In this article I have discussed about large internal combustion engines of diesel generator or diesel engines. Let us discuss this one by one.
First component of I.C engine is bedplate.
Bed plate acts as the main strong point member of the engine and supports main bearings and crankshaft. It can be understand from the view that it is rigid base which supports the heavy engine. It forms a platform over which other components such as columns, frames and guides are mounted which supports the entablature and other operational components. Bed plate is mounted either on a stool or directly on the stiffened tank top with holding down bolts and transmits the engine loads to the ship's structure. This reduces the vibration in main engine and thus provides strength. Being the underside most part of the engine, it forms part of the crank case and is used to collect the lubricating oil for returning to sump for reuse. Lubricating oil is used for lubrication main bearings or piston moving in liner. Bed plates are of different types depending on their structure.
1)Types of bed plates
Mainly two types of bed plates are generally in use.
a) Trestle type
This type of bed plate needs an elevated seating which must be very physically powerful and should have high degree of rigidity. This arrangement is suitable for a balanced engine of moderate length and it is not suitable for large engines.
b) Box and flate type bottom
This type of bed plate is most general as engine is bolted directly to tank top devoid of the need to provide an elevated seating. This saves money and easy to make. The present practice is to use the welded construction which reduces cost and the extra weight. Box type construction is more suitable for fabrication.
2) Construction of bed plate
Bed plate is normally a one piece structure which may be made of cast iron, cast steel, or may be of a hybrid design. One piece cast iron design is typically restricted to smaller engines which provide stress free structure with ease of mach inability. Large casting would become very heavy and costly and at same time may not provide homogeneity of strength, grain size, soundness, impurity segregation etc. Large bed plates are made in parts so that they are easy in making and easier to assemble. Bedplate consists of two main longitudinal girders provided with stiffening members and webs for additional rigidity. Lighting and access holes are cut with compensating sleeves to maintain lightness and strength. Holes are also made to reduce the stress concentration. Transverse members or girders of cast steel join the two boxes and withstand the fluctuating stresses while supporting the main bearings and tie bolts. They are fitted between each throw of the crankshaft and as close as possible. Bed plate carries the main bearings, each of which consists of two identical white metals ( a very soft metal used mainly for bush bearings) lined bearing shell.
Holding down bolts fasten the bed plate to the foundation plate. Holding down bolts are made of special material because there is large stress acting them. Conventional method consists of following operations. This method involves cutting of screw threads in the holes drilled on tank top. Chocks are used as their individual adjustment is possible in case of any distortion, chaffing or fretting takes place. Holding down bolts pass through the holes in the chocks and foundation plate. Coupling bolts are used to connect the crankshaft with engine flywheel, flywheel with shaft. Fly wheel is heavy disc which is fitted at one side of engine and this maintains the momentum. Coupling bolts are forged out of rolled bars of low alloy steels. They have reasonable grain structure, high toughness and high notch ductility. Longitudinal girder construction is the latest development for this structure. Advanced methods of prefabrication and welding are used today to form these structures for most of the engines. Cylinder blocks are bolted to each other longitudinally. All are then held with each other with tie bolts. To prevent the crankcase explosion, frame is equipped with large size spring loaded relief valves, one each for each cylinder on main engine and at least one on alternate cylinders on diesel generator engines.
Next part of the internal combustion engine is the connecting rod.
2) Connecting rod
Main function of connecting rod is to transmit the gas load from crosshead or gudgeon pin to the crankpin or crank web. Actually what happens when combustion takes place in internal combustion engine, the combustion material pushes the piston downward after firing of fuel. Additionally it has to convey the lubricating oil for lubrication of crosshead bearings / guides / crankpin bearing and for cooling of the piston. Connecting rod is subjected to high axial compressive force which is maximum at TDC ( top dead centre) due to upward inertia force and downward gas force. In two stroke engine, it is always subjected to high compression whereas in four stroke engine it is high compression and low tension. Due to abnormal working condition, connecting rod may be subjected to excessive stresses. Connecting rods have failed by buckling due to water hammer caused by water leakage inside the cylinder. Connecting rod may be highly stressed while starting or when piston is seized. Large diesel engine connecting rods are forged out of mild steel or medium carbon steel. It is machined all over and internally drilled to provide passage for lubricating oil. Top of the connecting rod is joined with piston rod at crosshead pin. Two bearings are fitted on each side of crosshead pin and bolted with flat end of connecting rod.
