Mechanical Technology Grade 12 Chapter 11 Heat Engines

The first reference to an engine can be traced back to 100 AD to a Greek physicist and mathematical genius, Heros of Alexandria, who described the principle of a basic steam engine.

In 1673 the Netherlands physicist Christiaan Huygens built the first internal combustion engine.

This engine ran on gunpowder. In Huygens’s design the power is generated by

atmospheric pressure on the piston. It was not a practical engine because of the

gunpowder.

In 1698 a British inventor, Thomas Savery, used the design of Papin and developed a pump engine without a piston.

This engine could pump water. In 1705 Thomas Newcomen built an improved steam engine

with a piston that operated in a cylinder. He used the engine to pump water. Later James Watt registered a patent on a steam engine

with a crankshaft. This engine converted the up and down motion into a rotary

motion. In 1782 Watt built his first double-action steam engine. The power was generated by steam and not by atmospheric

pressure.

In 1839 the American Isaac Babbitt invented a material for bearings.

This material allowed engines to run at high speeds.

On 9 May 1876 the public witnessed Nikolaus August Otto’s new four-stroke engine.

It produced 3 h.p. at 180 r.p.m. and was known as the ‘Otto Cycle’ engine.

In 1876 the American George Brayton built the first two-stroke engine.

Sir Dugald Clerk also invented a two-stroke engine, an improvement on Brayton’s engine.

Clerk is named the father of the two-stroke engine.

In 1885 Gottlieb Daimler built the first motor bike.

Later in that year he built the first motor boat and the first motor coach.

Daimler and Benz are often called the fathers of the motor vehicle.

After 14 years of contemplation, in 1893, Rudolf Diesel patented the first diesel engine.

It was manufactured by the engineering firm, Krupps.

The four-stroke engine is used in the following: motor cars motor bikes boats trucks lifting equipment water pumps generators.

Since the operation of each cylinder in a multi-cylinder engine is identical, we will focus only on a single-cylinder engine.

From this knowledge you will understand the operation of all four-stroke petrol engines.

The engine comprises a cylinder block in which a cylinder is bored to accommodate the piston and piston rings.

A crankshaft is mounted on one side of the cylinder block which is closed in an oil pan/sump.

The included space is called the crankcase. The oil pan contains a specified quantity of engine

lubricating oil. The piston is connected to the crankshaft by a

connecting rod and a gudgeon pin.

Sealing off the space above the piston on the other side of the cylinder, is a cylinder head.

The free volume in the cylinder when the piston is at the extreme top in the cylinder is called the combustion chamber.

The cylinder head contains the intake and exhaust valves. A valve mechanism opens and closes the valves. This valve mechanism is activated by the camshaft which is

driven by the crankshaft at half-crankshaft speed. An intake manifold with a carburettor is connected to the

intake port which leads to the intake valve and hence the cylinder, while the exhaust manifold is connected to the exhaust port.

You must know these terms and their abbreviations: Top Dead Centre (t.d.c.) – when the piston is at the end of

the upward stroke, that is the highest point which the piston can reach in the cylinder

bottom dead centre (b.d.c.) – when the piston is at the end of the downward stroke, that is the lowest point which the piston can reach in the cylinder

a stroke – the maximum distance of piston movement between extreme points; these extreme points are t.d.c. and b.d.c. and the crankshaft turns through 180°

a cycle – the four strokes of the piston; one cycle is completed during two crankshaft revolutions (720°)and one camshaft revolution.

The piston moves from t.d.c. toward b.d.c. When the piston descends from t.d.c., the intake

valve starts opening. With the exhaust valve closed during the stroke, a

partial vacuum or depression is created in the cylinder above the piston.

Atmospheric pressure fills this partial vacuum and, in doing so, passes through the carburettor barrel.

In the carburettor barrel petrol is mixed with air. This air-fuel mixture fills the cylinder via the intake

manifold and past the open intake valve. Just after the piston reaches b.d.c., the intake valve

is closed.

Both the intake valve and the exhaust valve remain closed during this stroke and the piston moves from b.d.c. to t.d.c.

The petrol mixture is compressed in the relatively small combustion chamber.

Just before the piston reaches t.d.c., a high tension spark is introduced into the combustion chamber by means of the sparkplug and the petrol mixture is ignited.

Both valves still remain closed and as a result of ignition, combustion of the petrol mixture takes place rapidly.

