1 Chonan Technical Service Training Center
Engine Principles
Chapter 1. What is the Engine?
1. What is the Engine?... 2. Kinds of Engine... 3. Engine layout... 4. Expansive force & Inertia Force... 5. Reciprocal Engine... 6. Exhaust & Intake Stroke... 7. Compression & Combustion Stroke... 8. Engine structure... 9. Diesel Engine... 10. Internal Combustion & Motor... 11. Lean Burn Engine... Chapter 2. Cylinder block and Moving parts
1. Cylinder Block... 2. Cylinder Liner... 3. Water Jacket... 4. Piston... 5. Piston Ring... 6. Connecting Rod... 7. Crankshaft... 8. Crank Case... 9. Journal Bearing... 10. Flywheel... 11. Balance shaft, Balancer for secondary
inertial force... Chapter 3. Cylinder Head
1. Cylinder Head... 2. Cam & Camshaft... 3. Driving the Camshaft... 4. Intake & Exhaust Valve... 5. Valve Driving System... 6. Valve Timing... 7. Variable Valve Timing... 8. Malfunction of Valve... 9. Overrun & Red Zone... Chapter 4. Intake System
1. Enhance the Volume Efficiency... 2. Intake Inertia Effect & Pulsation Effect....
3. Variable Intake System... 4. Intake System... 5. Throttle Valve & Manifold... Chapter 5. Exhaust System
1. Exhaust System... 2. Exhaust Inertia Effect & Pulsation Effect.. 3. Component of Exhaust Gas... 4. Air-Fuel Ratio & Exhaust Component... 5. Exhaust Purification System... 6. Blow-by Gas Recirculation Device... Chapter 6. Charger
1. The Kind of Charger... 2. Turbocharger... 3. Boost Pressure & Compression Ratio... 4. Turbo Lag... 5. Supercharging System & Heat... 6. Supercharger... Chapter 7. Lubrication System
1. Role of Engine Oil... 2. Lubrication Method... 3. Parts of Lubrication System... 4. Engine Oil... Chapter 8. Cooling System
1. Cooling System... 2. Radiator... 3. Cooling of the Cylinder Head... 4. Over Heat... Chapter 9. Fuel System
1. Carburetor... 2. Mechanical Fuel Injection System... 3. Electrical Fuel Injection System... 4. Fuel Supplying System... Chapter 10. Ignition System
1. Point type Ignition... 2. Full Transistor type Ignition... 3. Distributor-less Ignition...
4. Spark Plug... Chapter 11. Combustion and Combustion
Chamber
1. Combustion Process... 2. A/F ratio & Flame Velocity... 3. Ignition Timing... 4. Swirl Effect... 5. Knocking... 6. Abnormal Combustion... 7. Shape of Combustion Chamber... 8. Intake-Exhaust Valve & Combustion Chamber... 9. Piston & Combustion Chamber... Chapter 12. Performance, Fuel consumption,
Noise, and Vibration
1. Required Performance... 2. What is Output?... 3. Representing Method for Power... 4. What is Torque?... 5. To enhance Power... 6. S/B ratio & Output... 7. Compression Ratio & Output... 8. Enhancing Output by High rpm... 9. Transient Characteristic & Response... 10. Cylinder Array & Performance... 11. Fuel Consumption Ratio... 12. Output & Fuel Efficiency... 13. Fuel Efficiency of Vehicle... 14. Vibration of the Engine... 15. Noise of the Engine...
Chapter 1.
What is the engine?
1. What is the engine?
This book introduces about the automobile engine especially the gasoline engine.
However, it is very complicated to define about the engine, so called, what the engine is. In general concept, the engine is 『the devices driving something by changing the energy in the natural source such as fire, wind or electric material to the mechanical energy continuously』. There are many types of the engine and they are driven in different ways. Therefore, we can define the gasoline engine, as a kind of combustion device in other word, the 『device changing the heat acquired by combusting the gasoline to the mechanical force for driving the vehicles』.
How is the heat energy changed into the mechanical energy? For example, the bottle or pot. As they are heated, the covers move with a noise.
The heat acquired from the gas or electric energy boils the water so as to make the water vapor pushing up the cover of the bottle or pot.
