محركات احتراق داخلي معهد اعداد المدربين-مكائن ومعدات

121
تعليمية الحقيبة الساسية لمحركات احتراق اداخلي ال اعداد المهندس: ديودة العبي نزار فيصل ع1 يين التقنهد اعداد المدربين معئن والمعداتلمكا قسم ا

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internal combustion engine - second year machine & equipment dept. theoretical briefcase

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الحقيبة التعليمية

الداخلياالحتراق االساسية لمحركات

اعداد

نزار فيصل عودة العبيدي : المهندس

1

معهد اعداد المدربين التقنيين قسم المكائن والمعدات

Conversion of Thermal Energy

•Almost all of the mechanical energy produced today is produced from the conversion of thermal energy in some sort of heat engine.

• The operation of all heat-engine cycles can usually be approximated by an ideal thermodynamic power cycle of some kind.

•A basic understanding of these cycles can often show the power engineer how to improve the operation and performance of the system.

2

Conversion of Thermal Energy

•Almost all of the mechanical energy produced today is produced from the conversion of thermal energy in some sort of heat engine.

• The operation of all heat-engine cycles can usually be approximated by an ideal thermodynamic power cycle of some kind.

•A basic understanding of these cycles can often show the power engineer how to improve the operation and performance of the system.

3

P- and T-s Diagrams of Power Cycles

The area under the heat addition process on a T-s diagram is a geometric measure of the total heat supplied during the cycle qin, and the area under the heat rejection process is a measure of the total heat rejected qout. The difference between these two (the area enclosed by the cyclic curve) is the net heat transfer, which is also the net work produced during the cycle.

4

Reversible Heat-Engine Cycles

•The second law of thermodynamics states that it is impossible to construct a heat engine or to develop a power cycle that has a thermal efficiency of 100%. This means that at least part of the thermal energy transferred to a power cycle must be transferred to a low-temperature sink.

•There are four phenomena that render any thermodynamic process irreversible. They are:

Friction

Unrestrained expansion

Mixing of different substances

Transfer of heat across a finite temperature difference

5

• Thermodynamic cycles can be divided into two general categories: Power cycles and refrigeration cycles.

• Thermodynamic cycles can also be categorized as gas cycles or vapor cycles, depending upon the phase of the working fluid.

• Thermodynamic cycles can be categorized yet another way: closed and open cycles.

• Heat engines are categorized as internal or external combustion engines.

Categorize Cycles

6

Air-Standard Assumptions

To reduce the analysis of an actual gas power cycle to a manageable level, we utilize the following approximations, commonly know as the air-standard assumptions:

1. The working fluid is air, which continuously circulates in a closed loop and always behaves as an ideal gas.

2. All the processes that make up the cycle are internally reversible.

3. The combustion process is replaced by a heat-addition process from an external source.

4. The exhaust process is replaced by a heat rejection process that restores the working fluid to its initial state.

7

Air-Standard Cycle Another assumption that is often utilized to simplify the analysis even more is that the air has constant specific heats whose values are determined at room temperature (25oC, or 77oF). When this assumption is utilized, the air-standard assumptions are called the cold-air-standard assumptions. A cycle for which the air-standard assumptions are applicable is frequently referred to as an air-standard cycle.

The air-standard assumptions stated above provide considerable simplification in the analysis without significantly deviating from the actual cycles.

The simplified model enables us to study qualitatively the influence of major parameters on the performance of the actual engines.

8

Bore and stroke of a

cylinder

9

Mean Effective Pressure

Notice that the compression ratio is a volume ratio and should not be confused with the pressure ratio.

Mean effective pressure (MEP) is a fictitious pressure that, if it acted on the piston during the entire power stroke, would produce the same amount of net work as that produced during the actual cycle.

The ratio of the maximum volume formed in the cylinder to the minimum (clearance) volume is called the compression ratio of the engine.

TDC

BDC

min

max

V

V

V

Vr

minmax

net

VV

WMEP

10

Three Ideal Power Cycles

•Three ideal power cycles are completely reversible power cycles, called externally reversible power cycles. These three ideal cycles are the Carnot cycle, the Ericsson cycle, and the Stirling Cycle.

11

Three Ideal Power Cycles

•The Carnot cycle is an externally reversible power cycle and is sometimes referred to as the optimum power cycle in thermodynamic textbooks. It is composed of two reversible isothermal processes and two reversible adiabatic (isentropic) processes.

