AUTOMOTIVE CLUTCHES

AUTOMOTIVE CLUTCHES OPERATION AND SERVICE

CLUTCH CONSTRUCTION AND OPERATION 

PURPOSE OF THE CLUTCH 

The automotive drive train or power train- carries power from the engine to the drive wheel. In vehicles with a manual transmission or manual transaxle(transaxle includes a final drive and a differential, this is the difference between the transmission and transaxle), the power flows through a clutch. This device couples and uncouples the manual transmission and the engine. As we all know clutch are mostly operated by driver's foot. Some clutches have power assist device to reduce driver effort.
The clutch is located between the engine flywheel and the transmission or transaxle. Movement of the foot pedal operates the clutch pedal down, the clutch disconnects or disengages from the engine flywheel. In this condition no power flows from through the transaxle or transmission. When the driver releases the clutch pedal, the clutch engages. this allows the power to flow through.
Clutch is always engaged, when we push the clutch pedal clutch is disengaged(that is the clutch plate or friction disc is no more connected or coupled with flywheel or pressure plate).

FUNCTIONS OF THE CLUTCH 

The clutch has four functions:

1. It can be disengaged(when the clutch pedal is pressed). This allows engine cranking and permits the engine to run freely without delivering the power to transmission. 
2. While disengaged(clutch pedal down), it permits the driver to shift the transmission into various gears. This allows the driver to select the proper gear for the operating conditions.
3. While engaging (clutch pedal moves up), the clutch slips momentarily. This provides smooth engagement and lessens the shock on gears, shafts, and other drive train parts. 
4. When engaged(the clutch pedal up), the clutch transmits power from the engine to the transmission. All slipping has stopped.

CLUTCH CONSTRUCTION

The clutch consists of three basic parts engine flywheel, a friction disk, and a pressure plate.  The flywheel and pressure plate are driving members. They attach to and rotate with the engine crankshaft. The friction disc is the driven member. Friction disc is also called as driven plate or driven disc. It is splined to the clutch shaft or transmission input shaft. Flywheel and the pressure plate must turn together, but the friction disc can slide back and forth on the shaft splines. 

The pressure plate, with one or more springs, attaches to the clutch cover. This pressure plate assembly bolts to the flywheel and rotates with it. While the clutch is engaged, spring force holds the friction disc against the flywheel while the clutch is engaged.

The transmission input shaft has the same center-line as the engine crankshaft. The small end of the input shaft rides in a pilot bearing or bushing in the end of the crankshaft. The transmission front bearing or transmission-input shaft bearing supports the other end of the input shaft. To know about bearings http://en.wikipedia.org/wiki/Bearing_(mechanical) this will tell you about the basics of bearing.

CLUTCH OPERATION

Disengaging the clutch(pushing the pedal down) moves the pressure plate away from the friction disc(driven plate). Releasing the clutch pedal engages the clutch. Spring force clamps the friction plate between the pressure plate and flywheel(see above fig.). Then the friction disc and the transmission input shaft turns the flywheel.

Generally when the driver depresses the clutch pedal, linkage to the clutch pedal causes it to pivot. The fork pushes against the release bearing or throw-out bearing. This forces the release bearings inwards in the pressure plate assembly. These pivot to push the pressure plate away from the friction disc. As the pressure plate moves away from the friction disc, a slight air gap opens between the friction disc and the flywheel. These air gaps disengage the clutch so no power flows through it.

FRICTION DISC

It has a hub and plate, cushion springs, and dampening springs. The cushion springs attach to the plate. The friction facings attach to the cushion springs. When the clutch engages, the cushion springs compresses slightly to take up the shock of engagement.

The dampening springs or torsional springs are heavy coil springs set in a circle around the hub. The hub is driven through these springs. They help reduce the torsional vibration caused by the engine power impulses. This smoothes the power flow to the transmission.

There are grooves on both sides of friction-disc facings. These grooves prevent the facings from sticking to the flywheel face and pressure plate when the clutch disengages. The grooves break any vacuum that might form and cause the facing to stick. They also help cool the facings.

