Internal Combustion Engines

Internal Combustion Engines General Course Layout

SUBJECT: ME-448 INTERNAL COMBUSTION ENGINES

CREDIT HOURS: 3-0

CONTACT HOURS: 3 Hours per Week

TEXT BOOK:

Internal Combustion Engines: Applied Thermo sciences, Colin R.Ferguson, Allan T. Kirkpatrick, 2nd Edition, Wiley

REFERENCE BOOKS:

Edward F. Obert, Internal Combustion Engines and Air Pollution,Harper & Row NewYark.

Internal Combustion Engines by V.Ganesan

Internal Combustion Engine by Willard W. Pulkrabek

Internal Combustion Engine.(I. C. Engine)

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What is a Heat engine?

Just a device which converts one form of energy into another.

Normally heat energy is converted into mechanical energy. Efficiency of conversion is very important in an engine.

There are two broad categories;

Heat Engines

Internal Combustion

Engines (ICE)

External Combustion

Engines (ECE)

Heat Engine

Heat Engine Most commonly used are ICE, Gas Turbines and Steam

Turbines (now used rarely).

Advantages of ICE over others;

1. Absence of heat exchangers like boilers and condensers meansmore mechanical simplicity and higher efficiency.

2. Components work at an average temperature which is muchbelow the max temp of the working fluid. (Why? And benefit?)

3. Smaller units can be made and give reasonable thermalefficiency and cost.

4. The power to weight ratio of ICE is also high.

Major Disadvantages;

1. The disadvantages are that due to reciprocating parts, there isa lot of vibration.

2. Only liquid fuels and gases of given specifications can be used.

Classification of I. C. Engines

Nature of Thermodynamic Cycle 1. Otto cycle engine 2. Diesel cycle engine3. Dual combustion cycle engine

Type of the Fuel- used 1. Petrol engine2. Diesel engine 3. Gas engine 4. Bi-fuel Engine

Classification of I. C. Engines

Number of Strokes1. 2 - stroke engine2. 4 - stroke engine

Method of Ignition 1. Spark ignition engine, [S.I. Engine]2. Compression ignition engine, [C.I. engine]

Number of Cylinders 1. Single cylinder engine2. Multi-cylinder engine

Classification of I. C. EnginesPosition of the Cylinder

1. Horizontal engine 2. Vertical engine3. V- engine4. Radial engine

Method of Cooling1. Air cooled engine2. Water cooled engine

Speed of the Engine1. Low speed engine2. Medium speed engine3. High speed engine

Assignment

PARTS OF I.C. ENGINES

I C Engine Parts

NomenclatureDead Centre:The position of the working piston and the mechanical partswhich are connected to it, at the moment when the directionof piston motion is reversed at either end of the stroke iscalled the dead centre.

Top Dead Centre (TDC):It is the dead centre when the piston is farthest from thecrank shaft. Known as Inner Dead Centre (IDC) for horizontalengines.

Bottom Dead Centre (BDC):It is the dead centre when the piston is closest to the crankshaft. Known as Outer Dead Centre (ODC) for horizontalengines.

Nomenclature

Stroke:It is the linear distancetraveled by the pistonwhen it moves from oneend of the cylinder to theother end.

Bore:The nominal innerdiameter of the workingcylinder. It is usuallydenoted by d and isexpressed in mm.

Nomenclature

Stroke to Bore ratio:An important parameter inclassifying the size of theengine.Stroke to Bore Ratio: L/dd L Under Squared =L Squared L Over Square

Nomenclature

Swept volume or (Displacement volume)

It is the volume swept through by the piston in moving between TDC and BDC. (Vs)

Vs = A x L = (/4)d2 L

Clearance volume:It is the volume contained in the cylinder above the top of the piston, when the piston is at TDC. Denoted by Vc and expressed in cubic centimeters.

Total volume (VT)swept volume + clearance volume = Vs + Vc

Compression ratio: rIt is the ratio of total cylinder volume to clearance volume.

r = Total volumeclearance volume

r =

= 1+

Value of r for,

petrol engine lies between 6 to 10Diesel engine lies between 16 to 20

Cubic CapacityVS x K ( K= No of Cylinders)

Working of 4-Stroke Petrol/SI Engine

The petrol engines work on the principle of OTTO

CYCLE, also known as constant Volume cycle.

The engines operating on this cycle use either

petrol or other spirit fuels or the gases such as LPG

/ CNG as their fuels.

