IC Engines Units- 1 and 2 Lecture-2 Basic Components and Working Principles of IC Engines

Basic Components of Internal Combustion Engine

 Basic Engine Components: Nomenclature Cylinder Bore: The nominal diameter of cylinder (mm) Piston Area (A): The area of a cylinder of diameter equal to the cylinder bore (cm2) Note: Where cylinder rod passes through the combustion space as in a double acting engine, this area must be reduced by the area of cross-section of the piston rod. Stroke L: The nominal distance through which a working piston moves through two successive reversal of its direction of motion It is expressed in mm.

 Basic Engine Components : Nomenclature (contd.) Dead Centre: The position of the working piston top surface at the moment when the direction of the piston movement is reversed, at either end of the stroke, is called the dead centre. Top Dead centre T.D.C.: It is the dead center when the piston is at the farthest point from the crank shaft (Vertical engine) (Inner Dead Centre/ I.D.C.) for a horizontal engine Bottom Dead Centre B.D.C.): It
is the dead center when the piston is at the farthest point from the crank shaft (Vertical engine)

 Basic Engine Components : Nomenclature (contd.)

BDC (referred to as ODC or Outer Dead Centre) for Horizontal engine Displacement or Swept Volume (Vs): The nominal volume swept by the working piston when traveling from one dead centre to the other (cm3 or cc) Vs = pi/4* d2 * L Clearance Volume (Vc): The nominal volume of the combustion chamber above the piston when it is at the top dead centre. (cm3 or cc)

 Basic Engine Components: Nomenclature (contd.)

Cylinder volume (V): It is the sum of the swept volume plus the clearance volume V = Vs + Vc Compression ratio (r ): It is the ratio of cylinder volume (V) to the clearance volume r = V/ Vc = (Vc + Vs)/ Vc = 1 + Vs/ Vc

2. WORKING PRINCIPLES OF IC ENGINES

A. FOUR STROKE ENGINES FOUR STROKE SPARK IGNITION ENGINE (SI ENGINE) FOUR STROKE COMPRESSION IGNITION ENGINE (CI ENGINE) COMPARISON OF SI ENGINE AND CI ENGINE B. TWO STROKE ENGINES TWO STROKE SI ENGINES TWO STROKE CI ENGINES C. COMPARISON OF FOUR STROKE AND TWO STROKE ENGINES

1. IC ENGINES: CYCLE OF OPERATION A. CONSTANT VOLUME HEAT ADDITION CYCLE ENGINE:

OTTO CYCLE ENGINE - ALSO KNOWN AS SPARK IGNITION ENGINE/ SI ENGINE/ GASOLENE ENGINE

1. IC ENGINES: CYCLE OF OPERATION (Contd.) B. CONSTANT PRESSURE HEAT ADDITION CYCLE ENGINE: DIESEL CYCLE ENGINE DIESEL ENGINE ALSO KNOWN AS COMPRESSION IGNITION ENGINE / CI ENGINE

a. Intake Stroke c. Expansion Stroke

b. Compression Stroke d. Exhaust Stroke

Working Principle of a Four-stroke S I Engine

Four Stroke SI Engine

The four strokes of the cycle are Intake, Compression, Power and Exhaust. Each corresponds to one full stroke of the piston, therefore the complete cycle requires two revolutions of the crankshaft to complete the process.

Intake. During the intake stroke, the piston

moves downward, drawing a fresh charge of vaporized fuel/air mixture. The illustrated engine features a 'poppet' intake valve which is drawn open by the vacuum produced by the intake stroke. Some early engines worked this way, however most modern engines incorporate an extra cam/lifter arrangement as seen on the exhaust valve. The exhaust valve is held shut by a spring (not illustrated here).

As the piston rises the poppet valve is forced shut by the increased cylinder pressure. Flywheel momentum drives the piston upward, compressing the fuel/air mixture.
Compression. Power .

At the top of the compression stroke the spark plug fires, igniting the compressed fuel. As the fuel burns it expands, driving the piston downward.

Exhaust. At the bottom of the power

stroke, the exhaust valve is opened by the cam/lifter mechanism. The upward stroke of the piston drives the exhausted fuel out of the cylinder.