COIL SPRING CLUTCH

The pressure plate needs to be clamped. the clamping force is provided by a single diaphragm spring or by coil spring clutch. The coil spring clutch is the set in a circle between the cover and the pressure plate. Depressing the clutch pedal forces the release bearing against the release levers.Struts then carry the motion of pressure plate. It moves away from the springs against the clutch cover. This disengages the clutch.

When the clutch is engaged that is clutch pedal is released the coil springs expands and the friction disc clamps between the flywheel and the pressure plate. The coil spring must be strong enough to prevent the slippage. However, the stronger the plates, the harder the driver has to push the clutch pedal.

The solution for this is to use semi-centrifugal clutch. It has weights on the end of release levers. So when the speed increases, centrifugal force increases, causing the weights to add to the force of the springs. 

DIAPHRAGM-SPRING CLUTCH

The diaphragm spring supplies the force that holds the friction disc against the flywheel. The spring has tapered fingers pointing inward from a solid ring. The spring has tapered fingers pointing inward from a solid ring. These act as release levers to take up the spring force as the clutch disengages. There are different kinds of diaphragm spring, which will not be discussed.

This is just the basic of the. One should also know about the clutch linkage and clutch diagnosis.

VALVES AND VALVE TRAIN

VALVES AND VALVE TRAIN

In the previous blog we have learnt very less about the valve and valve train. So let us learn more about this topic. 

PURPOSE OF VALVES

Each cylinder has two valves intake valve and exhaust valve. The intake valve opens just before the intake stroke begins. This allows the air-fuel mixture to enter the cylinder( in case of diesel engine it is just air). The exhaust valve opens just before the exhaust stroke begins so that the burned gases can escape from the cylinder. 

The valve are operated by the valve trains. There are two basic types of valve train 

overhead camshaft with bucket tappets or rocker arms, and camshaft in block with pushrod.

Valves are arranged in many ways some of them are explained 

1. L-head engine
2. Overhead-valve engine
3. Overhead-camshaft engines
4. Multivalve engine

1. L-HEAD ENGINE

It is also known as flat head engine since the head is flat. Both the valves remain at the same side of the cylinder. It was used earlier in automotive engines, but now it is used only in small engines for lawn mowers and similar equipment. It has two major drawbacks. First, it cannot be designed to have a high compression ratio. The higher the compression ratio more power the engine produces. Second, the L-head had excessive exhaust emission. The exhaust gas contains  too much unburned and partly burned fuel. The reason is that the combustion chamber surfaces are large and relatively cool. This prevents complete combustion

2.OVERHEAD-VALVE ENGINE

In-line overhead engines

In an overhead-valve or pushrod engine, the camshaft is in the cylinder block and the valves in the cylinder head. overhead valve engines have a higher compression ratio than L-head engine. since the clearance volume is smaller. When the air-fuel mixture is compressed into a smaller space, the compression ratio is higher. This means more engine torque and power.

 

some overhead engines have valve reliefs cut into the piston head as shown in the diagram. The valve reliefs provide spaces into which the valves can open without striking the piston.

When the cam lobe comes up under the valve lifter, the lobe pushes the lifter up. This pushes the pushrod up. Pushrod movement causes the rocker arm to rock on its pivot. As one end of the rocker arm moves up, the other end moves down. The valve spring is compressed as the rocker arm moves pushes the valve stem down. This forces the valve off its seat to open the valve. This is how a overhead valve engine works.

3.OVERHEAD-CAMSHAFT ENGINE

In some engines the camshaft is in the cylinder head. This kind of engine is known as overhead camshaft engine. The advantage of the overhead engine is that it reduces the inertia due to pushrod and rocker arms. The rocker arms and pushrods resist changes in speed and direction. They resist motion and change in direction until sufficient force is applied to them. As a result, the rocker arm and especially the pushrod bend or flex slightly before they open the valve. So we have to replace the pushrod often. Also flexing increases as the the speed increases, and with the length of the pushrod. This causes increasing lag in valve action and thus limits the top speed. So this is also one of the factors which decide the top speed.