The connecting rods are drop forged from alloy steel in H sections. The parting faces between these two members are plain and at right angles to crankpin axis. Connecting rod with piston can be withdrawn in an assembled condition through cylinder liner from cylinder top. Care should be taken while taking the connecting rod because the liner is made of very smooth metal and any scratch will lead to improper functioning of system. Connecting rod is internally drilled for conveying lubricating oil through its body to gudgeon pin bearing for lubrication and cooling of piston crown.In earlier engines the water was used for cooling puposes but now oil has replaced the water.
The reason of the camshaft in a diesel engine is to time the sequence of opening and closing of intake and exhaust valves, and inject fuel at required timings. In four stroke engine, intake and exhaust valves are opened mechanically, the movement being transmitted from cam to roller follower which is connected to complex machinery for opening and closing of valves. Valve is closed by the spring force when roller , when process of intake or exhaust is finished. The push rod which is connected to the roller works against the spring force for working. But In two stroke engine, scavenge air is given through ports thus there is no need for intake valve. Exhaust valve is opened hydraulically by exhaust valve actuator. Two stroke engines are reversible type with directly coupled propeller or any load. One thing to note down here is that in a two stroke engine the exhaust valve timing is symmetrical about TDC (top dead centre), thus no change is required in ahead and astern directions.
Camshaft of a four stroke engine may be one piece solid built or may be semi built in two or more pieces, joined by bolts and nuts. In such a camshaft slight modification is possible by tappets for inlet and exhaust valves or by placing shims below fuel pump housing. Camshaft of a two stroke engine is usually semi-built and joined by muff coupling. Camshaft is held in position by journal bearings.
A flywheel is a mechanical device with heavy mass which provides significant moment of inertia and is used as a storage device for rotational energy. Since there are large disturbances in rotation of I.C engines, so the flywheel minimizes these disturbances with supplying stored energy. The inertia of the flywheel opposes fluctuations in the speed of the engine and stores the excess energy for intermittent use. In marine engines, the flywheel smoothens out the pulses of energy provided by the combustion in the cylinders and provides energy for the compression stroke of the pistons. In power presses the actual punching, shearing, and forming are done in only a fraction of the operating cycle. During the longer, non active period, the speed of the flywheel is built up slowly by a comparatively low-powered motor. When the press is operating, most of the required energy is provided by the flywheel.
Flywheel of the two stroke engine has gear teeth on its outer periphery for turning of the engine with the help or motor driven turning gear. This is done initially when we have to start the engine. It is used before starting the engine, during maintenance and inspection of the engine. The flywheel of a marine diesel generator engine has slots or holes provided on the outer periphery for turning the engine using a bar. This reduces the weight of flywheel and also the centrifugal force in it. On automobile engines, a gear teeth wheel is shrunk on the flywheel for starting the engine with electrical motor. Flywheel rim is marked with engine cylinder Top dead center or bottom dead centre positions and has graduations for checking/adjusting timings. Various pick-ups are mounted on modern engine flywheel used for measurement of speed, direction of rotation, engine power etc and automatic start.
5) Cylinder head
Cylinder head is a part of the combustion chamber and its main purpose is to close the end of cylinder and seal off the gases. It provides the close box for the fuel to ignite at high pressure. It is subjected to high pressure and temperature and is likely to crack. It is mounted on cylinder jacket by the long studs and nuts. Cylinder head accommodates various mountings such as:
a) inlet valve,
b) exhaust valve,
c) fuel injector,
d) relief valve,
e) indicator cock
f) Air starting valve.
Cylinder heads of earlier two stroke engines were simpler as there were no inlet or exhaust valves due to loop or cross scavenging. Modern diesel engines are equipped with a hydraulically operated exhaust valve located in centre of cylinder head as uniflow scavenging system is adopted now. To endure firing pressure and heat of combustion, cylinder head must be strong and cooled by water to prevent excessive temperature. Cylinder head is usually cast from alloy iron. Internal structure of cylinder head is very intricate due to various passages for the valves mounted on it which may also be water cooled. Cylinder head of a four stroke engine is much more complicated than a two stroke engine. Cylinder head usually had cracks between two exhaust valve pockets or between exhaust valve and fuel injector pockets. Cylinder head also failed due to burning, distortion or corrosion. To prevent these problems, cylinder head is provided with reasonable size of hand holes for inspection and cleaning. There is a provision of automatic lifting of cylinder in case the temperature and pressure increases too much.
