A Marine diesel engine consists of many working parts. Even different components make few modification in its design but their function remains the same. Each component has a definite function and location in its working. Chances are, you already know something about them even more for few; you already be used to reassembling them in a definite order.
For Those who already knew; It would be a brush up ( Revision ) of concepts while for others; its important to know different parts of an engine to understand “marine diesel engine”. But just in case if you want to go in details i will strongly recommend “Marine diesel Engine” a book by Nigel Calder; click over here to check its current price on amazon.
Different Components of Marine Diesel Engine
1 ) Bedplate
A bedplate is made of two parallel girders running across the length of the engine. They are connected together with another set of girders ” Transverse Girders “. It is positioned on the either side of the trust collar between the crank throw. A bearing support made of cast steel are embedded on these transverse girders.
Proper consideration is given towards the design and construction of aft-most transverse girders to maintain appropriate stiffness. This is the part subjected to variable engine thrust. Generally bedplate are constructed by welding cast steel transverse girders in between fabricated longitudinal girders along with bearings and tie bolt holes. But for small engines they are also made as a simple cast of cast iron.
It works as a foundation block for the two stroke marine diesel engine. They are right at the same time strong and flexible to support weight and handle fluctuating forces produced by the engine.
2 ) Crankshaft
A crankshaft is an engine component subjected to high torsion and fluctuating bending & shear stress. Material such as silicon ( 0.3% ), Carbon ( 0.2% ), Sulphur ( 0.02% ), Manganese ( 0.6% ) and Phosphorous ( 0.02% ) is used to build crankshaft. A crankshaft must have good bearing surface, resist wear and tear of journals and crank-pin, Good strength and unlikely for fatigue failure.
They can be constructed in either of the four way:
- Fully Buildup
- One Piece
- Semi buildup
- Welded Construction
A crankshaft is a key component of an engine transmitting cylinder power to the propeller shaft. Basically what it does is to convert the oscillating motion of connecting rod / reciprocating motion of the piston to rotatory motion of the shaft.
3 ) Camshaft
A camshaft consists of set of cams for each unit fitted on to the camshaft hydraulically. The whole shaft is supported by white metal line bearing with set of cams fitted made of steel. They have a separate set of lubrication to avoid any contamination with the fuel due to leakage.
A two stroke marine diesel engine have fewer cam on the camshaft than of four stroke engine due to the fact that; there is no inlet valve in two stroke engine which require cam movement for scavenging. To ensure the correct timing for exhaust and fuel injection; a camshaft is driven by crankshaft. They are connected together either by a chain or gear based on the engine design or so called company. ( Sulzer has gears while B&W uses chain ).
It is a control device which operates the three main valves ( Inlet, Exhaust and Fuel injector ). It operates these valves with the help of cam follower, rocker arm and push rod. Each cam profile is designed to produce desired speed and lift of the follower at appropriate time.
4 ) Frame Box
Frame box also known as “A frame” is a separately fabricated structure mounted on top of the bedplate. They carry crosshead guides while supporting the cylinder block. And are secured to the bedplate using bolts.
They support the cylinder block or popularly called as Entablature from bed plate.
5 ) Piston
A piston is a composite structure with crown and skirt that made the moving part of a combustion chamber. Piston Crown are subjected to fluctuating thermal and mechanical stresses while it transfer the combustion force to the piston rod or connecting rod depending upon type of engine ( 2 Stroke or 4 Stroke ). They are designed as a concave structure at the top to provide maximum combustion efficiency.
It is a general practice to use chromium – molybdenum alloy steel in its construction with a 8mm thick Inconel metal fitted on top of it to avoid burning of crown. It have four to five chromium plated grooves to accompany the piston rings. On other hand the skirt act as a guide for the piston to move along the cylinder liner.
They are subjected to much lower temperature and pressure and so face low thermal and mechanical stresses. A brass bands are fitted on to the skirt for better movement along the cylinder liner.
The function of a piston in marine diesel engine is to convert the force of expanding gases during combustion process to mechanical energy. While during the compression stroke it compresses the gas in between the crown and cylinder head with the energy provided by flywheel. A piston can be called as the heart of an engine as it transform all that energy under fluctuating thermal and mechanical stress.
6 ) Piston Rings
A piston ring have following general characteristics such as strength, resistance to wear and corrosion, elasticity and ability to transfer heat radially. They are made up of alloy cast iron with added minerals like molybdenum, chromium, titanium and nickel; Sometimes copper and vanadium are also added in their construction.
A piston ring provides sealing of combustion chamber by expending outwards. By doing this they prevent the combustion gases leak or escape from the space in between cylinder head and piston crown. It also provide proper heat transfer between the piston and liner while facilitating lubrication control; avoiding mixture of lube oil with the charge.
7 ) Liner
A liner is a thin metal cylinder inserted on top of a cylinder block and secured at the top by cylinder head. This allows cylinder liner to expand downward when subjected to heating. It is constructed of good quality alloy cast iron which can withstand high temperature and pressure of the cylinder.
