AUTOMOBILE ENGINEERING
Internal combustion engines—automotive engines are
called internal-combustion(IC) engines because the fuel that runs them
is burned internally, or inside the engines. There are two types, Reciprocating means moving up and down, or back and
forth. Almost all automotive engines are of the reciprocating type. In these
engines, pistons move up and down, or reciprocate, in cylinders. This type of
engine is called a piston engine. Rotary engines have rotors that spin, or rotate. The
only such engine now used in automobiles is the Wankel engine.
Two kinds of piston engines—there are two kinds of
piston engines—spark-ignition and compression-ignition. The differences between
the two are:
- The
type of fuel used
- The
way the fuel gets into the engine cylinders,compative
- The
way the fuel is ignited
The spark-ignition engines use a highly volatile
fuel which turns to vapor easily, such as gasoline or gasohol. The fuel is
mixed with air before it enters the
engine cylinders. The fuel turns into a vapor and mixed with the air to form a
combustible air-fuel mixture. This mixture then enters the cylinders and is
compressed. Next, an electric spark produced by the ignition system sets fire
to, or ignites, the compressed air-fuel mixture.
In
the compression-ignition, or diesel engine, the fuel is mixed with the air after the air enters the engine
cylinder. Air alone is taken into the cylinders of the diesel engine. The air
is then compressed as the piston moves up. The air is compressed so much that
its temperature goes up to 1000F or higher. Then the diesel-engine
fuel—a light oil—is injected (sprayed) into the engine cylinder. The hot air,
or heat of compression, ignites the
fuel. This is why the diesel engine is called a compression ignition engine.
Piston rings are of two types and they do two jobs:
- Form
a sliding seal between the piston and the cylinder wall. These are called
compression rings.
- Scrape
off most of the oil that is splashed on the wall so that it does not get
up into the combustion chamber where it would burn. These are called
oil-control ring.
Engine Operation
The complete cycle of
events in the engine cylinder requires four piston stroke—intake, compression,
power and exhaust. This requires two revolutions of the crankshaft. The four
strokes make the engine a four-stroke-cycle engine.
Diesel engine—there is
a second way to ignite the fuel—by compression ignition. This is the system
used in diesel engines. When air is compressed, it gets very hot. The more it
is compressed, the hotter it gets. Temperature increases with increasing
pressure. In the diesel engine, the air is compressed to as little as 1/22 of
its original volume. This is a compression ratio 22:1.
Difference between
spark-ignition-engine and diesel engine are:
- The
diesel engine compresses air alone.
- The
fuel is sprayed into the combustion chamber as the piston nears TDC on the
compression stroke.
- The
temperature of the air ignites the fuel.
Rocker arms—there are several different types of rocker
arms. Some rocker arms have a means of adjustment. The purpose is to provide a
minimum value-tappet clearance, or gap, in the valve train. Valve-tappet
clearance should be kept to a minimum to reduce noise and wear from parts
hitting together when the values are opened. The clearance should be large
enough to assure complete closing of the values.
Purpose of the fuel system
Both the carbureted
fuel system and the fuel-injection system have the same job. This is to supply
a combustible mixture of air and fuel to the engine. The fuel system must
change the properties of air and fuel for different operating conditions. When
the engine is cold, for example, the mixture must be rich (have a high
proportion of fuel). This is because the fuel does not vaporize readily at low
temperatures. Extra fuel must be added to the mixture so that enough will
vaporize to form a combustible mixture.
Carburetion—is the mixing of the gasoline fuel with air to
get a combustible mixture. The function of the carburetor is to supply a combustible
mixture of varying degrees of richness to suit engine operating conditions. The
mixture must be rich for starting, acceleration, and high-speed operation. A
less rich mixture is desirable at intermediate speed with a warm engine. The
carburetor has several systems through which air-fuel mixture flows during
different operating conditions. These systems produce the varying mixtures
required for the different operating conditions.
