Internal Combustion Engines Assignment

7/30/2019 Internal Combustion Engines Assignment

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Firing order in multi- cylinder engines:

In multi- cylinder engines; the expansion strokes for the different pistons must be arranged to

give suitable distribution of force, in this way the engine runs more quietly and smaller the

flywheel would be. The crank angle between any two explosions, ensuring the bestuniformity if crankshaft rotation should be as follows:

Four- stroke engines: = 720/ n

Two- stroke engines: = 360/ n

Where n is the number of cylinders.

The firing order in the multi cylinder engine is thus usually adjusted as 1-2-4-3 or 1-3-4-2 and

not as 1-2-3-4 as the sequence of engines. The reason for this is explained below:

Reason1. The firing order is of more importance in multi- cylinder engines, because the

Exhaust valves remain open for some interval of crank motion, so two exhaust valves of two

adjacent cylinders may open simultaneously. This overlapping will cause the exhaust of one

of the two adjacent cylinders to 'below- over' into the other in which the exhaust stroke is

nearly completing, thus interfering with the evacuation of the latter.

Blow- over can be minimized by using such a firing order that adjacent cylinders never fire in

succession.

2. Firing order is the order in which the power strokes are produced in various cylinders of a

multi cylinder engine. The firing order is essential to overcome the fluctuations produced due

to the impulses of pistons during power strokes. In a four cylinder engine all the four

cylinders does not have power stroke simultaneously and it produces vibrations so by

arranging the power strokes alternately these vibrations are reduced. Apart from this the

flywheel and vibration damper serves the same purpose. Generally the firing order for a fourcylinder engine is 1-4-3-2 (or) 1-4-2-3.

Carburetion

The process of mixture preparation in an S.I. Engine is called carburetion. This air-fuel

mixture is prepared outside cylinder in a device called carburettor. The carburettor atomizes

the fuel and mixes air in different proportions for various load conditions.

Functions The function of the carburettor is to measure out the correct proportions of liquid fuel

and air for the particular engine conditions.

It atomizes, vaporize and mix the fuel homogenously with air. It must supply correct amount of air-fuel mixture in correct proportion under all load

conditions and speed of the engine.

It must run the engine smoothly by supplying correct mixture strength.Types of Carburettors

1. Fixed Choke Type2. Constant Vacuum Type3. Submerged Jet Carburettor.


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1.Fixed Choke CarburettorIt is the most common and simplest type of carburettor being used in S.I.

Engines. It has the simplest construction and working which is depicted below in the figure.

It is a float type carburettor in which a float along with a needle valve is used to control the

amount of fuel in carburettor. Air enters from the venturi of a uniformly decreasing cross-

section which creates a region of pressure less than atmospheric pressure. As a result of this

pressure difference petrol is induced in the venture where it is mixed with air to form air-fuel

mixture.In this case the air flow is controlled by a choke valve. The choke or throat has a constant

area and the pressure changes with throttle opening and engine speed.

2. Constant Vacuum Type Carburettor

This type of carburettor is similar in function to that of fixed choke carburettor with a

difference that the pressure at the throat, and thus the air velocity remains constant but the

area of the throat is varied. Similarly the area of the petrol orifice or jet may also vary and

this is achieved by means of a tapered needle attached to the piston.

The needle moves in the orifice thus forming a discharge annulus for the petrol. It is shown in

the figure below:


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3. Submerged Jet CarburettorWith the elementary form of the carburettor described above the mixture would become

richer as the air flow is increased and this must be corrected. In some carburettors this is done

by fitting the main metering jet about 2.5 cm below the petrol level. This type of carburettor

is known as submerged jet carburettor.

The jet is situated at the bottom of a well, the sides of which are in communication with the

atmosphere. The second object of this arrangement is to achieve atomization which is not

obtained with elementary system described above. Air is drawn through the holes ion the

well, the petrol is emulsified, and the pressure difference across the petrol column is not as

great as in the elementary carburettor. On starting the petrol in the well is at the level of that

in the float chamber. On opening the throttle this petrol, being subject to the low throat

pressure, is drawn into the air. This continues with decreasing mixture richness as the holes in

the central tube are progressively uncovered. Normal flow then takes place in the main jet.

Types of Carburettor Based On direction of flow Up-draft Carburettors Down-draft Carburettors Side-draft carburettors

The figure shows updraft, downdraft and side-draft carburettors respectively.

Updraft Carburettors

This type is placed low in the engine and uses a gravity fed fuel supply. In other words, thetank is above the carburettor and the fuel falls to it. Even this carburettor uses gravity to

receive the fuel from the tank, the air-fuel mixture must be forced upward into the engine.

For special reasons of space and/or other considerations, some engines are fitted with

updraft carburettors. These need fairly high velocities to carry the fuel droplets in suspension

against the action of gravity.

