In this article we will discuss about:- 1. Introduction to Internal Combustion (IC) Engine 2. Thermodynamic Cycle Used for Internal Combustion Engine 3. Principles and Working 4. Valve Working and Valve Timing Diagram.
Introduction to Internal Combustion (IC) Engine:
Heat engine is a machine for converting heat, developed by burning fuel into useful work. It can be said that heat engine is an equipment which generates thermal energy and transforms it into mechanical energy.
Heat engine is of two types:
1. External combustion engine
2. Internal combustion engine
1. External Combustion Engine:
Here the combustion uses heat in form of steam, which is generated in a boiler, placed entirely separate from the working cylinder. In Internal combustion engine, the combustion of fuel takes place inside the engine cylinder and heat is generated within the cylinder of the engine.
2. Internal Combustion Engine:
It is the engine designed to derive its power from the fuel, burnt within the engine cylinder. It uses the expansive force of gases produced by burning the fuel within the cylinder. The generated heat is converted into useful power by a piston, constrained within the cylinder. The motion of the piston rotates a crankshaft with the help of a connecting rod. The heat that supplies the energy for working substance is generated within the cylinder. Hence the name is given as internal combustion engine.
There are two ways in which combustion takes place in the cylinder:
(a) By rapid explosion of air-fuel mixture within the cylinder, when it is ignited by a spark, is called constant volume combustion (C.V.C.).
(b) Combustion takes place by slow burning when the fuel is injected into highly compressed heated air contained in the cylinder. This is called constant pressure combustion (C.P.C.), because when the combustion takes place, the pressure in the cylinder is almost constant.
Thermodynamic Cycle Used for Internal Combustion Engine:
It is a series of events that repeat themselves in a regular sequence. The cycle consists of events taking place between two successive explosions in a cylinder of the engine.
There are several types of cycles but thermo-dynamic cycle, used for internal combustion engine is of two types:
1. Otto cycle
2. Diesel cycle
1. Otto Cycle:
In this cycle, the heat is taken in at one constant volume and rejected at another constant volume of the cylinder in the pressure volume diagram (Fig. 3.1) of the Otto cycle.
V1 = total cylinder volume.
V2 = clearance volume.
V1 – V2 = piston displacement.
The line MN represents atmospheric pressure level and AB represents the admission of the charge at a pressure slightly below the atmospheric pressure. BGC represents the compression of the charge in the cylinder, where the ignition occurs at the point C. The line CD represents the pressure rise in the cylinder, which occurs at constant volume V2. DE represents power stroke of the engine. Exhaust takes place at the point E and the pressure reduces nearly to atmospheric pressure during the exhaust stroke FA.
Analyzing the heat and the energy evolved in the cylinder, the thermal efficiency () of the engine is given by:
Where,
m = constant = Cp/Cv = 1.4
Cp = specific heat at constant pressure
Cv = specific heat at constant volume
Engines based upon this principle of otto cycle are called Otto engines or spark ignition engines.
2. Diesel Cycle:
In diesel cycle the heat is taken in at constant pressure and rejected at constant volume. In the pressure-volume diagram (Fig. 3.2) of diesel cycle, the line MN represents atmospheric pressure, AB represents the admission of the air in the cylinder, and BGC represents the compression of gases in the cylinder.
The injection of the fuel begins at C and stops at D. Combustion takes place during this interval and a condition of constant pressure is assumed. The ratio of the volume of D and C i.e. VD/VC is known as Cut off ratio. The expansion of gas occurs from D to E with exhaust valve opening at E and exhaust stroke FA.
Analysing the heat and energy evolved in the cylinder, the thermal efficiency of the engine is given as:
Where, ρ is the cut off ratio.
The engine based upon the principle of diesel cycle is called diesel engine.
Principles and Working of I.C. Engine:
Principle:
A mixture of fuel with correct amount of air is exploded in an engine cylinder which is closed at one end. As a result of the explosion, heat is released and this causes the pressure of the burning gases to increase. This pressure increase, forces a close fitting piston to move down the cylinder.
This movement of piston is transmitted to a crankshaft by a connecting rod so that the crankshaft turns a flywheel. To obtain continuous rotation of the crankshaft this explosion has to be repeated. Before this can happen, the used gases have to be expelled from the cylinder, the fresh charges of fuel and air must be admitted and the piston must be moved back to its starting position. This sequence of events is known as working cycle.
