and PC4-2B versions, which provided SEMT Pielstick’s main manufacturing thrust in the latter years. The rise in maximum firing pressure and reduction in specific fuel consumption for successive generations of 400mm-bore PC2 designs are illustrated (Fig. 1).

Experience gained with the PC2-2 design showed that the one-piece light alloy piston had to be abandoned in favour of the first two-piece design with steel crown and forged light alloy skirt. Developed by SEMT Pielstick in conjunction with a leading piston manufacturer, the new concept allowed higher mechanical and thermal loads, and secured better production quality. The PC2-5 engine, launched in 1973, benefited from this development in delivering a 30 per cent increase in power over the original PC2-2. Other refinements included a bore-cooled cylinder liner and a bevel-cut connecting rod allowing the crankpin diameter to be enlarged.

New turbocharging and fuel-optimised injection systems were applied to the PC2-6 engine, launched in 1982. The modular pulse converter (MPC) turbocharging system featured an almost constant pressure exhaust gas feeding to each turbine through a single exhaust pipe.

Longer stroke derivatives arrived in the mid-1980s to supplement the PC2 and PC4 series, these PC20 and PC40 models retaining the 400mm and 570mm-bore sizes but exploiting higher stroke–bore ratios, respectively increased from 1.15 to 1.375:1 and from 1.08 to 1.31:1. Significantly lower specific fuel consumptions were achieved. The PC20, with a stroke of 550 mm and rating of 607 kW/cylinder at 475 rpm, was subsequently dropped from the programme.

In 1996 SEMT Pielstick’s Japanese licensee Diesel United delivered the world’s most powerful medium speed engines for propelling longhaul coastal ferries. Each V18-cylinder PC4-2B model in the twin-engine installations yielded a maximum continuous output of 23.85MW at 410 rpm (Fig.2).

The last manifestation of SEMT Pielstick’s 400mm-bore PC2 medium speed engine, the 500mm-stroke PC2-6B design, developed 615 kW/cylinder at 500–520 rpm. An output range up to 11.34MW was covered by six, seven, eight and nine in-line and V45-degree 10-, 12-, 14-, 16- and 18-cylinder models (Figs. 3 and 4); a V20-cylinder version was also planned. The PC2-6B engine retained the same overall envelope for propulsion installations as the shorter stroke (460 mm) PC2-6 predecessor as well as its overhaul space requirements.

Planning the PC2-6B engine, the designers aimed to increase power output by at least 10 per cent over the established PC2-6 model without penalising specific fuel consumption and making maximum use of experience from the PC20 (400mm-bore/550mm-stroke) series which was introduced in 1985 (Fig. 5). A rise in power through a higher mean effective pressure rating was limited by:

• the pressure ratio of contemporary turbochargers
• a peak combustion pressure deliberately limited to 150 bar and a compression ratio set at 12.5 to allow trouble-free burning of heavy fuels; the 150 bar limit also reflected a commitment to a short development period for the new model (around one year) and the avoidance of costly production plant investment.

A second path to increased power was to raise the mean piston speed, which, for the PC2-6 engine, was conservative compared with its rivals. Two routes were available: increasing the rotational speed (the smallest possible step to 600 rpm is dictated by the synchronous speed requirement of genset applications); or increasing the stroke.

The latter alternative was pursued, seeking the longest possible stroke that still allowed the main dimensions of the PC2-6 engine crankcase to be retained and the cylinder heads to remain at the same position. These considerations determined a stroke of 500 mm. Engine component validation work was reduced since few new parts were used, mechanical and thermal stresses remained at the same levels as the PC20L, and the running speed was unchanged (hence also the dynamic behaviour of the valve train and injection pumps).

Manufacturing costs were cut by specifying a cast iron crankcase (Fig. 6) featuring an increased distance between seating paths to achieve high rigidity, and a modified water jacket to avoid cooling water circulation in the crankcase. Strain measurements made on the first V18-cylinder PC2-6B industrial engine reportedly verified the predicted values from a comprehensive 3D FEM investigation. An interchangeable welded crankcase was proposed as an alternative.

The cylinder liner (Fig. 7) is similar to that of the PC2-6 component, the smaller water jacket avoiding the direct contact of cooling water with the engine block and hence the possibility of corrosion.

The composite piston design (Fig. 8) is the same as that of the PC20 engine. Its steel head is without valve recesses to promote a uniform temperature distribution and thus limit thermal stresses; and the light alloy skirt has stepped bosses to reduce mechanical stresses under combustion pressure.

Only two compression rings (formerly three) are featured in the five-ring pack which, in conjunction with a honed liner, was designed to foster low lubricating oil consumption. An inspection of the pistons after endurance testing of a three-cylinder prototype engine revealed a ‘very good’ bearing picture for the skirt, without any contact marks between head and liner despite the reduction in skirt length under the piston pin.

An exhaustive study of the connecting rod design (Fig. 9) was dictated by the increased inertia forces resulting from the longer stroke. The key concerns were reliability and the prevention of fretting on the serration. The main differences with the PC2-6 rod design are a higher tightening force at the big end cap and larger bolts, which were moved 10mm outwards from the bore. The effectiveness of the modifications was confirmed by an endurance test on the prototype engine under extremely severe conditions (600 rpm). A subsequent inspection revealed no trace of fretting on the serration.

A modified crankshaf – with a main journal diameter increased to 350mm and crankpin diameter increased to 330mm – was also deemed necessary in view of the impact of a longer stroke and higher peak pressure on the bending and torsional stresses as well as on the bearing operating conditions. The larger diameter main journal maintains the oil film thickness and specific pressure at similar levels to those of the PC2-6. Despite the larger crankpin diameter, the connecting rod big end bearings are slightly more loaded than those of the PC2-6: the 15 per cent higher loading is largely compensated by improved bearing technology (Rillenlager-type shells).