Anti-icing—when fuel is sprayed into the air passing
through the air form, the fuel evaporates, or turns to vapor. During
evaporation, the fuel takes heat from the surrounding air and metal parts. In
the carburetor, spraying and evaporation of the fuel “rob” the surrounding air
and carburetor of heat. Under certain conditions, the surrounding metal parts
are so cooled that moisture in the air condenses and then freezes on the metal
parts. The ice can build up sufficiently, if conditions are right, to cause the
engine to stall. This is most apt to occur during the warm-up period following
the first start-up of the day.
To prevent such icing,
many carburetors have a special anti-icing system. During the warm-up period,
the manifold heat-control valve sends hot exhaust gases from one exhaust
manifold to the other. Part of this hot exhaust gas circulates around the
carburetor idle ports and near the throttle-value shaft. This adds enough heat
to prevent icing. Another system has coolant passage in the carburetor. A small
amount of engine coolant passes through a special manifold in the carburetor
throttle body. This adds enough heat to the carburetor to prevent icing.
Purpose of the lubricating system
The engine lubricating
system supplies lubricating oil to all engine moving parts.
Purpose of lubricating
oil----
- The
oil lubricants moving parts to minimize wear.
- The
oil minimizes power losses in the engine.
- The
oil serves as a cooling agent.
- The
oil helps to cushion the load.
- The
oil helps form a gastight seal between piston rings and cylinder walls.
- The
oil acts as a cleaning agent.
Properties of lubricating oil—viscosity, viscosity
index, viscosity number (SAE 10W W
means winter), multiple-viscosity oil (SAE
10E-30, this means that the oil is the same as SAE 10W when cold and SAE 30
when hot), resistance to carbon formation and oil oxidation, corrosion and rust
inhibitors, foaming resistance, detergent-dispersants, extreme-pressure
resistance, improved oil, synthetic oils etc.
Sludge formation—sludge is a thick, creamy, black substance
that sometimes forms in the crankcase. It clogs oil screens and oil lines,
preventing oil circulation, so the engine can fail from oil starvation.
How sludge forms—water collects in the
crankcase in two ways. First, water is formed as the product of combustion.
Second, the crankcase ventilating system carries air through the crankcase. The
air usually has moisture in it. This moisture condenses on cold engine parts.
The black comes from dirt and carbon.
Why sludge formation—if a car is driven for
long distance each time it is started, water in the crankcase quickly
evaporates. The crankcase ventilating system then removes the water vapor.
Therefore, no sludge will form. However, if the engine is operated when cold
most of the time, then sludge will form.
Preventing sludge—to prevent sludge, the
car must be driven long enough for the engine to heat up and get rid of the
water in the crankcase. This means the trips of 12 miles or 19 km or longer in
winter.
Purpose of cooling system—is to keep the engine at its most
efficient operating temperature at all speeds and under all operating
conditions. Cooling system are designed to remove about one-third (30 to 35
percent) of the heat produced in the combustion chamber by the burning of the
air-fuel mixture. So, the cooling system includes devices that prevent normal
cooling action during engine warm-up.
Two general types of
cooling system are used on automobile engines. They are air cooling and liquid
cooling. Most automobile engines are liquid-cooled.
Water jackets—are designed to keep the cylinder block and
cylinder head cool. The water jackets are open spaces between the outside wall
of the cylinder and the inside of the cylinder block and head. The coolant can
circulate freely around the engine hot spots. These include the value guides
and value seats, and the upper parts of the cylinder walls where the pistons
and rings slide up and down.
Radiator—is a heat exchanger that removes heat from
coolant passing through it. The radiator holds a large volume of coolant in
close contact with a large volume of air so that heat will transfer from the
coolant to the air.
Expansion tank—is partly filled with coolant and is connected
to the overflow tube from the radiator filler neck. The coolant in the engine
expands as the engine heats up. Instead of dropping out the overflow tube into
the street and being lost from the cooling system completely, the coolant flows
into the expansion tank. When the engine cools, a vacuum is created in the
cooling system. The vacuum siphons some of the coolant back into the radiator
from the expansion tank.
Thermostat—is placed in the coolant passage between the
cylinder head and the top of the radiator. Its purpose is to close off this
passage when the engine is cold.