Working:
I.C. engine converts the reciprocating motion of piston into rotary motion of the crankshaft by means of connecting rod. The piston which reciprocates in the cylinder is very close fit in the cylinder. Rings are inserted in the circumferential grooves of the piston to prevent leakage of gases from sides of the piston. Usually a cylinder is bored in a cylinder block and a gasket, made of copper sheet or asbestos is inserted between the cylinder and the cylinder head.
The combustion space is provided at the top of the cylinder head where combustion takes place. There is a rod called connecting rod for connecting the piston and the crankshaft. A pin called gudgeon pin or wrist pin is provided for connecting the piston and the connecting rod of the engine. The end of the connecting rod which fits over the gudgeon pin is called small end of the connecting rod.
The other end which fits over the crank pin is called big end of the connecting rod. The crankshaft rotates in main bearings which are fitted in the crankcase. A flywheel is provided at one end of the crankshaft for smoothening the uneven torque, produced by the engine. There is an oil sump at the bottom of the engine which contains lubricating oil for lubricating different parts of the engine.
Mechanical cycle of internal combustion engine can be completed in two ways:
1. When the cycle is completed in two revolutions of the crankshaft, it is called four stroke cycle engines.
2. When the cycle is completed in one revolution of the crankshaft, it is called two stroke cycle engines.
Four Stroke Cycle Engine:
In four stroke cycle engine, all the events taking place inside the engine cylinder are completed in four strokes of the piston. This engine has got valves for controlling the inlet of charge and outlet of exhaust gases. The opening and closing of the valve is controlled by cams, fitted on camshaft. The camshaft is driven by crankshaft with the help of suitable gears or chains. The camshaft runs at half the speed of the crankshaft.
The events taking place in I.C. engine are as follows:
1. Air or air-fuel mixture (charge) is taken in the cylinder.
2. The charge is compressed in the cylinder by the piston.
3. If charge is only air, the fuel is injected at the end of compression.
4. The charge is ignited at a predetermined time under specified pressure inside the engine cylinder.
5. The power developed due to expansive forces of gases inside the cylinder is transferred to the crank-shaft through the connecting rod.
6. Exhaust gases go out of the cylinder at regular interval of time.
The complete cycle covers all these events in systematic manner. Four stroke cycle engine completes all these events in four strokes of the piston, whereas the two stroke cycle engine covers all these events in two strokes of the piston.
The four strokes of the piston are:
1. Suction stroke
2. Compression stroke
3. Power stroke
4. Exhaust stroke
1. Suction Stroke:
During suction stroke, only air or mixture of air and fuel are drawn inside the cylinder. The charge enters the engine through the inlet valve which remains open during admission of the charge. The exhaust valve remains closed during this stroke. The pressure in the engine cylinder is less than atmospheric pressure during this stroke.
2. Compression Stroke:
The charge taken in the cylinder is compressed by the piston during this stroke. The entire charge of the cylinder is compressed to a small volume contained in the clearance volume of the cylinder. If only air is compressed in the cylinder (as in case of diesel engine), the fuel is injected at the end of the compression stroke. The ignition takes place due to high pressure and temperature.
If the mixture of air and fuel is compressed in the cylinder (as in case of spark ignition engine) the mixture is ignited by spark plug. After ignition, tremendous amount of heat is generated, causing very high pressure in the cylinder which pushes the piston backward for useful work. Both valves are closed during this stroke.
3. Power Stroke:
During power stroke, the high pressure developed due to combustion of fuel causes the piston to be forced forward or backward at regular intervals. The connecting rod with the help of crank shaft transmits the power to the transmission system for useful work. Both valves are closed during this stroke.
4. Exhaust Stroke:
Exhaust gases go out through exhaust valves during this stroke. All the burnt gases go out of the engine and the cylinder becomes ready to receive the fresh charge. The inlet valve is closed and exhaust valve remains open during this stroke.
Thus it is found that out of four strokes, there is only one power stroke and three idle strokes. The power stroke supplies necessary momentum for useful work.