Downdraft Carburettors

A downdraft carburettor is a one in which there is vertical venture tube with air flowing from

top to bottom. This carburettor operates with lower air velocities and larger passages. This is

because gravity assists the air-fuel mixture flow to the cylinder. The down draft carburettorcan provide large volumes of fuel when needed for high speed and high power output.


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Advantage

It is best in that gravity assists in keeping the fuel droplets flowing in the samedirection as the air flow.

A long runner (passage between throttle and intake manifold) that allows moredistance and time for evaporation and mixing is also good.

Demerits Both of these concepts were acceptable in early automobiles, which had large engine

components and on which a high hood was desirable for snob appeal. As automobiles

were built lower and engine compartments smaller, compromising was necessary and

carburettors were built with shorter barrels and runners. To further reduce engine

compartment height, side draft carburettors were developed with air flowing

horizontally.

Side-draft carburettorsIn this type of carburettor the air-fuel mixture flows horizontally.

Merits This carburettor has an advantage over downdraft carburettor that they are smaller and

much compact in the size thus most suitable for modern S.I. Engines.

Demerits

These generally need higher flow velocities to keep the fuel droplets suspended in theair flow, and with higher velocities come greater pressure losses.

Problems Associated with the Carburettors

A problem sometimes encountered with carburettors is icing, which usually occurs onthe throttle plate. Water vapour in the air will freeze when the air is cooled to lowtemperatures. Cooling occurs for two reasons: There is expansion cooling due to the

pressure reduction experienced by the air as it flows through the carburettor, and there

is evaporative cooling due to the just-added fuel droplets in the throat of the venturi.

Fuel additives and heating the carburettor are two possible solutions to this problem.

Another problem of carburettors is the splitting of the air flow around the throttleplate immediately after the fuel has been added. This makes it very difficult to get

homogeneous mixing and is a major reason why the air-fuel mixture delivered to the

cylinders is often non-uniform. This problem is more serious with later short barrel,

short runner carburettors.

At conditions other than WOT, the major pressure drop in an intake system will be atthe throttle plate of the carburettor. This may be as much as 90% of the total pressure

drop, or greater. The flow may become choked (sonic velocity) at a partially closed

throttle. When the throttle position is suddenly changed, it takes several engine

revolutions to re-establish steady-state flow through the carburettor.

A simple carburettor as described suffers from the fact that it provides the requiredair-fuel ratio only at one throttle position. At all other positions, the mixture is either

leaner or richer depending upon whether the throttle is opened less or more.

Engine Lubricating System


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The sides of a piston and the cylinder in the engine looks like this. The metal appears to be

rough. When two pieces of this rough metal rub together the peaks on the surfaces will

probably break or wear off. These broken off particles act like a sand paper. They cause or

increase friction, which increase temperature. They also wear away metal surfaces.

Roughness and dryness resist the movement of metal surfaces against one another. Friction

is the term for this resistance to movement. Engine efficiency decreases as friction increases.

Oil in the piston separates the two metal surfaces so that they do not rub together. In an I.C.

Engine, oil is used to reduce friction between moving parts.

It is also used to remove heat caused by the friction.

Oil has other functions, too. We know that when two surfaces rub together, small bits of

metal break off. These small bits are carried away by the oil.

Because there is an open space between the piston and cylinder wall, combustion gases can

leak between the piston and the cylinder wall. Since oil fills the space at this location, the oil

prevents the leaking of gases past the piston. Thus the oil acts as a seal.

Functions

Decrease friction Remove small bits of metal Cool the engine by removing heat caused by the friction Prevent gas from leaking around the piston

Lubricating the EngineThere are three basic types of oil distribution systems used in engines: splash, oil pump or

combination of both.

Oil must be delivered to all the metal parts of the engine that rub against other parts. The

lubricating system delivers the oil to these moving parts.

Splash Lubricating System

Here is an example of a splash lubricating system used on some small engines.

When the engine is not operating, the oil collects at the

bottom of the engine. Oil collects in the crankcase.

Connected to the crankshaft is a dipper. As the

crankshaft turns, the dipper dips into the oil in the

crankcase. Since the crankshaft rotates very fast, the

dipper moves through the oil with great force,

splashing oil over the engine parts. The oil splashesagainst the walls of the cylinder. The dipper also

splashes oil on the bearing and on the turning

crankshaft. Some of the oil is splashed inside the

piston and up to the tops of the connecting rod and its

wrist pin. The oil is sent to the piston where it

lubricates and seals the space between the piston and

the cylinder wall. After lubricating the cylinder wall

and bearings, the oil drips down to the crankcase again. Some of the oil from the dipper is

splashed into grooves on the upper block. This oil lubricates the wall rocker arm parts and the

camshaft before returning to the crankcase.


