A type of internal combustion engine which ignites fuel by injecting it into hot, high-pressure air in a combustion chamber. It has neither carburettor nor ignition system. The fuel is injected in the form of a very fine spray, by means of a nozzle, into the combustion chamber. There it is ignited by the heat of compressed air which the chamber has been charged with. The diesel engine operates within a fixed sequence of events, which may be achieved either in four or two strokes. The low-speed (i.e. 70 to 120 rev/min) two-stroke diesel is used for main propulsion units, since it can be directly coupled to the propeller and shafting. The medium speed four-stroke engine (250 – 1200 rev/min) is used for the auxiliaries such as alternators and also for main propulsion with a gearbox.
A four-stroke diesel engine resembles a gasoline engine as it works on the four-stroke cycle, that is: admission, compression, power and exhaust. When the piston gets down on the air admission stroke, the lower pressure in the cylinder allows a charge of air into the cylinder through the inlet valve which opens just before top dead centre.
Once the piston has passed the bottom dead centre and is beginning to ascend, the inlet valve closes and the upward movement of the piston compresses the air charge in the cylinder causing a quick rise of temperature. Before the second stroke is over, the charge of fuel oil is gradually injected into the cylinder by an injector.
The burning of the air-fuel charge makes the gases expand. They push the piston downwards and create the power stroke. Before the piston reached the bottom dead centre, the exhaust valve opens and, as the piston goes up again, the burnt gases are forced out through the exhaust valve. Just before top dead centre the inlet valve opens and the cycle begins again.
- High-speed diesel engine – Trunk piston type engine having a rated speed of 1400 rpm or above.
- Medium-speed diesel engine – Trunk piston type engine with speed range from 400 rpm to 1200 rpm.
- Low-speed diesel engine – Crosshead type engine with rated speed of less than 400 rpm.
From the Wärtsilä 46 Project Guide:
With a bore of 46cm and a stroke of 58cm, the rated output of the Wärtsilä 46F engine is 1250kW/cyl at 600rpm. Ancillary equipment such as pumps, thermostats and lubrication oil module can be either built on engine or separate. Al connections are concentrated at a few points to reduce installation work.
1. Engine block
The engine block is made of nodular cast iron in one piece for all cylinder numbers. The main bearing caps are fixed from below by two hydraulically tensioned screws. They are guided sideways by the engine block at the top as well as at the bottom. Hydraulically tensioned horizontal side screws support the main bearing caps.
The crankshaft is forged in one piece. Counterweights are fitted on every web. High degree of balancing results in an even and thick oil film for all bearings.
3. Connecting rod
The connecting rod of alloy steel is forged and machined with round sections. Thelower end is split horizontally to allow removal of piston and connecting rod throughthe cylinder liner. All connecting rod bolts are hydraulically tightened. The gudgeonpin bearing is of tri-metal type. Oil is led to the gudgeon pin bearing and to the piston through a bore in the connecting rod.
4. Main bearings and big end bearings
The big end bearings are of tri-metal type with steel back, lead bronze lining and a soft and thick running layer. Both tri-metal and bi-metal bearings are used as main bearings.
5. Cylinder liner
The centrifugally cast cylinder liner has a high and rigid collar to minimise deformations. The liner material is a special grey cast iron alloy developed for excellent wear resistance and high strength. Accurate temperature control is achieved with precisely positioned longitudinal cooling water bores. To eliminate the risk of bore polishing, the liner is equipped with an anti-polishing ring. The cooling water space between block and liner is sealed off by double O-rings. In the upper end the liner is equipped with an anti-polishing ring to eliminate bore polishing and reduce lube oil consumption.
6. Piston and piston rings
The piston is of composite design with nodular cast iron skirt and steel crown. The pistonskirt is pressure lubricated, which ensures a controlled oil distribution to the cylinder liner under all operating conditions. Oil is fed to cooling gallery in the piston top through the connecting rod. The piston ring grooves are hardened for good wear resistance. The piston ring set consists of two directional compression rings and one spring-loaded conformable oil scraper ring. All piston rings have a wear resistant chromium plating.
7. Cylinder head
The cylinder head is designed for easy maintenance with only four hydraulically tightened studs. No valve cages are used, which results in very good flow dynamics in the exhaust gas channel. The exhaust valve seats are water cooled and all valves are equipped with valve rotators. The seat faces of the inlet valves are Stellite-plated. In case the engine is specified for MDF operation only, also the exhaust valves are Stellite-plated. Engines that are intended for operation on HFO have Nimonic exhaust valves.
Further information: Project Guide Wärtsilä 46