LECTURE 2

MEC 2621- INTRODUCTION TO AUTOMOTIVE ENGINEERING

By: Dr. SANISAH SAHARIN

DEPARTMENT OF MECHANICAL ENGINEERING IIUM

Class 2 – fundamental of engine operation

Automo&ve  Engine  

Ø  Engine:   is   a   machine   that   converts   heat  energy  into  mechanical  energy.  

 Ø  The   heat   from   burning   a   air-­‐fuel  

mixture   produces   power   which   moves  the  vehicle.  

Ø  Automo>ve   eng ine   a re   i n te rna l  combus>on   (IC)   engine   because   the   fuel  that   runs   them   is   burned   internally,   or  inside  the  engines.  

Ø  There  are  two  types  of  engine  Ø Reciproca)ng   engine:   piston  moving  up  and  down,  or  back  and  forth  

Ø Rotary   (wankel)   engines:   have  rotor  that  spin  or  rotate  

Engine Components Ø  Cylinder block

Ø  Basic framework of the engine. Ø  It has cylindrical hole is called

engine cylinder, coolant jacket allows the coolant flow side of the cylinder, crankcase is the space of crankshaft, intake and exhaust port.

Ø  Piston Ø  A movable part, fitted into a

cylinder. Ø  It is moved based on the pressure

changing.

Ø  Piston valve Ø  Allows the engine breadth in

and breadth out.

Ø  Intake valve Ø  Allows the engine breadth

in the air fuel mixture.

Ø  Exhaust valve Ø  Allows the exhaust (CO,

NOx, PM and etc) to leave the cylinder.

Ø  Camshaft Ø  A shaft in the engine which has a

series of cams for operating the valve mechanism.

Ø  Combustion Chamber Ø  It is the space between the top of

the piston and the cylinder head. It is also called confined (closed) space of the cylinder in where air and fuel mixture burn.

Ø  Connecting rod Ø  Connects the crankshaft with the

piston. It is used to transfer the piston force to the CS.

Ø Essential for Engine Operation

Ø The automobile engine has cylinders.

Ø Piston moves up and down in each cylinder due to develop pressure in the Combustion chamber (CC) by the burning of air and fuel mixture.

Ø Piston force transfers to the crankshaft with the connecting road and develop torque at the crankshaft,

Ø The crankshaft torque transfers to the driving wheel of the car through the transmission.

Ø The car moves because the piston move.

Ø Engine  must  have  available  fuel  to  burn  in  Combus>on  Chamber  (CC)  for  moving  up  and  down  the  piston.  

Ø Fuel   gets   into   the   engine   due   to   the   pressure  difference.  

 Ø Vacuum   pressure  develops   inside   the   engine   cylinder  due  to  exhaust  out.  

 Ø Atmospheric  pressure    is  above  the  vacuum  pressure.  Ø Pressure  difference  =  Atmospheric  pressure-­‐Vacuum  

pressure  

Ø Gravity,  atmospheric  pressure,  and  vacuum  pressure  make  it  possible  for  the  fuel  to  get  into  the  engine  cylinders.  

 Ø Vacuum   is   the   absence   of   air  which   is   called   nega>ve  pressure.  

Ø When   a   piston   moves   down   in   a   cylinder,   the   pistons  creates  a  par>al  vacuum.  

Ø Air-­‐fuel  Mixture  burns  into  the  combus>on  chamber  of  the  cylinder  and  produce  power.  

Ø Power    of  the  engine  =  Pressure  x  Area  of  the  Cylinder  x  Piston  travelling  speed  

Ø Burning of air and fuel mixture into the cylinder combustion chamber is called combustion.

Ø Based on the burning characteristics of fuel into the combustion chamber, the engine is classified as:

Ø Spark Ignition engine: spark plug is used to burn the A/F mixture

Ø Compression ignition engine: fuel burn into the

combustion with compressed air.

Combustion in the engine

Ø  During burning of fuel, the burning gases become very hot. The temperature may go as high as 4500 -50000F (2500-27000C).

Ø  High temperature produces high pressure, based on ideal gas law,

PV=mRT, Ø  P is the pressure, V is the volume of fuel (consider as constant

for specific operation), m is the molar mass is constant, R is the gas constant, and T is the temperature. It is noted P and T are variables. If temperature increases, pressure will increase.

Ø  The pressure (P) will develop power which is called engine power.

Power of the engine = [P x Acylinder x Vpiston]

Ø Almost all cars currently use what is called a four-stroke combustion cycle to convert gasoline into motion.

Ø The four-stroke approach is also known as the Otto cycle, in honor of Nikolaus Otto.