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Oil Pump

In large industrial engines, since the splash system is not large enough to lubricate all the

moving parts, a forced lubricating system is used. Because oil will not flow up by itself and

oil pump forces the oil to circulate the whole length of this crankshaft has channels bored into

it. These channels carry the oil into different engine parts. The pump forces oil through thesechannels. Some of the oil passes through channels in the crankshaft to the bearings which

support the crankshaft. From the crankshaft, oil is forced up through the connecting rods to

the wrist pin. The oil in the piston flows out of the piston and down the cylinder wall. The

pump also forces oil through the channels to the top of the engine to lubricate the rocker arm

parts and the valves. A typical oil pump is shown in the figure.

It consists of rotating gears. The

pump is driven by the camshaft.

The inlet to the pump is

connected to the strainer.

Lubricating oil returning from

the engine parts gathers in the

crankcase. Heavy particles of

dirt collect during circulation

through the engine settle to the

bottom of the crankcase.

Therefore, the oil at the top of

the crankcase is cleaner. A float

on the strainer makes sure that

only surface oil goes into the

pump. Dirt, water and sludge on the bottom of the crankcase are prevented from getting intothe oil pump by this floating strainer. Sometimes dirt, sand, and small metal particles remain

in the circulating oil. The impurities are removed from the oil by oil filters. Many engines use

a filter in the lubricating system through which all of the oil from the pump must go before

going into the engine parts. This is called a full-flow system.

When a bypass filter is used, only a small part of the lubricating oil passes through the

filter each time the oil is circulated through the engine. All of the oil does not have to pass

through the filter in this system. After engines have been in operation for a while oil filters

may pick up enough impurities to fill them. Then these filters must be either cleaned or

replaced. Some cleanable filters only have to have a handle on them turned every few hours

to keep them clean. Lubricating oil, after continuous used, breaks down chemically into

acids, tars and, sludge. The oil must be changed at regular intervals to keep the oil fresh. Oiltreatment systems remove undesirable chemicals from the oil. But treating the lubricating oil,

the number of oil changes needed is reduced.

Oil CoolingIn small engines enough heat escapes through the outside engine parts to cool the lubricating

oil adequately. For example, the metal crankcase draws heat from the oil and the outside air

removes heat from the crankcase. In larger systems, this arrangement cannot remove enough

heat to keep the oil at a safe temperature.


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This drawing shows an oil cooling system for a large engine. Excess heat from the oil is

removed by an exchanger. Water flows through the heat exchanger and around the oil. This is

a water cool system.

Oil PressureThe lubrication system must build up

enough pressure to force oil through engineparts. Excessively low pressure is

undesirable. The engine may not get

enough oil. On the other hand excessively

high oil pressure also indicates trouble. A

clogged filter causes oil pressure to

increase. Oil that is too thick does not flow

properly. Thus pressure in the system

increases.

The figure shows a simplified drawing of

an oil-pressure safety valve or switch. This

valve is operated by the oil pressure.Lubricating oil enters at the top of the

valve. Oil pressure pushes down on the piston. A spring pushes up on the piston against the

oil pressure pushing down. The piston is connected to a shutoff valve. The shutoff valve, as

the name indicates, shuts off the fuel supply. As long as the oil pressure is high enough, it

holds the shutoff valve open. If the oil pressure falls too low, the spring closes the shutoff

valve. Closing the shutoff valve stops the engine by cutting off its fuel supply. Lubricating

system usually has a gage on the instrument panel which indicates oil pressure.

Engine Cooling Systems

1. Air-Cooled Engines

Many small engines and some medium-sized engines are air cooled. This includes most

small-engine tools and toys like lawn mowers, chain saws, model airplanes, etc. This allows

both the weight and price of these engines to be kept low. Some motorcycles, automobiles,

and aircraft have air-cooled engines, also benefitting from lower weight. Air-cooled engines

rely on a flow of air across their external

surfaces to remove the necessary heat to

keep them from overheating. On vehicles

like motorcycles and aircraft, the forwardmotion of the vehicle supplies the air flow

across the surface. Deflectors and

ductwork are often added to direct the flow

to critical locations. The outer surfaces of

the engine are made of good heat-

conducting metals and are finned to

promote maximum heat transfer.

Automobile engines usually have fans to

increase the air-flow rate and direct it in

the desired direction. Lawn mowers and

chain saws rely on free convection fromtheir finned surfaces. Some small engines


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have exposed flywheels with air deflectors fastened to the surface. When the engine is in

operation, these deflectors create air motion that increases heat transfer on the finned

surfaces.

It is more difficult to get uniform cooling of cylinders on air-cooled engines than on liquid-

cooled engines. The flow of liquid coolants can be better controlled and ducted to the hot

spots where maximum cooling is needed. Liquid coolants also have better...thermalproperties than air (e.g., higher convection coefficients, specific heats, etc).Cooling the front

of an air cooled engine which faces the forward motion of the vehicle is often much easier

and more efficient than cooling the back surface of the engine. This can result in temperature

differences and thermal expansion problems.

