Machinery Spaces
Home || Diesel Engines ||Boilers||Feed Systems ||Steam Turbines ||Fuel Treatment ||Pumps ||Refrigeration ||

Two-stroke Cycle Marine Diesel Engine

The two-stroke cycle is completed in two strokes of the piston or one revolution of the crankshaft. In order to operate this cycle where each event is accomplished in a very short time, the engine requires a number of special arrangements. First, the fresh air must be forced in under pressure. The incoming air is used to clean out or scavenge the exhaust gases and then to fill or charge the space with fresh air. Instead of valves holes, known as 'ports', are used which are opened and closed by the sides of the piston as it moves.

ships machinery spaces
container ships machinery info
A cross-section of a two-stroke cycle engine is shown in Figure below . The piston is solidly connected to a piston rod which is attached to a crosshead bearing at the other end. The top end of the connecting rod is also joined to the crosshead bearing.

Ports are arranged in the cylinder liner for air inlet and a valve in the cylinder head enables the release of exhaust gases.

The incoming air is pressurised by a turbo-blower which is driven by the outgoing exhaust gases. The crankshaft is supported within the engine bedplate by the main bearings. A-frames are mounted on the bedplate and house guides in which the crosshead travels up and down. The entablature is mounted above the frames and is made up of the cylinders, cylinder heads and the scavenge trunking.

2 stroke engine
A cross-section of a two-stroke cycle engine .


Comparison between two stroke cycle diesel engine and a four stroke engine

The main difference between the two cycles is the power developed. The two-stroke cycle engine, with one working or power stroke every revolution, will, theoretically, develop twice the power of a four-stroke engine of the same swept volume. Inefficient scavenging however and other losses, reduce the power advantage .

For a particular engine power the two-stroke engine will be considerably lighter—an important consideration for ships. Nor does the two-stroke engine require the complicated valve operating mechanism of the four-stroke. The four-stroke engine however can operate efficiently at high speeds which offsets its power disadvantage; it also consumes less lubricating oil.

Each type of engine has its applications which on board ship have resulted in the slow speed (i.e. 80— 100 rev/min) main propulsion diesel operating on the two-stroke cycle. At this low speed the engine requires no reduction gearbox between it and the propeller.

The four-stroke engine (usually rotating at medium speed, between 250 and 750 rev/ min) is used for auxiliaries such as alternators and sometimes for main propulsion with a gearbox to provide a propeller speed of between 80 and 100 rev/min.

The reason for circulating fuel in large two-stroke engines is to ensure the system is fully primed and at temperature. Oil that has been recirculated in a diesel engine fuel system normally returns to a buffer/vent/mixing tank.

2 stroke engine

Fig: Two stroke engine

4 stroke engine

Fig: Four stroke engine


Auxiliary blowers – On two-stroke diesels, electrically-driven auxiliary blowers are usually provided because the exhaust gas-driven turbo-charger cannot provide enough air at low engine speed. Auxiliary blowers are internal parts of the main engine.


Marine diesel engine related other useful articles:
  1. Four stroke cycle diesel engines operational guideline

    2. The four-stroke cycle is completed in four strokes of the piston, or two revolutions of the crankshaft. In order to operate this cycle the engine requires a mechanism to open and close the inlet and exhaust valves
    More .....

  2. Two stroke cycle diesel engines operational guideline

    3. The two-stroke cycle is completed in two strokes of the piston or one revolution of the crankshaft. In order to operate this cycle where each event is accomplished in a very short time, the engine requires a number of special arrangements.
    More .....

  3. Power measurement for marine diesel engine - The engine indicator

    4. There are two possible measurements of engine power: the indicated power and the shaft power. The indicated power is the power developed within the engine cylinder and can be measured by an engine indicator. The shaft power is the power available at the output shaft of the engine and can be measured using a torsionmeter or with a brake.
    More .....

  4. Supply of fresh air and removal of exhaust gases by a gas exchanger

    5. A basic part of the cycle of an internal combustion engine is the supply of fresh air and removal of exhaust gases. This is the gas exchange process. Scavenging is the removal of exhaust gases by blowing in fresh air.
    More .....

  5. The fuel oil system for a diesel engine

    6. The fuel oil system for a diesel engine can be considered in two parts—the fuel supply and the fuel injection systems. Fuel supply deals with the provision of fuel oil suitable for use by the injection system.
    More .....

  6. Lubricating oil system for a marine diesel engine - how it works

    7. The lubrication system of an engine provides a supply of lubricating oil to the various moving parts in the engine. Its main function is to enable the formation of a film of oil between the moving parts, which reduces friction and wear. The lubricating oil is also used as a cleaner and in some engines as a coolant.
    More .....