As shown in the figure the camshaft is at he cylinder head. The cam lobe pushes the bucket tappet which will push the valve inside, hence the valve opens as the cam lobe moves away from the bucket tappet the valve spring moves the bucket tappet back to its original position.this closes the valve. We can use one camshaft or two camshaft as per the will. In many engines we have one camshaft for intake valves and one for exhaust vlave.
The figure also shows the chain tensioners. The reason for using chain tensioner is that, the use of the chain for long time makes it loose and hence he timing of the camshaft and other things are disturbed which is very dangeous. The chain tensioner or the timing belt continually applies a slight force to the outside surface of the chain or belt. This takes up the slack.

4.MULTIVALVE ENGINES 

Engines with more than two valve in a cylinder is known as multivalve engines. The additional valve allows more air fuel mixture to enter and the exhaust gas to escape more easily. This improves the volumetric efficiency of the engine. Also the valve head diameter reduces and the valve weight is also less. This reduces the affect of inertia and reduces the valve spring force needed to close a larger valve at high engine speed.

DRIVING THE CAMSHAFT

in a generalised way two gears can be diven by many ways such as by timing gears or by sprockets and timing chain. In the figure of overhaed camshaft engine we have camshaft driven by sprockest and timing chain. Sprockets are like gears, except that the teeth are shaped to fit into the chain links. A timing chain and the sprockets  usually run more quietly than gears. It is obvious that when the timing chain and sprockets are used the driving and driven gear move in the same direction unlikely the timing gears.

V-belt used for many purpose 

Timing gears

CAMSHAFT TIMING 

Camshaft timing is the relationship between the camshaft and the crankshaft. The camshaft is driven by the crankshaft. Anything that affects this relationship may affect engine operation. There are four stroke cycle engine. The crankshaft must turn two times to turn the camshaft once, which opens each valve once. This 1:2 geat ratio is achieved by making the camshaft gear or sprocket twice as large as the crankshaft gear or sprocket.

VALVES

Now here is some more information about the valves. The intake valve is usually larger than the exhaust valve. The reason is that when the intake valve is open, the only force moving air-fuel mixture into the cylinder is atmospheric pressure. When the exhaust valve opens on the exhaust stroke, there is still high pressure in the engine cylinder. A smaller exhaust valve provides enough space for the high-pressure exhaust gases to get out of the cylinder. Some engines have three valves per cylinder. Two valves for intake and one for exhaust.
Lead was used to be used in the gasoline to increase the compression ratio. But due to its harmful effects it was banned. this lead was used to act as a lubricant. Without lead some faces and seats wear rapidly and valve seat recession occurs. To prevent this, the valves are coated with special coatings such as stellite (a very hard metal)
some valves are chrome plated stems and a hard alloy tip welded on the stem end. This reduces wear on the two areas. other valves have a hollow stem to reduce valve weight. Lighter the valve less is the inertia and hence more the power and efficient engine.

VALVE COOLING

The intake valve runs relatively cool, since it passes only the air-fuel mixture. But the exhaust valve passes the very hot exhaust gases. The exhaust valve may become red hot in operation, with temperature of up to 871 degree C.
The valve stem is the coolest. The area between the stem and the face is the hottest. The valve stem transfers heat to the valve guide to help cool the stem. The valve face transfers heat to the valve seat each time the valve closes. This helps cool the face.

Valves installed in cylinder head, showing the
coolant passages and types of valve guides and seat

The valve stem and the seat are cooled by the engine cooling system.Coolant circulates through the water jackets or coolant passages around the valve seat and the valve guide in the cylinder head. Some heads have nozzles that force coolant around the valve seats. Others use deflectors in the water jackets to improve coolant circulation around the seats.
To help cool exhaust valves, some have hollow stems partly filled with the metal sodium. Sodium melts at 208 degree F. When the engine is running, the sodium is a liquid. Valve movement throws the sodium up and down in the stem. This circulation takes heat from the valve head and carries it up to the cooler stem.



some other topics are there

INTERNAL COMBUSTION ENGINE

INTRODUCTION

First of all the question arises that what is an engine? An engine is a machine that converts heat energy into mechanical energy. Now the engines can be classified as Internal combustion engine and external combustion engine. In this blog we will focus on internal combustion engines only.