There is an important thing. The moving force is not come from the heat energy, but the hot air or vapor by the heat works.
That is, the media is need for changing the energy form. This media is the working fluid in the technical terminology. The working fluid for
the gasoline engine is the air in-taken with the gasoline into the engine and then combusted and exhausted. Fuel Combustion Heat Generation Air Expansion Pressure Generation Piston Movement
The procedure for transmitting the fuel to the mechanical energy in the vehicle engine will be explained in the following sections.
In this process, the working fluid is the air. If there is no working fluid, the energy transformation shall not be performed.
In contrary, think about the transforming the mechanical energy to the heat energy. In the vehicle, the brake is the represented example. The principle is the friction heat coming from the rubbing or striking the two materials. We can warm our hands by rubbing each other, that is, the moving force(rubbing) can easily transform to the heat energy(warming hands). At this time, there is no working fluid. The force is changed into heat directly.
However, when the heat energy is transformed into the mechanical energy, there must be a working fluid. Being the media for the
transforming the energy, there should be a lot of loss of energy, at any case. Therefore, how much energy of the heat can be transformed into the mechanical energy, the efficiency, is an important factor in the engine.
2. Kinds of engine
There are many kinds of the engines.
The engines can be classified by the number of cylinder, the array style of the cylinder or the equipped method at the vehicle, and so on. As you know, the engine makes the driving force by the reciprocal movement of the piston in the cylinder so that the power is decided by the number of the cylinder.
Therefore, the engine is mainly classified by the number of the cylinder. The commercial vehicles are classified into the 2, 3, 4, 5, 6, 8, and 12 cylinders. More displacement volume has the more cylinders.
According to the array of the cylinder, there are three types including the In-line type with serial arraying of the cylinder, the V type with V-shaped arraying of the cylinder and the opposed type in which the cylinders are arrayed facing each other.
According to the engine installation type, there are two types; the one is the lengthwise type and the breadthwise type. When the engines are arrayed in length direction of the vehicle is called as the lengthwise type, when the engines are arrayed in width direction of the 5 Chonan Technical Service Training Center
vehicle is called as the breadthwise type.
Lengthwise type Breadthwise type For example, the FR type car having the engine at front and driving the rear wheels has the lengthwise type engine. The reason is that the propeller shaft transmitting the driving force to the rear wheels shall be run underneath the floor. In the case of FF type car, the most small car, having the engine at front and driving the front wheels, the engine is breadthwise type because that the rotation axis of the engine and the driving axis rotating the wheels should be are parallel. However, when the FF type car has the 6-cylinder engine, if the serial engine is installed in width direction, then the width of the car is too wide. Therefore, in that case, the engine rather is the V-type. In this manner, considered with the displacement volume and vehicle type, the engine array type and the installation type are selected for the best combination in design.
3. Engine layout
FF : Front Engine Front Drive FR : Front Engine Rear Drive MR : Midship Engine Rear Drive RR : Rear Engine Rear Drive
It is possible to assume that the engine will be
installed at the front of the car. However, the all vehicles do not have their engine at front side. In 1770, the origin of the car, the engine of the steam car of Cugnot was located at the front ends of the body. In 1885, the first car equipped the gasoline engine, the engine of the Daimler’s car is located at the under the seat and the front of the rear wheel axis.
In the history, many researches had been performed to find the best condition where the engine was installed. In 1891, the FR (front engine rear drive) car was in France. The FR type is that the engine was installed at the front side and the rear wheels were driven. After that, this type would be the set as the standard layout of the engine. Until now, the most large passenger cars and the sports cars accept this type.
The typical characteristics of the FR type car is that the cabin space is located to rear side, the steering is controlling the front wheels and the driving is contributed to the rear wheels so that the operation and weight load are evenly divided to the front side and rear side and the movement performance and passenger convenience are balanced as well as the vibration and noise are less than other types.
In the layout of the relatively small public car, there had been two big developments for about hundred years. The first one is the Volkswagen in 1936 having the engine at the rear side for driving the rear wheel. After the World War II, this type was leading the worldwide design of the mainstream the passenger car of rear engine and rear drive (RR) type. The second one is the Mini of England in 1959. The Mini accepted the front engine and front drive (FF) type having the breadthwise type engine at front side for driving the front wheels. Nowadays, this type is applied to the small cars as well as the middle sized sedans. The FF type car has the engine and driving devices at the front side so that it should not applied to the car of which weight is concentrated to the front side. The defect of this type is that it is not easy to steer the car. However, it has the relatively large space of indoor and trunk, and the safety is better than other types. Therefore, it is the best design for the layout of the utility cars.