High temperatures are developed as a result of this combustion and cause the gases to expand.

This expansion exerts considerable pressure on the piston and the force is transmitted to the crankshaft via the connecting rod, thereby giving a rotary motion (rotation) to the crankshaft.

As the piston reaches b.d.c., the exhaust valve is opened. The crankshaft rotates as a result of momentum in the

crankshaft, assisted by the flywheel, and the piston moves from b.d.c. to t.d.c.

The piston forces out the burnt gases past the open exhaust valve to the exhaust manifold from where it is fed to the atmosphere by pipes.

When the piston reaches t.d.c., the engine is ready to start the following intake stroke and cycle.

A running engine emits the exhaust gas, carbon monoxide.

The gas is very harmful to both people and the environment.

Leaded petrol has now been phased out and more and more cars have catalytic converters that convert the carbon monoxide to less harmful carbon dioxide.

Engcycle.wmv

The two-stroke principle was invented by Sir Dugald Clerk in 1880.

Still today two-stroke engines are widely used by motorcycles, lawn mowers and motorboats.

The construction of this engine is similar to that of the four-stroke petrol engine.

However, the three-port two-stroke engine does not have valves. Instead there are the intake port, the exhaust port and the transfer port,

all of which are closed and opened by the moving piston. The transfer port has no direct connection with the atmosphere, but

connects the crankcase with the cylinder above the piston. A second important feature is the crankcase which is air-tight and which

contains no lubricating oil as the oil is added to the petrol. A third feature is the piston crown; its design promotes scavenging of the

cylinder. (Scavenging occurs when the new gases from the bypass port push the burnt gases out the outlet port and so clean the cylinder.)

Note that the transfer port is situated on one side of the cylinder while the intake and exhaust ports are on the opposite side of the cylinder.

The concepts, t.d.c., b.d.c. and stroke, are the same as with the four-stroke petrol engine.

A cycle, however, is completed during only two piston strokes.

During this cycle, the crankshaft completes one revolution, or 360°.

The two phases of the cycle are completed during two piston strokes.

Scavenging is the most important aspect of the two-stroke petrol engine.

Because the direct pumping action of the piston cannot be used in this case, the burnt gases are scavenged by the incoming petrol mixture.

With the piston at b.d.c., the intake port is closed and the other two ports open.

At this stage, the cylinder above the piston is filled with the petrol mixture.

During ‘piston-travel’ from b.d.c. to t.d.c., the transfer port is first closed by the piston and then the exhaust port.

As the piston ascends, the petrol mixture is compressed in the combustion chamber while a vacuum is created in the crankcase.

With the upward movement of the piston, the intake port is opened and the petrol mixture fills the vacuum in the crankcase.

Just before the piston reaches t.d.c., the compressed petrol mixture is ignited by a high tension spark.

After ignition of the petrol mixture, combustion causes high temperatures to develop, and as a result the gases expand.

These expanding gases force the piston towards b.d.c. and a powerful rotational movement is given to the crankshaft.

As the piston moves towards b.d.c., the intake port is closed and the petrol mixture is compressed in the crankcase.

With the further movement of the piston, the exhaust port is opened first and the burnt gases start escaping.

Just after this the transfer port is opened and the compressed petrol mixture in the crankcase enters the cylinder under pressure and assists in driving out the burnt gases.

Note that the piston crown is designed specially so that the incoming petrol mixture is deflected.

This design is necessary to prevent the petrol mixture from escaping together with the burnt gases.

If this doesn’t happen, the petrol mixture will be wasted to a large extent while the upper section of the cylinder will still be filled with burnt gases.

PG. 174

Compression ignition (C.I.) engines are often called diesel engines.

These engines are named after the German engineer, Rudolf Diesel.

In 1892 he registered a patent on an engine that relied on the heat generated during compression to ignite a fuel of coal dust.

This fuel was forced into the cylinder by air pressure at the end of the stroke.

A British engineer, Herbert Ackroyd-Stuart, improved the C.I. Engine.

His version involved the induction and compression of air and the timed injection of liquid by a pump.

Diesel fuel is crude oil which has not been refined to the extent petrol has, and is therefore cheaper to manufacture.

C.I. engines operate as a result of fuel burning in the cylinder which is dependent on the amount of heat generated by the burning fuel.

