Marine Diesel Engine is an Internal Combustion(IC) Engine, that uses Heavy Fuel Oil(HFO), and Compressed air to generate the power output. The marine diesel engine that is used for the propulsion of the ship is called as “Main Engine“. And the marine diesel engine that is used for the generation of Electric Power onboard ships is called “Auxiliary Engine“.
In this article, I’ll explain the most frequently asked questions on Marine Deisel Engines.
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How does a marine diesel engine work?
Marine diesel Engines work on the principle of the “Dual Combustion Cycle“. But, to understand the “Dual Combustion Cycle”, you must study the basic combustion process.
The combustion process in every Marine Diesel Engine or any other CI (Compression Ignition) engine involves 4 steps.
- Suction Stroke: Air intake to the Engine.
- Compression Stroke: Air is compressed inside the cylinder to reach compression pressure.
- Ignition/Power stroke: Fuel is injected that burns in the presence of compressed air to generate power output.
- Exhaust Stroke: The unburnt gases are blown out of the engine.
These 4 stages complete one cycle of the combustion process. But, as per the requirement, the combustion cycles are classified into 3 types.
- Otto Cycle(Constant Volume): Spark Ignition (SI) Engines and High-Speed Engines
- Diesel Cycle(Constant Pressure): Compression Ignition (CI) Engines.
- Dual combustion cycle: In most of the modern Compression Ignition (CI) Engines.
How many types of marine diesel engines are there?
Generally, there are 2 types of marine diesel engines that can be found onboard ships.
- Two-Stroke Engines(2S): Most of the Main Engines are 2s Engines.
- Four-Stroke Engines(4S): Auxiliary engines are normally 4s engines.
What are the Differences between Two-Stroke and Four-Stroke Engines?
|Two Stroke Engine||Four Stroke Engine|
|Every revolution of the crankshaft generates 1 power stroke||Two revolutions of the crankshaft generate 1 power stroke.|
|Lighter Flywheel||Heavier Flywheel|
|Low/Medium speed Engines||High-Speed Engines|
|Simple reversing mechanism||Reversing is complicated|
|Slow response for Load changes||Quick response for load changes|
|Auxiliary Blowers are required||Not required|
|Longer Stroke length||Shorter Stroke length|
|Crosshead type Engines||Trunk type engines|
Basic parts of a Two-Stroke Marine Diesel Engine?
The basic components of a 2S marine diesel engine are,
- Connecting rod
- Cylinder block
Basic Construction of 2S Crosshead Marine Diesel Engine
The majority of 2 stroke engines encountered at sea are of the “crosshead” type. In this type of engine the combustion space (formed by the cylinder liner, piston, and cylinder head), and the scavenge space are separated from the crankcase by the diaphragm plate.
The piston rod is bolted to the piston and passes through a stuffing box mounted in the diaphragm plate.
The stuffing box provides a seal between the two spaces, stopping oil from being carried up to the scavenge space and scavenge air leaking into the crankcase.
The foot of the piston rod is bolted to the crosshead pin.
The top end of the connecting rod swings about the crosshead pin, as the downward load from the expanding gas, applies a turning force to the crankshaft.
To ensure that the crosshead reciprocates in alignment with the piston in the cylinder, guide shoes are attached on either side of the crosshead pin. These shoes are lined with white metal, a bearing material and they reciprocate against the crosshead guides, which are bolted to the frame of the engine.
The crosshead guides are located in between each cylinder.
Using the crosshead design of the engine allows engines to be built with very long strokes – which means the engine can burn a greater quantity of fuel/stroke and develop more power.
The fuel used can be of a lower grade with a higher sulphur content than that used in a trunk piston engine.
High alkalinity cylinder oils with a different specification to that of the crankcase oil are used to lubricate the cylinder liner and piston rings and combat the effects of acid attack.
Disadvantages of using two-stroke trunk type engine over crosshead type engine.
The 2 stroke diesel crosshead engine works on exactly the same principle and cycle as the 2 stroke diesel trunk piston engine. The disadvantages of the two-stroke diesel trunk piston engine are:
- Though it has a low overall height, lubricating oil splashed up from the crankcase to lubricate the liner can find its way into the scavenge space, causing fouling and a risk of a scavenge fire.
- There is also the likelihood of liner and piston skirt wear, allowing air into the crankcase.
- This can supply the required oxygen for a crankcase explosion should a hot spot develop.
- The crankcase oil must have additives that can cope with contamination from products of combustion, and the acids formed during combustion due to the sulphur in the fuel
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What are the basic systems of the marine diesel engine?
Four important systems of the marine diesel engine are,
- Air System: That supplies combustion air to the engine. The air system includes various parts such as Air compressors, Air receivers, Starting air valves, Air distributors, Turbochargers, Scavenge manifold, etc.
- Fuel Oil system: The fuel supply system includes Fuel Oil purifiers, Fuel supply pumps, Fuel Injectors, Fuel supply units, etc.
- Lubricating System: It is a very important system for the safe working of the engine. Includes Lube Oil Purifiers, L.O pumps, L.O supply unit, etc.
- Cooling Water System: It is important to keep the engine cool because of the very high temperatures generated during the combustion process.
What are the safeties on Main Engine?
Main Engine Slowdown
- Main L.O pressure low.
- PCO inlet L.O press low.
- PCO low flow.
- PCO non-flow.
- Cylinder L.O. non-flow.
- JCW inlet press low.
- JCW outlet temperature is high.
- Main L.O. inlet temperature is high.
- Cam L.O. inlet temperature high.
- Scavenge airbox temperature high.
- Exhaust gas outlet temperature high.
- Oil mist in crankcase high.
- Control air pressure low.
- Alpha lubricator failure.
Main Engine Alarm
- Emergency stop
- Slowdown canceling
- Shutdown canceling
- Wrong-way alarm
- Critical speed
- Pickup sensor failure
Few important terminologies of marine diesel engine.
- Compression Ratio: It is the ratio of the volume of air at the start of the compression stroke to the volume of air at the end of the compression stroke. Compression Ratio for CI engines 12.5 to 13.5
- Scavenge Efficiency: It is the ratio of the quantity of air drawn inside the cylinder to the total volume of the cylinder.
- Specific Fuel Oil Consumption(SFOC): It is the amount of fuel oil consumed (in grams) to produce per unit power in per unit time. It’s a very important parameter to calculate the performance of the main engine.
- Fuel air ratio: Theoretical – 14.5 Kg per Kg of Fuel, Actual – 29 to 44 kg per kg of fuel.
- Brake-Power: Power output at the drive end shaft of an engine without the power loss by gears, transmission, friction, etc.
- Indicated-Power: Theoretical maximum output power of the Engine. Indicated Power(I.P) = Brake Power(B.P) + Frictional power(F.P)
- Mechanical Efficiency: Brake Power / Indicated Power.
- Mean Effective Pressure: It is the mean effective power that may be assumed to act on the piston during its power stroke.
- Maximum Continuous Rating(MCR): Maximum Continuous output at which the engine can run safely and continuously.
- Continuous Service Rating(CSR): Continous Service Rating is the output at continuous service speed.
- Overload Rating: The extent of overload above MCR that can be safely exerted onto the engine for a short period of time. (10% overload for 1hr)
- Astern Output: Maximum Output of the engine in the Astern Direction
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This was a brief introduction to the marine diesel engine. Which covered the working of the diesel engine, types, differences, advantages & disadvantages, basic parts, construction, Safety devices, important terminology, etc. Detailed read here
Have any questions? Drop them in the comments.
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Very good article and nice explanation, hope useful to every cadet who were joining to start their career in Marchent Navy.