The midship engine and rear drive (MR) type is focused on the performance rather than the convenience of the passenger so that it is mainly applied to the sports cars. When the main parts of the engine are located at front side than the rear wheel type, it is called the midship type. When the main parts are located
at rear side, it is called the rear engine type. 4. Expansive Force and Inertia Force
Almost of the commercial gasoline engines for the vehicles are the Recipro-engines except for the rotary engine of which working principle is different. The word “Recipro” comes from the “Reciprocating”. The reciprocation is the mechanical movement of go and back so that the Recipro-engine is the device transforming the reciprocal movement to the rotational movement using the crank which is the device having the prominence and depression shape.
7 Chonan Technical Service Training Center Engine Layout & Vehicle Features
The cross sectional view of the Recipro-engine shows that there is a piston reciprocal moving in the cylinder at the upper side and there is a crankshaft at the lower side and the piston and the crankshaft are connected by the connecting rod.
The gasoline engine uses the air as the media of working fluid for transforming the heat energy to the mechanical energy. The air is mixed with the mist of gasoline in the cylinder. When the mixture of air and gasoline are compressed using the piston and combusted, then the expansive gas will press the piston. The expansive force pressing the piston shall drive the car.
At this time, except for the expansive force, there is an inertia force which we are considering. This force is somewhat strange because it is not generated intentionally; however, this force is followed in the moving part of the engine naturally. Therefore, it can affect to the performance of engine or is the source of the vibration or noise.
Considering the reciprocal movement of the piston, the piston starts moving from the halt state at the highest position of the stroke, has the maximum speed at the middle of the stroke, After that, the speed is reduced and stops to the lowest position of the stroke, and then goes back to the highest position again. During performing these movements, the inertia forces shall be generated when the speed of movements are changed. For example, from the highest position of the piston to the middle of the stroke, there is an ascending inertia force. After that there is a descending inertia force from the middle of the stroke to the lowest position of the piston.
When this inertia force is resonated with the other inertia forces from the other pistons, there may be vibrations or noises.
The Recipro-Engines are classified into two types, the 2-cycle type and the 4-cycle type. After developed by Daimler of Germany in 1883, the 4-cycle type which had been completed its standard system until 1900 is the mainly used in the most of vehicles.
The principle of the reciprocal engine is that the mixture of the air and the gasoline are injected into the cylinder, the mixture is combusted using the electric spark to explode, the combustion force drives the piston in reciprocal movement, and the reciprocal movement is changed to the rotational movement by the crankshaft.
The operation of the 4-cycle engine is like that. When the piston is at the highest position, the intake valve is opened. Being down the piston, the mixtures of fuel gas is injected into the cylinder and then the intake valve is closed [Intake Stroke]. Next, the piston will go to upward to compress the mixtures of gas [Compression Stroke]. Then, the compressed mixtures of gas will be burnt by an electrical ignition [Combustion Stroke]. The burnt gas having the high pressure and temperature will press the piston downward. At
this time, the exhaust valve shall be open to take out the burnt gas [Exhaust Stroke]. These strokes are repeated.
Among these 4 strokes, only at the combustion stroke, the engine makes the power for work. Therefore, there are needed the additional forces, for intake and exhausting the gas at the intake and exhaust stroke, and for compressing the mixture at the compression stroke. To do so, a flywheel is installed to the crankshaft to make these additional forces using the inertia force to ensure the continuous rotational movement.
The 2-cycle engine comprises of the two strokes. The operation of this engine comprises the four components such as the intake, the compression, the combustion, and the exhaust as the 4-cycle engine.
Compression and Expansion
Exhaust and Intake
However, before and After the piston is located at the highest position, the compression and combustion operations are performed, and before and After the piston is located at the lowest position, the exhaust and intake operation at the same time. So, through the two strokes, the one cycle of engine operation is completed. The 4-cycle engine performs the combustion stroke at one time per two cycles of the crankshaft, but, the 2-cycle engine performs the combustion stroke at every cycle of crankshaft. Therefore, the 2-cycle has higher efficiency.