This heat energy is then converted into useful work. The construction of the C.I. Engine is the same as

that of the spark-ignition engine and can also be divided into the four-stroke and the two-stroke type.

The engine consists mainly of the following components (similar to petrol engine): cylinder – an integral part of the cylinder block piston – has a perfect fit in the cylinder connecting rod – coupled to the piston by means of a

gudgeon pin and which facilitates a hinge action crankshaft – into which the connecting rod is coupled

by the big-end bearing main bearings – keep the crankshaft in position in the

cylinder block and at the same time allow it to rotate freely

It also consists of: crankshaft timing gear – fitted to the front end

of the crankshaft camshaft – housed in the camshaft bearings

in the cylinder block valve mechanism – consisting of a cam

follower, push rod, rocker and a valve kept closed by a stiff spring. For each

cylinder, two of each of the above components are required – one set for the intake function, and one set for the outlet function.

It is similar to the petrol engine, except that : The compression ratio is approximately 20:1

compared to a petrol engine which has a compression ratio of about 10:1.

o On the compression stroke the fuel is injected and mixes with the compressed air a few degrees before the piston reaches t.d.c..

h The mixture is ignited by the high temperature that results from the higher pressure of the compressed air/fuel on the compression stroke.

Types of two-stroke C.I. Engine1. Engines with ports in the bottom end of the cylinder

which are used to scavenge exhaust gases and admit the fresh air charge.

2. Engines where the intake port is in the bottom end of the cylinder and the exhaust port is in the cylinder head. The closing and opening of the exhaust port in this case is controlled by poppet valves.

All two-stoke C.I. engines are fitted with blowers which force pure air into the cylinder.

These engines complete their full working cycle within two strokes of the piston.

The cylinder is designed with ports at the side of the cylinder, more or less at the end of the stroke of the piston.

The intake ports are designed so that the fresh air charge is directed in a spiral movement towards the cylinder head.

The exhaust port is cut so that it will open first as the piston moves downwards on its stroke.

Driven by the engine, a blower forces air under pressure into the cylinder when the intake port is uncovered by the piston.

A few degrees before the piston reaches t.d.c., fuel is injected and mixes with the compressed air after which it is ignited.

Combustion follows ignition and as a result the temperature is raised which causes the gases to expand.

The expanding gases force the piston downwards in the cylinder.

When the piston has moved through three-quarters of its downward stroke, the exhaust port is uncovered.

The spent gases, still under pressure, escape through the exhaust port.

The piston moves further downwards and uncovers the intake port.

Fresh air under pressure from the blower at approximately 150 kPa enters the cylinder.

As a result of the design of the intake port, the spent gases are forced out by the incoming fresh air charge.

As the piston moves upwards, it first closes the intake port and thereafter the exhaust port.

The piston compresses the fresh air charge to a high pressure and as a result a high temperature is developed.

The mixture is ignited by the high temperature that results from the higher pressure of the compressed air/fuel on the compression stroke.

The cylinder is designed with only intake ports which are cut around the whole bottom end of the cylinder.

When the piston is at b.t.c., the ports are exposed.

The cylinder head is equipped with two exhaust ports which are controlled by poppet valves.

A camshaft and valve mechanism are required to operate the valves.

Just before the piston reaches t.d.c., fuel is injected into the cylinder.

The fuel mixes with the air and is ignited by the hot, compressed air.

It burns and this raises the temperature. The high temperature causes the gases to

expand which force the piston downwards in the cylinder.

When the piston has moved through three-quarters of its downward stroke, the exhaust valves in the cylinder head are opened and the spent gases, still under pressure, start escaping.

With slight movement of the piston further downwards, the intake port is exposed.

Fresh air, under pressure from the blower at approximately 150 kPa, enters the cylinder.

The fresh air forces out the spent gases through the exhaust port in the cylinder head.

As the piston moves upwards again, the exhaust valves are closed.

The intake ports are now closed by the piston with the result that the fresh air charge is trapped.

With the further movement of the piston upwards the air charge is compressed to a high pressure and high temperature develops.

Use sketches to describe the following:1. The four-stroke compression ignition engine.2. The operation of the power stroke of the four-

stroke C.I. engine.3. The operation of the ‘Uniflow’-type two-stroke

engine.4. The operation of the port-type two-stroke engine

during the exhaust and intake strokes. Pg 178

A cylinder block is cast in a single unit and may be either a single-cylinder or a multi-cylinder block.

