Wednesday, 19 September 2012

ENGINE PRICIPAL CHAPTER TWO Cylinder Block & Moving parts

ENGINE PRICIPAL


 Chapter 2.
Cylinder Block & Moving parts

1. Cylinder block

The cylinder block is the basic part of the engine. It is made of cast iron or aluminum. It comprises of the cylinder in which the piston shall be moving reciprocally, the water jacket for circulating the cooling water maintaining the temperature of the cylinder, and the crankshaft installed underneath.
The role of cylinder is for guiding the reciprocal movement of the piston accepting the force and high temperature from the combustion of the mixtures, for cooling the cylinder properly, and for supporting the crankshaft. As the basis of the engine, it should have enough strength for enduring the supporting the attached all parts of engine.
For these purposes, the cylinder is generally made of cast iron because that the iron is easy to be processed mechanically and has the characteristics of good resistance against the ware and corrosion.
Recently, instead of the cast iron, the aluminum alloy is more popular. The aluminum is lighter and transmitting the heat easier than steel so that it is deemed as the ideal material for engine. It is not easy to apply the aluminum to the engine because it has different heat expansion coefficient with the steel, the main material of other parts and it is complicated to design the engine structures, as well as it is more expansive than steel.
For the passenger’s car, the weight of the engine is about 1015% of the total weight of the car. The 1520% of the engine weight is come from the cylinder block. It is very important to be light maintaining the strength of it as possible. Therefore, the skeleton structure of the cylinder block has different thickness such that the thickness is thicker of the portion applied heavy force or having possibility of deformation and the thickness is thinner of the other portions. To design the cylinder block regarding these factors, the structure analysis is performed by the finite element method in which the engine is divided into triangular or rectangular cells and the each element is established in simultaneous equations to calculate by numerical analysis using computer.
In side of the block, there should be the water jacket for circulating the cooling water so it should be precisely manufactured for the complicate structure. To prevent from cracking at the bottle neck point of different thickness or to enhance the resistance against wear, it should be heat treated.

2. Cylinder Liner
The inside wall of the cylinder block is the frictional face with the piston with a lubricant oil there-between. Therefore, it satisfies the strict requirements that it endures at the high temperature and wear, that its changes of the dimension by the heat expansion coefficient shall be within the tolerance, and that it should not be adhesive with each other by the high temperature.

Generally, when the block material is the steel, this part is made by polishing the cast iron cylinder, so called as the linerless type. When the block material is the aluminum alloy, the inside wall of the cylinder having a cylinder liner made of cast ion for preventing the ware of the side wall. The liner is the thing which is attached the inside of the cylinder. The cylinder liner may be made with the cylinder block or separately and joined after that with the cylinder.
For the aluminum cylinder block, the cast iron is used. It is heavier than the aluminum alloy as well as it has the lower heat transmission ratio than the aluminum. Therefore, for the engine of racing car or high efficiencies, the special liner made of the silicon alloy based on the aluminum or having special treatment on the aluminum surfaces are utilized.
These special liners are so expensive and hard to manufacture. Also, there are some tries to develop the linerless cylinder with aluminum alloy cylinder block. Even though the linerless cylinder is more expensive, the engine can be lighter and compacted so that it is mainly accepted to the high performance engines.
The gap between the cylinder liner and the piston is depended on the material. When the liner is the cast iron and the piston is the aluminum alloy, considering that the heat expansion ratio of the aluminum is almost twice than that of the steel so that the gap will be reduce at the high temperature of the engine, the gap shall be 3040 microns (0.030.04mm) at the room temperature. If the liner and the piston are all the aluminum, then the gap shall be 10 microns because there is no difference of the heat expansion between them.
The around of the cylinder liner is formed as a shape of path for cooling water, the water jacket, to maintain the temperature of the engine to certain value by absorbing the heat energy come from the remained energy of the combustion.