•The Ericsson power cycle is another heat-engine cycle that is completely reversible or “externally reversible.” It is composed of two reversible isothermal processes and two reversible isobaric processes (with regenerator).

•The Stirling cycle is also an externally reversible heat-engine cycle and is the only one of the three ideal power cycles that has seen considerable practical application. It is composed of two reversible isothermal processes and two reversible isometric (constant volume) processes.

12

Carnot Cycle and Its Value in Engineering

The Carnot cycle is composed of four totally reversible processes: isothermal heat addition, isentropic expansion, isothermal heat rejection, and isentropic compression (as

shown in the P- diagram at right). The Carnot cycle can be executed in a closed system (a piston-cylinder device) or a steady-flow system (utilizing two turbines and two compressors), and either a gas or vapor can be used as the working fluid.

H

LCarnot,th

T

T1

13

Internal-Combustion Engine Cycles

• Internal-combustion (IC) engines cannot operate on an ideal reversible heat-engine cycle but they can be approximated by internally reversible cycles in which all the processes are reversible except the heat-addition and heat-rejection processes.

•In general, IC engines are more polluting than external-combustion (EC) engines because of the formation of nitrogen oxides, carbon dioxide, and unburned hydrocarbons.

•The Otto cycle is the basic thermodynamic power cycle for the spark-ignition (SI), internal-combustion engine.

14

The Ideal Air Standard Otto Cycle

15

Otto Cycle: The ideal Cycle for Spark-Ignition Engines

Figures below show the actual and ideal cycles in spark-ignition (SI) engines and their P- diagrams.

16

Ideal Otto Cycle

The thermodynamic analysis of the actual four-stroke or two-stroke cycles can be simplified significantly if the air-standard assumptions are utilized. The T-s diagram of the Otto cycle is given in the figure at left.

The ideal Otto cycle consists of four internally reversible processes:

12 Isentropic compression

23 Constant volume heat addition

34 Isentropic expansion

41 Constant volume heat rejection

17

Thermal Efficiency of an Otto Cycle The Otto cycle is executed in a closed system, and disregarding the changes in kinetic and potential energies, we have

1

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min

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kk

and; Where,

18

Example IV-4.1: The Ideal Otto Cycle

determine a) the maximum temperature and pressure that occur during the cycle, b) the net work output, c) the thermal efficiency, and d) the mean effective pressure for the cycle. <Answers: a) 1575.1 K, 4.345 MPa, b) 418.17 kJ/kg, c) 52.3%, d) 574.4 kPa>

Solution:

An ideal Otto cycle has a compression ratio of 8. At the beginning of the compression process, the air is at 100 kPa and 17oC, and 800 kJ/kg of heat is transferred to air during the constant-volume heat-addition process. Accounting for the variation of specific heats of air with temperature,

19

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22

Diesel Cycle: The Ideal Cycle for Compression-Ignition Engines

The diesel cycle is the ideal cycle for CI (Compression-Ignition) reciprocating engines. The CI engine first proposed by Rudolph Diesel in the 1890s, is very similar to the SI engine, differing mainly in the method of initiating combustion. In SI engines (also known as gasoline engines), the air-fuel mixture is compressed to a temperature that is below the autoignition temperature of the fuel, and the combustion process is initiated by firing a spark plug. In CI engines (also known as diesel engines), the air is compressed to a temperature that is above the autoignition temperature of the fuel, and combustion starts on contact as the fuel is injected into this hot air. Therefore, the spark plug and carburetor are replaced by a fuel injector in diesel engines.

23

The Ideal Air Standard Diesel Cycle

24

Ideal Cycle for CI Engines (continued)

In diesel engines, ONLY air is compressed during the compression stroke, eliminating the possibility of autoignition. Therefore, diesel engines can be designed to operate at much higher compression ratios, typically between 12 and 24.

The fuel injection process in diesel engines starts when the piston approaches TDC and continues during the first part of the power stroke. Therefore, the combustion process in these engines takes place over a longer interval. Because of this longer duration, the combustion process in the ideal Diesel cycle is approximated as a constant-pressure heat-addition process. In fact, this is the ONLY process where the Otto and the Diesel cycles differ.