Ø Four Strokes are : Intake, Compression, Combustion and Exhaust

Engine Operation

Ø Intake valve opens and exhaust valve closes.

Ø Piston goes down to BDC.

Ø Air-fuel mixture (A/F) is drawn into the cylinder.

Intake stroke

Ø Both valve are closed.

Ø  Piston goes to TDC.

Ø A/F is compressed.

Ø  Increase the pressure and temperature.

Compression stroke

Ø Both  valves  are  closed.  

Ø Spark   plug   sparks   and  ignites   the   high   pressure  and   temperature   A/F  mixture.  

Ø Explosive   force   pushes  down  the  piston.  

Ø Torque   developed   at   the  crankshaN.    

Power  stroke  

Ø  Intake valve closes and exhaust valve opens.

Ø Piston goes up. Ø  H i g h p r e s s u r e a n d

temperature exhaust leave the cylinder.

Ø  Va c u u m p r e s s u r e i s incurred into the cylinder.

Exhaust stroke

In the Engine Cylinder�

Ø  The intake valve closes after the piston passes it bottom position and starts to move up again

Ø  The bottom position of the piston into the cylinder is called bottom dead center (BDC).

Ø  The piston moves up, compresses the A/F mixture into a confined space called combustion chamber.

Ø  The piston reaches the top position and spark plug fire.

Ø  The top position of the piston into the cylinder is called top dead center (TDC).

Ø  The spark sets the fire to ignite the compressed A/F mixture.

Ø  The temperature of the burning A/F mixture goes up as high as 3000 °C.

Ø  The high temperature makes the pressure as high as 4140 kPa (1 psi = 6.9 kPa or 1 kPa = 1 kN/m^2).

Ø  The 4140 kPa push down the piston of up to 17,792 N

Ø This 17,792 N pushes the piston down. Ø The downward movement, carried through the connecting

rod, rotates the crankshaft. Ø The crankshaft turns the gears and drive shafts to move

the car.

OTTO  Cycle  

THERMODYNAMIC  

P-V AND T-S DIAGRAMS

THERMAL EFFICIENCY

•  Thermal efficiency has been defined as ‘ the relation between power produced and the energy in the fuel burned to produce that power’.

•  Thermal efficiency is the percentage of energy taken from the combustion which is actually converted to mechanical work.  In a typical low compression engine, the thermal efficiency is only about 26%.  In a highly modified engine, such as a race engine, the thermal efficiency is about 34%. 

Thermal efficiency (air-standard efficiency) of Otto Cycle,

o  For a cylinder of an engine, the crankshaft, connecting rod, piston, and head assembly can be represented by the following geometry: o  B = Bore of the cylinder o  r = connecting rod length o  a = crank radius o  S =stroke length o  Ư̆ = Crank angle, degree

Ø  The top dead centre (TDC) of an engine refers to the crankshaft being in a position such that Ư̆ = 0º.

Ø  The volume in this position is called clearance volume (VC)

Ø  Bottom dead centre (BDC) refers to the crankshaft being Ư̆ = 180º.

Ø  The total volume VL is maximum at bottom dead centre (BDC).

COMPRESSION  RATIO  

•  Compression ratio is defined as the ratio of maximum to minimum volume.

•  VL is total volume (volume at BDC) and VC is clearance volume(volume at TDC) .

r = VLVC

Ø  What is significant of the compression ratio for the engine on vehicle performance ?

Ø Heavy vehicle needs high torque- what about its engine compression ratio?

Ø Lighter passenger car needs low torque- what about compression ratio for its engine?

o  A 3-litre SI V6 engine that operates on a four-stroke cycle at 3600 rpm. The compression ratio is 9.5 and the engine is square (B=S).

o  Cylinder bore and stroke length o  Average piston speed o  Clearance volume of one cylinder

Example 1

Example 2

•  A two stroke gas engine has piston diameter of 150 mm, length of stroke 400 mm and indicated mean effective pressure 5-5 bar. The engine makes 120 explosions per minute. Determine the mechanical efficiency of the engine, if its bhp is 5 kW�

Ø  Energy input to an engine Qin comes from the combusting fuel. Ø  Fuel is a HC.

Ø  Air is used to supply the oxygen needed for this chemical reaction.

Ø  For combustion Ø  the proper relative amounts of air (oxygen) and fuel must

be presented.

Ø  Air-fuel ratio (AF) are parameters used to describe the mixture ratio.

Air-Fuel Ratio

•  A frequently used quantity in the analysis of combustion process is the air-fuel ratio A/F. it is defined as the ratio of the mass of air to the mass of fuel for a combustion process.

