Wednesday, 19 September 2012

ENGINE PRINCIPAL CHAPTER 3 Cylinder Head


ENGINE PRINCIPAL

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 can not 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 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 determinded 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 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 800and 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 7585. This difference in their sizes is for balancing of the gas flow. The intake is performed by the decreased pressure resulting from the downing of the piston, on the other hand, the exhaust is performed by the high pressure resulting from the combustion. To balance the flow of the intake and exhaust, the intake valve size may be larger than exhaust valve.
The valve stem is designed, considering the flow, such as that of the intake valve is thinner as possible to reduce the flow resistance and that of the exhaust valve is thicker as possible to transmit the heat from the extrude portion to the stem. The heat is transferred through the path of the valve stem the valve guide the cylinder head the cooling water. Some high performance engine uses the hollow stem securing sodium for enhancing the valve cooling.
The portion of the port contacting with the valve face is called as the valve seat. If the cylinder head is casting iron, then the valve seat has double structure. If the cylinder head is made of aluminum alloy, then the seat is made of heat resistance steel.
The valve spring always press the valve to attach to the cam so that the valve spring is preferable to be soft to reduce the frictional resistance generated when the cam nose presses the valve. Additionally, in order to enlarge the amount of intake and exhaust gas, the valve may be enlarged and the lift of the cam may be extended as well as the valve spring may be softened to operate faster. Then, it may make the surging problem and it is hard to be balanced.

5. Valve Driving System
As the valve controls the gases for inhaling into and the taking out from the cylinder, its driving method can give an important affects on the engine performances. There are various type of valve driving system. It has been developed from the side valve type, OHV, OHC to the DOHC type.

Side valve type
In the side valve type, the camshaft installed near the camshaft presses the long valve system to open and close the valve. The combustion chamber is large and it has long time interval for combusting the mixture of fuel so that it can not make high output power. Nowadays this type is not used.

OHV (Over head valve) type
In the Over Head Valve (OHV) type, the valve like in the side valve type is attached on the  cylinder to open and close the valve using a long rod, the push rod. The shape and structure is similar with that of engines used now to enhance the performance.

OHV (Over Head Camshaft) type
After that, the OHC (Over Head Camshaft) type is developed. As know from the name, the camshaft is located over the head, exactly to say, in the middle of the cylinder head. In the OHC type, there are two types, the one is the in-line type in which the intake valve and the exhaust valve are alternatively arrayed along the linear direction, and the other is the V-shaped arrange type in which the intake valve and the exhaust valve are faced each other in opposite side to form the V-shape. The later has more enhanced efficiencies and higher performance.

DOHC (Double Over Head Camshaft) type
Developing this V-shaped arrange type more and more, the DOHC (Double Over Head Camshaft) type, in which the intake valve and the exhaust valve are independently driven by the different camshaft, is mainly used nowadays for the high performance engine. As we see from the name, there are two camshafts so we call it as the twin cam system also. Additionally, in the V-type engine, it has two cylinder head so the camshaft shall be four.

Direct type   Swing arm type  Rocker arm type
The driving method for the intake and exhaust valves is divided into the direct type, in which the cam directly drives the valves, and the rocker arm type, in which the cam drives the valve using a lever. The rocker arm means the lever connecting the leverage point and the cam. Using the lever, it can drive the valve faster than the cam lift.
The direct type has less components and high strength. Using the oil pressure for the hydraulic tappet, the valve can trace the cam profile always.

6. Valve Timing
The valve timing is the time at the opening and closing the intake and exhaust valve. Each time indicating when the valve starts opening and when the valve finishes closing is represented by the rotational angle of the crankshaft about the highest point or the lowest point of the piston as the standard time.

As simply saying about the opening and closing the valves, the exhaust valve will open when the piston is at the lowest point. After taking out the gas, when the piston is at the highest point, the exhaust valve will be closed. At the same time, the intake valve will be opened to take in the mixture of fuel. When the piston is at the lowest point, the intake valve will be closed.
However, this is just concept for the operation of valves. The mixture of fuel and the used gas have mass so the flow of the mixture or gas is not completed at a moment but need some time intervals. Furthermore, the valves can not open and close at an instant moment, too. For example, the intake valve need some time interval to open fully, and the mixture is not inhaled at a moment into the cylinder but inhaled during some time interval by the inertia of flow.
Therefore, the intake valve shall open prior that the piston is reached at the highest point. When the piston starts to go down, then the valve is already little opened to take into the mixture of fuel into the cylinder. By that the intake open little early, the valve will be fully opened when the piston is reached at the lowest point, so that the mixture of fuel can be inhaled enough.

Intake valve timing
When the piston is passed the lowest point, the intake valve is not closed thoroughly. To do so, the mixture will be more inhaled into the cylinder by the inertia of the flow of the mixture.
At the end of the combustion stroke, the exhaust valve will be open just before the piston is reached at the lowest point(BDC).

Exhaust valve timing
It is for exhausting the combusted gas as fast as possible by the remained expansion force in the cylinder. In the same manner of the intake valve, even the piston is passed the highest point(TDC), the valve is still opened to exhaust the burnt gas fully using the inertial of exhausting flow.

According to these operating process of the valves, there is some period in which the intake valve and the exhaust valve are opened at the same time, because the exhaust valve is closed after passing through the highest point(TDC) and the intake valve is opened before reaching to the highest point(TDC). At this time, the vacuum inertia force generated from the exhausted gas can accelerate the inhalation of the fuel mixtures. This period is called the valve overlap.


