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 10∼15% of the total weight of the car. The
15∼20% 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 30∼40
microns (0.03∼0.04mm) 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.
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