Two Stroke Cycle Engine:
In such engines, the whole sequence of events i.e. suction, compression, power and exhaust are completed in two strokes of the piston and one complete revolution of the crankshaft. There is no valve in this type of engine. Gas movement takes place through holes called ports in the cylinder. The crankcase of the engine is gas tight in which the crankshaft rotates.
First Stroke (Suction + Compression):
When the piston moves up the cylinder it covers two of the ports, the exhaust port and the transfer port, which are normally almost opposite to each other. This traps a charge of fresh mixture in the cylinder and further upward movement of the piston compresses this charge.
Further movement of the piston also uncovers a third port in the cylinder suction port. More fresh mixture is drawn through this port into the crankcase. Just before the end of this stroke, the mixture in the cylinder is ignited as in the four stroke cycle.
Second Stroke (Power + Exhaust):
The rise in pressure in the cylinder caused by the burning gases forces the piston to move down the cylinder. When the piston goes down, it covers and closes the suction port, trapping the mixture drawn into the crankcase during the previous stroke then compressing it. Further downward movement of the piston uncovers first the exhaust port and then transfers port.
This allows the burnt gases to flow out through exhaust port. Also the fresh mixture under pressure in the crankcase is transferred into the cylinder through transfer port during this stroke. Special shaped piston crown deflect the incoming mixture up around the cylinder so that it can help in driving out the exhaust gases.
When the piston is at the top of its stroke, it is said to be at the top dead centre (TDC). When the piston is at the bottom of its stroke, it is said to be at its bottom dead centre (BDC). In two stroke cycle engine, both the sides of the piston are effective which is not the case in four stroke cycle engine.
Scavenging:
The process of removal of burnt or exhaust gases from the engine cylinder is known as scavenging. Entire burnt gases do not go out in normal stroke, hence some type of blower or compressor is used to remove the exhaust gases in two stroke cycle engine.
Comparison between 4 Stroke and 2 Stroke Engine:
4 Stroke Engine:
i. No of Power Stroke:
One stroke for every two revolutions of crankshaft
ii. Power for the Same Cylinder Volume:
Small
iii. Valve Mechanism:
Present
iv. Construction & Cost:
Complicated, expensive
v. Fuel Consumption:
Little
vi. Removal of Exhaust Gases:
Easy
vii. Durability:
Good
viii. Stability of Operation:
High
ix. Changeability of rpm:
High (with large fly wheel)
x. Lubrication:
Equipped with an independent lubricating oil circuit
xi. Oil Consumption:
Little
xii. Carbon deposit inside cylinder:
Not so much
xiii. Noise:
Suction & exhaust is noiseless but other working is noisy
xiv. Air Tightness of Crankcase:
Unnecessary
xv. Cooling:
Normal
xvi. Self-Weight and Size:
Heavy & large
2 Stroke Engine:
i. No of Power Stroke:
One stroke for every one revolution of crankshaft
ii. Power for the Same Cylinder Volume:
Large (about 1.5 times of 4 stroke)
iii. Valve Mechanism:
Ports are there instead of valves
iv. Construction and Cost:
Simple, cheap
v. Fuel Consumption:
High (about 15% more)
vi. Removal of Exhaust Gases:
Difficult
vii. Durability:
Poor
viii. Stability of operation:
Low
ix. Changeability of rpm:
Low (with small flywheel)
x. Lubrication:
Using fuel, mixed with lubricating oil
xi. Oil Consumption:
Much
xii. Carbon Deposit inside Cylinder:
Much because of mixed fuel
xiii. Noise:
Suction & exhaust is noisy but other working is noise less
xiv. Air Tightness of Crankcase:
Must be sealed
xv. Cooling:
Chances of overheating
xvi. Self-Weight and Size:
Light & small
Valve Working and Valve Timing Diagram of Internal Combustion (IC) Engine:
Valve:
A valve is a small mechanical device, used for opening and closing the passage leading to the engine cylinder. Inlet valve of an internal combustion engine allows air or air-fuel mixture to go into the combustion chamber. The exhaust valve allows burnt gases to go out of the engine cylinder.
Each valve is opened or closed once during each cycle. A strong spring with the help of retainer and a key holds the valve tightly against the seat and thus prevents leakage on the compression and power stroke. The common face and seat angle of valve is 45° but 30° angle is also used for intake valves.
The most common type of valve is called poppet valve.