Advantages

When compared with liquid-cooled engines, air-cooled engines have the following

advantages:

(1) Lighter weight,

(2) Less costly,(3) No coolant system failures (e.g., water pump, hoses),

(4) No engine freeze-ups

(5) Faster engine warm up.

Disadvantages

Disadvantages of air-cooled engines are that they

(1) Are less efficient,

(2) Are noisier, with greater air flow requirements and no water jacket to dampen noise, and

(3) Need a directed air flow and finned surfaces. Standard heat transfer equations for finned

surfaces can be used to calculate the heat transfer off of these engine surfaces.

2. Water-Cooled Engines

The engine block of a water-cooled engine is surrounded with a water jacket through which

coolant liquid flows (Fig). This allows for a much better control of heat removal at a cost of

added weight and a need for a water pump. The cost, weight, and complexity of a liquid

coolant system make this type of cooling very rue on small and/or low-cost engines.


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The coolant system of a typical automobile engine is shown in Fig. above. Fluid enters the

water jacket of the engine, usually at the bottom of the engine. It flows through the engine

block, where it absorbs energy from the hot cylinder walls. The flow passages in the water

jacket are designed to direct the flow around the outer surfaces of the cylinder walls and past

any other surface that needs cooling. The flow is also directed through any other component

that may need heating or cooling (e.g. heating of the intake manifold or cooling of the oil

reservoir). The flow leaves the engine block containing a high specific enthalpy because of

the energy it absorbed in engine cooling. Exit is usually at the top of the engine block. This

complete process is illustrated

below in the figure.

Enthalpy must now be removedfrom the coolant flow so that the

circulation loop can be closed

and the coolant can again be used

to cool the engine. This is done

by the use of a heat exchanger in

the flow loop called, for some

unknown reason, a radiator.

Radiator

The radiator is a honeycomb heat exchanger with hot coolant flowing from top to bottom

exchanging energy with cooler air flowing from front to back, as shown in Figure.

Air flow occurs because of the forward motion of the automobile, assisted by a fan located

behind the

radiator and

either drivenelectrically or off

the engine

crankshaft. The

cooled engine

coolant exits the

bottom of the

radiator and re-

enters the water

jacket of the

engine,

completing aclosed loop. A


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water pump that drives the flow of the coolant loop is usually located between the radiator

exit and engine block entrance. This pump is either electric or mechanically driven off the

engine. Some early automobiles had no water pump and relied on a natural convection

thermal flow loop.

Air leaving the automobile radiator is further used to cool the engine by being directed

through the engine compartment and across the exterior surfaces of the engine. Because ofthe modern aerodynamic shape of automobiles and the great emphasis on cosmetics, it is

much more difficult to duct cooling air through the radiator and engine compartment. Much

greater efficiency is needed in rejecting energy with the modern radiator heat exchanger.

Modern engines are designed to run hotter and thus can tolerate a lower cooling air-flow rate.

Steady-state temperature of the air within the engine compartment of a modern automobile is

on the order of 125C.

To keep the coolant fluid temperature from dropping below some minimum value, and thus

keeping the engine operating at a higher temperature and efficiency, a thermostat is installed

in the coolant loop, usually at the engine flow entrance.

ThermostatThermostat is a thermally activated go-no go valve. When the thermostat is cold, it is closed

and allows no fluid flow through the main circulation channel. As the engine warms up, the

thermostat also warms up, and thermal expansion opens the flow passage and allows coolant

circulation. The higher the temperature, the greater the flow passage opening, with the greater

resulting coolant flow. The coolant temperature is, therefore, controlled fairly accurately by

the opening and closing of the thermostat.

Prevention of Vaporization of Coolant

It is desirable for the coolant to remain mostly liquid throughout the flow loop. If boiling

occurs, a small mass of liquid becomes a large volume of vapour, and steady-state mass flow

becomes almost impossible to sustain. By using ethylene glycol in a pressurized system, high

temperatures can be achieved without large scale boiling. Localized boiling in small hot spots

does occur within the engine water jacket. This is good. The very hottest spots within the

engine (either momentary or almost steady state) require the greatest heat removal and

cooling. The phase change that is experienced when boiling occurs at these local hot spots

absorbs a large amount of energy and supplies the necessary large cooling at these spots. The

circulating convection flow carries the resulting vapour bubbles away from the hot spots back

into the main stream of the coolant. Here they condense back into liquid due to the cooler

fluid temperature, and bulk flow is not interrupted. As hot engine coolant leaves the engine

block, it can be used to heat the passenger compartment of an automobile, when desired. This

is done by routing a portion of the coolant flow through an auxiliary system that supplies thehot side of a small liquid-to-air heat exchanger. Outside or re-circulated air is heated as it

passes through the other half of the heat exchanger and is ducted into the passenger

compartment and/or onto the cold windows for defrosting. Various manual and automatic

controls determine the flow rates of the air and coolant to supply the desired warming results.