Scavenging Additionally, it has no intake and exhaust valves so that it has simple structure and low cost. However, this merit can be a defect.
The exhaust gas is exhausted by the entering new mixtures of gas when the piston is located at the lowest position. Therefore, some new mixtures will be mixed the combusted gas and unburned gases will be exhausted. It makes the air pollutions and the large fuel consumptions.
6. Exhaust & Intake Stroke
This content will explain about the engine on focusing the 4-cycle gasoline engine, the most used type.
To understand the 4 strokes, the intake, the compression, the combustion, and the exhaust, it is helpful to refer the figures, the indicator diagram (P-V diagram), showing the process of the engine operations.
It looks somewhat complicated, but it is easy to understand just following the figures.
The indicator diagram is the graph consisting of the horizontal axis representing the pressure of the chamber and the vertical axis representing the volume of the chamber. At the left ends of the graph, C and F, the piston is located at the highest position of the cylinder, and at the right ends of the graph, G and H, the piston is located at the lowest position of the cylinder. By comparing the lines of the graph to the 4 strokes, the line of A-B is the intake stroke, the line of B-C is the compression stroke, the line of C-D is the combustion stroke and the line of D-E is the exhaust stroke.
In act, the cycle of the strokes is starting from the intake stroke. To understand the engine operation, it is easy to start from the exhaust stroke. To inhale more air as possible, the engine uses also the force from the exhausted gas outgoing to the outside of the engine through the exhaust port.
The exhaust stroke is for pushing the combusted gas to outside of the chamber by moving the piston from the lowest position to the highest position with being opened exhaust valve. In principle, it is think that the exhaust valve will be opened when the piston is reaching at the lowest position.
Exhaust stroke
However, actually, the exhaust valve prefers to be opened before the piston reaches to the lowest position, that is, at the position of D in the drawing. Remaining the pressing force of the burent gas, the exhausting the used gas is more effective by opening the exhaust valve in advance. After that, the piston will push out the remained used gas thoroughly to finish the exhaust stroke.
At the intake stroke, the intake valve is opened, and the piston goes down from the highest position to the lowest position so that the mixtures of fuel gas and the air are inhaled into the cylinder from the intake port. At that time, the intake valve shall be opened just before the piston reaches the highest position, that is, the E in the drawing. Doing so, the intake operation is enhanced somewhat because of the exhausting force of the used gas will pull the intake gases.
Intake stroke
With the same manner, the intake valve shall be closed when the piston is at the B. Doing so, more mixtures of gas shall be inhaled into the cylinder by the inertia force of the intake gas. To inhale air into the engine is performed by the difference of air pressure. When the piston goes down, the air pressure in the cylinder is lower than that of the outside of the cylinder, so that the air around the intake valve shall be inhaled to the cylinder. The opening timing of the valve is different from the stroke position is to use this force to inhale more air as possible
7. Compression and Combustion Stroke
At the compression stroke, the mixtures of gas are compressed by the piston, so that the pressure is increased and the temperature is high by adiabatic compression. Therefore, the gasoline is vaporized by the compressive heat of the air, ready to be combusted. The injected gasoline in the cylinder like the misty with the air is vaporized to be gas state by the heat from the adiabatic compression. Then it is ready to be combusted easily. This space for combustion is called the combustion chamber.
The reason of that it is hard to start the engine in winter is that it is hard for the gasoline to be vaporized. To solve this problem, one method
is to mix more gasoline with the air. Another problem is that; when the gasoline is vaporized, the ambient heat is used to the vaporization, so the temperature of the chamber is lowered somewhat. It may decrease the fuel efficiency of the engine. To prevent from lowering the fuel efficiency, the amount of the gasoline is reduced. However, to do so, the temperature of the chamber is so high that the combustion may be performed prior to the ignition, so called the abnormal combustion.
The most important fact from the intake stroke to the compression stroke is the flow of the mixtures of the gasoline and the air. It is not proper that the flow is too strong to be ignited. The little particles of gasoline should be mixed with air to be the mixtures. Therefore, many manufacturers research and develop the shape of the intake port and flow pattern of the mixtures so that the strength and the pattern of the mixture flow are maintained until the combustion stroke for the best efficiency of the engine.