Working of a Four Stroke SI Engine This animation also illustrates a simple ignition system using breaker points, coil, condenser, and battery.

Suction Or Intake Stroke (0 1) Compression Stroke ( 1 2) + Burning (2 3) Expansion Or Power Stroke (3 4) Exhaust Stroke (4 5) + (5 0)

Ideal p-V Diagram of a Four-Stroke S I Engine

FOUR STROKE COMPRESSION IGNITION ENGINE (CI ENGINE)

a. Intake Stroke b. Compression Stroke c. Expansion Stroke d. Exhaust Stroke Cycle of Operation of a C I

WORKING PRINCIPLE OF FOUR STROKE CI ENGINE:

i Suction Stroke Air Alone Inducted (0 1) ii. Compression Stroke Air Compressed Into Clearance Volume (1 2) iii. Expansion Stroke Fuel Injection Maintaining Constant Pressure During Combustion + Expansion (2 3) + (3 4) iv. Exhaust Stroke Exhaust Gases Pushed Out

1. IC ENGINES: CYCLE OF OPERATION (Contd.)

Actual Indicator Diagrams of Two-Stroke and Four-Stroke SI Engines

1. IC ENGINES: CYCLE OF OPERATION (Contd.)

Actual Indicator Diagrams of Two-Stroke and Four-Stroke SI Engines

CYCLE 0-1 1-2 2-3 3-4 4-0

TWO STROKE ENGINE Suction And Scavenging Compression Heat Addition Expansion Exhaust

FOUR STROKE ENGINE Suction Compression Heat Addition Expansion Exhaust

COMPARISON OF SI AND CI ENGINES

COMPARISON OF SI AND CI ENGINES Sl.No DESCRIPTION 1 Basic cycle SI ENGINE CI ENGINE

Otto cycle (const.Vol heatDiesel cycle (const. addition) Press. Heat addition) Gasoline (petrol) Diesel oil Highly volatile Non-volatile Self ignition temp. High Self ignition temp. Comparatively low Gaseous mixture of fuel Fuel directly injected as +air introduced during droplets into Comb. suction stroke Chamber at high pressure at the end of comp. Stroke

Fuel

Introduction of fuel

Sl.No 4

DESCRIPTION Introduction of fuel (contd.)

SI ENGINE

CI ENGINE

Carburetor and Ignition Fuel pump and Injector system are required. are necessary Modern engines have gasoline injection Throttle controls the air-fuel mixture introduced. Quantity of fuel is regulated. Air quantity is not regulated.

Load Control

Ignition

Requires an ignition Self ignition occurs due system with spark plug to high temperature of in the combustion air because of chamber. Primary compression. Ignition voltage provided by system and spark plug battery or a magneto are not required.

Sl. no. 7

DESCRIPTION

SI ENGINE

CI ENGINE 16 to 20. Upper limit is set by the weight increase of the engine.

Compression ratio6 to 10. Upper limit is fixed by anti-knock quality of the fuel. Speed

Due to light weight and Due to heavy weight and homogeneous due to heterogeneous combustion, they are combustion, they are low high speed engines speed engines. Because of the lower compression ratio, the max. value of thermal efficiency that can be obtained is lower. Lighter construction due to lower peak pressures. Because of higher compr. ratio, the max value of thermal efficiency that can be obtained is higher Heavier due to higher peak pressures

Thermal efficiency

10

Weight

TWO STROKE ENGINE

Douglas Clarke Invented The Two Stroke Engine In 1878 Power Stroke In Each Revolution Of The Crank Shaft The Suction And Exhaust Stroke Achieved By Alternate Arrangement Theoretically, Power Output Of The Engine Can Be Doubled For The Same Speed As Compared To A Four Stroke Engine Cycle Is Completed In One Revolution Of The Crank Shaft

METHOD OF FILLING FRESH CHARGE AND REMOVING BURNT GASES FROM THE ENGINE The piston acts in a four stroke engine during suction and exhaust strokes respectively. In a two stroke engine, the filling is accomplished by the charge compressed in the crank case or by a blower. Simultaneously, the products of combustion are moved out through the exhaust ports. No separate piston strokes are required. Two strokes are sufficient for the cycle. One for compressing the fresh charge Two for expansion or power stroke.

