Mec2621_lecture 3- Engines-part 2

7/24/2019 Mec2621_lecture 3- Engines-part 2

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LECTURE 3


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! An internal combustion engine is an engine in which thecombustionof a fueloccurs in a combustion chamberinsideand integral to the engine.

! In an internal combustion engine it is always theexpansion of the high temperature and pressure gasesthat are produced which apply force to the movablecomponent of the engine, such as the pistons or turbineblades.


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! The term internal combustion engine usually refers to anengine in which combustion is intermittent, such as the

familiar four-stroke and two-stroke engines, along with avery few more exotic variants, such as theWankel engine.

! These engines almost invariably use reciprocatingpistons, with crankshafts, connecting rods and most ofthem now use camshafts with cams.


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! A second class of internal combustion engines usecontinuous combustion - Jet engines (including

gas turbines) and most rockets, each of which areinternal combustion engines on the same principle aspreviously described.

! The internal combustion engine (or ICE) contrasts with

the external combustion engine, such as a steam or

Stirling enginein which the energy is delivered within aworking fluid heated in a boiler by fossil fuel, wood-burning, nuclear, solar etc.


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Engine Types and Classifications

! Automobile engine can be classified according to:

! Ignition

! Reciprocating or rotary

! Number of cylinders

!Arrangement of cylinders

!

Arrangement of valve and valve trains

! Types of fuel burnt

! Number of stroke per cycle

! Type of cooling


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IGNITION


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!Spark ignition (SI): high-voltage electrical

discharge between two electrodes ignites air-fuelmixture in combustion chamber surrounding spark

plug

!

Compression ignition (CI): air-fuel mixture self-ignites due to high temperature in combustion

chamber caused by high compression, Diesel engine


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DESIGN: RECIPROCATING

AND ROTARY


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Reciprocating Engine

! Engine has one or more

cylinders in which pistonsreciprocate back and forth

! Combustion chamber in

closed end of cylinders

!

Power delivered to rotating

output crankshaft by

mechanical linkage withpistons


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Rotary Engine

! Engine made of block

(stator) built around

large non-concentric

rotorand crankshaft

!

Combustion chambersare built into the

nonrotating block


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NUMBER AND ARRANGEMENT

OF CYLINDERS


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!

A passenger car engines can have three, four, five,

six, eight or ten (or even 12) cylinders.


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! Typical four cylinder engine can be arranged:

!In a row (in line)

!

In a line engine has set of cylinders are arranged inline.

!In two rows or banks set at an angle V (V type)

!V type engine has set of cylinders in two rows.

!The two rows are separated by an angle to eachothers.

!The angle could be 60 degrees or 90 degrees

! In two rows or banks opposing each other (flat type)

! The rows of two cylinders oppose to each other. They

are in the same plane, separated by 180 degrees.

! Three banks of cylinders on same crankshaft (W type,not common)

Four cylinder engines


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In-line V Flat


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! The straight-4 engine is much smoother than one, two, and

three cylinder engines, and this has resulted in it becoming theengine of choice for most economy cars, although it can befound in some sports cars as well.

! However, the straight-4 is not a fully balancedconfiguration.

! While an even-firing straight-4 engine is in primary balance

because one pair of pistons is always moving up at the sametime as the other pair is moving down, piston speedas with allinternal combustion enginesis higher through the top 180 ofthe crankshaft rotation than the bottom 180.


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! Therefore, across the 4 cylinders two pistons are alwaysaccelerating faster in one direction while two others are

accelerating slower in the other, which leads to a secondarydynamic imbalancean up-and-down vibration at twice thecrankshaft's speed.

! This imbalance is tolerable in a small, low-displacement, low-power configuration, but the vibrations get worse with

increasing size and power.


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VALVE AND VALVE TRAIN

ARRANGEMENT AND LOCATION


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Valves arrangement

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!

Types of valve train

!Two basic types of valve train

!

Overhead camshaft

!Camshaft in block with pushrod


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!

Number of valves per cylinder

!Normally each cylinder of the engine has two valves

!