Compression stroke
As processing the compression stroke, the piston reaches the highest position. When the piston is at the C of the drawing, the ignition will be performed by the electric spark generated from the spark plug. The timing to make the spark is very important. The mixture
is not fully combusted at the ignition time, but the combustion is started from the ignition. It is needed some time interval between the time of the ignition and the time to be maximum pressure of the chamber.
Combustion stroke
The ignition timing is determined by considering that the combustion shall be completed between the highest position and the almost half position of the combustion chamber. Additionally, this combustion speed is proportion to the rotational speed of the engine so that the ignition timing should be adjusted with the engine speed.
Beginning the combustion, the vaporized mixtures are combusted in a short time so that the pressure and the temperature are increased. At this time the expanded gas by the combustion shall press the piston. This pressing force should be stronger as possible. The time period of combustion is preferable to be short to strength the force. If the time period for combustion is longer, then the combustion force is not leading the pressing the piston but following the piston. Therefore, the engine efficient is worst. The combustion time period is affected by flow of the mixtures defined by the size and shape of the combustion chamber and the component of the
mixture and so on. 8. Engine structure
The gasoline engine is a complicated machine having comprising of parts. Look into how the engine is structured.
The engine is similar with the 3 floors building. The first floor is the crank case including the crankshaft transforming the reciprocal movement to the rotational movement. The second floor is the cylinder block including the cylinder of which a piston is moving with reciprocal movement. The third layer is the cylinder head.
In this structure, the moving parts of the first layer and the second layer are called as the main moving part. It includes the piston, the crankshaft and the connecting rod.
In the third layer, there are the valves controlling the intake and exhaust of the mixtures gas and the used gas and the camshaft operating the valves. These are called the cylinder head system.
On the cylinder head, there are intake manifold sending the gasoline and the air to the cylinder and the exhaust manifold taking out the burnt 13 Chonan Technical Service Training Center
gas. These are called the intake-exhaust system. The manifold consists the “many” and the “fold”, that is, many materials are combined. In actual, they are many branched pipes distributing the air and gasoline to each cylinder or merging the exhausted gases in one place.
There is the fuel system including the fuel pump taking the gasoline from the fuel tank and the carburetor or fuel injector to make a air fuel mixture.
There is the lubrication system including the oil pump supplying the oil for reducing the frictions and the oil filter for filtering the oil. Also there is the cooling system including the radiator and water pump for maintaining the temperature of the engine properly.
To drive the engine, the electric power is needed. There are electric devices including igniting spark plug, the alternator generating the electric power and the start motor supplying the initial movement to the engine.
Additionally, there are the auxiliaries such as the oil pump for the power steering the air-con compressor and so on.
9. Diesel Engine
The diesel engine has similar shape and structure with those of the gasoline engine. The different point is the ignition method. The gasoline engine ignites the mixture of fuel with the electric spark. Contrary, in diesel engine, the fuel is injected into the compressed air having the high temperature. When the air is compressed, the temperature of compressed air is increased. The gasoline engine compresses the mixture of fuel gas up to 1/10 of the initial volume.
The diesel engine compresses the air about 1/20 of the initial volume to increase the temperature of the air over 600℃, and injects the fuel compressed with over 100 atm at the injection pump during 1/1000∼2/1000 seconds. The output shall be controlled by the amount of injected mixtures of fuel and air for the gasoline engine. On the other hand, the output of the diesel engine can be controlled by the amount of the injected fuel without controlling the air (the fixed amount of the air). To combust the fuel perfectly by increasing the temperature of the air, the compression ratio shall be increased. However, doing so, the expansion power will be increased also. Therefore, the engine should be stronger to endure the increased force. Additionally, the high quality of fuel injecting pump may be needed. Then the engine is heavier and the cost is expensive. So diesel engine is not proper to apply to the passenger’s car.
In the diesel engine, because almost constant amount of the air about the volume of the cylinder is inhaled, the load applying to the engine is relatively light. When the fuel amount is small at the low speed, the fuel shall be almost perfectly combusted. However, with the full load, the diesel engine needs more amount of fuel so that the air amount is respectively small. Therefore, it may exhaust a lot of black smoke.
