02-ic engine operation

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IC Engine Operation Internal Combustion Engines Classification:

1. Application

2. Basic Engine Design3. Operating Cycle

4. Working Cycle

5. Valve Port Desi n and Location

ME C411 AUTOMOTIVE VEHICLES 1 Tuesday, 22 January 2013

6. Fuel

7. Mixture Preparation

8. Ignition

9. Stratification of Charge10. Combustion Chamber Design

11. Method of Load Control

12. Cooling


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1. Application1. Automotive: (i) Car (ii) Truck/Bus (iii) Off-highway

2. Locomotive

3. Light Aircraft4. Marine: (i) Outboard (ii) Inboard (iii) Stern


5. Power Generation: (i) Portable (Domestic) (ii) Fixed (Peak Power)

6. Agricultural: (i) Tractors (ii) Pump sets7. Earthmoving: (i) Dumpers (ii) Tippers (iii) Mining Equipment

8. Home Use: (i) Lawnmowers (ii) Snow blowers (iii) Tools

9. Others


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2. Basic Engine Design1. Reciprocating:

(a) Single Cylinder

(b) Multi-cylinder In-line, V, Radial, Opposed Cylinder, Opposed Piston

2. Rotary:

(a) Single Rotor

(b) Multi-rotor



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3. Operating Cycle Otto (For the Conventional SI Engine)

Atkinson (For Complete Expansion SI Engine)

Miller (For Early or Late Inlet Valve Closing Type SI Engine

Diesel (For the Ideal Diesel Engine)

Dual (For the Actual Diesel Engine)



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4. Working Cycle (Strokes)1. Four Stroke Cycle: (a) Naturally Aspirated (b) Supercharged/Turbocharged

2. Two Stroke Cycle:

(a) Scavenging: Direct/Crankcase/Crossflow ; Backflow/Loop; Uniflow

(b) Naturally Aspirated or Turbocharged



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Compression

(ports closed)

Air Taken Into

Crankcase

Combustion

(ports closed)

Exhaust

(intake port closed)

Air compressed in crankcase

Scavenging

and Intake

(ports open)


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5. (a) Valve/Port Design1. Poppet Valve

2. Rotary Valve

3. Reed Valve

4. Piston Controlled Porting


.

1. The T-head

2. The L-head

3. The F-head

4. The I-head: OHV,OHC


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6. Fuel Conventional: (a) Crude oil derivatives: Petrol, Diesel

(b) Other sources: Coal, Bio-mass, Tar Sands, Shale

Alternate: (a) Petroleum derived: CNG, LPG

(b) Bio-mass Derived: (i) Alcohols (methyl and ethyl)

(ii) Vegetable oils

(iii) Producer gas and biogas


(iv) Hydrogen Blending

Dual fueling


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7. Mixture Preparation1. Carburetion

2. Fuel Injection



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8. Ignition1. Spark Ignition: (a) Conventional: Battery or Magneto

(b) Other methods

2. Compression Ignition


CI Engine


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9. Charge Stratification1. Homogeneous Charge (Also Pre-mixed charge)

2. Stratified Charge: (i) With carburetion, (ii) With fuel injection

10. Combustion Chamber Design1. Open Chamber: (i) Disc type

(ii) Wedge


(iv) Bowl-in-piston

(v) Other design

2. Divided Chamber: (For CI): (i) Swirl chamber

(ii) Pre-chamber(For SI) (i) CVCC

(ii) Other designs


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11. Method of Load Control1. Throttling: To keep the mixture strength constant. Also known as charge

control. Used in the Carbureted S.I. Engine.

2. Fuel Control: To vary the mixture strength according to the load. Used in the

C.I. Engine

3. Combination: Used in the Fuel-injected S.I. Engine.

12. Cooling


1. Direct Air-cooling2. Indirect Air-cooling (Liquid Cooling)

3. Low Heat Rejection (Semi-adiabatic) engine.

Liquid Cooling


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An Overview of Reciprocating Engines The reciprocating engine, basically a

piston-cylinder device, has a wide range of

applications.

The piston reciprocates between two fixedpositions called TDC and BDC.

Stroke: Distance between TDC and BDC

Bore: Diameter of the iston c linder


Clearance Volume: Cylinders minimumvolume when the piston is at TDC.