3. Water Jacket
When casting the cylinder block, the cylinder is surrounded by the core made of sand to form vacant spaces. These spaces are the water jacket for circulating the cooling water to take down the temperature of the cylinder head and cylinder to the proper temperature for operating.
The water circulating inside the water jacket goes into the engine from the lower outlet port of the radiator cooling the heated water. The water flows from the lower part of the engine to the upper part of the engine. After cooling the cylinder head, the heated water is taken out from the engine and goes into the upper inlet port of the radiator. During circulating inside the water jacket, it is important to cool down the each cylinder equivalently. The design of the water jacket is focused on the flow method to spread the water smoothly over the all parts with smaller volume of the water as possible. The heated water is cooled in the radiator and then return to the water jacket again. In winter, the heated water selectively flow into the another radiator for heating the cabin.

Generally, the water jacket is surrounding the cylinder thoroughly. In order to reduce the path length along to the cylinder array, the water jacket is surrounding the out side of the cylinder so that the water does not flow into the adjacent space the cylinders. This type is called the Siamese type. Like the Siamese twins, some portions of the jacket surrounding the each cylinder are merged into one body. The conventional jacket is called the full jacket type.

For the engine having the liner, the type is divided into two kinds by whether the water is contacting with the liner or not. When the cylinder liner is surrounded by the wall of the cylinder block so that the outside of the liner can not contact with the cooling water, it is called the dry type liner. When the most portions of the liner contact with the cooling water directly, it is called the wet type liner.
The wet liner has better cooling efficiency. It should be sealed with an O-ring between the liner and block to prevent leaking the cooling water. In HMC, the most engines having the liner are equipped with the dry type liner because HMC have not any problem come from the heat adhesive of the engine yet and we worry about the leaking the cooing water.

4. Piston

The piston moving inside the cylinder reciprocally transmits the weight force of 3~4 tons (5 tons for diesel engine) according to the combustion of the fuel mixture gas having over temperature of 2000 at the combustion stroke to the connecting rod. The first thing to be considered in design of the piston is that the piston should be made of light materials to reduce the inertia force of the reciprocal movement. The next point is that its material should have the strength enough to endure the combustion force. And then, the material of the piston shall have the good heat-trance and not be distorted or deformed by the high temperature.

At first, the aluminum or aluminum alloy can be considered for lightening and strengthening. Then, for enhancing the heat resistance to prevent from changing in dimension, the heat treatment shall be performed.
The upper part of the piston is called as the piston head or the piston crown. It is very important part forming the combustion chamber between the cylinder head. To enhance the combustion efficiency by combusting the fuel mixture instantly, the shape of piston head prefer to be flat. To enhance the compression ratio, the middle portion may be upraised or there are some recessed positions, the valve recess, to take the intake and exhaust valves not to touch with the piston. The under portion of the piston is the piston skirt stabilizing the reciprocal movement of the piston. The some front portions of the skirt looks like being cut out because that the balance weight is passing these areas when the piston goes down.
There is a gap between the piston and the cylinder. This gap shall be sealed with the piston ring. When the piston is moving in reciprocal, some portions of the skirt may be touching the cylinder wall. To reduce this touch, the shape of the skirt shall be changed. The shorter length of the skirt has fewer noises from the friction with the piston and lighter weight. However, it is preferable for designing the skirt to be balanced with the size of the piston.
The piston is connected with the connecting rod by a piston pin. So, the most forces of combustion are applied to this pin. As the piston pin is the shape of hollow cylindrical structure, the larger of outer diameter, in a same weight, is the more strength against the bending force. However, when the diameter of the piston pin is enlarged, the piston pin boss shall be also enlarged. Therefore, the compression height, the length from the pin to the piston head, is also longed so the weight of the engine is heavier. So, the diameter should be designed by considering the balance with the piston size.

5. Piston Ring
The main roles of the piston ring, the wheel shaped steel surrounding the head part of the piston, are to prevent from leaking the gas by sealing between the piston and the cylinder, to prevent from remaining the lubricant oil in the combustion chamber by gathering the oil down from the cylinder wall and to prevent from transmitting the heat from the piston to the cylinder.
Generally, the piston ring comprises of three rings. The two rings near to the piston head are called the compression rings, and the one ring near to the skirt is the oil ring. The top ring of the compression rings is used for sealing the gas, the oil ring is used for removing the lubricant oil, and the second ring of the compression rings is used for helping the sealing and for controlling the thickness of the lubricant oil film.