25

Ideal Cycle for CI Engines (continued)

1

11111

123

14

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232323

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kc

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in

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rk

r

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TT

q

q

q

w

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2

3

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cr

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and

Where,

26

Thermal efficiency of Ideal Diesel Cycle

Under the cold-air-standard assumptions, the efficiency of a Diesel cycle differs from the efficiency of Otto cycle by the quantity in the brackets. (See Slide #26) The quantity in the brackets is always greater

than 1. Therefore, th,Otto > th, Diesel when both cycles operate on the same compression ratio.

Also the cuttoff ratio, rc decreases, the efficiency of the Diesel cycle increases. (See figure at right)

27

Internal-Combustion Engines

The two basic types of ignition or firing systems are the four-stroke-cycle engines, commonly called four-cycle engines, and the two-stroke-cycle engines, commonly called two-cycle engines.

The four-cycle engines has a number of advantages over the usual two-cycle engine, including better fuel economy, better lubrication, and easier cooling.

The two-cycle engine has a number of advantages, including fewer moving parts, lighter weight, and smoother operation. Some two-cycle engines have valves and separate lubrication systems.

28

Cylinder Arrangements for Reciprocating Engines

Figure below shows schematic diagrams of some of the different cylinder arrangements for reciprocating engines.

29

• Vertical in-line engine is commonly used today in four- and six-cylinder automobile engines.

• The V-engine is commonly employed in eight-cylinder (V-8) and some six-cylinder (V-6) automobile engines.

• The horizontal engine is essentially a V-engine with 180o between the opposed cylinders. This system was used as the four-cylinder, air-cooled engine that powered the Volkswagon “bug”.

• The opposed-piston engine consists of two pistons, two crankshafts, and one cylinder. The two crankshafts are geared together to assure synchronization. These opposed-piston systems are often employed in large diesel engines.

30

• The delta engine is composed of three opposed-piston cylinders connected in a delta arrangement. These systems have found application in the petroleum industry.

• The radial engine is composed of a ring of cylinders in one plane. One piston rod, the “master” rod, is connected to the single crank on the crankshaft and all the other piston rods are connected to the master rod. Radial engines have a high power-to-weight ratio and were commonly employed in large aircraft before the advent of the turbojet engine.

• When the term “rotary engine” is used today, it implies something other than a radial engine with a stationary crank.

31

Engine Performance

There are several performance factors that are common to all engines and prime movers. One of the main operating parameters of interest is the actual output of the engine. The brake horsepower (Bhp) is the power delivered to the driveshaft dynamometer.

The brake horsepower is usually measured by determining the reaction force on the dynamometer and using the following equation:

00033

2

,

FRNBhp d

Where F is the net reaction force of the dynamometer, in lbf, R is the radius arm, in ft, and Nd is the angular velocity of the dynamometer, in rpm.

32

Horsepower

For a particular engine, the relationship between the mean effective pressure (mep) and the power is:

minute. per strokes pow er of number the is and

w here

ep

dis

minmax

net

pdis

CNN

strokeboreV

VV

Wmep

,

NVmepBhp

4

00033

2

Where C is the number of cylinders in the engine, Ne is the rpm of the engine, and is equal to 1 for a two-stroke-cycle engine and 2 for a four-stroke-cycle engine.

33

Brake Thermal Efficiency

The brake thermal efficiency of an engine, th, unlike power plants, is usually based on the lower heating value (LHV) of the fuel. The relationship between efficiency and the brake specific fuel consumption (Bsfc) is:

Bhp

Bsfc

LHVBsfcth

lbm/h rate, fuel

w here

2545

Note that the brake specific fuel consumption (Bsfc) of an engine is a measure of the fuel economy and is normally expressed in units of mass of fuel consumed per unit energy output.

34

External-Combustion Systems

External-combustion power systems have several advantages over internal-combustion systems. In general, they are less polluting. The primary pollutants from internal-combustion engines are unburned hydrocarbons, carbon monoxide, and oxides of nitrogen.

In external-combustion engines, the CHx and CO can be drastically reduced by carrying out the combustion with excess air and the NOx production can be markedly reduced by lowering the combustion temperature. By burning the fuel with excess air, more energy is released per pound of fuel.

There are three general ideal external-combustion engine cycles, the Stirling and Brayton are ideal gas-power, and vapor power cycles.

35

The Brayton cycle was first proposed by George Brayton for use in the reciprocating oil-burning engine that he developed around 1870.