A cylinder block designed for an indirect air-cooled engine contains channels, called water jackets, through which the coolant is circulated by the water pump.

Cast iron and aluminium alloy metals are generally used to make cylinder blocks.

The cylinder block’s function is to house the crankshaft, the camshaft and the pistons.

It is also a mounting piece for all other components on the inside and outside thereof.

1. How does coolant circulate through the cylinder block?

2. Name the metals from which cylinder blocks and cylinder heads are manufactured.

3. State the function of the cylinder block.

If the engine is indirectly air-cooled, the cylinder head is also cast as a single unit and contains water jackets.

In the case of direct air-cooled engines, the cylinder head has no water jackets, but fins on the outside which increase the area subjected to the cooling air.

Cylinder heads are also manufactured from cast iron or aluminium alloy.

The cylinder head serves as a lid for the cylinders so that the petrol mixture may be compressed in the cylinders without a loss of pressure.

In most cases the cylinder head contains a section of the combustion chamber.

With an overhead valve engine, the head houses the valve mechanism.

In the case of the side-valve engine, this is the function of the cylinder block.

The cylinder head also accommodates the spark plugs.

1. State the function of the cylinder head.2. State two ways by which cylinder heads of

direct and indirect air-cooled engines may be identified.

Crankshafts are made from steel alloys with nickel-chrome and chrome vanadium.

The crankshaft is the main shaft of the engine. It is housed in the cylinder block in bearing

liners. The crankshaft is provided with as many crank

journals as there are cylinders in the engine.

The crankshaft is also provided with counterweights for the purpose of static and dynamic balancing.

The flywheel is bolted to the rear of the crankshaft while the crankshaft pulley and the crankshaft gear are fitted to the front.

The crankshaft gear is for driving the camshaft gear. Steel alloys with nickel-chrome and chrome-

vanadium are used in the manufacture of crankshafts to resist shock.

The function of the crankshaft is to convert the reciprocating movement of the pistons into a rotary movement.

Assessment 1. Name the metal from which crankshafts

are manufactured. 2. State the function of the crankshaft.

A piston is a hollow, cylindrical unit, open at one end and closed at the other.

The closed end (or upper end) is called the piston crown.

A piston fits into a very accurately machined cylinder with the minimum of clearance.

The piston contains a hole for fitting the gudgeon pin and grooves to accommodate the piston rings.

The number of grooves is the choice of the designers, but when pistons are replaced, this should be kept in mind.

Cast iron and aluminium alloy are popular metals used to make pistons.

A piston serves as a pump and thereby: causes a vacuum in the cylinder during the intake

stroke forces out the burnt gases during the exhaust

stroke compresses the petrol mixture in the combustion

chamber during the compression stroke.

A piston also transmits the force due to the expanding gases to the crankshaft via a gudgeon pin and the connecting rod. This causes the crankshaft to rotate.

1. Which metals are pistons made from?2. What is the purpose of a piston in an engine?

Piston rings are applied together with the piston to act as a pump.

An average of three piston rings per piston is popular with modern engines, except where high engine performances are required.

On smaller engines usually only two piston rings per piston may be used, but never less than two.

There are two types of piston ring, namely compression rings and oil control rings.

The general tendency is to apply all piston compression rings above the position of the gudgeon pin.

The oil control ring is applied below the gudgeon pin. Compression rings are those nearest the piston crown

with the oil control ring further below. When three piston rings are used, only one of them is an

oil control ring. In the case of two-stroke engines, no oil control rings are

applied.

Cast iron is commonly used in the manufacture of piston rings, although chrome may be added to the first compression rings.

Function The function of the compression ring is to provide for a

gas-tight seal between the piston and the cylinder wall. This is so that pressure from above the piston will not

escape to the crankcase. Oil control rings control the quantity of oil on the cylinder

walls, that is they remove excess oil from the cylinder walls but leave sufficient behind for lubrication purposes.

State the function of the compression and oil control rings.

A gudgeon pin is a hollow, cylindrical object which fits through holes in the piston and the small-end bearing of the connecting rod.

In this way the piston is connected to the connecting rod.

When the gudgeon pin is pressed into the small-end or clamped thereto, the bearing surface is in the piston.

When the gudgeon pin fits loosely in the small-end, circlips are used in the piston bosses to keep the gudgeon pin in position.