Fuel Pump


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Typical fuel pump operation is illustrated in the figure. The pumping element is a

plunger which reciprocates ion a barrel. A hole is bored through the centre of the plunger for

a short length. An angular groove is cut on the outside of the plunger and extends for only

part of the way round the plunger. This groove and the central hole are connected by a hole

drilled between them. Two ports are drilled in the barrel. The left-hand port is slightly higher

than the right-hand port. The left-hand port is the main fuel supply and the right-hand port isthe spill way. In figure the pump plunger has just descended and both ports are uncovered.

Fuel has thus been

sucked into the barrel

and fills the space

above the plunger, the

central hole and the

angular groove. In next

figure the plunger is

ascending. Both ports

are now closed and the

fuel oil above theplunger has now been

compressed into the

delivery top the

injector.In figure high pressure

fuel has been delivered

to the injector and the

plunger, still ascending,

reaches the point where

the top edge of the

angular groove meets the spill way port. This port is in direct communication, via the angular

groove and the central hole, with the oil above the plunger. The pressure of this oil

immediately drops, so delivery to the engine is terminated. The plunger continues to rise,

completing its stroke, but it will not deliver any further fuel to the engine. The distance from

the top of the angular groove to the top of the plunger varies. This varying distance is used to

control the engine. By rotating the plunger, anyone of these distances can be selected to

match up with the spill way port. If a short distance is selected, a small quantity of fuel is

delivered. If a long distance is selected a large quantity of fuel is delivered. The rotation of

the plunger is controlled either by an arm or by a rack and pinion at the bottom of the

plunger. A control rod actuates either the arm or the rack. The plunger reciprocation is

constant in stroke and is usually cam operated. The plunger is usually forced-closed on to thecam surface using the spring. The engine is stopped by rotating the plunger as shown in

figure. The plunger is now in such apposition that the angular groove meets the spill way port

just before it covers the fuel port on the left. No pressure is build-up is possible for the

remainder of the stroke because the oil above the plunger is at all time in contact with the

spill way port, via the central whole and angular groove. Under these conditions no high

pressure oil is delivered to the engine, so the engine stops.

A complete pump unit to feed a four cylinder oil engine is illustrated in the figure.


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It shows that each cylinder has its own pump

and all pumps are enclosed in one pump unit.

The operating cams are located on one cam

shaft at the bottom of the pump unit. A supply

fuel pump is mounted on the side to pump

fuel from the main tank to the pump unit fuelgallery at the top. The control rod in this

pump rotates lever on the bottom of the pump

plunger. This controls the fuel quantity

pumped to the engine cylinders. The engine is

governed by a pneumatic governor fitted on

the front of the pump unit. The pneumatic

governor is connected to the engine intake and

is controlled by intake depression.

In the case of four stroke cycle engine, the

camshaft of the pump is motored at half

engine speed. In the two-stroke cycle enginethe camshaft of the pump is motored at engine speed.

The pump illustrated in figure is not the only possible multi-cylinder pump

arrangement. Another arrangement is to have a pump with a single plunger and a delivery

distribution system whereby the engine cylinders are separately fed with fuel at the correct

time. The arrangement is called the distributor pump. Distributor pumps are commonly

fitted on the smaller, high speed, motor vehicle diesel engines.

Fuel Injector Nozzle

Fuel injectors are nozzles that inject a spray of fuel into the intake air. They arenormally controlled electronically, but mechanically controlled injectors which are cam

actuated also exist. A metered amount of fuel is trapped in the nozzle end of the injector, and

a high pressure is applied to it, usually by a mechanical compression process of some kind.

At the proper time, the nozzle is opened and the fuel is sprayed into the surrounding air.

Each cylinder of a diesel engine has an injector nozzle which protrudes into the

combustion space of the cylinders. Like

the spark plug, the injector nozzle is made

gas-tight with the cylinder. A typical

injector nozzle is shown in the figure

below.


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High pressure fuel passes into the injector at the fuel inlet. It passes down the injector body

and reaches the needle valve seat. The pressure operates against the area of the spindle

above the seat, so the valve needle is lifted against the action of the spring at the top of the

injector. High pressure oil is then forced through very small holes into the engine cylinder.

Because of the high pressure and the small whole size, the fuel is very finely atomised as itenters the engine cylinder. This greatly assists the rapid and successful burning of the fuel.

The pressure is set by using the spring at the top of the injector nozzle.

A small amount of fuel leaks between the body of the injector and the needle-valve

stem. This provides the necessary lubrication. Eventually the fuel will find its way out of the

top of the injector through a leak off pipe; it will return to the lain supply.

Types of fuel injectorsVarious kinds of fuel injectors are available.