In the gasoline engine, the ignition is performed by the electric spark to the mixture so the combustion time period is very short. However, in the diesel engine, the fuel diesel is sprayed into the compressed air, so it needs some time period to be vaporized. Therefore, the maximum speed of engine is limited relatively lower and the output shall be lower than the gasoline engine.
Compared with the gasoline engine, the expansion force and the inertia force of the moving part are bigger so it makes bigger noise and more vibration. It has merits of easy maintenance resulted from not having the delicate parts such as the ignition system and of good fuel efficiency so that it is used for commercial or business purpose rather than passenger’s car.
10. Internal Combustion & Motor
The force of the 4-cycle gasoline engine, as one of the internal combustion, is changed according to the RPM (Revolutions Per Minute) of the engine compared by the electric motor or the steam engine. So, it is impossible to drive with the lower revolutions than certain RPM value. Therefore, the clutch and the transmission should be equipped when the gasoline engine is used for vehicles.
For the 4-cycle engine, using the four strokes, it makes moving force by combusting the mixture of fuel and air in the cylinder. It is very different with the electric motor used in electric vehicles which can start just by applying the electric power. If the mixtures are not supplied into the cylinder at idle condition, the engine can not continue to run. In order that the engine should be operating continuously when the vehicle is stopped, the device for connecting or disconnecting the moving force of the engine to 15 Chonan Technical Service Training Center
the wheels such as clutch should be equipped. In general, the vehicles needs larger power when it is started or accelerated, but when it is driven in constant speed, it does not need larger power. For the motor, it outputs larger force when it rotates with lower RPM, and when the RPM is increased, the output will be lower. Therefore, the electric motor can be applied to the engine of the vehicles without any transmitting device.
However, for the gasoline engine, the power is determined according to the RPM of the engine. The range of the RPM is limited within certain ranges. For example, the RPM of the gasoline engine is about 700∼7000 revolutions per minute, and the RPM for getting the maximum power (torque) is about 4000 revolutions. Therefore, when the vehicles are running with various speeds, it is necessary to control the speed and power of the vehicle by inserting a transmission between the engine and the wheels.
At simple sight, the motor may be the best engine for the vehicles. The important thing is the fuel, the source of power. The gasoline is easy to store during operating the engine, but it is hard for the motor to store the electric power effectively.
To develop the electric vehicles, it is essential to develop the batteries having the high efficiency for charging and maintaining the rechargeable batteries in equivalent state. Many companies are trying developing the method for maintaining the rechargeable batteries. Even though the basic performance has been developed in field test, the manufacturing cost is very high. However there are being suggested some patents and technologies for utilizing.
11. Lean Burn Engine
The exhaust purification system using the 3way catalysts has the characteristics of maintaining the actual air-fuel ratio to the ideal valve to perform the oxidation and reduction of the harmful compounds simultaneously. To do so, the purification of the exhausted gas will be limited, and the fuel amount used for the engine is decided by the engine driving status. Therefore, the engine shall not be developed no more to get higher driving force with less amount of the fuel.
The lean burn system is developed for enhancing the fuel efficiency with the good purification of exhaust gas. To enhance the fuel efficiency is most important point for future. The lean burn engine is one of the most attracted public attention technologies.
With the high A/F ratio by reducing the gasoline in the mixture, how are the three major harmful materials, carbon monoxide, hydrogen carbon, and nitrogen oxide. The oxygen is more plentiful than fuel, so the amount of carbon monoxide will be less or the most carbon monoxide will be changed into carbon dioxide, harmless gas. The hydro carbon will also be completely combusted and changed into carbon dioxide and water. Now we will concern only the last one, nitrogen oxide.
If the A/F ratio is higher, then the temperature will be increase by the plentiful of the oxygen and the amount of the nitrogen oxide will be
increased. At about 16 of A/F ratio, the nitrogen oxide will be maximized. If the A/F ratio is higher than 16, then the combustion temperature will be down so the torque is also down. If the A/F ratio is higher and higher, the combustion is not stable, and torque is very unstable, finally combustion will not be performed.