Some piston comprises of the two rings, the compression ring and the oil ring. In this case, the roles of rings are somewhat loss, but the fuel efficiency can be enhanced by reducing the loss of force from the friction between the piston ring and cylinder wall. Some racing cars accept the two ring system for shortening the piston height to reduce the engine weight.
The compression ring is made of the spring steel by casting iron, and the surface should be heat treated to reduce the friction and to enhance the lubricant of the piston. To insert the ring into the grooved portion of the piston and to ensure the tensile force for compressing to the cylinder, one portion of the ring shall be opened. This open portion is called the end gap. The combusted gas is leaked out through this end gap a little. This blow-by gas is returned to the combustion chamber by the returning device not to leak out.
The grooved portion of piston for the compression ring has slightly lager then the width of the rings. When the piston is moving up and down, the rings are rotating to prevent the end gap of the three rings from being aligned each other. If the rings have not enough strength, the rings are fluttering within the grooves at the high speed of the engine so it can not seal the gas properly.

The cross section of the oil ring has the shape of reversed “C”. The gathered oils by the rings are returned to the inside of the piston through the hole located at the lower portion of the C-shaped ring. When the engine has high speed, the ring can not gather the oil only with the its tensile force, so an additional spring, the expander, shall be attached to enforce the compressing force of the ring to the cylinder.

6. Connecting Rod
The connecting rod is the rod for connecting the piston and the crankshaft. It transfers the reciprocal movement to the rotation movement. The connecting rod moves very complicatedly with swing movement about the piston pin and the linear movement up and down. So, there is a balance weight to control the inertia force generated by the complicating movements.

The contribution ratio of the connecting rod weight to the inertia force is about 2 to the one reciprocal movement. To lightening the load to the bearing and the vibration by reducing the inertia force, the connecting rod should be light as possible. However, it has enough strength to transmit the combustion force to the crankshaft.
The connecting rod is made of the special steel by casting or forging. The forging is preferably used for ensuring the strength. For the racing cars, the expensive but very light and strong material, titanium alloy, is used also.

The types of the rod can be divided into two types according to the cross sectional shape of the rod, I type and H type.
If the strength is same, then the I-type is lighter than H-type. Therefore, general cars accept the I-type rod. The H-type has stronger structure against the bending force to the direction of the pin axis.
As the Connecting rod is longer, the lateral vibration is smaller. The reason is that, considering the force applied to the piston at the rotation of the crank by divided into lateral direction and longitudinal direction, the longer connecting rod can reduce the ratio of the force to the lateral direction than the shorter connecting rod so that the vibration and friction also shall be reduced. However, if the connecting rod is so long, the engine weight is heavier so it is not preferable. Generally, the length from the center of the piston pin to the crank pin, is about twice than the length of the stroke.

The end portion of the connecting rod to the piston side is called as the small end, and the end portion to the crank pin side is called as the big end. The small end is connected to the piston with the piston pin, and the big end is attached to the crank pin by inserting a bearing.

7. Crankshaft

The crank means the bended handle as the transfer from the reciprocal movement to the rotational movement as have mentioned until now. At the early time of the vehicles history, the engine is started with the crank. After the electric motor was used for the starting the engine, until 1950s, Some cars had have a crank at the front of the engine for emergency device at the malfunction of the motor.

The crankshaft connects the cranks of each cylinder. The main shaft is called the crank journal and the attaching part to the big end of the connecting rod with the crank is called the crank pin. The other side, the attaching part to the small end of the connecting rod with the piston is called the piston pin. The connector connecting the crank journal and the crank pin is called the crank arm. The sector formed pendulum at the front of the crank arm is called the counter weight or the balancing weight.