Certain Bores are grooved inside the liner for better colling and transfer of heat from the piston. This helps maintain the strength of the metal at extremely high temperature. Cooling water is placed called jacket, a space in between the cylinder block and the liner. It is then sealed at the bottom with help of ‘O-Rings’ with a tell tell hole to indicate any leakage.
Space for scavenge air is cut and then machined in lower parts of the liner to generate rotary motion in scavenge air for better efficiency. A number of holes are also drilled for cylinder lubrication in the liner to provide points to inject cylinder lub oil with non return valve to avoid blow backs.
The function of a cylinder liner in marine diesel engine is to provide durable and heat resistant combustion chamber. It also provide area for cooling, lubrication, scavenging and assist in sealing the combustion chamber. This helps prevent compressed gas and combustion product to escape from the engine around the walls of the cylinder.
8 ) Connecting Rod
A connecting rod is fitted in between a crosshead and crankshaft in two stroke marine diesel engine; while in between gudgeon pin and crankshaft in four stroke engine. They are constructed of forged steel with shaped structure at both ends to accommodate bearings. In older designs, white metal bearings were used as bearing; while modern engines have different type of white metal used. When the clearance of these bearings reach manufacturer limit, they are replaced by a new one.
A hole is grooved inside the connecting rod to allow passage of oil for bearings and under piston space cooling. An ideal length of connecting rod is designed to be as less as possible to reduce the size of the engine while dealing with increased angularity and side thrust with short length.
A connecting rod does the function of transforming the reciprocating motion of a piston into rotary motion of crankshaft. It also do the work of transferring power produced by the piston to the crankshaft.
9 ) Cylinder Head
A cylinder head is a structure which accompany important engine parts like inlet valve ( 4 Stroke ), Exhaust valve and fuel injector. They are exposed to maximum temperature and pressure so provided with adequate cooling arrangement. They sit on top of the liner flange and secured with a series of nut and bolts with the cylinder block.
As it is subjected to high temp and pressure it must radially transfer heat, be symmetrical, have high coefficient of thermal expansion and resist bending. Above all they must also have space to install exhaust, inlet and fuel injectors. Internal passage are bored into its design for cooling water to increase cooling efficiency.
To avoid thermal shock due to temperature gap in-between the cylinder and cooling water; reasonably hot water is used for cooling cylinder head. Normally, cooling water for cylinder head is connected in series with liner jacket water.
It’s main function is to form the top part of the combustion space; while supporting all necessary valves required for the operation such as inlet, exhaust and fuel injector.
10 ) Inlet and Exhaust Valves
Large two stroke marine diesel engine have only exhaust valves mounted on the cylinder head. A exhaust valve opens inward to the cylinder to have positive closing due to internal pressure. The valve is operated by the cam profile of cams mounted on the camshaft. Modern engines use air spring rather than mechanical springs as in older designs.
A valve rotor ( 2 Stroke ) or rotocap ( 4 Stroke ) is provided on the valve steam of a exhaust valve to turn it by a short degree on each operation. It ensures even temperature through the valve reducing chances of failure. In some designs high quality heat resistant steel alloy are used for its construction; while newer designs valves made of nimonic metal.
An inlet valve is the larger valve of the two as the compressed air is forced into cylinder. Its large diameter also helps reduce scavenge air temperature avoiding risk of early ignition and knocking. They are constructed by low quality fabricated alloy steel as they are subjected to less corrosion and heat than exhaust valve.
The function of inlet valves in a four stroke marine diesel engine is to inject a charge of fresh air in the combustion space; while the work of exhaust valves all marine engines is to eject out all the byproduct of the combustion.
11 ) Turbocharger
One of the revolutionary development in the diesel engine ( Including Marine Diesel Engine ) that have increased the efficiency of a plant / Ship / Vehicle drastically. It helps increase the power output for the same engine without change in its design or size. It is also known as the supercharger in many automobile; but have a significant importance in the marine industry.
A turbocharger consists of two main parts; blower and the turbine. Both turbocharger and the blower is mounted on the same shaft separated by labyrinth and gland seals. The turbine blades rotates by the energy of the flue gas passing through the nozzle rings. High velocity flue gas coming out of the nozzle rings strike these heat resistant blades of turbine. Which in turn rotate the shaft connected to it. Adequate cooling arrangements are made to prolong its maintenance free ( Break down maintenance ) operation.
A blower is fitted on the other side of the shaft with a filter and inducer to direct the flow of air towards the center. This helps avoid any shock load to the blower blades. These blades are made of light aluminum alloy driven by shaft movement. The impeller sucks in the fresh air axially while delivers it radially through a diffuser.
The function of a turbocharger is to increase the overall power and efficiency of an engine. It has two main parts: A blower and a turbine. Turbine rotates with the shaft by the kinetic energy of flue gas produced by nozzle rings. On other hand a blower rotates by the rotating shaft which in turn produce air suction and then compression to desired pressure.
#Note: I will look forward for your helpful comment and recommendations to improve this Article ( Marine Diesel Engine – Parts And Functions )
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