Antifreeze
solution—water
freezes at 0°C. If water freezes in the engine cooling
system, it stops coolant circulation. Some parts of the engine will overheat.
This could seriously damage the engine. To prevent freezing of the water in the
cooling system, antifreeze is added to form the coolant. The most commonly used
antifreeze is ethylene glycerol, although alcohol-base antifreeze has been used
in the past. A mixture of half water and half ethylene glycerol will not freeze
above -34°F.
Electrical system—the major components of the automobile
electrical system are:
- Starting
system—battery, starting motor, wiring, and switches.
- Charging
system—alternator and regulator, with wiring.
- Ignition
system
- Accessory
system—horns, lighting, instrument-panel warning systems.
Sulfation—the active materials in the plates are converted
into lead sulfate during discharge. This lead sulfate is reconverted into
active material during recharge. However, if the battery stands for long
periods in a discharged condition, the lead sulfate is converted into hard,
crystalline substance. The substance is difficult to reconvert into active
materials by normal charging processes. Such a battery should be charged at
half the normal rate for 60 to 100 hours. Even though this long charging period
may reconvert the sulfate to active material, the battery may still remain in a
damaged condition. The crystalline sulfate, as it forms, tends to break the
plate grids.
Purpose of charging system—the charging system
has two jobs:
- To
put back into the battery the current used to start the engine.
- To
handle the load of the lights, ignition, radio, and other electrical and
electronic equipment while the engine is running.
Purpose of ignition system—supplies high-voltage
surges as high as 47000 volts, to the spark plugs in the engine cylinder. These
surges produce electric sparks across the spark-plug gaps. The heat from the
spark ignites, or sets fire to, the compressed air-fuel mixture in the
combustion chambers. When the engine is idling, the spark appears at the plug
gap just as the piston nears TDC on the compression stroke. When the engine is
operating at higher speeds, or with part throttle, the spark is advanced. It is
moved ahead and occurs earlier in the compression stroke. This gives the
compressed mixture more time to burn and deliver its energy to the pistons.
Atmospheric pollution—to reduce these pollutants, automotive
vehicles is equipped with several emission controls:
- Positive
crankcase ventilation (PCV)
- Evaporative
emission control systems
- Heated-air
systems
- Exhaust-gas
re circulation (EGR)
Purpose of clutch—the clutch is used on cars with transmissions
that are shifted by hand. It allows the driver to couple the engine to, or
uncouples the engine from, the transmission. The clutch is linked to the clutch
pedal in the passenger compartment. When the driver presses down on the clutch
pedal, the linkage causes the clutch to disengage. This uncouples the engine
from the transmission. When the driver releases the clutch pedal, springs in
the clutch cause it to engage again. Now power can flow from the engine,
through the clutch, to the transmission and power train.
Synchronizers—to prevent gear clash during shifting while
the car is in motion, synchronizing devices are used in automotive
transmission. These devices ensure that gears that are about to mesh will be
rotating at the same speed, so they will engage smoothly.
Universal joints—the universal joint allows driving power to be
carried through two shafts that are at an angle to each other.
Slip joints—has outside splines on one shaft and matching
internal splines on a mating hollow shaft.
Front suspension—a system allows the front wheels to move up
and down and absorb road shocks. But the system must also allow the front
wheels to swing from side to side so the car can be steered. Coil, leaf, and
torsion-bar springs are used in seven basic arrangements:
- Coil
spring between lower control arm and a seat in the car frame.
- Coil
spring between the upper control arm and a seat in the car body.
- Coil
spring between an I-beam axle and a seat in the frame.
- Coil
spring between a seat on a strut rod which is attached to the lower
control arm and a seat in the car body. There is no upper control arm.
This is called McPherson-strut suspension.
- Torsion
bar connected longitudinally between the lower control arm and the car
frame.
- Torsion
bar connected transversely between the lower control arm and the car body.
- Leaf
springs between and I-beam axle and a seat in the frame.
Purpose of steering system—allows the driver of
guide the car along the road and turn left or right as desired. The system
includes the steering wheel, which the driver controls, the steering gear,
which changes the rotary motion of the wheel into straight-line motion, and the
steering linkages.