Most operate by trapping a small amount of fuel behind the nozzle orifice. Thenozzle is closed by a needle valve held against its seat by a spring or magnetic force.On lower pressure nozzles, injection is initiated by increasing pressure and pushing

opens the valve, allowing flow to occur. On high-pressure nozzles, flow is initiated

by lifting the valve needle off its seat by action of an electric solenoid. Spray

duration, and sometimes pressure, is generally controlled electronically.

Some systems add air to the fuel in front of the injectors, and the actual injectionconsists of an air-fuel mixture. This greatly enhances the evaporation and mixing

processes after injection. Modern experimental two-stroke cycle automobile engines

use this process to assure proper mixing in the very short time available between

injection and combustion.

Some fuel injection systems, including most very early ones, consist of throttle bodyinjection. This consists of one or more injectors mounted near the inlet of the intakemanifold, usually just downstream of the throttle plate. This injector or set of

injectors supplies fuel for all cylinders, allowing the distribution to be controlled by

the intake manifold. This is simpler technology than multipoint injection and a fair

amount cheaper to manufacture. Fewer injectors are needed and coarser nozzles can

be used, as there is longer flow duration to evaporate and mix the larger fuel droplets.

A greater variation in cylinder-to-cylinder AF can be expected. Controls are simpler,

with some injectors giving a continuous spray under some operating conditions.

Throttle response time is slower with throttle body injection than with port injection.

Some SI and all CI engine fuel injection systems have the injectors mounted in thecylinder head and inject directly into the combustion chamber. This gives veryconstant fuel input cycle-to-cycle and cylinder-to-cylinder. Modern experimental two-


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stroke cycle automobile engines use this system to avoid losing fuel out of the exhaust

system during scavenging and valve overlap. This type of system requires very

precise injectors giving extremely fine droplets of fuel. Fuel is added during the

compression stroke, which allows an extremely short period of time for evaporation

and mixing, less than 0.008 second at 3000 RPM. High turbulence and swirl are also

important.

The fuel injection types used in newer cars include:

* Single-point or throttle body injection (TBI)

* Port or multi-point fuel injection (MPFI)

* Sequential fuel injection (SFI)

* Direct injection

Single-point or throttle body injection (TBI)The earliest and simplest type of fuel injection, single-point simply replaces the

carburettor with one or two fuel-injector nozzles in the throttle body, which is the

throat of the engines air intake manifold. For some automakers, single-point injection

was a stepping stone to the more complex multi-point system. Though not as precise

as the systems that have followed, TBI meters fuel better than a carburettor and are

less expensive and easier to service.

Port or multi-point fuel injection (MPFI)Multi-point fuel injection devotes a separate injector nozzle to each cylinder, right

outside its intake port, which is why the system is sometimes called port injection.

Shooting the fuel vapour this close to the intake port almost ensures that it will be

drawn completely into the cylinder. The main advantage is that MPFI meters fuel

more precisely than do TBI designs, better achieving the desired air/fuel ratio and

improving all related aspects. Also, it virtually eliminates the possibility that fuel will

condense or collect in the intake manifold. With TBI and carburettors, the intake

manifold must be designed to conduct the engines heat, a measure to vaporize liquid

fuel. This is unnecessary on engines equipped with MPFI, so the intake manifold can

be formed from lighter-weight material, even plastic. Incremental fuel economy

improvements result. Also, where conventional metal intake manifolds must be

located atop the engine to conduct heat, those used in MPFI can be placed more

creatively, granting engineers design flexibility.

Sequential fuel injection (SFI)Sequential fuel injection, also called sequential port fuel injection (SPFI) or timed

injection, is a type of multi-port injection. Though basic MPFI employs multiple

injectors, they all spray their fuel at the same time or in groups. As a result, the fuel

may hang around a port for as long as 150 milliseconds when the engine is idling.

This may not seem like much, but its enough of a shortcoming that engineers

addressed it: Sequential fuel injection triggers each injector nozzle independently.

Timed like spark plugs, they spray the fuel immediately before or as their intake valve

opens. It seems a minor step, but efficiency and emissions improvements come in

very small doses.


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Direct injectiondirect injection takes the fuel injection concept about as far as it can go, injecting fuel

directly into the combustion chambers, past the valves. More common in diesel

engines, direct injection is starting to pop up in gasoline engine designs, sometimes

called DIG for direct injection gasoline. Again, fuel metering is even more precise

than in the other injection schemes, and the direct injection gives engineers yetanother variable to influence precisely how combustion occurs in the cylinders. The

science of engine design scrutinizes how the fuel/air mixture swirls around in the

cylinders and how the explosion travels from the ignition point. Things such as the

shape of cylinders and pistons; port and spark plug locations; timing, duration and

intensity of the spark; and number of spark plugs per cylinder (more than one is

possible) all affect how evenly and completely fuel combusts in a gasoline engine.

Direct injection is another tool in that discipline, one that can be used in low-

emissions lean-burn engines.