A manufacturer focused on the torque variations according to the lean burn. By adapting the combustion pressure sensor detecting the combustion pressure in the cylinder, the engine is operated with the A/F ratio just before the torque variation is occurred. Therefore, they can make next generation lean burn engine having the low fuel consumption and the less amount of nitrogen oxide. In that system, the lean burn is performed at the condition in which the driving is not hindered by the low torque at low load. When the vehicle in accelerating or high load, the combustion is performed with the theoretical A/F ratio and the exhaust gas is purified by 3way catalyst. Many manufacturers continue to research for enhancing the fuel consumption by focusing on the intake system and combustion chamber with 16∼20 of A/F ratio. Many new engines satisfying this combustion requirement and having less exhaust gas problem are shown more and more.
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 cannot 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 crankcase is the part covering from the cylinder of the cylinder block to the crankshaft. In the crankcase, 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 crankcase. As the crankcase 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 crankcase 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 crankcase is covering to the center of the crankshaft. In the deep skirt type, the crankcase 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 crankcase. 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 25 Chonan Technical Service Training Center
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 crankcase.
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 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.
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 27 Chonan Technical Service Training Center
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 shaft 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.
Chapter 3.
Cylinder Head
1. Cylinder Head
The cylinder head is attached on the cylinder block with being inserting a gasket to prevent from leaking the combusted gas. The bottom of the cylinder head is also the roof of the combustion chamber. Therefore, the shape of the cylinder head is very complicated. The rectangular box shaped portion located upper position has the valve driving system inhalation the mixture of fuel to the engine and exhausting the combusted gas, and the ignition plug so that the shape and operation of this part can decide the engine performance such as the combustion of the mixture of fuel.
The structure of the cylinder head is various according to the type of engine. The most cylinder head has the common structure as this. At the upper part, there is the valve driving system in which the intake port inhaling the mixture of fuel into the combustion chamber and exhaust port taking out the combusted gas in the longitudinal direction. Inside the cylinder head, there is a water jacket circulating
the cooling water from the cylinder block. The combustion chamber is very important part to decide the engine performance. So the shape and the size are the important factors. If the combustion chamber is big, the time interval for combusting the mixture of fuel is long even the mixture can be compressed enough. So the bigger power cannot be ensured. Therefore, it is prefer for the sized of the combustion chamber to be compact.
Additionally, the shape of the combustion chamber has less uneven surfaces as possible to enhance the combustion of the mixtures. If the shape of the combustion chamber is complicated, then the heat from the combustion shall be easily lost because the surface of the chamber is too large about the volume of it. So, the force pressing the piston shall be lowered.
The intake port is also important part because that the flow of the mixture is defined by the size and the shape of it. Considering just about the flow, the smoother inner surface is better for reducing the resistance against the flow and the straight shape of port is the better. However, the shape of the port is helpful for the inhaling mixture into the cylinder to be form the swirled flow in order to be combusted at the combustion stroke as well as possible. The water jacket will absorb the remained heat after the combustion until finishing the exhaust stroke as fast as possible to prevent from increasing the temperature of the next inhalation mixtures. Especially, the around portions having the high possibility of increasing the temperature such as the exhaust valve and spark plug should be cooled mainly to prevent from making a trouble by the over heat.
At the cylinder head, there is bearing for 29 Chonan Technical Service Training Center
supporting the valve driving system including the cam shaft. The bearing is lubricated and cooled by the engine oil.
2. Cam and Camshaft
The cam drives the valves operation opening and closing the intake port for inhaling the mixture of fuel into the combustion chamber and the exhaust port for taking out the combusted gas. For the OHC or DOHC engine, the cam is attached at the camshaft installed at the middle part of the cylinder head.
The camshaft has the cams with the same number of the valves for intake and exhaust which are arranged with angles according to the timing of the opening and closing. For the 4-cycle engine, the opening ratio of the intake and exhaust valves is one about the two revolutions of the crankshaft. Therefore, the camshaft revolves with the ratio of one turn about the two revolutions of the crankshaft. The extrusion portion of the cam is called as the cam nose or the cam robe. The height is called cam lift. The “lift” means that the cams lift the valve so that the opening status is determined by the cam profile. The opening and closing timing of the valves are determined by the operation angle, the angle from the start point to the end point of the nose.
