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·
¨©§ ���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
• 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.
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