The reason of the shape of the counter weight being spreading from the center (root) to the circumferential portion (outer portion) is that it can have larger inertial force when it rotates about the root part; even the counter weight has the same weight density.
In the reciprocal engine, the piston is press the crank journal with the connecting rod at every combustion stroke. The crankshaft affected by the complicated bending and distorting force. Therefore, the crank journal shall have strength enough to endure these forces so it is made of the casting or forging steel. For the high performance engine or the racing car engine, the forging steel is most used for ensuring the strength. For the commercial or general purpose vehicle, the casting steel is used because the forging process is more expensive. Even though the casting steel has less strength than forging steel, it is not so critical point because that it is possible to manufacture the counter weight precisely.
The counter weight balances the weights force between from the reciprocal movement of the piston and from the rotational movement of the crankshaft. Simply think, to balance the weight is to match the inertia forces from the piston and the counter weight as the ratio of 1:1. The counter weight should be small as possible within the requirement load range of the journal in order to reduce the weight of the crankshaft.

8. Crank Case
The crank case is the part covering from the cylinder of the cylinder block to the crankshaft.
In the crank case, there are some auxiliary devices such as the alternator (the alternative current generator), generating the electric power, the compressor of the air conditioner and the oil pump for the power steering. And the engine mount brackets installing the engine to the vehicle body are also attached to the crank case. As the crank case is one part of the cylinder block, it is always vibrated by the reciprocal movement of the piston and the rotational movement of the crankshaft. Therefore, the material of the crankshaft should satisfy the requirement of the resistant against the shocking force and vibration.
The types of the crank case are divided into two types according to the covering range over the crankshaft, the half skirt type and the deep skirt type. In the half skirt type, the front portion of the crank case is covering to the center of the crankshaft. In the deep skirt type, the crank case is covering over the bearing cap.

Because the half skirt type has the short length, it is possible for the block to make be light. However, the joint strength shall be weaker than the deep skirt type, because the joining area is small when the transmission is attached to the engine. It is easy to make some vibrations so that it is necessary to be assisted by supporters. Additionally, the space for attaching the auxiliary devices shall be smaller.
To secure the crankshaft to the cylinder block and to reinforce the strength of the block, a supporting device might be formed with the bearing of the crankshaft at the lower part of the crank case. According to the type of this supporting device, there are the ladder frame style and bearing beam style.
At the lower part of the cylinder block, an oil pan is attached also. This is for gathering the oil completing the lubricating and cooling role. It is made of a pressed steel sheet and attached by rubber packing like the head cover. The oil pan is easy to make a noise so that it is made of the vibration resistance steel plate. The vibration steel plate is manufactured by inserting a resin plate between the two steel plates to prevent from vibrating.

9. Journal Bearing
The bearing is for helping the smooth rotation of the rotating axis and supporting the rotation axis. There are various types of the bearing including the plain bearing supporting the axis with the flat and wide side, and the bearing supporting the axis and the around of the axis with balls or rollers. Generally, for the crankshaft of the engine, the plain bearing is more used.

The reason that the roller bearing type is not applied to the crankshaft is that the load can be concentrated at the contacting portions of the ball or roller in a point or linear type. In the plain bearing the load is applied on the lubricated side, the larger contacting area than the ball or roller bearing so that the plain bearing can support large force.
As the plain bearing is also called as a sliding bearing, the shaft is sliding on the bearing with the lubricant oil. Even if the surface of the solid metal body is applied the smoothing surface treatment precisely and carefully, it should have roughness somewhat. Therefore, when the two solid bodies are directly contacted, they should be worn.
The lubricant oil inserting between the plain bearing and the axis can make the rough surface of these two solid bodies to be smoothly. The two solid bodies are not contacted directly even they are so closed.
The thickness of the oil film, that is the gap with the bearing, is changed by the load or heat expansion. When it is so small, it may be adhered by friction heat, otherwise, when it is so big, it may make vibrations and noise.
The bearing is made by welding the bearing alloy having light weight and good fatigue resistance such as the copper or aluminum. On the surface, a special metal basis on the lead is coated. The bearing has the oil hole and oil groove for supply the lubricant oil to lubricate the contact portion between the connecting rod and the crank pin and between the crankshaft and the crank case.
The crank journal, the rotational axis of the crankshaft, is attached at the lower part of the cylinder block by the bearing cap with the plain bearing. For the serial engine, this bearing should be attached at the front side and rear side of the cylinder. If it is the 4-cylinder, it has 5 bearings and if it is the 6-cylinder, it has 7 bearings, that is, it called as 5-bearing and the 7-bearing, respectively. Certain old style engine of 4-cylinder might have 3-bearing structure. This type is not used because the crankshaft is easy to be bent and make vibrations.
10. Flywheel