Automotive engines are internal combustion engine (I.C engine)because the fuel that runs them is burned internally or inside the engine. I.C engine are further classified as reciprocating and rotary engine. Now a days reciprocating engines are mainly used in automotive engines. Reciprocating means moving up and down, or back and forth. These engines have pistons that move up and down, or reciprocate, in cylinders. These are also known as piston engines.

Rotary engines have rotors that spin, or rotate. Rotary engines are not used in automobiles widely. 

The basic difference between reciprocating engine and rotary engine is that in rotary engines crankshaft remains stationary, while cylinders, crankcase and other units rotate in this operation.

TYPES OF RECIPROCATING ENGINES

There are two types of reciprocating engines
1. spark-ignition
2. compression-ignition 

They differ in 3 ways
a. The type of fuel used.
b. The way the fuel gets into the cylinders.
c. The way the fuel is ignited.

SPARK-IGNITION ENGINE

1. It usually runs on gasoline or an alcohol blend which are liquid fuels.
2. The fuel must be highly volatile so that it vaporizes quickly.
3. Fuel vapor mixes with air before entering the engine cylinders.
4. Combustion occurs due to spark produced in the cylinder.

COMPRESSION-IGNITION ENGINE

1. Mostly diesel is used in this kind of engines.
2. Fuel vapor is directly sprayed into the cylinder, not mixes with air before entering into the cylinder.
3. Combustion occurs due to pressure in the cylinder.

ENGINE CONSTRUCTION

Both types of piston engines are similar in construction. They have following main parts:-

1. Combustion chamber
2. Cylinder 
3. Cylinder Head
4. Camshaft
5. Valves
6. Crankshaft
7. Connecting rod

The figure shows a three-cylinder spark-ignition engine. The cylinder head covers the top of the cylinder. The bottom of the cylinder is open. As the engine runs, the piston slides up and down in the cylinder. The piston at the lowest position is said to be at bottom dead center(BDC), while at the top most point is said to be at top dead center(TDC) as shown in the diagram below.

WORKING OF S.I ENGINE

Engine could be two stroke or four stroke engine. Let us see four stroke engine first

The movement of piston can be divided into four parts or four strokes 

1. Intake stroke 
2. Compression stroke
3. Power stroke 
4. Exhaust stroke

These strokes are movement of piston from TDC to BDC. The crankshaft makes two complete revolutions to complete the four strokes. This makes the engine a four stroke engine.

INTAKE STROKE 

During the intake stroke of a spark ignition engine, the piston is moving down. The intake valve is open. Air-fuel mixture flows through the intake port and into the cylinder. The fuel system supplies the mixture.

As the piston moves down, air fuel mixture enters the cylinder. This is because the piston creates a partial vacuum above it. Due to pressure difference(outside is atmospheric pressure) the air gets in the cylinder. So intake stroke is the movement of the piton from top dead center to bottom dead center. (It should be noted that it is only ideal not in practical.)

COMPRESSION STROKE 

As the piston moves from BDC to TDC the air fuel mixture is compressed. The space which is there after compression is combustion chamber. In this stroke both the valves are kept closed so that proper pressure can be developed. The amount the mixture is compressed is the compression ratio. It is the ratio between the original volume and the compressed volume in the combustion chamber. If the mixture i compressed to 1/8 of its original volume than the compression ratio is 8:1

POWER STROKE 

As the piston is approaching at TDC at the end of compression stroke, an electric spark is produced by the spark plug. Due to spark the air fuel mixture gets burnt. It produces high temperature upto 6000°F. Due to very high temperature it cause very high pres sure which pushes the piston down. The connecting rod carries the motion to the crankshaft, which turns the wheels.