CRANK CASE SCAVENGED TWO STROKE ENGINE.

a. Compression/ Ignition

b. Expansion and

c. Exhaust

Working of a Two-stroke Gasoline Engine

Four Events of 2-S Engine

Intake. The fuel/air mixture is first drawn into the crankcase by the vacuum created during the upward stroke of the piston. The illustrated engine features a poppet intake valve, however many engines use a rotary value incorporated into the crankshaft. During the downward stroke the poppet valve is forced closed by the increased crankcase pressure. The fuel mixture is then compressed in the crankcase during the remainder of the stroke.

Transfer/Exhaust. Towards the end of the stroke, the piston exposes the intake port, allowing the compressed fuel/air mixture in the crankcase to escape around the piston into the main cylinder. This expels the exhaust gasses out the exhaust port, usually located on the opposite side of the cylinder. Unfortunately, some of the fresh fuel mixture is usually expelled as well. Compression. The piston then rises, driven by flywheel momentum, and compresses the fuel mixture. (At the same time, another intake stroke is happening beneath the piston).

Two Stroke Engine (contd.)

Power. At the top of the stroke the spark plug ignites the fuel mixture. The burning fuel expands, driving the piston downward, to complete the cycle.

Two Stroke Engine (contd.) The two stroke engine employs the crankcase as well as the cylinder to achieve all the elements of the Otto cycle in only two strokes of the piston. Intake. The fuel/air mixture

IDEAL INDICATOR DIAGRAM OF A TWO STROKE SI ENGINE

TWO STROKE ENGINE - CYCLE OF OPERATIONS

Air charge simultaneously inducted into the crank case through spring loaded inlet valve, as the pressure in the crank case drops due to the upward motion of the piston during the compression stroke. After the compression and ignition, the expansion follows in the usual way. During the expansion stroke, the charge in the crank case is compressed. Near the end of the expansion stroke, the piston uncovers the exhaust ports and the cylinder pressure drops to atmospheric pressure, as the combustion products leave the cylinder.

TWO STROKE ENGINE - CYCLE OF OPERATIONS (contd.) Further movement of the piston uncovers the transfer ports, thus permitting the slightly compressed charge in the crank case to enter the engine cylinder. The top of the piston has usually a projection to deflect the fresh charge towards the top of the cylinder before flowing through the exhaust ports. This has the dual purpose of A. Scavenging the upper part of the cylinder of the combustion products and B. Preventing the fresh charge from flowing directly towards the exhaust ports.

TWO STROKE ENGINE - CYCLE OF OPERATIONS (contd.)

Same objective can be achieved without piston deflector through proper shaping of the transfer port. During the upward motion of the piston from BDC, the transfer ports close first and then the exhaust ports close when the compression of the charge begins and the cycle is repeated.

COMPARISON OF 2 AND 4 STROKE ENGINES Features Of Two Stroke Engine . Developed to get a greater power output for a given engine size Eliminates valves (only ports; some have an exhaust valve) hence mechanically simpler construction Cheaper to produce Easier maintenance Theoretically should develop twice the power

Features Of Two Stroke Engine (contd.) Actual power output is higher by only about 30% than a comparable to four stroke engine - due to Reduced effective expansion stroke Increased heating because of increased number of power strokes This limits the maximum operating speed It gives more uniform torque on crank shaft and smaller fly wheel is sufficient. Less exhaust gas dilution With these inherent disadvantages, Two stroke SI engines are presently suitable for only smaller engines.