A steel alloy of high quality is used in the manufacture of gudgeon pins and they are case-hardened.

Function The gudgeon pin acts as the linkage (crank

and slider) between the piston and connecting rod.

State the function of the gudgeon pin.

Connecting rods are manufactured from high quality cast steel with chrome-vanadium.

The function of the connecting rod is to connect the piston with the crankshaft whereby the force exerted on the piston is transmitted to the crankshaft.

At the one end, the connecting rod has the big-end bearing which is fitted to the crankpin of the crankshaft in bearing liners.

The other end of the connecting rod is called the small end.

This end of the connecting rod is connected to the piston by the gudgeon pin.

The gudgeon pin may fit loosely in the small end or may be press-fitted or clamped

The camshaft is housed in the cylinder block except in engines with overhead camshafts.

In all cases the camshaft is driven at half the number of crankshaft revolutions.

Except for the bearing journals the camshaft contains pear-shaped cams which are responsible for opening the engine valves.

In most cases the camshaft also contains an eccentric cam for operating the mechanical fuel pump, as well as a helical gear for driving the distributor and the oil pump.

An alloy of cast iron, copper and chrome is commonly used in the manufacture of camshafts.

Function The functions of the camshaft are:

to convert the rotary movement of the crankshaft in the reciprocating movement of the valves

to open the valves at a pre-determined time to drive the distributor, oil pump and mechanical

fuel pump.

1. State the function of the camshaft.2. At which frequency (relative to the

crankshaft) does the camshaft rotate?3. Which components are driven by the

camshaft and how is this possible?

The timing gears comprise the crankshaft gear and the camshaft gear.

These gears are found at the front end of the engine.

The camshaft consists of exactly twice the number of teeth as that on the crankshaft gear.

This design is necessary so that the camshaft is driven at half the number of crankshaft revolutions.

The timing gears are marked by the manufacturers.

These marks are used to determine the valve timing.

When the gears are fitted, care should be taken that the timing marks are opposite each other and in a straight line with the centres of the gears.

The function of the timing chain is to transmit the rotary movement of the crankshaft to the camshaft.

It performs the same function as timing gears. When the engine design is such that the

timing gears do not mesh directly, a timing chain is applied.

One disadvantage of this application (of a timing chain) is the increase in wear.

There is also the tendency of the chain to stretch which results in noisy operation, but this can be limited to a great extent by the application of a tensioner.

Note that when the gears mesh directly, the direction of rotation of the two gears is opposite, while the direction of rotation is the same when a timing chain is applied.

Most modern vehicles use toothed timing belts to control the valve timing.

The timing belts operate in the same way as chains do, but run on toothed pulleys instead of sprockets.

Timing belts have the advantage of not requiring lubrication and also operate more quietly than belts and gears.

They do however need to be replaced more frequently to prevent breakages and serious engine damage.

1. What are timing gears used for?2. Why do manufacturers mark timing gears?3. What is used in conjunction with timing

chains to counteract the effect of chain elongation or stretching?

4. How is the valve timing controlled on modern engines?

Material Valve lifters are

manufactured from steel alloys and are case-hardened.

The function of a valve lifter is to follow the shape of the pear-shaped cam on the camshaft by which a reciprocating movement is obtained.

Valve lifters are available in various designs, but the most important aspect is the difference in operation of the overhead valve engine and of the side-valve engine.

On side-valve engines and some engines with overhead camshafts, the valve lifter is placed directly between the camshaft and the valve stem.

Provision for the adjustment of valve clearance is made on the valve lifter.

This type is therefore known as the adjustable valve lifter.

Other types of valve lifter are the tubular type, mushroom type, roller type and the hydraulic type, all of them being applied on overhead valve engines.

The latter types are applied directly between the camshaft and the pushrod and are not adjustable.

Between which two components of the side-valve engine is the valve lifter applied?

The pushrod has the simple function of transmitting the reciprocating movement of the valve lifter to the rocker.

The pushrod is a long, slender metal shaft or rod which is positioned between the valve lifter and the rocker on overhead valve engines.

The one end is rounded to fit into a recess of equal design in the valve lifter while the other end contains a round socket in which the ball-end of the adjusting screw on the rocker fits.

1. State the function of the pushrod.2. Make a neat sketch depicting how the

pushrod acts between the cams and rocker arm assembly.