Brayton Cycle:

The Ideal Cycle for Gas-Turbine Engines

Fresh air at ambient conditions is drawn into the compressor, where its temperature and pressure are raised. The high-

pressure air proceeds into the combustion chamber, where the fuel is burned at constant pressure. The resulting high-temperature gases then enter the turbine, where they expand to the atmospheric pressure, thus producing power. (An open cycle.)

36

Brayton Cycle (continued)

The open gas-turbine cycle can be modeled as a closed cycle, as shown in the figure below, by utilizing the air-standard assumptions.

The ideal cycle that the working fluid undergoes in this closed loop is the Brayton cycle, which is made up of four internally reversible processes:

12 Isentropic compression (in a compressor)

23 Constant pressure heat addition

34 Isentropic expansion (in a turbine)

41 Constant pressure heat rejection

37

T-s Diagram of Ideal Brayton Cycle

Notice that all four processes of the Brayton cycle are executed in steady-flow devices (as shown in the figure on the previous slide, T-s diagram at the right), and the energy balance for the ideal Brayton cycle can be expressed, on a unit-mass basis, as

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TTChhq

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pout

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w here

38

P- Diagram and th of Ideal Brayton Cycle

Then the thermal efficiency of the ideal Brayton cycle under the cold-air-standard assumptions becomes

k/kp

p

p

in

out

in

netBrayton,th

r

T/TT

T/TT

TTC

TTC

q

q

q

w

1

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23

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11

1

111

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ratio. pressure the is and , w here1

2

4

3

1

4

3

1

1

2

1

2

P

Pr

T

T

P

P

P

P

T

Tp

k/kk/k

39

Thermal Efficiency of the Ideal Brayton Cycle

Under the cold-air-standard assumptions, the thermal efficiency of an ideal Brayton cycle increases with both the specific heat ratio of the working fluid (if different from air) and its pressure ratio (as shown in the figure at right) of the isentropic compression process. The highest temperature in the cycle occurs at the end of the combustion process, and it is limited by the maximum temperature that the turbine blades can withstand. This also limits the pressure ratios that can be used in the cycle.

40

With the demise of the steam powered tractor in the late 1800’s, most modern tractors are equipped with internal combustion engines. Internal combustion engines are identified by the number of strokes in the cycle and by the fuel that is used to run them.

Common Tractor Classifications: 4 stroke cycle

- gasoline - diesel

- LP

41

42

43

Intake

Exhaust

Lubricating

Electrical

Cooling

Fuel

Hydraulic

Drive Train

44

Parts:

1. Pre-Cleaner

2. Air Cleaner

3. Intake Manifold

4. Intake Valve

5. Turbocharger (if used)

6. Intercooler (if used)

45

Parts:

1. Exhaust Valve

2. Exhaust Manifold

3. Muffler

4. Cap

46

Parts:

1. Crankcase Oil Reservoir (Oil Pan)

2. Oil Pump

3. Oil Filter

4. Oil Passages

5. Pressure Regulating Valve

Oil goes to:

1. Camshaft Bearings

2. Crankshaft Main Bearings

3. Piston Pin Bearing

4. Valve Tappet Shaft

47

Parts:

1. Battery

2. Ground Cable

3. Key Switch

4. Ammeter

5. Voltage Regulator

6. Starter Solenoid

7. Starter

8. Distributor * Gasoline Only

9. Coil

10. Alternator

11. Spark Plug

12. Power Cable

48

Cooling System

Liquid & Air

Parts:

1. Radiator

2. Pressure Cap

3. Fan

4. Fan Belt

5. Water Pump

6. Engine Water Jacket

7. Thermostat

8. Connecting Hoses

9. Liquid or Coolant

49

Cooling System

Air cooled Fins are used to dissipate heat

Liquid cooled Coolant is used to dissipate heat.

50

Gasoline

Diesel

Liquid Propane (LP)

Alternate Fuels

51

Parts:

Fuel Tank

Fuel Pump

Carburetor

Fuel Filter

Fuel Lines

52

Diesel Fuel System

Parts: 1. Fuel Tank

2. Fuel Pump

3. Fuel Filters

4. Injection Pump

5. Injection Nozzles

53

Power Transmission

Mechanical & Hydraulic

Parts:

1. Clutch Pedal

2. Clutch

3. Shift Controls

4. Transmission

5. Differential

6. Differential Lock Pedal

7. Final Drives

8. Power Take Off (PTO)

54

CONVENTIONAL INTERNAL

COMBUSTION ENGINES

TWO STROKE ENGINES

Migrating Combustion Chamber Engine (MCC)

FOUR CYCLE ENGINES

Conventional Four Cycle (OTTO ENGINE)

Rotary Engine (WANKEL)

Rotating Cylinder Valve Engine (RCV)

55

TWO STROKE ENGINES

Two-stroke engines do not have valves,

which simplifies their construction and

lowers their weight.