When the valve is closed to the valve seat, it is preferred that the impact shall be little as possible, so that the cross sectional shape of the cam is the egg shape.
The valve is always applied a force in the closing direction by the valve spring. By pressing the spring with the cam nose, the valves will be opened. If the cam velocity becomes fast to enlarge the inertia force of the valve, the reciprocal movement of the valve is not balanced with the rotational movement of the cam. The engine speed generated by this limited speed of the valve opening and closing is the maximum speed of the engine. So the cam profile is very important.
The cam nose should be treated special surface treatment to ensure the strength for enduring from the frictions with the valve lift and rocker arm installed at the valve. To do so, the camshaft is made of cast iron and the cam nose should be treated with a cooling treatment such as the chilling method to strength the textures of the surface, when it is cast.
Small cam lift Big cam lift
There are two methods for supplying the lubricant oil to the cam journal supporting the cam nose and the camshaft, from outside and from inside. In the outside supplying method, the oil will be supplied from the journal. In the inside supplying method, the camshaft has a hole for supplying the lubricant oil from the middle portion of the journal. In addition, the camshaft is made by hollow tube type and the lubricant oil may be supplied through the hollow tube.
3. Driving the Camshaft
For the OHC engine, the crankshaft is located under the cylinder block and the camshaft is located on the cylinder head, so there should be a chain or belt for transforming the rotational movement of the crankshaft to the camshaft. As the driving for the intake and exhaust valves should be exactly timed with the rotation of the crankshaft, for some case such as racing car, a gear is used for transforming the rotational movement exactly.
In the chain system for transforming, the tooth wheel for the chain is called the sprocket. The one attached at the crankshaft is the crankshaft sprocket, and the other attached at the camshaft is the camshaft sprocket.
In the system in which the camshaft is driven by the chain, the ratio the teeth numbers between the crankshaft sprocket and the camshaft sprocket is 1:2. In order to maintain the tension of the chain, a chain tensioner is attached, and the chain guide prevents from shaking the chain during rotating.
If this method is applied to the DOHC engine, the sprocket shall have the large diameter according to the number of teeth ratio. Therefore, the gap of camshaft and gap of valves of intake and exhaust will be enlarged. Consequently, this method is not applied to the compact type engines. To solve this problem, an additional sprocket is inserted there between to transmit the rotational movement to the camshaft sprocket.
The timing belt type uses belt having toothed surface and pulley instead of the chain and the sprocket, respectively. In this type, the 31 Chonan Technical Service Training Center
sprocket and pulley attached at the end of the camshaft have the timing mark indicating the opening and closing timing of the valve. So we use the term of “timing” to the name of the parts. The pulley attached at the crankshaft is the crankshaft timing pulley and the one attached at the camshaft is the camshaft timing pulley. As well as the chain type, in this type, there are direct reduction type and double reduction type.
Even though the OHC engine can accept the long chain to drive the camshaft, the belt driving method is mainly used. The reason is that the long chain can make mismatching the timing and big noises, and the chain system needs the lubricating equipment. However, the belt is made of fiber and rubber so it can be easily broken by the heat or oil. It is prefer that it should be exchanged at every 90,000km running.
4. Intake and Exhaust Valve
The cylinder head includes the intake port inhaling the mixture of fuel to the cylinder and the exhaust port taking out the burnt gas. The valves at the ports are the intake valve and the exhaust valve, respectively. According to the valve shape of mushroom, we call poppet valve.
The poppet valves consist of the valve head and the valve stem. The valve stem supports the valve guide and the valve spring. The valve is opened by the pressing operation of the cam nose, and closed by the elastic force of the valve spring.
The combustion temperature is over 2000℃, and the temperature of the burnt gas passing through the valve is over 1000℃. Therefore, the temperature of the exhaust valve is over 800℃ and that of the intake valve is over 300℃. So the material of the valve should be heat resistance steel.
The valve size is represented by the diameter of the head portion. The intake valve is larger than the exhaust valve. Representing by the area of the head portion, when the intake valve is 100, the exhaust valve is about 75∼85. This difference in their sizes is for balancing of the gas flow. The intake is performed by the decreased pressure resulting from the downing
