Mec2621_lecture 4- Engines-part 3

7/24/2019 Mec2621_lecture 4- Engines-part 3

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CYLINDER BLOCK

! The cylinder block is the foundation of the engine.

!

The parts are attached to the cylinder blocks are mainly:

! Crankshaft with main bearings,

! Piston with piston rings and connecting rods,

!

Valves with camshaft,

!Intake and exhaust manifold

! Cylinder head

!

Flywheel


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!

Most blocks are cast from gray iron (cast iron) or iron mixedwith other metals such as nickel and chromium.

! Some blocks are cast from aluminum alloy.

!

As the aluminum alloy is too soft and wear rapidly.

! Either cast iron or cast from aluminum alloycontaining silicon particles cylinder liners are used.

!

Aluminum weight much less and conducts heat morerapidly than cast iron.

!

Aluminum alloy reduces vehicle weight and improvesfuel economy.


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CYLINDER HEAD

!It sits above the cylindersontop of the cylinder block.

!

It closes in the top of thec y l i n d e r , f o r m i n g t h ecombustion chamber.

!This joint is sealed by ahead gasket. In most engines,

the head also provides spacefor the passages that feed airand fuel to the cylinder, andthat allow the exhaust toescape.

!The head can also be a place

t o m o u n t t h e v a l v e s ,s p a r k p l u g s , a n dfuel injectors.


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PISTON AND PISTON RINGS


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!

Piston and piston ring

!

Piston: are made of aluminum alloy. Its diameter isslightly smaller than the cylinder which allows thepiston to slide up and down

!It is called sliding fit

!Piston rings are installed on the pistons ring grooves for

preventing excessive blow-by. There are two types ofpiston ring:

!Compression ring: these form a sliding sealbetweenthe piston and the cylinder wall.

!So that the gases which is developed due to theburning of A/F mixture will not able to go down tothe crankage.


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Oil control ring (oil rings) :These scrape off most of thelubricating oil splashed on the cylinder wall and return the oilto the crankcase.


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! Pistons clearance

!

Pistons clearance is the distance between the cylinder walland the skirt.

!

The skirt is the lower part of the piston

!Piston clearance is usually in the range of0.025-0.12 mm.

!In the running engine, the piston and rings moveon films of oil that fill the piston clearance.

!

If the clearance is too small, there is a loss ofpowerfrom high friction and sever wear.

!If the clearance is too large, there is a chance ofpiston slap.

!

Piston slap may collapse pistons skirt.


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!A device that can be opened or closed to allow or to stop theflow of liquid or gas.

!Allows the IC engine cylinder to breath-in and breath-out.

! Each engine has at least two valves

!An intake valve opens just before the intake stroke

begins.!Allows the air-fuel mixture to enter the cylinder

!

A exhaust valve opens just before the exhaust strokebegins

!Allows the burn gases to leave the cylinder


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! The valve for the IC engine is usually a flat disk of metalwith along rod known as the valve stem.

!The stem is used to push down on the valve and open it,with a spring generally used to close it when the stem isnot being pushed on.

! The valves are operated by a valve trainwhich is comprised ofvalves, thevalve stem, camand cam followers.

! The engine normally operates the valves by pushing on thestems with cams (rotating or sliding piece in amechanical linkage used especially in transforming rotarymotion into linear motion or vice-versa) and cam followers.

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! The shapeand position of the cam determines the valvelift and when and how quickly (or slowly) the valve is

opened.

! The cams are normally placed on a fixed camshaftwhichis then geared to the crankshaft, running at halfcrankshaft speed in a four-strokeengine.


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!What are the advantage and disadvantage of usingbigger shape exhaust valve.

!What are the merit and demerit of using bigger size ofintake valve ?

!

Advantage: Increasedvolumetric efficiencydue tovery large port openings . This is important when

an engine operates over a wide RPM range.

!Disadvantage: perfect sealing is difficult to achieve


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! Arrangement of valves and valve trains

!Engine valve allows the engine to breath.

!

The intake valve open to admit the A/F mixture to theengine cylinders.

! The exhaust valves open to allow burned gases to exit orexhaust from the engine cylinders.

! Cam on rotating camshaft operate the valve train, which

open the valves.

! Location of the camshaft

! Either on the cylinder head or in the cylinder block.

! Types of camshaft drive

!

Camshaft are driven by timing gears, sprockets andtiming chain or sprockets and toothed timing belt


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!

Types of valve train

!

Two basic types of valve train

!