The flywheel is equipped to the transmission side of the crankshaft to maintain the smooth rotation using inertia force and to reduce the irregularity of the rotational force. The crankshaft is rotated twice per one of the combustion. At the other strokes, the reversed directional force shall be needed for the compression, the intake and exhaust. If there is no the flywheel, then the rotational force of the crankshaft shall be reduced at these strokes. Therefore, when the intervals of the each combustion stroke are long like in the idling state, the engine may be stopped.
Around the flywheel, a ring gear is attached to rotate the crankshaft by being teethed with the pinion gear. The clutch disk can attach to the flat side of the flywheel by the spring to transmit the driving force to the transmission.

The magnitude of the torque is calculated by multiple the magnitude of the force to the distance between the center of the axis to the point at which the force is applied. The magnitude of the force is proportional to the inertia mass so that if the fly wheel is heavy and the outer diameter is large, or if the outer portion is heavy, then the force of the flywheel might be large.
In general engine, the half of the total inertia mass is distributed at the flywheel. Therefore, when the rotation of the engine is low or when the engine is in the idling state, the inertia mass of the flywheel should be large to rotate the engine regularly. However, with the large inertia mass of the flywheel, the rotation of the engine can not be changed easily. It is hard to increase the engine rotation by pressing the accelerator, or to take the engine brake by releasing the accelerator. That is, the engine response will be worse. So, the fuel efficiency shall be worst too.
Some engines use the 30% of the torque generated from the engine to increase the rotation of engine itself when the accelerating is performed with the lower speed shift. The size and the weight of the flywheel are decided by the purpose of the vehicles. For example, the engine for the racing car uses small size one, and that of the family car uses large size one.  For the general purpose, the flywheel is made of the cast iron, and for the special purpose such as the racing car, it is made by cutting the steel material having high strength.

11. Balance shaft, Balancer for the secondary inertial force
The piston, the Connecting rod and the crank make inertia force according to the reciprocal and rotational movement. For this reason, if the one-cylinder engine has not the counter weight for balancing between the inertial force and the weight of the piston, the connecting rod and the crank, then the engine may be severely vibrated by the unbalancing.
For the serial 4-cylinder engine, the four pistons are connected to the crankshaft with being paired the first, forth and the second , third, in facing each other. When the crankshaft is rotating, the inertial forces are offset so that the counter weight might be not needed.
In actual movement structure of the piston-crank system of the 4-cylinder engine, the inertia force shall not be offset. This is come form the structure in which the piston in reciprocal movement is connected to the crank in rotational movement with the connecting rod. For example, in the half rotation of the crankshaft when the piston moves from the highest point (TDC, Top Dead Center) to the lowest point (BDC, Bottom Dead Center), the piston has the maximum speed at the near of the highest point of the stroke rather than at the middle of the stroke. The rotation of the crank is regular so that the inertia force of the crank of each cylinder (the first inertia force) is easily offset. However, the inertial force of the piston is not. For example, the upper inertia force generated when the first and forth pistons are moving from the highest point to the lowest point is larger than the lower inertia force generated when the second and third pistons are moving from the lowest point to the highest point.

By representing these relationship on the graph with the inertia force at the vertical axis and the rotation angle of the crankshaft at the horizontal axis, when the upper inertia force of the first and fourth pistons is maximum value, the lower inertia force of the second and third pistons is minimum value, and vice versa after the crankshaft with 180°. From this relationship, we know that the inertia force is generated with the ratio of 2 times per one rotation of the crankshaft. This inertia force is called as the secondary inertia force. It is easy to be generated when the engine is in the idling state.
The four-cylinder engine is equipped in the small passenger car generally. For the convenience of the passengers, a balance shaft having the half circle shape in the cross sectional view shall be attached at the both side of the engine to reduce the vibration from the secondary inertia force. This balance shaftt is designed to rotate with two times of speed in reverse direction against the crankshaft. The additional inertia force generated from the balance shaft will offset the vibration from the secondary inertia force.

No comments:

Post a Comment