Features of Two Stroke Engine (contd.) INCOMING CHARGE CONSISTS OF FUEL AND AIR
DURING SCAVENGING, BOTH INLET AND EXHAUST PORTS ARE OPEN SIMULTANEOUSLY FOR SOME TIME. THERE IS A POSSIBILITY FOR SOME FRESH CHARGE CONTAINING FUEL MAY ESCAPE WITH THE EXHAUST. THIS RESULTS IN A HIGHER FUEL CONSUMPTION AND A LOWER THERMAL EFFICIENCY AT PART THROTTLE OPERATION, THE AMOUNT OF FRESH MIXTURE ENTERING IS NOT ENOUGH TO CLEAR ALL THE EXHAUST GASES. A PART OF IT REMAINS IN THE CYLINDER TO CONTAMINATE THE FRESH CHARGE RESULTS IN IRREGULAR OPERATION

Two Stroke Diesel Engine More Advantageous Than Two Stroke SI Engine!
No Loss Of Fuel With Exhaust Gases As The Intake Charge Is Only Air. Hence Many Of The High Output Diesel Engines Work On This Cycle. A General Disadvantage Common To Both Two Stroke Gasoline And Diesel Engines Is Greater Cooling And Lubricating Oil Requirements Due To One Power Stroke Per Crank Shaft Rotation And Higher Temperatures. Results In Higher Consumption Of Lubricating Oil.

Comparison Of Four And Two Stroke Cycle Engines Sl.No. 1 Four-stroke Engine Thermodynamic Cycle Is Completed In Four Strokes One Power Stroke In 2 Crank Shaft Revolutions Two Stroke Engine Thermodynamic Cycle Is Completed In Two Strokes One Power Stroke In Each Crank Shaft Revolution

Hence Turning Moment Is Not Hence Turning Moment Is More So Uniform- Needs A Larger Uniform- Needs A Lighter Fly Wheel Fly Wheel Power Produced For The Same Size Engine Is Less, Or For Same Power, Engine Is Heavier And Bulkier. Power Produced For The Same Size Engine Is Twice, Or For The Same Power, The Engine Is Lighter And More Compact.

COMPARISON OF FOUR AND TWO STROKE CYCLE ENGINES (CONTD.) Sl.No. 4 Four-stroke Engine Lesser Cooling And Lubrication Requirements. Lower Rate Of Wear And Tear Because Of One Power Stroke/ Two C.S Revolutions Require Valves And Valve Actuating Mechanisms For Opening And Closing Of The Intake And Exhaust Valves Two Stroke Engine Greater Cooling And Lubrication Requirements. Higher Rate Of Wear And Tear Because Of One Power Stroke/ C.S Revolution

Two Stroke Engines Have No Valves But Only Ports (Some Engines Have Exhaust Valve Or Reed Valve).

Initial Cost Of The Engine Is Initial Cost Of The Engine Is Less Higher

Comparison Of Four And Two Stroke Cycle Engines (Contd.) Sl. No. 7 Four-stroke Engine Two Stroke Engine

Volumetric Efficiency Is MoreVolumetric Efficiency Is Less Due Due To More Time ForTo Less Time For Induction Of Induction Of Charge Charge Thermal Efficiency Is Higher.Thermal Efficiency Is Lower. Part-load Efficiency Is Better. Part-load Efficiency Is Poor. Used Where Efficiency Is Used Where Low Cost, Important, viz., In Cars, Compactness And Light Weight Are Buses, Trucks, Tractors, Important, Viz., In Mopeds, Industrial Engines, Scooters, Motor Cycles, Hand Aeroplanes, Power Generation Sprayers Etc. Etc.

ENGINE PERFORMANCE PARAMETERS

Sl. No. i. ii iii iv v vi vii viii ix x xi xii

Parameter Indicated Thermal Efficiency Brake Thermal Efficiency Mechanical Efficiency Volumetric Efficiency Relative Efficiency/ Efficiency Ratio Mean Effective Pressure Mean Piston Speed Specific Power Output Specific Fuel Consumption Inlet Valve Mach Index Fuel-Air or Air-Fuel Ratio Calorific value of the Fuel

Notation

ith bth m v rel

pm
sp
Ps

sfc Z
F/A or A/F CV (HCV/ LCV)

Valve Timing I.V.O I.V.C E.V.O E.V.C

Low Speed 100 Before. TDC 100 After. BDC 250 Before. BDC 50 After. TDC

High Speed 100 Before. TDC 600 After. BDC 550 Before. BDC 200 After TDC

Port Timing Diagram of a Two Stroke Engine