The arrangement of the valve on the engine is of two types:
1. L-head type and
2. Over-head type
‘L-head’ arrangement is used quite extensively on tractor engines and automobile engines. In ‘Overhead’ arrangement, the valve stem is surrounded by a removable guide and a spring which holds the valve tight in its seat.
Valve Head:
It is made of special alloy which can withstand high temperature and hammering action due to expanding gases.
Valve Stem:
It is a round steel rod attached with the valve head.
Valve Seat:
It is the place in the cylinder head where the valve head sits well. It may be made in cylinder head or in the engine block. Sometimes removable valve seats are also used.
Valve stem guide. It is a small guide, which fits into the cylinder block. It is usually made of cast iron. In some cases, a reamed hole in the block also serves as valve guide.
Valve Operating Mechanism:
Valve operating mechanism consists of several components such as:
(a) Crankshaft gear
(b) Cam gear
(c) Camshaft
(d) Push rod
(e) Tappet and
(f) Rocker arm
The crankshaft gear operates the cam gear which is fixed at one end of the camshaft. Consequently, camshaft rotates and moves the tappet, which pushes the push rod in proper time. Thus push rod opens or closes the valves at predetermined intervals. The camshaft gear is double the size of the crankshaft gear, so there is one revolution of the camshaft for every two revolutions of the crankshaft in case of four stroke engine.
Crankshaft gear:
A gear fixed at the end of the crankshaft which meshes with the gear of the camshaft is called crankshaft gear.
Cam gear:
A gear fixed at the end of the camshaft to mesh with the crankshaft gear is called cam gear.
Tappet:
Tappet is also called valve lifter. Tappet raises or lowers the valves. It receives motion from the cams, mounted on the camshaft. It opens or closes the valves at proper time. It is usually made of hardened steel.
Valve Lifter Guide:
It guides the tappet in motion.
Rocker Arm:
It is an arm used to change upward motion of push rod to downward motion for opening an engine valve. It is a small rod, one end of which touches the end of the valve stem and the other end touches the upper end of the tappet rod.
Tappet Clearance:
It is the clearance between rocker arm and valve stem to enable the valves to sit properly.
Valve Timing Diagram:
A valve timing diagram is a diagram of crank rotation on which the time of opening closing of inlet valve, exhaust valve are shown.
Valve timing mechanism is concerned with relative closing and opening of valves and their duration with respect to the cylinder position and the degree of crankshaft rotation. Top dead centre (TDC) is the instant when a piston is at the top of its stroke i.e. it is on the point of changing from upward to downward motion. Bottom dead centre (BDC) is the instant when a piston is at the bottom of its stroke i.e. it is on the point of changing from downward to upward motion.
Theoretically the intake valve should open on top dead centre (TDC) and close at bottom dead centre (BDC), whereas the exhaust valve should open on bottom dead centre and close on top dead centre, but in actual practice these angles differ. Valve timing is a function of engine speed.
The best valve timing for any given engine can be determined only by actual test, as it depends greatly on the design of the intake and exhaust passage. For most of the average tractor engines of four stroke cycle, the inlet valve opens about 5° before TDC and closes at about 30° after BDC the exhaust valve opens about 40° before BDC and closes at about 5° after TDC.
Firing Order:
The sequence in which the power stroke in each cylinder of an engine occurs is called firing order. The arrangement of the crank pin on the crankshaft and design of the camshaft both determine the firing order. For four cylinder engine the most commonly used firing orders are 1-3-4-2 and 1-2- 4-3. For six cylinder engines, firing order may be 1-4-2-6-3-5 or 1-5-3-6-2-4.
Firing Interval (FI):
The interval between successive power strokes in different cylinders of the engine is called firing interval and is determined as below.
During the first 180° rotation of crankshaft, the cylinder No. 1 has got the power stroke, cylinder No. 2 has got the compression stroke, cylinder No. 3 has got the exhaust stroke and cylinder No. 4 has got the suction stroke. Similarly during 360° rotation of the crankshaft, the first cylinder has got exhaust stroke, second cylinder power stroke, third cylinder intake stroke and fourth cylinder compression stroke.
Similar are the cases for 540° rotation of crankshaft and 720° rotation of crankshaft. At one time, each cylinder has got different strokes and after two revolutions of the crankshaft, each cylinder has got only one power stroke for a four stroke four cylinder engine.