7. Variable Valve Timing

Variable camshaft angle type
Being overlapped in the opening time of intake and exhaust valve, the enhanced intake efficiency of the mixtures is as high as the enhanced exhaust efficiency of the used gas, that is, the rotation of the engine. On the other hand, when the engine is in the rotating state with low speed such as the idling state, the engine efficiency may be lowered by the lowered gas flow.
Especially, for some high performance engine having large overlap, in the low speed, the intake valve will open with large amount of burnt gas so that the burnt gas will be adhered to the intake port. Therefore, the combustion will be unstable or incompletely combusted. In the 4-valve engine, if the valve overlap is too large, then the engine will be easily unstable at the idling state. So, the overlap time for the 4-valve engine shall be very short, or some cases have the zero overlap time, that is, the intake valve is opened according that the exhaust valve is closed.

Selective camshaft-lobe type
As we mentioned, the valve timing is different according to the rotation speed of the engine. That is, the intake valve should be opened with little lately at the lower rotation, and with little early at the higher rotation. So, an additional switch operated by oil pressure is installed at the intake cam sprocket in order that the camshaft can be rotated somewhat when the engine is rotating over certain RPM so that the cam presses the intake valve early. This is the variable valve timing system.
In the variable valve timing system, the cam shape is not changed, so the valve shall be also closed early when it is opened early. Early closing the valve, the amount of the inhaled mixture is reduced. Therefore, the choice of the valve timing is decided by not only the rotation speed of the engine but also the load of the engine.
Consequently, the cam system will include the two kinds of cams, the one for the low speed of the engine and the other for the high speed of the engine. For the low speed cam, the valve will be opened lately and closed early, and the lift will be small and then the inhaled mixture of fuel will be reduced so that the fuel efficiency can be enhanced. For the high speed cam, the valve will be opened early and closed lately and the lift will be large to inhaled more mixtures so that the engine output shall be enlarged. This system also called variable valve timing system but this system controls the valve lift also. It is more advanced system.

8. Malfunction of the Valve
The valve is opened by the cam but closed by the spring. Actually, the valve attached by the spring to the valve seat is opened with the pressing by the cam nose. At that time, the force rotating the camshaft is rather small as possible. So the soft spring is more proper. However, for some high performance engine having large size of valve or lift, the spring shall be harder and the balance of the spring strength is important factor.
Even not occurring in the normal driving situation, the hardness of the spring with the weight and the strength of the valve can make an abnormal operation such as the valve jump, the valve bounce or the valve surge, when the engine is rotating over the limitation rpm.
The valve jump is that the inertial force of the valve is too large so that the cam can not press the spring and then the valve is jumped up from the cam nose when the camshaft is rotating with high speed. The valve can return back to the original position but the other dynamic components including cam, rocker arm, valve lifter, etc are damaged by friction each other.

The valve bounce is that the valve face is not contacted to the valve seat (contacted portion with the valve) but bounced from the valve seat when the valve is closed by the spring. The dynamic components shall be damaged by this bouncing. As increasing the rotation of the engine, the bouncing is occurred over than limited rpm. This limited rpm is called as the crush speed or limit rpm of the engine.

The valve surge is the abnormal vibration of the spring. As the natural frequency of the spring is corresponding with to the elastic timing by the cam, the spring can make a high movement by the self-excited vibration. When the engine is rotated in force. If it is continued then the spring shall be broken.
These abnormal operations of valve can be easily made when the valve is heavy and the lift is large. When the most used engine was the 2-valve OHC engine, they were main problems in engine. After the 4-valve DOHC engine is mainly used, these problems are scarcely occurred. As the 2-valve system became to the 4-valve system, the valve area would be enlarged so the intake and exhaust flow would be smoother. Therefore, the lift has not to be enlarged. Furthermore, the valve is to be lightened so that the spring has not to be strengthened even if the rpm is increased.

9. Overrun and Red Zone
In the tachometer of the engine, there is certain rpm range colored in red called as the red zone. Some meter has the yellow zone just before the red zone.

The rpm at the beginning of the red zone is the maximum allowable rpm decided by the characteristics and endurance requirement of the dynamic components including valve and valve spring and the main system components including piston and connecting rod when the engine is operated with the maximum speed having the maximum output.
To operate the engine over the maximum allowable rpm is called as the overrun or over-revo meaing the revolution. The over-revo may occur when the shift is down to the lower speed gear at the driving with high speed. When the engine is in the idling state, if the rpm is increased in force, then the engine will be overrun state.
When the engine is overrun, the valve shall be abnormally operated such as the valve surge, jump or bounce. In this case, the valve and spring may be damaged or in some cases, the piston may be damaged by bumping with the head of the piston and the valve. In order to prevent bumping between the piston and valve, a recessed portion is made at the piston. However, if the piston is jumped up over the recessed portion, then the piston will be bumped to the valve.
If the average speed of piston movement is abnormally fasted by the overrun, the gap between the piston ring and the cylinder will be damaged and the oil film of the bearing at the piston pin and the crankshaft will be broken so that the temperature will be increased or some parts will be adhered by this heat. When the engine is operating with high speed, the combustion speed is also fast and the around temperature of the combustion chamber will be increased. So, these problems might be easily occurred, so it needs to be careful.
Additionally, by the overrun, the engine has the increased inertia force being results of the abnormal vibration. The parts shall be broken or cracked.
The maximum allowable rpm is set with 300~1300 rpm higher than the maximum output rpm. In some engine, in the red zone the fuel injection will be cut to prevent the rpm from increasing over the maximum rpm and to prevent from making problems by the overrun.

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