Displacement / Swept Volume: Volumebetween TDC and BDC

Intake Valve and Exhaust Valve Compression Ratio (rk) is a volume ratio

and not pressure ratio.

rk = Vmax/Vmin = VBDC/VTDC


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Mean Effective Pressure (mep): It is a fictitious pressure, such that if it actedon the piston during the entire power stroke, would produce the same

amount of net work as that produced during the actual cycle.

MEP (pm) = Wnet/(Vmax Vmin)

MEP can be used as a parameter to compare theperformance of reciprocating engines of equal

size. Engine with a larger value of mep will

deliver more net work per cycle.


Reciprocating engines are classified base on thecombustion process as either spark-ignition (SI)

engine or compression-ignition (CI) engine.


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Air Standard Cycles IC engines in which the combustion of fuel occurs in the engine cylinder itself

are non-cyclic heat engines.

The working fluid, the fuel-air mixture (charge) undergoes permanent

chemical change due to combustion and the products of combustion afterdoing work are thrown out of the engine, and a fresh charge is taken in.

Therefore, the working fluid does not undergo a complete thermodynamic

c cle.


To simplify the analysis of IC engines, air standard cycles are conceived. In an air standard cycle, a certain mass of air operates in a complete

thermodynamic cycle, where heat is added and rejected with external heat

reservoirs, and all the processes in the cycle are reversible.

Air assumed to behave as an ideal gas, and its specific heats are assumed tobe constant.

Therefore, the air standard cycles correspond to the operation of IC engines.


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Otto Cycle It consists of four strokes (by the piston in the cylinder) and two revolutions

(by the crankshaft) for each thermodynamic cycle.

Stroke is a single traverse of the piston in a cylinder from TDC to BDC or vice

versa. One revolution of crankshaft is equal to two strokes of piston.



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Valve Timing Diagram



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It is seen that Wnet is directly proportional to pressure ratio, rp. For given values

ofrp and , pm increases with rp. For an Otto cycle, an increase in rk leads to an

increase in pm

, Wnet

and .


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Diesel Cycle In SI engine, a mixture of air and fuel is compressed during compression

stroke, and the compression ratios are limited by the onset ofautoignition or

knocking.

In CI engines, only air is compressed during the compression stroke.Therefore, diesel engines can operate at much higher compression ratios,

typically between 12 and 24.

Spark plug and carburetor are replaced by a fuel injector in diesel engines.


The limitation on compression ratio in the SI engine can be overcome bycompressing air alone, instead of fuel-air mixture, and then injecting fuel into

the cylinder in spray form when combustion is desired.

The temperature of air after the compression must be high enough so thatfuel burns spontaneously when it is injected in.

The burning rate can be controlled by the fuel injection rate into the cylinder.

An engine operating in this way is called a compression ignition (CI) engine.


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The efficiency may be expressed in terms of any two of the following three ratios.

It is seen that,


As rc

> 1, is also greater than unity. For the same rk

, therefore, Diesel

< Otto

.


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Limited Pressure Cycle, Mixed Cycle or Dual CycleThe air standard Diesel cycle does not simulate exactly the pressure volume

variation in an actual CI engine, where the fuel injection is started before the

end of compression stroke.

A closer approximation is the limited pressure cycle in which a part of heat isadded to air at constant volume, and the remainder at constant pressure.


The efficiency may be expressed in terms the following ratios.

It is seen that,


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Substituting the values of T1, T2, T3 and T5 in the expression for efficiency yields,



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Comparison of Otto, Diesel and Dual CycleThe three cycle can be compared on the basis of either compression ratio or

the same maximum pressure and temperature.

For the same rk and Q2, the cycleefficiency will be higher for greater Q1.

Otto > Dual > Diesel


For the same maximum pressure and

temperature and Q2, the cycleefficiency will be higher for greater Q1.

Diesel > Dual > Otto


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Atkinson Cycle Atkinson cycle is an ideal cycle for an Otto engine exhausting to a gas turbine.

In this cycle the isentropic expansion (3-4) of an Otto cycle is allowed to furtherexpand to the lowest possible cycle pressure (3-5) so as to increase work output.