Two-stroke engines fire once every

revolution, while four-stroke engines fire

once every other revolution. This gives

two-stroke engines a significant power

boost.

56

TWO STROKE ENGINES

These advantages make two-stroke engines lighter, simpler and less

expensive to manufacture.

Two-stroke engines also have the potential to pack about twice the

power into the same space because there are twice as many power

strokes per revolution.

The combination of light weight and twice the power gives two-stroke

engines a great power-to-weight ratio compared to many four-stroke

engine designs.

57

TWO STROKE ENGINES

Two-stroke engines don't last nearly as

long as four-stroke engines. The lack of

a dedicated lubrication system means

that the parts of a two-stroke engine

wear a lot faster.

Two-stroke oil is expensive, and you

need about 4 ounces of it per gallon of

gas. You would burn about a gallon of

oil every 1,000 miles if you used a two-

stroke engine in a car.

58

TWO STROKE ENGINES

Two-stroke engines do not use fuel

efficiently, so you would get fewer miles

per gallon.

Two-stroke engines produce a lot of

pollution

so much, in fact, that it is likely that you won't

see them around too much longer.

59

FUEL

INTAKE

60

COMPRESSION

61

COMBUSTION

&

EXHAUST

62

TWO STROKE OPERATION

TWO STROKE OPERATION

63

FOUR CYCLE ENGINES

conventional Otto engines

64

FOUR CYCLE ENGINE OPERATION

65

FOUR CYCLE ENGINE

CHARACTERISTICS

FOUR STROKE ENGINES LASTS LONGER THAN TWO STROKE ENGINES. The

lack of a dedicated lubrication system means that the parts of a two-stroke engine

wear a lot faster.

FOUR STROKE ENGINES DON’T BURN OIL IN COMBUSTION CHAMBER. Two-

stroke oil is expensive, and you need about 4 ounces of it per gallon of gas. You

would burn about a gallon of oil every 1,000 miles if you used a two-stroke engine in

a car.

FOUR STROKE ENGINES ARE MORE FUEL EFFICIENT. Two-stroke engines do

not use fuel efficiently, so you would get fewer miles per gallon.

FOUR STROKE ENGINES ARE CLEANER. Two-stroke engines produce a lot of

pollution

INVERTED FLIGHTS MAY NOT BE EASY IN FOUR STROKE ENGINES. Two-

stroke engines can work in any orientation, which can be important in acrobatic

flights. A standard four-stroke engine may have problems with oil flow unless it is upright, and solving this problem can add complexity to the engine.

66

Unusual Four stroke engines

applications

ROTARY CYLINDER VALVE ENGINE RCV ENGINE

ROTARY ENGINES WANKEL ENGINE

67

ROTARY ENGINES

Wankel Engine

Rotary engines use the four-stroke combustion

cycle, which is the same cycle that four-stroke

piston engines use. But in a rotary engine, this is

accomplished in a completely different way. 68

The heart of a rotary engine is the rotor. This is roughly the

equivalent of the pistons in a piston engine. The rotor is

mounted on a large circular lobe on the output shaft. This

lobe is offset from the centerline of the shaft and acts like the

crank handle on a winch, giving the rotor the leverage it

needs to turn the output shaft. As the rotor orbits inside the

housing, it pushes the lobe around in tight circles, turning three times for every one revolution of the rotor.

69

How Rotary Engines Work

For every three

rotations of the

engine shaft

corresponds to

one complete

piston rotation

(360 degrees)

WANKEL ENGINE OPERATION

70

How Rotary Engines Work

If you watch carefully, you'll see the offset

lobe on the output shaft spinning three times

for every complete revolution of the rotor.

As the rotor moves through the

housing, the three chambers

created by the rotor change size.

This size change produces a

pumping action. Let's go through

each of the four stokes of the

engine looking at one face of the rotor.

71

Four Stroke Gas Engines

The four strokes of a internal combustion engine are:

•Intake

•Compression

•Power

•Exhaust

Each cycle requires two revolutions of the crankshaft (720˚ rotation), and

one revolution of the camshaft to complete (360˚ rotation).