Spark Ignition engines are also called petrol

engines.

In a 4-Stroke petrol engine, the charge is admittedto the engine cylinder is a homogeneous mixture

of petrol and air.

Depending on the load on the engine, the fuel andair is mixed in proper proportions and sent in to

the cylinder by a popular device known as

carburetor.


In a 4-stroke petrol engine there are four main events

taking place

1. Suction

2. Compression

3. Power or expansion

4. Exhaust

Each event is performed during a single stroke of the

piston


INTAKE [Suction]: During the intake stroke, the piston movesdown ward, drawing a fresh charge of vaporized fuel-airmixture, This operation is represented by the line AB on theP-V diagram.

Volume [V]

Pressure [P]

AB

TDC

BDC

2. Compression Stroke: During compression stroke, the piston movesfrom BDC to TDC, thus compressing air petrol mixture. Due tocompression, the pressure and temperature are increased and isshown by the line BC on the P- V diagram. Just before the end of thisstroke the spark - plug initiates a spark which ignites the mixture andcombustion takes place at constant volume as shown by the line CD

Volume [V]

Pressure [P]

AB

TDC

BDC

C

D

3. Working Stroke: The expansion of gases due to the heatof combustion exerts a pressure on the piston. Under thisimpulse, the piston moves from TDC to BDC and thus thework is obtained in this stroke as shown by the line DE

Volume [V]

Pressure [P]

AB

TDC

BDC

C

D

E

4. Exhaust Stroke: At the end of the power stroke, the exhaustvalve is opened & greater part of the burnt gases escapesbecause of their own expansion. The drop in pressure at constantvolume is represented by the line EB. During this stroke thepiston moves from BDC to TDC and pushes the remaining gasesto the atmosphere. This stroke is represented the line BA on theP-V diagram.

Volume [V]

Pressure [P]

AB

TDC

BDC

C

D

E

P V diagram for SI Engine / Otto cycle engine

TDC

BDC

C

A B

E

D

Pressure

Volume

Theoretical Otto cycle

Diesel Engines

Diesels come in two stroke and four stroke versions

and operate much like the gasoline driven engines.

Diesels have a greater compression ratio than

gasoline engines.

Diesel 16:1 20:1

Gasoline 6:1 10:1

Working of 4-S Diesel/CI Engine

The basic construction of a four stroke diesel engine is

same as that of four stroke petrol engine.

Except that instead of a spark plug, a FUEL INJECTOR is

mounted in its space.

Fuel injector injects the fuel in to the cylinder as a fine

spray at very high pressure

In case of diesel engine, the air enters the inside the cylinder

during suction, and it will get compressed during the

compression stroke. (i.e.. charge is only air)

At the end of the compression stroke the diesel is injected in

to the cylinder in the form of fine spray

When this fine spray diesel comes in contact with hot air in

the cylinder, it auto ignites and results in a combustion of

injected diesel fuel.

Since ignition in these engines is due to the temperature of

the compressed air, they are also called compression ignition

engines.

1. INTAKE [Suction]:

During the intake stroke, the piston moves down ward,drawing a fresh charge [AIR]. This operation isrepresented by the line AB on the P-V diagram.

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TDC

BDC

Volume [V]

Pressure [P]

AB

2. Compression Stroke: During compression stroke, thepiston moves from BDC to TDC, thus compressing air.Due to compression, the pressure and temperature areincreased and is shown by the line BC on the P- Vdiagram. Just before the end of this stroke, a meteredquantity of Diesel is injected into the hot compressedair in the form of fine sprays by means of fuel injector.The fuel starts burning at constant pressure shown bythe line CD.TDC

BDC

Volume [V]

Pressure [P]

AB

CD

3. Working Stroke: The expansion of gases due to theheat of combustion exerts a pressure on the piston.Under this impulse, the piston moves from TDC to BDCand thus the work is obtained in this stroke as shown bythe line DE

TDC

BDC

Volume [V]

Pressure [P]

AB

CD

E

4. Exhaust Stroke: At the end of the power stroke, theexhaust valve is opened & greater part of the burntgases escapes because of their own expansion. Thedrop in pressure at constant volume is represented bythe line EB. During this stroke the piston moves fromBDC to TDC and pushes the remaining gases to theatmosphere. This stroke is represented the line BA onthe P-V diagram.