Rockers are mounted on a rocker shaft. There is one rocker to each valve in the

engine. The rocker is placed between the pushrod and the valve stem and is

therefore applied only on overhead valve engines. The rocker provides for the

adjustment of valve clearance by means of adjusting screws. These adjusting

screws may be the self-locking type or lock nuts.

Material Rockers are manufactured from superior

quality steel alloy.

Function The function of the rocker is to transmit the

reciprocating movement of the pushrod to the engine valve.

1. How many rockers are there in a four-cylinder engine?

2. Between which two components are rockers applied?

3. In Chapter 8 you learnt about various linkages. Which particular linkage does the rocker arm represent?

Intake valves are manufactured from steel with a high chrome content while exhaust valves are manufactured from silicon-chrome steel to cope with high temperatures.

Each cylinder in the engine has an intake valve and an exhaust valve.

These valves can be distinguished from each other by the diameter over the valve head of the intake valve which is usually more than ‒that of the exhaust valve.

The valve stem fits in a valve guide in the cylinder head in the case of the overhead valve engine and in the cylinder block in the case of the side-valve engine.

Engine valves require valve springs to close them and to keep them properly closed to prevent the loss of pressure.

The modern tendency is to indicate the tension of valve springs by means of a colour code.

The valve spring is kept onto the valve by means of a valve spring retainer and two half-moon cotters.

Material The valve spring is manufactured from a

superior quality spring steel.

Function Valve springs are used to ensure that valves

are drawn tightly into their valve seats, to ensure a gas-tight seal.

1. Name two types of adjusting screw which are used for adjusting valve clearance.

2. How can intake valves be distinguished from exhaust valves?

3. Name the metals from which engine valves are manufactured.

4. State the function of engine valves.5. State the function of the valve spring.

The exhaust manifold is manufactured from cast iron and the intake manifold from cast iron or aluminium alloy.

All internal combustion engines require at least one intake and one exhaust manifold which are two separate components.

Mounting of the manifolds depends on the engine design. The two manifolds may be bolted together before they are

mounted as a unit on the engine, or bolted to the engine independently of each other.

A V-type engine uses two independent exhaust manifolds and one intake manifold.

The carburettor is bolted to the intake manifold while the exhaust pipes and silencer are connected to the exhaust manifold.

The functions of the intake manifold are to convey the petrol mixture from the carburettor to the various engine cylinders and to promote vaporisation of the petrol mixture before admitting it to the cylinders.

The function of the exhaust manifold is to direct the exhaust gases from the various engine cylinders to a common point.

1. Name the metals from which manifolds are manufactured.

2. State the function of manifolds.

Gaskets are placed between two surfaces to prevent leakage of the following substances: gases, water, oil and petrol.

Gaskets are very important for application between castings which are temporarily bolted together.

No specific design can be referred to because the gaskets vary from engine to engine.

Gaskets are applied to obtain gas-tight and oil-tight joints.

Manufacturers usually supply a complete set of gaskets for each type of engine, but they may also be made by a motor mechanic.

One of the materials used in the manufacture of gaskets is cork which, in some cases, is impregnated with a synthetic liquid rubber to make it less brittle.

Other materials are solid synthetic rubber or neoprene rubber, paper, aluminium alloy and thin copper sheet metal.

In the case of the latter, two plates are applied with asbestos between them.

The following materials are used: cylinder head gasket – asbestos

coated with copper intake and exhaust manifold gaskets

– treated asbestos oil pan and rocker cover gaskets –

cork or rubber other – treated fibre (vellumoid).

A seal prevents the leakage of water, oil or grease.

It also prevents dust or water from penetrating from the outside.

When revolving shafts on the inside of an engine are extended to the outside, it is necessary to seal off the shaft against the casting to prevent losing oil.

This is the main function of the seal.

The most important positions for the application of seals are at the ends of the crankshaft.

Seals may comprise a metal housing, with a spring-loaded neoprene rubber seal, asbestos impregnated with graphite or even felt being applied.

Seals are used for fuel pumps, crankshafts, gearboxes, rear axles and front wheels.

1. State the function of gaskets.2. Name the materials from which gaskets are

manufactured.3. Name four places where gaskets are used.4. State the function of oil seals.5. Name three types of oil seals.6. Name four places where oil seals are used.7. State the most important positions for oil

seals on an engine.