6) Inlet and outlet valve
As already said that Four stroke diesel engines are provided with over-head, mushroom type poppet valves which are opened by cams through push rod and rocker arm and closed by springs once during each working cycle. They consist of two parts namely valve and stem. Valve should be seated smoothly as valve and seal can not survive hammering at high temperature. These are subjected to high thermal and mechanical stresses thus they should have high strength, impact resistance, hardness at surface and resistance to corrosion and oxidation at high temperature. Inlet valve is not subjected to so rigorous temperature so they are made of Chrome Nickel alloy steel whereas exhaust valve are made of Nickel Chromium Molybdenum steel. Exhaust valve and seat mating area may be coated with heat resistant Nimonic alloy. Valve seats are shrinking fitted on cylinder head. Exhaust valve seats are usually water cooled. During overhaul, seats are reconditioned by lapping them with valves. Seats with extensive damage are removed from cylinder cover by intense heating usually given by an arc welding run on the entire mating surface and pulling apart.
Usually two inlet and two exhaust valves are used to perk up flow of gas with a central fuel injector. Tappet clearance should be maintained as per maker instructions from time to time when engine is in cold state. This ensures positive closure of the valves when they are hot. The seal of combustion material will be best when valves are seated on perfectly. Well basic purpose valve drive is to cause and control the opening and closing of inlet and exhaust valves. Valve operating gear consists of rocker arms which actuate the valves, push rods which connect the rocker arm and the cams on the camshaft. Push rods are usually made hollow to reduce weight with high degree of stiffness. Lower end of push rod carries a follower which rides on the cam. To reduce friction a roller acts as the cam follower. Upper end of push rod fits into a cup on one end of rocker arm. Rocker arms actuate the valves in the cylinder head. A cam driven from crankshaft actuates the rocker arm through cam follower and pushrod tappet.
7) Fuel pump drive
Jerk type fuel pump is mounted just above the fuel cam on its housing. In this fuel pump a plunger type mechanism is there which forces the fuel to pass at rapid rate. Fuel pump drive consists of a slide which moves in its guide. Bottom part of the slide has a roller mounted on its bottom which makes contact on the periphery of the fuel cam .Spring of the fuel pump keeps the plunger down which in turn exerts force on the slide such that roller is always in contact with the cam. As long as the roller remains on the base circle of the cam, there is no movement of the plunger. As the cam flank pushes the roller, plunger starts to move up against the spring force until roller is on the nose of the cam which is the highest position of the plunger. When the cam moves further, spring force pushes the plunger and its drive downwards until roller touches the base circle again. Jerk type fuel pumps are equipped with an arrangement to lift the fuel pump drive off it's seat such that fuel cam can no longer operate the fuel pump. This is required when one of the engine's units is operated unloaded.
This is called the brain of the engine. It controls the supply of fuel as per need.
Marine diesel engines used for propulsion and power generation are equipped with speed governors. The work of the governor is to maintain the speed from no load to full rated load between specified limits. The governor of a slow speed engine with directly coupled propeller may have simple over speed regulation but the governor of diesel generator engine must provide a precise speed regulation. The governor controls the speed of engine by constantly positioning the fuel control linkage thereby varying the amount of fuel injected in each cycle of engine. Speed droop is a governor feature which allows a prime mover to run at a proportionally lower speed as load is increased while at the same time increasing fuel to compensate for additional load. It permits a stable division of load between two prime movers connected electrically or mechanically to a common service.
On the other hand if the speed droop is slight, response will be faster with many fluctuations and thus the governor will be unstable. The centrifugal force on the rotating flyweights is directly opposed by a spring pressure. At every position of flyweights, balance must be made between centrifugal force and the opposing spring force. When the governor changes it position on occurrence of variable speed and it will keep on acquiring different position. As the speed changes, the flyweights sense it and compare it with spring load on governor spindle, an error signal is sent by moving the governor spindle. Speed droop linkage varies the compression of speeder spring as the terminal shaft rotates to correct the fuel supply. The linkage automatically changes the governor speed setting by reducing the speeder spring force a slight amount as a function of travel of power piston in increased direction. Its function is same like tying a stone in a string and revolving it in circular path.
Modern propulsion engine governors are equipped with scavenge pressure limiter which enables the governor to release fuel in accordance with available scavenge pressure at prevent black smoke. This may be bypassed in emergency.
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