  7. Cooling of ships engine - how it works , requirement of fresh water & sea water cooling system

    8. Cooling of engines is achieved by circulating a cooling liquid around internal passages within the engine. The cooling liquid is thus heated up and is in turn cooled by a sea water circulated cooler. Without adequate cooling certain parts of the engine which are exposed to very high temperatures, as a result of burning fuel, would soon fail.
    More .....

  8. Starting air system for diesel engine - how it works

    9. Diesel engines are started by supplying compressed air into the cylinders in the appropriate sequence for the required direction. A supply of compressed air is stored in air reservoirs or 'bottles' ready for immediate use. Up to 12 starts are possible with the stored quantity of compressed air.
    More .....

  9. Governor-Function of governors controlling speed of marine diesel engine

    10. The principal control device on any engine is the governor. It governs or controls the engine speed at some fixed value while power output changes to meet demand. This is achieved by the governor automatically adjusting the engine fuel pump settings to meet the desired load at the set speed.
    More .....

  10. Cylinder relief valve of a marine diesel engine - operational guideline

    11. The cylinder relief valve is designed to relieve pressures in excess of 10% to 20% above normal. The operation of this device indicates a fault in the engine which should be discovered and corrected.
    More .....

  11. Explosion relief valve of a marine diesel engine

    12. As a practical safeguard against explosions which occur in a crankcase, explosion relief valves or doors are fitted. These valves serve to relieve excessive crankcase pressures and stop flames being emitted from the crankcase. They must also be self closing to stop the return of atmospheric air to the crankcase.
    More .....

  12. Turning gear operational guideline
    The turning gear or turning engine is a reversible electric motor which drives a worm gear which can be connected with the toothed flywheel to turn a large diesel. A slow-speed drive is thus provided to enable positioning of the engine parts for overhaul purposes.
    More .....

  13. Couplings, clutches and gearboxes of a marine diesel engine

    14. The principal control device on any engine is the governor. It governs or controls the engine speed at some fixed value while power output changes to meet demand. This is achieved by the governor automatically adjusting the engine fuel pump settings to meet the desired load at the set speed.
    More .....

  14. MAN B&W diesel engine - Basic principles and operational guideline

    15. It is one of the MC series introduced in 1982, and has a longer stroke and increased maximum pressure when compared with the earlier L-GF and L-GB designs.
    More .....

  15. Crankcase oil mist detector of a marine diesel engine

    16. It is one of the MC series introduced in 1982, and has a longer stroke and increased maximum pressure when compared with the earlier L-GF and L-GB designs.
    More .....

  16. Various Heat exchanger for running machinery on board cargo ships

    17. Shell and tube heat exchangers for engine cooling water and lubricating oil cooling have traditionally been circulated with sea water. The sea water is in contact with the inside of the tubes, tube plates and water boxes.
    More .....

  17. Guideline for Turbochargers safety and operational requirement

    18. Shell and tube heat exchangers for engine cooling water and lubricating oil cooling have traditionally been circulated with sea water. The sea water is in contact with the inside of the tubes, tube plates and water boxes.
    More .....

  18. Function of Piston & piston rings

    19. Piston forms the lower part of the combustion chamber. It seals the cylinder and transmits the gas pressure to the connecting rod. The Piston comprises of two pieces; the crown and the skirt.The crown of a piston is subject to mechanical and thermal stresses.
    More .....




Marine machineries - Useful tags

Marine diesel engines ||Steam generating plant ||Air conditioning system ||Compressed air ||Marine batteries ||Cargo refrigeration ||Centrifugal pump ||Various coolers ||Emergency power supply ||Exhaust gas heat exchangers ||Feed system ||Feed extraction pump || Flow measurement || Four stroke engines || Fuel injector || Fuel oil system || Fuel oil treatment ||Gearboxes || Governor || Marine incinerator || Lub oil filters || MAN B&W engine || Marine condensers || Oily water separator || Overspeed protection devices || Piston & piston rings || Crankshaft deflection || Marine pumps || Various refrigerants || Sewage treatment plant || Propellers || Power Plants || Starting air system || Steam turbines || Steering gear || Sulzer engine || Turbine gearing || Turbochargers || Two stroke engines || UMS operations || Drydocking & major repairs || Critical machinery || Deck machineries & cargo gears || Control and instrumentation ||Fire protection ||Engine room safety ||





Machinery Spaces.com is about working principles, construction and operation of all the machinery items in a ship intended primarily for engineers working on board and those who working ashore . For any remarks please Contact us

Copyright © 2010-2016 Machinery Spaces.com All rights reserved.
Terms and conditions of use
Read our privacy policy|| Home page||