Each stroke = 180˚ of crankshaft revolution.

72

Intake Stroke

First Stroke

The piston moves down the cylinder from TDC (Top Dead Center) to BDC (Bottom Dead Center).

This movement of piston causes low air pressure in the cylinder (vacuum)

Mixture of Air and Fuel in the ratio of 14.7 : 1 (air : fuel) is drawn into the cylinder.

Intake valve stays open and the Exhaust valve stays closed during this stroke.

73

This starts at the highest point known as top dead center and ends at bottom dead center

The intake stroke allows the piston to suck fuel and air into the combustion chamber through the intake valve port.

74

Compression stroke

Second stroke

The piston moves from BDC to TDC

Intake and exhaust valves stay closed

Air and fuel mixture is compressed 8:1 to 12:1

The pressure in the cylinder is raised

75

Compression starts at bottom dead center and ends at top dead center.

The second motion of the stroke takes all the fuel and air that was stored and compresses it into one tenth its original sizes. Making the air/fuel mixture increase in temperature preparing it for the next stage in its combustion cycle. 76

Power stroke Third stroke

At the end of compression stroke the sparkplug fires, igniting the air/fuel mixture.

Both the valves stay closed in this stroke.

The expanding gases from the combustion in the cylinder (with no escape) push the piston down.

The piston travels from TDC to BDC.

77

Force acting from pressure

• In engines the amount of force exerted on the top of a piston is determined by the cylinder pressure during the combustion process.

P r e s s u r e

A r e a

78

The power stroke starts as soon as the piston reaches top dead center allowing the spark plug to ignite.

This electric current created by the spark plug ignites the fuel and air mixture sending the piston back down the cylinder with a pressure reaching high as 600 PSI.

79

Exhaust stroke

Fourth and last stroke

The momentum created by the Counter-weights on the crankshaft, move the piston from BDC to TDC.

The exhaust valve opens and the burned gases escape into the exhaust system.

Intake valve remains closed.

80

The final stage of the stroke releases all the burned fuel through the exhaust valve.

As the piston moves from bottom dead center to top dead center it takes all the burned fuel and pushes it out of the cylinder, preparing it for the next cycle of strokes.

81

Indicator Diagrams and Internal Combustion Engine Performance Parameters

• Much can be learned from a record of the cylinder pressure

and volume. The results can be analyzed to reveal the rate at

which work is being done by the gas on the piston, and the rate

at which combustion is occurring. In its simplest form, the

cylinder pressure is plotted against volume to give an indicator

diagram.

82

Pressure-Volume Graph 4-stroke SI engine

One power stroke for every two crank shaft revolutions

1 atm

Spark

TC

Cylinder volume

BC

Pressure

Exhaust valve

opens

Intake valve

closes

Exhaust

valve

closes

Intake

valve

opens

83

Exhaust Valve : Valve Timing Diagram

Pcyl

Patm

84

Inlet Valve : Valve Timing Diagram

Pcyl Patm

85

Valve Timing for Better Flow

86

Efficiency

• In general, energy conversion efficiency is the ratio between the useful output of a device and the input. For thermal efficiency, the input, to the device is heat, or the heat-content of a fuel that is consumed. The desired output is mechanical work, or heat, or possibly both. Because the input heat normally has a real financial cost, a memorable, generic definition of thermal efficiency is;

87

• When expressed as a percentage, the thermal efficiency must be between 0% and 100%. Due to inefficiencies such as friction, heat loss, and other factors, thermal engines' efficiencies are typically much less than 100%. For example, a typical gasoline automobile engine operates at around 25% efficiency. The largest diesel engine in the world peaks at 51.7%.

88

• Work done on the piston due to pressure

89

• The term indicated work is used to define the net work done

on the piston per cycle

• the indicated mean effective pressure (imep),can be defined

by;

90

• The imep is a hypothetical pressure that would produce the same indicated work if it were to act on the piston throughout the expansion stroke. The concept of imep is useful because it describes the thermodynamic performance of an engine, in a way that is independent of engine size and speed and frictional losses.

• Unfortunately, not all the work done by the gas on the piston is available as shaft work because there are frictional losses in the engine. These losses can be quantified by the brake mean effective pressure (bmep,), a hypothetical pressure that acts on the piston during the expansion stroke and would lead to the same brake work output in a frictionless engine.

91

Mechanical Efficiency

Some of the power generated in the cylinder is used to overcome engine

friction and to pump gas into and out of the engine.