Carburettor

A carburettor has three main functions: to mix the fuel and air in the proper

ratio to suit the particular circumstances, for example, cold starting

to vaporise the fuel mixture so that it can burn better

to control the speed of the engine by increasing or decreasing the fuel mixture.

1. State the functions of the carburetor. 2. Sketch a carburettor and indicate on the

sketch how it operates.

The ignition coil transforms the 12 volts of the battery to the high voltage required (between 16 000 volts and 22 000 volts) to make the current jump the spark plug gap.

The coil has two circuits, a primary circuit and a secondary circuit.

Materials used for spark plugs are: electrode – nickel alloy isolator – ceramic casing – steel.

Spark plugSpark plug

The function of the spark plug is to provide a gap in the combustion chamber over which the high voltage spark can jump in order to ignite the air–fuel mixture at the end of the compression stroke.

State the function of the spark plug.

These materials are used for distributors: distributor sleeve – aluminium alloy steel alloy distributor driving shaft with cams contact-breaker plate – soft steel contact breaking points – tungsten rotor – synthetic material distributor cover – synthetic material

The distributor distributes the high voltage spark from the coil to the different spark plugs in the correct order, and at the right time.

The starter motor makes the crankshaft rotate at a pre-determined speed so that ignition of the air–fuel mixture can take place, in order to start the engine.

An alternator generates electrical power in order to charge the battery and keep the battery charged.

It should not be used to charge a flat battery.

State the function of the following auto-electrical components:1. ignition coils2. spark plugs3. distributors4. starter motors5. alternators

Bearings are made of bronze or brass with a working surface of a thin layer of white metal or an alloy consisting of lead, antimony and radium.

Ball bearings may also be used in engine components.

A bearing provides a reduced-friction surface on which the wearing face of a rotating part rests, for example crankshaft bearings, gudgeon pin bushes and camshaft bearings.

Bushes are usually made from phosphor bronze which is porous and impregnated with oil to reduce friction.

1. What is Babbitt metal?2. Describe the material that is used for bushes

and state why it is used.

Fly wheels are made of cast iron and steel.

The fly wheel contains a ring gear, clutch plate and pressure plate.

The flywheel absorbs energy during the power stroke to help the engine run during the three idle strokes.

Name the functions of the flywheel.

The fuel pump provides fuel from the tank to the carburettor at a pre-determined pressure and at all engine speeds.

Mechanical fuel pumps are operated by the rotation of the camshaft.

Electrical fuel pumps are usually used on modern vehicles with fuel injection engines.

Describe the function of the fuel pump.

The water pump is mounted in front of the engine between the block and the radiator.

waterpump.wmv

The water pump circulates the coolant continuously within the engine block and radiator to ensure that the coolant temperature stays even.

This is necessary to avoid overheating around the combustion chambers and cylinders.

Why are water pumps important components in engine cooling systems?

The first attempt to make practical use of steam seems to have been made by Heros of Alexandria in ancient times.

He was the first person to describe the principles of a steam engine in 100 years AD.

Heros is also celebrated for other amazing devices, one of which powered the self-opening doors of a temple.

He lit a fire beneath an altar that caused the air to warm and expand, increasing the pressure in a water reservoir below.

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This forced water through a siphon into a hanging bucket.

The descent of the bucket pulled on ropes which opened the doors.

When the fire was extinguished, the water returned into the vessel and the counter-weight shut the doors.

A steam engine is based on the fact that steam made by heating water occupies more than a thousand times as much space as the water from which it comes.

The expansion of water into steam exerts a force.

It is this force that is being used when a steam engine performs work.

The water to make steam is heated in a boiler. Any kind of fuel – coal, oil, gas, wood, or coke – may be burned to boil the water.

The resulting steam flows through a pipe into the engine itself.

Since the fuel is burned outside the engine, a steam engine is called an external combustion engine.

We will now discuss three inventors of the steam engine, namely:1. Thomas Savery (1650–1715)2. Thomas Newcomen (1663–1729)3. James Watt (1736–1819)

Thomas Savery was an English military engineer and inventor who in 1698, patented the first crude steam engine, based on Denis Papin’s Digester or pressure cooker of 1679.

Thomas Savery had been working on solving the problem of pumping water out of coal mines.

His machine consisted of a closed vessel filled with water into which steam under pressure was introduced.

This forced the water upwards and out of the mine shaft. A cold water sprinkler was then used to condense the steam. This created a vacuum which sucked more water out of the

mine shaft through a bottom valve.