•  The mass m of a substance is related to the number of moles n through the relation: m = nM, where M is the molar mass. The reciprocal of A/F ratio is called the fuel-air ratio.

• 

Mohammedali Abdulhadi & A. M. Hassan ˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰

26

fuelofmassairofmass

mmFA

f

a

222224

762.324

762.34

NmnOHmnCONmnOmnHC mn ¸¹·

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A frequently used quantity in the analysis of combustion process is the air-fuelratio

A/F. it is defined as the ratio of the mass of air to the mass of fuel for a combustion

process.

The mass m of a substance is related to the number of moles n through the

relation: m = nM, where M is the molar mass. The reciprocal of A/F ratio is called the

fuel-air ratio.

The minimum amount of air needed for the complete combustion of a fuel is

called the stoichiometric or theoretical air. In actual combustion processes, it is

common practice to use more air than the stoichiometric amount. The amount of extra

air than the stoichiometric is called (excess air). Amount of air less than

stoichiometric amount is called (deficiency of air). Equivalence ratio is the ratio of

the actual fuel- air ratio to the stoichiometric fuel-air ratio. Sometimes this ratio is

given in term of A/F ratio and called mixture strength.

RatioFAActualRatioFAtricStoichiomestrengthMixture

I ratioAFStoichratioAFActualratioeEquivalenc

)(.

)(

actual)FA(

stoich)FA(

stoich)AF(

actual)AF( I

Where: stoichiometric :1 = ࢥ

.lean (week) mixture- excess of air :1 >ࢥ

.rich mixture- deficiency of air :1 <ࢥ

A general reaction equation of a hydrocarbon fuel for stoichiometric condition

with air is given by:

The composition of a hydrocarbon fuel CnHm are carbon and hydrogen, n and m

can be determined for 1 kg of fuel as follows:

mnn

mn

nCfuelofWeight

fuelinCofWeight�

�

12

12

202.212

12

1

Exhaust and Flue Gas Analysis: The products of combustion are mainly gaseous. When a sample is taken for

analysis it is usually cooled down to a temperature which is below the saturation

temperature of the steam present. The steam content is therefore not included in the

•  The minimum amount of air needed for the complete combustion of a fuel is called the stoichiometric or theoretical air. In actual combustion processes, it is common practice to use more air than the stoichiometric amount.

•  The amount of extra air than the stoichiometric is called (excess air). Amount of air less than stoichiometric amount is called (deficiency of air).

•  Equivalence ratio is the ratio of the actual fuel- air ratio to the stoichiometric fuel-air ratio. Sometimes this ratio is given in term of A/F ratio and called mixture strength.

Mohammedali Abdulhadi & A. M. Hassan ˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰˰

26

fuelofmassairofmass

mmFA

f

a

222224

762.324

762.34

NmnOHmnCONmnOmnHC mn ¸¹·

¨©§ ���o¸

¹·

¨©§ ��¸

¹·

¨©§ ��

A frequently used quantity in the analysis of combustion process is the air-fuelratio

A/F. it is defined as the ratio of the mass of air to the mass of fuel for a combustion

process.

The mass m of a substance is related to the number of moles n through the

relation: m = nM, where M is the molar mass. The reciprocal of A/F ratio is called the

fuel-air ratio.

The minimum amount of air needed for the complete combustion of a fuel is

called the stoichiometric or theoretical air. In actual combustion processes, it is

common practice to use more air than the stoichiometric amount. The amount of extra

air than the stoichiometric is called (excess air). Amount of air less than

stoichiometric amount is called (deficiency of air). Equivalence ratio is the ratio of

the actual fuel- air ratio to the stoichiometric fuel-air ratio. Sometimes this ratio is

given in term of A/F ratio and called mixture strength.

RatioFAActualRatioFAtricStoichiomestrengthMixture

I ratioAFStoichratioAFActualratioeEquivalenc

)(.

)(

actual)FA(

stoich)FA(

stoich)AF(

actual)AF( I

Where: stoichiometric :1 = ࢥ

.lean (week) mixture- excess of air :1 >ࢥ

.rich mixture- deficiency of air :1 <ࢥ

A general reaction equation of a hydrocarbon fuel for stoichiometric condition

with air is given by:

The composition of a hydrocarbon fuel CnHm are carbon and hydrogen, n and m

can be determined for 1 kg of fuel as follows:

mnn

mn

nCfuelofWeight

fuelinCofWeight�

�

12

12

202.212

12

1

Exhaust and Flue Gas Analysis: The products of combustion are mainly gaseous. When a sample is taken for

analysis it is usually cooled down to a temperature which is below the saturation

temperature of the steam present. The steam content is therefore not included in the

Il ne faut pas fair ces choses a moitié.