The term friction power, , is used to describe collectively these power

losses, such that:

gi

f

gi

fgi

gi

bm

W

W

W

WW

W

W

,,

,

,

1

fW

Friction power can be measured by motoring the engine.

The mechanical efficiency is defined as:

bgif WWW ,

92

• Mechanical efficiency depends on pumping losses (throttle position) and frictional losses (engine design and engine speed). • Typical values for automobile engines at WOT are: 90% @2000 RPM and 75% @ max speed. • Throttling increases pumping power and thus the mechanical efficiency decreases, at idle the mechanical efficiency approaches zero.

Mechanical Efficiency, cont’d

93

• Brake Specific Fuel Consumption (BSFC) is a measure of fuel

efficiency within a shaft reciprocating engine. It is the rate

of fuel consumption divided by the power produced. Specific

fuel consumption is based on the torque delivered by the

engine in respect to the fuel mass flow delivered to the engine.

Measured after all parasitic engine losses is brake specific fuel

consumption [BSFC] and measuring specific fuel consumption

based on the in-cylinder pressures (ability of the pressure to do

work) is indicated specific fuel consumption [ISFC].

94

• The final parameter to be defined is the volumetric efficiency of the engine; the ratio of actual air flow to that of a perfect engine is

• In general, it is quite easy to provide an engine with extra fuel; therefore, the power output of an engine will be limited by the amount of air that is admitted to an engine.

95

Volumetric Efficiency

• Volumetric efficiency a measure of overall effectiveness of engine and its intake and exhaust system as a natural breathing system.

• It is defined as:

• If the air density ra,0 is evaluated at inlet manifold conditions, the

volumetric efficiency is a measure of breathing performance of the

cylinder, inlet port and valve.

• If the air density ra,0 is evaluated at ambient conditions, the volumetric

efficiency is a measure of overall intake and exhaust system and other

engine features.

• The full load value of volumetric efficiency is a design feature of entire

engine system.

NV

m

da

a

v

0,

2

r

96

• Systems which are thermally insulated from their surroundings undergo processes without any heat transfer; such processes are called adiabatic. Thus during an isentropic process there are no dissipative effects and the system neither absorbs nor gives off heat.

• A reversible process, is a process that can be "reversed" by means of infinitesimal changes in some property of the system without loss or dissipation of energy.

• Isentropic process is a process which is a process is both adiabatic and reversible .

97

• A closed cylinder with a locked piston contains air. The pressure inside is equal to the outside air pressure. This cylinder is heated to a certain target temperature. Since the piston cannot move, the volume is constant, while temperature and pressure rise. When the target temperature is reached, the heating is stopped. The piston is now freed and moves outwards, expanding without exchange of heat (adiabatic expansion). Doing this work cools the air inside the cylinder to below the target temperature. To return to the target temperature (still with a free piston), the air must be heated. This extra heat amounts to about 40% more than the previous amount added. In this example, the amount of heat added with a locked piston is proportional to CV, whereas the total amount of heat added is proportional to CP. Therefore, the heat capacity ratio in this example is 1.4

98

99

Efficiencies of Real Engines

• The efficiencies of real engines are below

those predicted by the ideal air standard cycles

for several reasons. Most significantly, the

gases in internal combustion engines do not

behave perfectly with a ratio of heat capacities.

100

Ignition and Combustion in Spark Ignition and Diesel Engines

• Spark ignition (SI) engines usually have pre-mixed combustion, in which a flame front initiated by a spark propagates across the combustion chamber through the unburned mixture. Compression ignition (CI) engines normally inject their fuel toward the end of the compression stroke, and the combustion is controlled primarily by diffusion.

• Whether combustion is pre-mixed (as in SI engines) or diffusion controlled (as in CI engines) has a major influence on the range of air-fuel ratios (AFRs) that will burn.

• In pre-mixed combustion, the AFR must be close to stoichiometric-the AFR value that is chemically correct for complete combustion. In practice, dissociation and the limited time available for combustion will mean that even with the stoichiometric AFR, complete combustion will not occur.

• In diffusion combustion, much weaker AFRs can be used (i.e., an excess of air) because around each fuel droplet will be a range of flammable AFRs.

• Typical ranges for the (gravimetric) air-fuel ratio are as follows:

101

Diesel engines have a higher maximum

efficiency than spark ignition engines for three

reasons:

• The compression ratio is higher.