Some have multi-valve cylinder of the engine!PURPOSE of these added valves is to allow the

engine to breathe more freely.


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TYPE OF FUEL BURNT

Di l E i


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Diesel Engine

!A diesel engine is known as a

"compression ignition" engine.

!

Since the air is compressed to veryh i g h p r e s s u r e r a i s i n g i t s

temperature and then diesel is

injected in a very fine spray which

causes the diesel to ignite and

explode.!Whereas a gasoline engine is known as

a "spark ignition" engine.

! Since a spark plug is required to

ignite the mixture of gasoline and

air in the combustion chamber.

O ti l P i i l


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! Diesel engine operation completes like gasoline engine

with four stroke

!

Intake stroke

!

The intake valve opens and exhaust valve closescompletely.

! The position of the piston into the cylinder is inbottom dead center (BDC).

! Draw the air in to the cylinder.

!

Compression Stroke

! Both of the valves close

!

The piston moves up, compresses the air into aconfined space is called combustion chamber untilreach to the TDC.

!

Pressure and temperature increase to its maximumlevel.

Operational Principle


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!

Combustion Stroke

! Both of the valves close

!

Injector injects the right amount of fuel to the hightemperature air into the combustion chamber.

! Fuel burn instantly due to the high temperature of air.

! Develop the high pressure and explosive force topush the piston down

!

Create the torque at the crankshaft to propel thecar.

! Exhaust Stroke

! Intake valve remain closed and exhaust valve opens

! Leave the emission from the combustion chamber.

! Make vacuum the cylinder


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! Fuel delivery continues as long as the valve open.

!

The fuel flow time and amount is controlled by ECM

! When the proper amount of fuel has sprayed out, the ECM

turns off the injector.

! The carburetor which is called the air-fuel mixing device isreplaced now by ECM control fuel-injection system

O ti l P i i l


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Operational Principle


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Injector (Electromagnetic)


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Diesel Engine operation:


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Gasoline Engine and Diesel Engine -

comparison

o Major types of engines are gasoline and diesel.

o The engines required to run with burning gasoline anddiesel are different because gasoline and diesel aredifferent types of fuel.

o gasoline is a highly volatile fuel and gets ignited veryeasily whereas diesel is comparatively heavier anddirtierfuel.


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Gasoline Engine and Diesel Engine

Comparison

!Inlet stroke

! In gasoline engines the mixture of air and gasoline isdrawn in by the falling piston.

! In diesel engines only air is drawn in by the falling piston.

!

Compression stroke

! In gasoline engine, the mixtureis compressed.

! In diesel engine, only air is compressed.


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! Combustion stroke

!

In gasoline engine, the air and fuel mixture is ignitedusinga spark plug and burns expanding and forcing the pistondown.

! In diesel engine, fuel is injected at a high pressure into thehot, compressed air in the cylinder, causing it to burn andforce the piston down. No spark is required.

!

Exhaust stroke

! In both gasoline and diesel engines, the burned mixture ofair and fuel is pushed out of the cylinder by the risingpiston.


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! Gasoline engines are lighterthan diesel engines (heavier and

oiler).

! Gasoline engine is called spark ignition (SI) engine while thediesel engine is called compression ignition(CI) engine.

! Diesel engines have higher torque than gasoline engines.

!

a diesel engine would pull heavy loads easily than agasoline engine.

! the diesel engine would be steady and carry heavier loadsto longer distances.


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! Diesel engines have better fuel efficiency as compared to

gasoline due to the fact that they have higher compressionratio.

! The most important part of diesel engine, fuel economy.

! Diesel engines give better mileage than gasoline engines.

! So running on diesel would make you go farther at a lower

cost than running on gasoline.


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Why gasoline, not diesel?

! Diesel engines;

!Higher compression ratios (20:1); tend to be heavier thanequivalent gasoline engine

! tend to be more expansive

! lower maximum RPM, i.e. higher torque than high

horsepower, slow in acceleration

!

produce more smoke and smell funny

! harder to start in cold weather

! Noisier and tend to vibrate

! Diesel fuel less available


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NUMBER OF STROKE PER

CYCLE

F t k T t k


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Four-stroke vs Two-stroke

! Piston engines operate on either

!