Front-end geometry—is the relationship of the angle among the
front wheels, the front-wheel attaching parts, and the ground. The various
factors that enter into front-end geometry are:
Front-suspension
height, Camber, Steering-axis inclination, Caster, Toe, Turning radius
Purpose of tires—a tire has two functions. First, they are
air-filled cushions that absorb most of the shocks caused by road
irregularities. The tires flex, or give, as they meet these irregularities.
Therefore they reduce the effect of the shocks on the passengers in the car. Second,
the tires grip the road to provide good traction. Good traction enables the car
to accelerate, brake, and make turns without skidding.
Tire size and markings—tire size is marked on the sidewall of
the tire. An older tire might be marked 7.75-14. This means that the tire fits
on a wheel that is 14 inches [356 mm] in diameter at the rim where the tire
bead rests. The 7.75 means the tire itself is about 7.75 inches [197 mm] wide
when it is properly inflated.
Tires carry several
markings on the sidewall. The markings include a letter code to designate the
type of car the tire is designed for. D means a lightweight car. F means
intermediate. G means a standard car. H, J and L are for large luxury cars and
high-performance vehicles. For example, some cars use a G78-14 tire. The 14
means a rim diameter of 14 inches. The 78 indicates the ratio between the tire
height and width. The tire is 78 percent as high as it is wide. The ratio of
the height to the width is called the aspect ratio or profile ration. Four
aspect ratios are 83, 78, 70, and 60. The lower the number, the wider the tire
looks. A 60 tire is only 60 percent as high as it is wide.
IMPORTANT TERMS USED IN
I.C. ENGINES
Supercharging—is the process of supplying the intake air to
the engine cylinder at a pressure greater than the pressure of the surrounding
atmosphere.
Scavenging—is the process of removing the burnt gases in
I.C. engines from the combustion chamber of the engine cylinder.
Auto-ignition—is the phenomenon by which a fuel catches fire
without external flame. Iso-octane helps to resist auto-ignition and normal
heptane accelerates auto-ignition.
Pre-ignition—is the ignition of the charge in spark
ignition engine before the spark occurs in the spark plug. Or, pre-ignition is
the spontaneous combustion of the mixture before the end of the compression
stroke.
Detonation—the rapid auto-ignition of a portion of a fuel
causes a pressure wave of high intensity to be set up in the cylinder of an
I.C. engine.
Octane number—measures of the resistance of a fuel to
combustion knock using standardized engine tests. The research octane number is
determined using ASTM Method D 2699; the motor octane number is determined
using ASTM Method D 2700. The Antiknock Index is the average of the Research
and Motor numbers. Octane numbers are determined using n-heptane that has an
octane number of 0, and isooctane that has an octane number of 100.
Cetane number—the ignition quality of a diesel fuel measured
using an engine test specified is ASTM method D613. Cetane number is determined
using two pure hydrocarbon reference fuel: cetane, which has a cetane rating of
100; and heptamethylnonane (also called isocetane), which has a cetane rating
of 15.
Top or Inner Dead Centre—the top most position towards cover end
side of the cylinder is known as top dead centre (i.e. T.D.C) or inner dead
centre (i.e. I.D.C)
Bottom or Outer Dead Centre—the lowest position of
the piston towards the crank end side of the cylinder is known as bottom dead
centre (i.e. B.D.C) or outer dead centre (i.e. O.D.C)
Clearance volume—when the piston is at top dead centre there is
some volume between the piston and cylinder head. This volume is known as
clearance volume which is represented by Vc.
Swept volume—the volume corresponding to the piston
displacement from T.D.C (or B.D.C) is known as swept volume.
Stroke—it is the distance traveled by the piston from one of its
dead centre position to the other dead centre position.
Compression ratio—is the ratio of the total volume of the cylinder
to clearance volume.
A four stroke engine is one which requires four strokes of the piston
to complete the cycle or the engine which requires two revolutions of the crank
shaft to complete the cycle is known as four stroke engine.
A two stroke engine is one which requires two strokes of the piston
or one revolution of the crank shaft to complete the cycle.