• During the initial part of compression, only air

is present.

• The air-fuel mixture is always weak of

stoichiometric.

102

Simple Combustion Equilibrium

• For a given combustion device, say a piston engine, how

much fuel and air should be injected in order to completely

burn both? This question can be answered by balancing the

combustion reaction equation for a particular fuel. A

stoichiometric mixture contains the exact amount of fuel

and oxidizer such that after combustion is completed, all the

fuel and oxidizer are consumed to form products.

103

• Combustion stoichiometry for a general hydrocarbon fuel, with

air can be expressed as;

• The amount of air required for combusting a stoichiometric

mixture is called stoichiometric or theoretical air.

104

Methods of Quantifying Fuel and Air Content of Combustible Mixtures

• In practice, fuels are often combusted with an amount of air

different from the stoichiometric ratio. If less air than the

stoichiometric amount is used, the mixture is described as fuel

rich. If excess air is used, the mixture is described as fuel lean.

For this reason, it is convenient to quantify the combustible

mixture using one of the following commonly used methods:

• Fuel-Air Ratio (FAR): The fuel-air ratio, f, is given by

105

• Equivalence Ratio: Normalizing the actual fuel-air ratio by the

stoichiometric fuel air ratio gives the equivalence ratio,

• The subscript s indicates a value at the stoichiometric

condition. f <1 is a lean mixture , f¼1 is a stoichiometric

mixture, and f >1 is a rich mixture

• Lambda is the ratio of the actual air-fuel ratio to the

stoichiometric air-fuel ratio defined as

106

Fuel Requirements

• Gasoline is a mixture of hydrocarbons (with 4 to

approximately 12 carbon atoms) and a boiling point range of

approximately 30-200°C. Diesel fuel is a mixture of higher

molarmass hydrocarbons (typically 12 to 22 carbon atoms),

with a boiling point range of approximately180-380°C. Fuels

for spark ignition engines should vaporize readily and be

resistant to self-ignition, as indicated by a high octane rating.

In contrast, fuels for compression ignition engines should self-ignite readily, as indicated by a high cetane number.

107

• Octane number is a standard measure of the anti-knock

properties (i.e. the performance) of a motor or aviation fuel.

The higher the octane number, the more compression the fuel

can withstand before detonating. In broad terms, fuels with a

higher octane rating are used in high-compression engines that

generally have higher performance.

• Knocking (also called knock, detonation, spark knock, pinging

or pinking) in spark-ignition internal combustion engines

occurs when combustion of the air/fuel mixture in the cylinder

starts off correctly in response to ignition by the spark plug,

Effects of engine knocking range from inconsequential to

completely destructive.

.

108

• Cetane number or CN is a measurement of the combustion

quality of diesel fuel during compression ignition. It is a

significant expression of diesel fuel quality among a number of

other measurements that determine overall diesel fuel quality.

109

• The octane or cetane rating of a fuel is established by

comparing its ignition quality with respect to reference fuels in

CFR (Co-operative Fuel Research) engines, according to

internationally agreed standards. The most common type of

octane rating worldwide is the Research Octane Number

(RON). RON is determined by running the fuel in a test engine

with a variable compression ratio under controlled conditions,

and comparing the results with those for mixtures of iso-octane and n-heptane.

110

Engine Knock and thermal Efficiency of an Engine

The thermal efficiency of the ideal Otto cycle increases with both the compression ratio and the specific heat ratio.

When high compression ratios are used, the temperature of the air-fuel mixture rises above the autoignition temperature produces an audible noise, which is called engine knock.

(antiknock, tetraethyl lead? unleaded gas)

For a given compression ratio, an ideal Otto cycle using a monatomic gas (such as argon or helium, k = 1.667) as the working fluid will have the highest thermal efficiency.

111

Charge Stratification

112

Combustion Chamber Designs

113

Combustion Chamber Design

114

Combustion Chamber Design

115

Combustion Chamber Design

116

Combustion Chamber Design

117

Combustion Chamber Design

118

• A turbocharger, or turbo, is a centrifugal compressor powered by a turbine that is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine (forced induction), thereby resulting in greater performance (for either, or both, power and efficiency). They are popularly used with internal combustion engines (e.g., four-stroke engines like Otto cycles and Diesel cycles).

Turbocharging

119

Engine Artificial Respiratory System: An Inclusion of CV

Turbo-Charged Engine 120

Turbo -Charger

121