For 1 kg gas: Q1 = Cv (T3 T2) and Q2 = Cp (T5 T1);



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2 Stroke EngineCylinder Still uses a flywheel

(not shown)Combustion

chamberExhaust port

Crankcase


PistonCrankshaftConnecting

RodIntake port

Reed valve

Transfer port


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TDC BDC Reed Valve

Is sucked

open


Piston moves from

BDC to TDC

Air/Fuel/Oil mixture is sucked into crankcase


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Two Stroke

Piston BDC to TDC

A/F/O mixture taken intocrankcase

Four Stroke

Piston TDC to BDC

A/F mixture taken intocylinder

Compare Intake Stroke



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TDC BDC Reed Valve

Shuts


Piston gets to

TDC

Air/Fuel/Oil mixture is now trapped in crankcase


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TDC BDC Reed Valve

sealing

Crankcase Compression (primary compression)


Piston moves backTo BDC

Air/Fuel/Oil mixture is now pressurized in crankcase.


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Crankcase Compression Is only a few pounds of pressure per square inch (psi) [very weak]

Cylinder compression in a four stroke engine was several psi [very strong]

The crankcase in a two stroke engine has to be very small so we can build

some pressure when the piston is moving to BDC.



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What is going to happen when the top of thepiston gets to here?


Crankcase

Compression


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Cylinder fills withA/F/O mixture

Piston reaches BDC



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Intake to Crankcase/Compression in Cylinder


Another A/F/O mixture is sucked into crankcase while the First one

is compressed in cylinder.


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TDC BDC

Piston is pounded


Piston gets to

TDC

Air/Fuel/Oil mixture is ignited in cylinder.


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Blow helps crankcase compression


Compression

What is going to happen when the piston gets here?


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TDC BDC

Power/Exhaust Stroke (TDC-BDC)


Still closed

Exhaust port is uncovered and exhaust starts leaving.


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Every time the piston

reaches TDC, there isanother power stroke!



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Piston

Position Activity

BDC TDC

1. Fresh A-F-O comes into the crankcase as the reed valve opens.

2. Admission of already compressed A-F-O (from crankcase) into the

cylinder nears the end.

3. Exhaust of burnt gases completes upon closure of the exhaust port.4. Secondary compression commences in the cylinder as the both ports

are closed.

5. As the piston nears the TDC, reed valve of the crankcase closes and the


secondary compression in the cylinder ends.

6. Immediately combustion takes place.

TDC BDC

1. Expansion/Power process begins and continues until piston uncovers

the exhaust port.

2. Primary compression begins in the crankcase as its volume decreases

due to inward piston movement.

3. Exhaust process begins once the exhaust port is uncovered.4. Primary compression in the crankcase completes as the piston uncovers

the inlet port.

5. Admission of crankcase compressed A-F-O into the cylinder begins as the

inlet port is uncovered.


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CI Engine Fuel System



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Diesel Engine Direct Injection



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Diesel Engine Indirect Injection



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Fuel Injection Systems Electronic distributor pump

Electronic unit injector (EUI)

High-pressure common rail



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Electronic Distributor Pump



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High-Pressure Common RailCommon Rail

Spill

Control

Valve


ECU

Injectors

High-Pressure Pump

Fuel

Return to

Tank


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SELF STUDY4 Stroke vs 2 Stroke

Comparisons & Differences

Advantages & Disadvantages Applications

SI Engine vs CI Engine


Comparisons & Differences

Advantages & Disadvantages

Applications


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Engine Performance Evaluation Commonly used terms to compare the engine performance are as follows: Engine speed

Mean piston speed

Mean effective pressure

Indicated power

Friction power

Brake power


Torque

Specific fuel consumption A/F ratio or F/A ratio

Mechanical efficiency

Brake thermal efficiency

Indicated thermal efficiency Volumetric efficiency


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Engine Speed (N): No. of revolutions made by the crank per unit time. It isnormally expressed in revolutions per minute (rpm).

Mean piston speed (MSP): IfL is the stroke and N is the rotational speed, themean piston speed is equal to 2LN.

Mean Effective Pressure (MEP): It is a fictitious pressure that, if acted on the

piston during the entire power stroke, would produce the same amount of

net work as that produced during the actual cycle.

It is the avera e ressure inside the c linder based on the calculated or


measured output.

If the MEP goes up, the cylinder volume can go down and still achieve thesame power output.