A modified cylinder head design was introduced to improve the pressure distribution on the gasket, the main modifications involving fireplate local reinforcements and repositioning of the side wall closer to the gasket circle. The highest temperature measured on the fireplate (between the exhaust valves) during tests of the prototype was, at 280°C , identical to that found on the PC20 engine.

PC2-6B design details

Frame: a stiff nodular cast iron crankcase construction.

Cylinders: each cylinder, including a water jacket and bore-cooled liner, is fitted into the crankcase. Only the upper part of the liner is cooled, avoiding any flow of water in the crankcase and hence possibility of corrosion.

Crankshaft: the one-piece forged unit rests on underslung main bearings fitted with thin bearing shells. A temperature sensor is fitted on each main bearing, preventing the development of shell bearing abnormality.

Connecting rods: the bevel cut big end cap is secured by serrations on the rod; in V-type engines the connecting rods of a cylinder pair are mounted side-by-side on the same crankpin.

Piston: the composite-type piston comprises a steel crown (‘shaker’ oil cooled) and a light alloy skirt of the stepped boss type to reduce stresses under peak combustion pressure. The floating piston pin permits free rotation. The five-ring pack includes two compression rings while two spring-loaded scraper rings control lube oil consumption, their position in the upper part of the skirt facilitating easier piston and liner lubrication.

Cylinder head: a vermicular cast iron component attached to the water jacket and to the liner by eight tie-bolts anchored in the crankcase bosses.

Valves: both inlet and exhaust valves have a tight guide bush and the exhaust valves feature a turning device. The entire water flow across the cylinder head passes through the exhaust valve cages, substantially decreasing the valve seat temperature.

Fuel injection: the injectors are cooled by a separate freshwater system. The fuel pumps provide a pressure exceeding 1,000 bar.

Camshaft: the bearings are secured with four bolts directly below the injection pump supports, an arrangement which avoids the transmission of injection stresses to the crankcase. Each camshaft and its bearings can be removed laterally from the side of the engine. The fuel pump driving gear design secures a low contact pressure between cam and roller.

Turbocharging: a patented modular pulse converter (MPC) system represents a compromise between impulse and constant pressure systems, and also fosters easier maintenance of piping and expansion bellows.

In creating a low pollutant version of the PC2-6B engine, SEMT Pielstick aimed to find a compromise between low emission levels, first cost and running cost using proven solutions. The following NOx emission reduction techniques, investigated earlier on other prototype engines, were applied:

• increased compression ratio: raised to 14.8 instead of 12.7:1 on the standard engine
• retarded fuel injection: 4 degrees before TDC instead of 11 degrees
• water/fuel emulsion injection in a 30 per cent/70 per cent ratio
• injection rate modification: an injector nozzle with nine 0.76mm-diameter holes instead of the standard nine 0.72mm holes
• turbocharger matching modification: rematched to increase slightly the combustion air excess.

PC4-2B design

The last version of the 570mm-bore PC4 series, the PC4-2B design, had a 660 mm stroke and developed 1,325 kW/cylinder at 430 rpm on a mean effective pressure of 22 bar. The production programme embraced V10-, 12-, 16- and 18-cylinder models covering a power band up to 23.85MW (Figs. 2 and 10). A specific fuel consumption of 176 g/kWh was quoted.

Progressive improvements benefited the PC4 design after the first example entered service in 1977, the refinements focusing on the exhaust and inlet manifolds, exhaust valves, fuel injection system, crankshaft and camshafts. The PC4-2 engine featured a 620 mm stroke and the PC40L (Fig. 11) a stroke of 750 mm and a running speed of 375 rpm.

The key features of the PC4-2B engine included:

• a one-piece welded steel engine frame with a steel plate oil sump mounted on the bottom
• a bore-cooled cylinder liner of special cast iron located inside a cast iron water jacket, avoiding contact between the cooling water and the engine frame; cooling of the liner’s upper part was calculated to reduce thermal stresses
• a one-piece underslung crankshaft of chromium molybdenum forged steel; each main bearing is fixed by two vertical and two horizontal tie-bolts, fostering a weight reduction for the frame; a temperature sensor is installed on each main bearing to prevent shell bearing abnormalities
• composite-type pistons embracing a steel crown and light alloy skirt, with a floating-type piston pin; the crown is oil cooled by the shaker effect
• forged alloy steel connecting rods with a wide big end and stepped boss small end to secure low bearing pressures
• a cast iron cylinder head fixed to the water jacket and to the liner by eight studs screwed into the frame bosses; it incorporates two inlet and two exhaust valves, a fuel valve, an indicator valve, a safety valve and a starting valve
• heat-resistant steel inlet valves exploit the two-guides solution while the Nimonic exhaust valves have sealed guides; all water flowing across the cylinder head runs through the exhaust valve cages, significantly reducing the valve seat temperature; all the valves are provided with Rotocap rotating devices
• camshaft bearings fixed directly under the fuel injection pump brackets avoiding the transmission of injection stresses to the frame
• a large fuel injection pump diameter and short piping underwrite a high injection pressure achieving fine fuel pulverisation even at low load; complete and clean combustion of the heaviest residual fuels with a high asphaltene content is promoted; the fuel pump plunger and barrel can be exchanged through the pump’s upper part without dismantling the pump body; the weight of the dismountable equipment is 20kg; the fuel injectors are cooled by a separate freshwater circuit

• the patented modular pulse converter (MPC) turbocharging system was the best compromise between pulse and constant pressure systems, according to SEMT Pielstick; air coolers could be supplied with two banks of tubes, one of which could be deployed to heat air when the engine operates at low load. MP