Engine Transmission System

Engine Transmission System can be of following types:

Front Wheel Drive Rear Wheel Drive Four wheel Drive Two wheel Drive

Rear Wheel Drive (Driving through a propeller shaft)

In a front-engine rear-wheel drive car, power is transmitted from the engine through the

clutch and the gearbox to the rear axle by means of a tubular propeller shaft.

The rear axle must be able to move up and down on the suspension according to variations of

the road surface.

The movement causes the angle of the propeller shaft, and the distance between the gearbox

and the rear axle, to change constantly.

To allow for the constant movement, splines on the front end of the propeller shaft slide in

and out of the gearbox as the distance changes; the shaft also has universal joints at each end,

and sometimes in the middle.The universal joints allow the propeller shaft to be flexible, while constantly transmitting

power.

The last part of the transmission is the final drive, which incorporates the differential and is

sometimes called the differential.

The differential has three functions:

1. to turn the direction of drive through 90 degrees to the rear wheels

2. To allow either rear wheel to turn faster than the other when cornering

3. To effect a final gear reduction

a pinion gear inside the differential is driven by the propeller shaft and has its gears bevelled -

cut at an angle. It meshes with a bevelled crown wheel so that the two gears form a 90 degreeangle.
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The crown wheel usually has about four times as many teeth as the pinion gear, causing the

wheels to turn at a quarter the propeller-shaft speed. The drive is transmitted from the

differential to the rear wheels by means of half shafts, or drive shafts. At the differential end

of each half shaft, a bevelled pinion gear is connected to the crown wheel by means of an

intermediate set of bevel pinions.

Rear engine driving rear wheels

Some cars, such as VW Beetles and smaller Fiats, have rear-mounted engines and gearboxes,

driving the rear wheels. Power is transmitted through the clutch to the gearbox, passing to the

wheels through drive shafts. The layout is similar to some front wheel-drive cars, except that

no allowance need be made for steering movement of the wheels. Sometimes the shafts are

connected to the flanges at the gearbox by `doughnut' couplings. The shafts and flanges are

bolted on either side of the couplings, and drive is transmitted through the flexible rubber.

AdvantagesThe standard design has a series of advantages on passenger cars and estate cars:

There is hardly any restriction on engine length, making it particularly suitable for more

powerful vehicles (in other words for engines with 812 cylinders)

Under full load most of the vehicle mass is on the driven rear axle (important for estate cars

and trailers

A long exhaust system with good silencing and catalytic converter configuration.

Simple and varied front axle designs are possible irrespective of drive forces. More even tyre wears thanks to function distribution of steering/drive.

Uncomplicated gear shift mechanism.

Good cooling because the engine and radiator are at the front; a power-saving fan can be

fitted.

Front Wheel Drive

Front-wheel-drive cars use the same transmission principles as rear-wheel drive cars, but the

mechanical components vary in design according to the engine and gearbox layout.Transverse engines are normally mounted directly above the gearbox, and power is


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transmitted through the clutch to the gearbox by a train of gears. In-line engines are mated

directly to the gearbox, and drive passes through the clutch in the normal manner. In both

cases, drive passes from the gearbox to a final-drive unit. In a transverse-mounted engine, the

final-drive unit is usually located in the gearbox. In an in-line engine, it is usually mounted

between the engine and the gearbox. Power is taken from the final-drive unit to the wheels by

short drive shafts. To cope with suspension and steering movement in the wheels, the driveshafts use a highly developed type of universal joint called a constant-velocity (CV) joint. A

CV joint uses grooves with steel ball bearings in them instead of the `spider' found in a

universal joint, and transmits power at a constant speed, regardless of the angle and the

distance between the final-drive unit and the wheels. Some cars, such as earlier Minis, also

have drive-shaft couplings which are 'spider' joints, and do the same job as universal joints in

rear-wheel-drive cars, allowing up-and-down movement of the suspension. They are usually

made of rubber bonded to metal.

Advantages

there is load on the steered and driven wheels; good road-holding, especially on wet roads and in wintry conditions the car is

pulled and not pushed

simple rear axle design good engine cooling (radiator in front), and an electric fan can be fitted; exhaust system with long path (important on cars with catalytic converters)

Four Wheel Drive

In four-wheel drives, either all the wheels of a passenger car or commercial vehicle are

continuouslyin other words permanentlydriven, or one of the two axles is always linked

to the engine and the other can be selected manually or automatically. This is made possible

by what is known as the centre differentiallock. If a middle differential is used to distribute

the driving torque between the front and rear axles, the torque distribution can be established

on the basis of the axleload ratios, the design philosophy of the vehicle and the desired

handling characteristics. That is why Audi choose a 50%:50% distribution for the V8 Quattro

and Mercedes-Benz choose a 50%:50% distribution for M class off-road vehicles, whereas

Mercedes-Benz transmits only 35% of the torque to the front axle and as much as 65% to the

rear axle in vehicles belonging to the E class.