EXHAUST STROKE 

As the piston approaches BDC on the power stroke, the exhaust valve opens. After passing through BDC, the piston moves up again. The burned gases escape through the open exhaust port. As the piston nears TDC, the intake valve opens. The sequence is repeated till the engine is operating  

PISTON AND PISTON RINGS 

Pistons are made of Aluminum alloy. The size of the piston is slightly smaller than the cylinder, which allows the piston to slide up and. This is sliding fit. But there is a problem if we keep some gap between piston and cylinder. Can u think???

Well the problem is that if we keep some gap than there will be pressure loss, leakage of coolant and lubricating oil also. So we should make piston of the same size of a cylinder(in diameter). if we do so the piston will wear very fast and replacement will be needed again and again. This is another problem. So we find an another solution, this solution is piston rings.

Due to pressure difference there is a leakage of compressed gas this leakage is called lowby. Excessive blowby reduces engine power, wastes fuel, and pollutes the air.

To prevent excessive blowby, piston rings are installed on the pistons. The rings are split at one point. This allows them to be expanded slightly, slipped over the head of the piston, and into ring grooves cut in the piston.

There are two types of piston rings

Compression rings These form a sliding seal between the piston and the cylinder wall.

Oil-control rings These scrape off most of the lubricating oil splashed on the cylinder wall, and return the oil to the crankcase.

CLASSIFICATION OF ENGINE

Engine can be classified on the basis of 

1. Number of cylinder. 
2. Arrangement of cylinder. 
3. Arrangement of valves and valve trains.
4. Type of cooling. 
5. Number of strokes per cycle.
6. Types of fuel burned. 
7. Method of ignition.
8. Firing of order. 
9. Reciprocating or rotary.

Engine can has many number of cylinder like three four six or eight cylinder. Now the question is of how they are arranged. Arrangement could in many ways. two major ways are inline and V shape. Other ways are in two rows or banks opposing each other, like spoke on a wheel(radial type).

We won't go deep into these types.

ARRANGEMENT OF VALVES AND VALVE TRAIN

if we notice an engine it is closed from all the sides then how air enters into the cylinder. a point to think. we can't just keep the cylinder open because a pressure should be developed inside the cylinder. so it is supposed to closed. So we should have mechanism which let air in and let air outs so that fresh air is there for combustion and pressure should also be build. Valve are used to do this.

VALVES AND VALVE TRAIN

A valve is a device that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. In an engine a valve is used to control the inlet and outlet of air and fuel. The valve train consists of the valves and a mechanism that opens and closes them. The opening and closing system is called a camshaft. The camshaft has lobes on it that move the valves up and down, as shown in.
The red and blue thing are lobes while the shaft on which lobes are there is called camshaft. The whole thing is known as valve train(assembly of valve, lobes camshaft).

 this is a diagram of camshaft.





There are several different arrangements of valve and valve train. Classification is based on the following basis:-

1. location of camshaft.
2. how the camshaft is driven.
3. type of valve train.
4. number of valve per cylinder.

On the basis of type of cooling engine is classified as air cooled or liquid cooled. Most of the engines are liquid cooled. Some of the engine are air cooled like Volkswagen Beetle engine.
Classification of engine is based on the number of strokes. we have already learnt that.
Engine are classified on the basis of fuel also. If engine is using gasoline or diesel or LPG.
Method of ignition- spark-ignition or compression ignition engine.

FIRING ORDER

engines can also be classified on the basis of firing order. Firing order is the sequence in which the cylinders deliver their power strokes. It is decided by crankpin and crankshaft. For a four stroke engine the complete firing order represents two complete revolution of the crankshaft, that is 720 degrees of crankshaft rotation. Most of the engines are even firing(think of the reason). For example in an inline six cylinder engine firing impulse occurs every 120 degrees of crankshaft rotation. Firing order differs for different engines. Several firing orders are 1-3-4-2, 1-5-3-6-2-4, 1-2-3-4-5-6 and many others. Firing order may differs for a V shape or inline whether they have same number of cylinders or not.

RECIPROCATING AND ROTARY(FIND IT YOURSELF)


This was the basic outlet of the reciprocating engine. Read other articles to know more about engines and other parts of a car.