TDC

BDC

Volume [V]

Pressure [P]

AB

CD

E

CA B

E

D

Pressure

Volume

Theoretical Diesel cycle

P V diagram for C.I. Engine / Diesel Cycle Engine

D

TDC

BDC

Working of the Two Stroke Engine

In a two stroke engine, a cycle is completed by the two

strokes of the piston.

Out of the four strokes, the two strokes that are

eliminated are, suction and exhaust strokes.

However, the exhaust process is achieved by the

admission of charge which is extremely compressed,

which drives out the burnt gases out and this process is

popularly called as SCAVENGING.

In case of the two stroke engines instead of valves,ports are used.

Ports in the cylinder liner, opened and closed by thepiston motion itself

Working of the Two Stroke Engine

Working of Two Stroke Petrol Engine

First stroke Second stroke

Working of Two Stroke Petrol Engine

First stroke (Downward) As soonas the charge is ignited, the hotgases force the piston to movedownwards, rotating thecrankshaft, thus doing the usefulwork. During this stroke the inletport is covered by the piston andthe new charge is compressed inthe crank case as shown in the fig.

As soon as the transfer port opens, thecompressed charge from the crankcaseflows into the cylinder.

As the compressed charge enters intothe cylinder, it pushes out the exhaustgases from the cylinder.

The process of removal of exhaustgases by the fresh incoming charge isknown as scavenging.

Further downward movement of the piston uncovers firstthe exhaust port and then the transfer port.

The burnt gases escape through the exhaust port.

Second stroke: (upward)

Here the piston moves from BDCto TDC, during the process theexhaust port and transfer port arecovered and the charge in thecylinder is compressed.

Simultaneously, vacuum is createdin the crankcase, and a new chargeis drawn into the crankcasethrough the uncovered inlet port.

The compressed charge isignited in the combustionchamber by a sparkprovided by the spark plugand the cycle of events isthen repeated.

Working of Two Stroke Diesel Engine

First stroke Second stroke

First stroke (Downward)Combustion starts once the dieselis injected in to the hotcompressed air, the hot gases forcethe piston to move downwards,rotating the crankshaft, thus doingthe useful work. During this strokethe inlet port is covered by thepiston and the new charge [air] iscompressed in the crank case asshown in the fig.

As soon as the transfer port opens, thecompressed charge from the crankcaseflows into the cylinder.

As the compressed charge enters into thecylinder, it pushes out the exhaust gasesfrom the cylinder.

The process of removal of exhaust gasesby the fresh incoming air is known asscavenging.

Further downward movement of the piston uncoversfirst the exhaust port and then the transfer port.

The burnt gases escape through the exhaust port.

Second stroke: (upward)

Here the piston moves fromBDC to TDC, during the processthe exhaust port and transferport are covered and the freshair in the cylinder is compressed.Simultaneously, vacuum iscreated in the crankcase, and anew charge [air] is drawn intothe crankcase through theuncovered inlet port.

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At the end of the compression diesel is injected to thecompressed air which is at a temperature higher thanthe self ignition temperature of diesel. Hence, theinjected diesel auto ignites when it comes in contactwith hot air. And the cycle of events is then repeated.

Disadvantages of Two Stroke Engine as Compared to Four Stroke Engine

Reduced Effective Expansion Stroke

For SI Engines only:

During Scavenging both the inlet and exhaust valvesare open simultaneously there is a lot of fuelwastage this use limited to only smaller units.

Lower thermal efficiency for the same reason.

At part throttling conditions, the amount of chargeentering might not be enough to remove all theexhaust gases.

More power strokes more heat production greater cooling and lubricating required.

More noise

More pollution

Performance Parameters

PERFORMANCE PARAMETRES

Before we can move on with the parametersparameters, we have to understand the energy flow(energy balance) of the system.

Efficiencies

Indicated thermal efficiency () =

=

Brake thermal efficiency () =

=

Mechanical Efficiency () =

=

=

+

Relative Efficiency or Efficiency Ratio

Relative efficiency is the ratio thermal efficiency ofactual cycle to ideal cycle

rel = Actual Thermal Efficiency

Air Standard Efficiency


Volumetric efficiency:

Volumetric efficiency is a ratio or percentage of whatquantity of air-fuel mixture enters actually thecylinders during suction process to the actualcapacity of the cylinder under static conditions.

Your engine will not use the full volume 100%because of friction losses and leakage.