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Power in engine is produced by burning the fuel which gives heat. The heatsupplied by fuel is divided into various accounts as follows:

Heat supplied by fuel = Heat converted into work + Heat rejected to coolingwater + Heat rejected to surroundings + Heat carried away by exhaust gases.



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Indicated Power (IP): It is the power developed in the engine cylinder. This isequal to the gross area given by the indicator diagram and calculated based

on the following formula. ; where Pm - indicated mean

effective pressure, L - stroke, A - piston cross-sectional area, N - crank

rotational speed and k - number of cylinders.

Friction Power (FP): It is the power required to (i) overcome the friction invarious parts of the engine and (ii) pump the charge during admission and

burnt gases during the exhaust.

60/LANkPIP m=


ra e ower : t s t e actua power rea ze at t e eng ne s a t. t s a so

equal to the difference of cylinder work (IP) and friction power (FP).Brake power of the engine is measured in several ways but the easiest is

applying a belt round a pulley on the engine shaft. The tension in the belt can

be increased or decreased. If higher tension is T1, lower tension is T2 and D is

the diameter, N rpm, then

Torque (T): It is the net force acting through a radius.

( )

60

2

60

2

221

TNND)TT(BP

=

=


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Specific Fuel Consumption (sfc): It is the rate of fuel consumption per unitpower either BP or IP.

Mechanical Efficiency (m): It is the ratio of brake power to indicated power.

Indicated Thermal Efficiency (ith): It is the ratio of indicated power to thepower/heat release from the fuel.

Brake Thermal Efficiency (bth): It is the ratio of brake power to thepower/heat release from the fuel.


vol

inducted into the engine cylinder during the suction stroke to the mass of theair corresponding to the swept volume of the engine at atmospheric pressure

and temperature.

Volumetric efficiency of an engine is an indication of the measure of the

degree to which the engine fills its swept volume.

For supercharged engine the volumetric efficiency has no meaning as it

comes out to be more than unity.


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Engine Performance Curves



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A 4-stroke SI Engine runs at 2400 rpm. The cylinder bore is 100 mm and crankradius is 100 mm. From indicator diagram the MEP is found to be 100 kPa. If

mechanical efficiency is 80%, find B.P.

Stroke (L) = 2 crank radius = 2 100 = 200 mm = 0.2 m

Piston area (A) = (bore)2/4 = 7.854 10-3 m2

Indicated power (IP) = PmLANk/60 = 3.1416 kWBrake power (BP) = IP*mech = 2.5133 kW

If the s ecific fuel consum tion of the above en ine is 315 k BP hr and


calorific value of fuel is 46000 J/kg, find brake thermal efficiency. If the

compression ratio is 6, find relative efficiency. Use = 1.4.

Cycle efficiency (Otto) = 1 1/(rk)-1 = 51.16 %

Fuel consumption (mf) = bsfc*BP = (315/3600)*2.5133 = 0.22 kg/s

Brake thermal efficiency (bth) = BP/(mf*CV) = 2.5133/10.12 = 24.85 %

Relative efficiency (rel) = bth/Otto = 48.57 %


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A single cylinder engine operating at 2000 rpm develops a torque of 8 N-m.The indicated power of the engine is 2.0 kW. Find loss due to friction as the

percentage of brake power.

Brake power = 2NT/60 = 1.6755 kW

Friction power = 2.0 1.6755 = 0.3245 kW

% loss = (0.3245/1.6755)*100 = 19.37 %

A 6-cylinder, 4-stroke gasoline engine has a bore of 90 mm, stroke of 100 mmand com ression ratio of 8. The relative efficienc is 60 %. If the is c is 300.9


g/kWh, estimate CV of the fuel and corresponding fuel consumption. Take

that imep is 8.5 bar and speed is 2500 rpm.

Air standard efficiency (Otto) = 1 1/(rk)-1 = 56.47 %

Indicated thermal efficiency (ith) = Otto*rel = 33.88 %

CV of the fuel = 1/(isfc*ith) = 35313.2 kJ/kg

IP = PmLANk/60 = 67.59 kWFuel consumption (mf) = isfc*IP = 20.34 kg/hr


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For additional problems:


Please refer to the chapters 1 and 3 of the Reference Book 1

02-ic engine operation

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