Two-stroke cycle or

!Four-stroke cycle

! Four-stroke-cycle engine

!Power stroke is in every fourth stroke

!A power stroke during two crankshaft revolution


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! Two-stroke-cycle engine

!

Every other stroke has a power stroke

!

A power stroke during each crankshaft revolution

!Two stroke-cycle engine produces twice asmany power stroke at the same crankshaftspeed (rpm) as the four-stroke-cycle engine.

!Piston opens the transfer (inlet) and exhaustport.

!

Therefore, always mixing some A/F mixturewith the exhaust gases and leave the engine.

!Only part of the downward strokes producespower.

Mode of Operation of Two Stroke Engine


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Mode of Operation of Two- Stroke Engine


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! FIRST STROKE

!

The piston is at the upward of the cylinder.

!A pipe at the left side is opened and air-fuel mixture enterinto the cylinder

!A/F mixture is already compressed a bit when the A/Fmixture flows from lower to the upper part of the cylinder.

! The fresh gases leaves the cylinder with the exhaustthrough an ejection pipe.


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! SECOND STROKE

! After being hurried upward, the piston now covers the pipe on

the left side and the ejection pipe.

!

Because there is no way out any more, the upper, fresh gasmixture gets compressed now.

! At the same time in the part below fresh gas is taken in bythe piston driving upward through the open suction pipe.

! At the top dead-center, the compressed fuel mixture is

ignited by the sparking plug, the piston is presseddownward while it compresses at the same time the freshgas below.

! The process begins again as soon as the piston arrives at itslowest point.

FIRST STROKE


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! Intake

! The A/F mixture is first drawn into thecrankcase by the vacuum created duringthe upward stroke of the piston.

! Transfer/Exhaust

! Toward the end of the 1st stroke, piston

exposes the intake port allowing thecompressed fuel/air mixture in the crankcaseto escape around the piston into the maincylinder.#

! This expels the exhaust gasses out theexhaust port, usually located on theopposite side of the cylinder.

#

! Unfortunately, some of the fresh fuelmixture is usually expelled as well.

FIRST STROKE

SECOND STROKE


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! Compression

!

The piston then rises, driven by flywheelmomentum, and compresses the fuelmixture.#(At the same time, another intakestroke is happening beneath the piston).#

! Combustion Stroke

!

At the top of the stroke the spark plugignites the fuel mixture. The burning fuelexpands, driving the piston downward, tocomplete the cycle.

SECOND STROKE


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The Two-stroke cycle

1=TDC

2=BDC

A: intake/scavenging

B: Exhaust

C: Compression

D: Expansion(power

Two-stroke engine loses fresh fuel because of the "overlap" of

the valve times (both valves are open for an instant).


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! Differences between two-stroke engine and four-stroke engine

! The two-stroke internal combustion engine differs fromthe more common four-stroke engine by completing thesame four processes (intake, compression, combustion,exhaust) in only two strokes of the piston rather than four.

! This allows a power stroke for every revolution of thecrank, instead of every two revolutions as in a four-stroke

engine.

! Two-stroke engines provide high specific power (Specificpower = power/weight), so they are valued for use inportable, lightweight applications.


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o Two - stroke engine is always worse than in the four-stroke

engine, which loses fresh fuel only because of the "overlap"of the valve times (both valves are open for an instant).

o Two stroke engine pollutes the environment with highemission. So it is not for friendly environment.

o The fuel mixture of the two-stroke engine often gets

shifted with a certain quantity of oil because of thenecessary lubrication.

o Unfortunately the oil gets burnt partly, too, and harmful

gases are expulsed by the engine.


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FUEL INPUT METHOD


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!

Carbureted: Air and fuel

mixed at throat


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! Fuel injection

! Multipoint port fuel injection:

one or more injectors at eachcylinder intake

! Throttle body fuel injection:

injectors upstream of intake

manifold


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TYPE OF COOLING

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Mec2621_lecture 3- Engines-part 2