Volumetric efficiency above 100% can be reached byusing forced supercharging or turbocharging.

Remember more air means that more fuel can beburnt and more power can be derived from the samestroke.


Volumetric efficiency:

Remember that irrespective of whether you engine is CI, SI or even gasengine for that matter, the volumetric rate of flow of air is to beconsidered only.

Mean Effective Pressure

=

ip = Indicated power [kW]pim = Indicated mean effective pressure [N/m

2 ]L = Length of the stroke [m]A = Area of the piston [m2]N = Speed in revolutions per minute [rpm]n = Number of strokes N/2 (4S) & N (2S)K = Number of cylinder

=

PERFORMANCE PARAMETRESIndicated mean effective pressure is

= ( 6000)/

Break mean effective pressure is = ( 6000)/

ip = Indicated power [kW]pim = Indicated mean effective pressure [N/m

2 ]L = Length of the stroke [m]A = Area of the piston [m2]N = Speed in revolutions per minute [rpm]n = Number of stroke N/2 & NK = Number of cylinder

Mean Piston Speed

Sp = 2LN

L = Length of the stroke [m]N = Crank Speed in revolutions per minute [rpm]

An important parameter for correlating enginebehavior as a function of speed.Resistance to air inflow and inertia due to movingparts restrict the value of this parameter to within5-20m/s.Higher end values for automobiles and lower endvalues for large diesel units (marine, industry).

Specific Power Output

Ps = bp/A

= constant x pbm x Sp

Power output per unit piston area.

Designer has to use available piston area regardless of the engine size.

Specific power is proportional to the product of mean effective

pressure and mean piston speed.

Either increase the speed or the bmep.

Specific Fuel Consumption

sfc = Fuel Consumption per unit Time

Power

Kilograms of fuel per kilowatt-hour.

Engine Performance Parameter.

Inversely proportional to the thermal efficiency of theengine.

Calorific Value ( CV)

Calorific value of a is the thermal energy released perunit quantity of the fuel when the fuel is burningcompletely and the products of combustion arecooled back to the initial temperature of thecombustible mixture.

When water vapors resulting from the combustionprocess are condensed, the heating value so obtainedis called the higher or gross calorific value.

Lower or net calorific value.

Air/Fuel or Fuel/Air Ratio

A mixture which contains just enough air for complete

combustion of all the fuel in the mixture is chemically

correct or stoichiometric fuel-air ratio.

A mixture having more fuel than the chemically correct

mixture is termed as rich mixture.

Similarly, a mixture that contain less fuel or more air is

called a lean mixture.


=( )

(h )

Where

= 1 means stoichiometric ( chemically correct) mixture

1 means lean mixture

1 means rich mixture


These ratios are critical for combustion and efficiency of

engine standpoints.

In a SI engine, the fuel air ratio remains practically constant

over a wide range of operations (15:1).

In a CI engine, at a given speed, the air flow does not vary

with the load; rather, the fuel flow varies directly with load.

Therefore the term fuel-air ratio is considered rather than

the air fuel ratio.

Combustion Efficiency

Combustion efficiency is defined to account for the fractionof fuel that burns in a combustion process.

A small fraction of fuel does not burn and is wasted with theexhaust.

Typical values range from 0.95-0.98 when an engine isoperating properly.

For one engine cycle in one cylinder the heat added is;

= For Steady State

=

= mass flow rate of fuel for one cycle

= heating value of fuel

Combustion Efficiency

The thermal efficiency will be

= W/ = 0/

0

= 0/

0

Inlet Valve Mach Index (Z)

In a reciprocating engine the flow of intake charge takesplace through the intake valve operation which is varyingduring the induction process.

Also the maximum gas velocity through the area is limited bythe local sonic velocity.

Thus gas velocity is finally chosen by the given equation.

u = gas velocity through the inlet valve

= Piston Area

= Intake valve opening area

Ci = Inlet Valve Flow Coefficient

Vp = Mean Piston Speed


=

= (

)

=

Where

b = cylinder diameter

Di = inlet valve diameter

Vp = mean piston speed

= inlet sonic velocity

C = inlet valve average flow coefficient

Z = inlet valve Mach index


There is a relationship between volumetricefficiency and Z.

Maximum Volumetric efficiency can be achieved atInlet Mach No of 0.55 (Experimentally).

This adds up as an important design criteria.

Documents

Internal Combustion Engines