Thomas Newcomen was the English blacksmith who invented the atmospheric steam engine, an improvement over Thomas Savery’s previous design.

The Newcomen steam engine used the force of atmospheric pressure to do the work.

Thomas Newcomen’s engine pumped steam into a cylinder.

The steam was then condensed by cold water which created a vacuum on the inside of the cylinder.

The resulting atmospheric pressure operated a piston, creating downward strokes.

In Newcomen’s engine the intensity of pressure was not limited by pressure of the steam, unlike what Thomas Savery had patented in 1698.

In 1712, Thomas Newcomen together with John Calley built their first engine on top of a water-filled mine shaft and used it to pump water out of the mine.

The Newcomen engine was the predecessor to the Watt engine and it was one of the most interesting pieces of technology developed during the 1700s.

James Watt was a Scottish inventor and mechanical engineer, born in Greenock.

He was renowned for his improvement of the steam engine. In 1765, James Watt, while working for the University of Glasgow, was assigned the task of repairing a Newcomen engine, which was deemed inefficient but the best steam engine of its time.

That started the inventor working on several improvements to Newcomen’s design.

Most notable was Watt’s 1769 patent for a separate condenser connected to a cylinder by a valve. Unlike Newcomen’s engine, Watt’s design had a condenser that could be cool while the cylinder was hot.

Watt’s engine soon became the dominant design for all modern steam engines and helped bring about the Industrial Revolution.

A unit of power called the Watt was named after James Watt.

The symbol for watts is W, and it is equal to 1⁄746 of a horsepower.

In a modern piston steam engine, steam from the boiler enters a thick-walled metal chamber called a steam chest.

There are three holes, or ports, in the floor of the steam chest.

The centre port opens into a pipe, the steam outlet or exhaust; the other two ports, one on either side of the centre port, open into another thick-walled metal chamber, the cylinder.

Moving back and forthacross the floor of the steam chest is a rectangular box that has no bottom.

This is the D-slide valve, which always covers the steam outlet port and alternately covers one of the two intake ports.

Steam from the boiler rushing through one of the intake ports strikes one side of the piston and forces it towards the opposite end of the cylinder.

The movement of the piston is called a stroke.

As the piston moves, the D-slide also moves so that, when the piston reaches the end of its stroke, the port through which the steam entered the cylinder is closed.

At the same moment, the D-slide valve opens the other port so that incoming steam is guided to the other side of the piston.

Steam striking this side pushes the piston back towards the opposite end of the cylinder.

As the D-slide valve allows steam to enter alternately one intake port and then the other, this valve also keeps open a channel to the steam outlet, where the steam that has just finished pushing the piston escapes.

The outlet may allow the steam to escape into the air, or it may channel the steam to a chamber called the condenser.

There the steam is cooled and condenses into water, which may be sent back to the boiler where it is again heated into steam.

The process of entering steam pushing the piston back and forth, the movement of the D-slide valve, and the escape of steam first from one side of the piston and then the other, goes on continually as long as steam enters the chest at pressure that is high enough.

A rod, called the piston rod, is attached to one side of the piston.

The piston rod passes through a steam-tight seal at one end of the cylinder.

As the piston moves back and forth, the piston rod moves in and out of the cylinder.

The outer end of the rod is attached to a device called a cross-head, which moves within a wide metal tube extending outwards from the cylinder.

The crosshead transmits the movement of the piston to a series of devices that turn a large heavy wheel, the flywheel.

The back-and-forth motion of the piston is translated into the rotary motion of the flywheel.

If the shaft of the flywheel is attached to a machine, the energy in the fuel is finally changed into work performed by the machine.

Attached to the turning axle of the flywheel is the eccentric, a thick metal disc with an off-centre hole through which passes the axle of the flywheel.

The eccentric translates the rotary motion of the axle into back-and-forth motion.

A rod transmits this motion through a steam-tight seal to the D-slide valve inside the steam chest, moving the valve back and forth over the steam ports.

1. Name the person who first explained the principles of the steam engine.

2. Name three inventors of the steam engine and state the years in which they lived.

3. Define a stroke of a steam engine.4. What is the function of the condenser of the steam

engine?5. Define the function of a flywheel.6. Define the function of the eccentric on the steam engine.7. Use a drawing to illustrate the operation of a simple

steam engine with a piston.