Overhead camshaft- SOHC and DOHC

! Camshaft in block with pushrod

! Number of valves per cylinder

!

Normally each cylinder of the engine has two valves

! Some have multi-valve cylinder engine

! PURPOSEof these added valves is to allow the engineto breathe more freely.


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! Camshaft timing is the relationship between the

operation of camshaft and crankshaft

! Camshaft is driven by crankshaft

! Crankshaft makes two complete revolutions (720

degree) to complete the four piston strokes

! This makes the engine a four cycle engine

! Word cycle means a series of events that repeat

themselves

Camshaft timing


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! Two crankshaft revolutions make one revolution of the

camshaft

! One revolution of camshaft opens each valve once.

! 1:2 gear ratio is needed to complete the rotation of the

camshaft by the crankshaft

! Therefore, the camshaft diameter must be twice time

larger than the crankshaft sprocket diameter (i.e.

2dcrankshaft= Dcamshaft )


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!

Methods of camshaftdriving

!

T i m i n g g e a r : d r i v e

camshaft in cylinder block

! T i m i n g c h a i n : d r i v e

camshaft in cylinder block

! T i m i n g c h a i n : d r i v e

camshaft on cylinder head

! T i m i n g b e l t : d r i v e

camshaft on cylinder head

CRANKSHAFT


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CRANKSHAFT

! The crankshaft is a one piece casting or forging of heat-treated alloy steel.

!Counter-weights placed opposite the crankpins balancethe crankshaft.

!The output end of the crankshaft has the flywheel or

drive plate attached to it.

!

The front end has a sprocket drives the camshaft, thevibration damper and the pulleys.

!

Electromagnetic pick-up sensor is placed on thecrankshaft in order to calculate the engine speed (rpm)and to maintain the piston position and firing order.


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HARMONIC BALANCER/


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HARMONIC BALANCER/

VIBRATION DAMPER! Vibration damper is a component that prevents the

unwanted vibrationor bunchingor twistingaffect.

!

The power strokes tend to twist the crankshaft;

!Each power stroke applies a very high force (18,000 N)on the crankpin and it;

!might pushes the rest of the crankpins of the

crankshaft and crankshaft untwists.

!

this twist-untwist action repeats in every power stroke

!this action tends to create an oscillating (back andforth) motion in the crankshaft

!

TORSIONAL VIBRATION which prevents the

crankshaft from breaking.


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INTAKE MANIFOLD


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INTAKE MANIFOLD

! The intake manifold is a set

of tubes.

!These tubes carry A/F

mixture from throttle

valve to the intake

ports in the cylinder

head.

EXHAUST MANIFOLD


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EXHAUST MANIFOLD

!

The exhaust manifold is aset of tubes.

!

It carries exhaust gasfrom the cylinder headto the exhaust system

!The exhaust gas passesthrough the catalyticconverter, the mufflerand the resonator.

!

T h e c a t a l y t i cconverter convertsm o s t o f t h epollutants in thee x h a u s t g a s t oh a r m l e s s

substances.!The muffler and the

resonator reduceexhaust noise.

FIRING ORDER


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FIRING ORDER

! The firing order is the sequence of power delivery of eachcylinder in a multi-cylinder reciprocating engine.

!

This is achieved by sparking of the spark plugs in agasoline engine in the correct order, or by the sequenceof fuel injection in a Diesel engine.

!When designing an engine, choosing an appropriate firing

order is critical!To minimizing vibration and for achieving smooth

running,

!Correct firing order also provide long engine fatiguelifeand user comfort.


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! In a gasoline engine, the correct firing order is obtained bythe correct placement of the spark plug wires on the

distributor.

! In a modern engine with an engine management systemand direct ignition, the Engine Control Unit (ECU) takescare of the correct firing sequence.


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! The crankpin and crankshaft arrangement determine thefiring order.

!

Firing orders for the same type of engine may differ.

!

Two firing orders for in-line

!

4-cylinders engines are 1-3-4-2 and 1-2-4-3


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! 6-Cylinders engines 1-5-3-6-2-4

The complete firing order of a four-cycle engine representstwo complete revolutions of the crankshaft.

!This is 720 degrees of crankshaft rotation

! Firing impulse occurs every 120 degrees of

crankshaft rotation (720/6=120).

!

When piston number 1 is at TDC on the end ofcompression stroke, piston number 6 is at TDC onthe end of the exhaust stroke.


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Mec2621_lecture 4- Engines-part 3

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Transcript of Mec2621_lecture 4- Engines-part 3