Two Wheel DriveTwo-wheel drive (2WD) describes vehicles with a drive train that allows two wheels to

receive power from the engine simultaneously.

Spark Timing

Most four-stroke engines have used a mechanically timed electrical ignition system. The

heart of the system is the distributor. The distributor contains a rotating cam driven by the

engine's drive, a set ofbreakerpoints, a condenser, a rotor and a distributor cap. External to

the distributor is the ignition coil, the spark plugs and wires linking the distributor to thespark plugs and ignition coil.
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The system is powered by a lead-acid battery, which is charged by the car's electrical

system using a dynamo or alternator. The engine operates contact breaker points, which

interrupt the current to an

induction coil (known as the

ignition coil).

The ignition coil consistsof two transformer windings

sharing a common magnetic

corethe primary and

secondary windings. An

alternating current in the

primary induces alternating

magnetic field in the coil's

core. Because the ignition

coil's secondary has far more

windings than the primary, the

coil is a step-up transformerwhich induces a much higher

voltage across the secondary

windings. For an ignition coil,

one end of windings of both

the primary and secondary are connected together. This common point is connected to the

battery (usually through a current-limiting ballast resistor). The other end of the primary is

connected to the points within the distributor. The other end of the secondary is connected,

via the distributor cap and rotor, to the spark plugs.

The ignition firing sequence begins with the points (or contact breaker) closed. A steady

charge flows from the battery, through the current-limiting resistor, through the coil primary,

across the closed breaker points and finally back to the battery. This steady current produces

a magnetic field within the coil's core. This magnetic field forms the energy reservoir that

will be used to drive the ignition spark.

As the engine turns, so does the cam inside the distributor. The points ride on the cam so that

as the engine turns and reaches the top of the engine's compression cycle, a high point in the

cam causes the breaker points to open. This breaks the primary winding's circuit and abruptly

stops the current through the breaker points. Without the steady current through the points,

the magnetic field generated in the coil immediately and rapidly collapses. This change in the

magnetic field induces a high voltage in the coil's secondary windings.

At the same time, current

exits the coil's primarywinding and begins to charge

up the capacitor

("condenser") that lies across

the now-open breaker points.

This capacitor and the coils

primary windings form an

oscillating LC circuit. This

LC circuit produces a

damped, oscillating current

which bounces energy

between the capacitorselectric field and the ignition
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coils magnetic field. The oscillating current in the coils primary, which produces an

oscillating magnetic field in the coil, extends the high voltage pulse at the output of the

secondary windings. This high voltage thus continues beyond the time of the initial field

collapse pulse. The oscillation continues until the circuits energy is consumed.

The ignition coil's secondary windings are connected to the distributor cap. A turning rotor,

located on top of the breaker cam within the distributor cap, sequentially connects the coil'ssecondary windings to one of the several wires leading to each cylinder's spark plug. The

extremely high voltage from the coil's secondary -often higher than 1000 voltscauses a

spark to form across the gap of the spark plug. This, in turn, ignites the compressed air-fuel

mixture within the engine. It is the creation of this spark which consumes the energy that was

stored in the ignition coils magnetic field.

What is the difference between Spark Ignition and

Compression Ignition?

Spark ignition uses petrol as the fuel, but Compression ignition uses diesel.

SI works on Otto cycle while CI works on diesel cycle.

SI is used in petrol engines while CI is used in diesel engines.

CI has more efficient than SI.

CI produces more noise than SI when it works.

CI produces more hydrocarbons at the exhaust stroke of the engine than SI engines.

SI engine has a spark plug, but CI does not have one.

SI air-fuel mixture enters into the combustion chamber, but in CI, air and fuel enter

separately in to the combustion chamber.

CI has higher compression ratio than SI.

SI is more harmful due to the pre-detonation things than CI. In SI spark plug is timed to start combustion at the ideal moment, usually somedegrees before the piston reaches the top (TDC or Top Dead Centre). The burning

mixture then drives the piston down for the power stroke.

But in the CI fuel is injected (sprayed) into the cylinder at the ideal moment - this too

is usually some degrees before TDC, and the red-hot air in there then starts the fuel

burning. This then drives the piston down for the power stroke.

SI is limited in maximum compression ratio (the amount the air or mixture iscompressed as the piston rises in the cylinder), or it can start to behave like CI

(Dieseling).

CI uses the highest compression ratio possible to improve ignition - and - improveefficiency too. The expansion ratio is the inverse of the compression ratio, and the
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more that the burning gas is allowed to expand whilst doing work, the better the

efficiency.

CI is also an 'excess air' cycle, potentially. A full cylinder of air is compressed eachtime, and the amount of fuel injected is varied to change power - this is beyond the

lean burn that can be achieved with SI.

Documents

Internal Combustion Engines Assignment