S20 represent modern state-of-the-art in diesel engine technology for continuous, reliable operation on heavy and marine fuel oil.
Cylinder head and exhaust valves
The water-cooled cylinder head has two inlet and two exhaust valves. It is held down by four hydraulically tightened studs.
The S20, AL20H,ATL25 cylinder heads represents the state-of-the-art in diesel engine design for heavy fuel oils, with its key design features of:
Bore cooling for extreme rigidity. In fact, the AL20H, ATL25 was the first four-stroke engine to have bore cooling in a four-valve cylinder head. Smaller deformations, whether thermal or mechanical, greatly improve seating of the exhaust valves.
Intensively cooled seat inserts for the exhaust valves ensure low and uniform temperatures around the valve seating faces. Insulating air gap in each seat insert prevents overcooling and thus low-temperature corrosion in the exhaust gas passage above the valve.
Rotators on the exhaust valves equalize the circumferential temperature distribution and keep the seats clean to maintain good valve sealing.
Exhaust valves in Nimonic alloy for high corrosion resistance.
The result is long intervals between exhaust valve inspections, up to 9000 h on heavy fuel oil. So combined overhauls of exhaust valves and pistons are practicable and cut time out of service.
The engine casing combines the crankcase and cylinder block in a single-piece grey iron casting. The main bearing caps for the underslung crankshaft are secured by conventional vertical and transverse bolts. The casing is designed to have low stresses and to transmit forces directly between the cylinder heads and crankshaft. Its high stiffness maintains good alignment for the crankshaft and also allows the engine to be carried on resilient mountings.
The charge air manifold is incorporated within the engine casing: between the cylinder banks of Vee-form engines and on the back of in-line engines. Crankcase doors provide easy access to the running gear from both sides and are fitted with crankcase relief valves.
Forged in alloy steel, the crankshaft is characterizes by large-diameter pins and journals giving a high torsional stiffness. It is machined all over and fitted with counterweights to give complete balance of rotating masses.
Thin-walled shell bearings are used for both main and bottom-end bearings. Following extensive and successful field trials, S20, AL20H, ATL25 engines are now fitted with a new aluminium-alloy bearing shells with better wear resistance than the previous tri-metal type. Furthermore, main bearings of in-line engines are equipped with modern white-metal bearing shells
The wet-type cylinder liner is supported on the casing through a relatively elastic, symmetrical sleeve which greatly reduces liner bore distortion for the best piston running. Liner wall temperatures are matched for heavy-fuel operation to prevent both low-temperature corrosion and lubricating oil deterioration.
Piston and connecting rod
The two-part piston comprises a forged steel crown and skirt. Particular features of its design include:
- appropriate temperatures in the oil-cooled crown for reliable operation on heavy fuel oil, and in particular with low ring-groove temperatures;
- carefully chosen ring pack for the best piston-running behaviour. It consists of two chromium-plated piston rings with barrel-shaped faces to promote initial running in, a stepped piston ring to help stability, and two spring-loaded oil-scraper rings. Flanks of the rings and grooves are also chromium-plated for heavy fuel oil service.
The bottom end of the connecting rod is diagonally split to allow a large bearing diameter yet with the piston and its rod still able to be withdrawn through the cylinder bore. The two bottom-end studs are hydraulically tightened.
Each cylinder has an individual jerk-type fuel injection pump for its single fuel injection valve. This injection system is specifically designed for heavy fuel oil operation:
- Stepped helix edge on the pump plunger to dampen hydraulic oscillations and prevent secondary injection or cavitation damage thereby extending working life of the nozzle.
- Oil-barrier groove at the lower end of the pump plunger prevents any fuel leakage from contaminating the lubricating oil system.
- Declining-velocity cam allows a high delivery rate, but avoids unnecessary high injection pressure peak.
- Oil-cooled injection nozzles have appropriate tip temperatures for heavy-fuel duty.
- Rounded inner edges to the nozzle holes give more stable behaviour over a longer period with less scatter of injection rates.
- The high-pressure fuel pipe is as short as possible, and is carried through the cylinder head to ensure segregation from the engine lubrication system.