Normal and Abnormal Combustion · Web viewThe principle behind any reciprocating internal combustion engine: If you put a tiny amount of high-energy fuel (like gasoline) in a small,

Mechanical Engineering DepartmentInternal Combustion Lab. (0620520) Eng. Azad F. Otoum

INTERNAL COMBUSTION ENGINES

1.Introduction

1. ) AIM : Study of IC Engine models

The internal combustion engine is an engine in which the combustion of a fuel occurs with an

oxidizer (usually air) in a combustion chamber. In an internal combustion engine the expansion of

the high temperature and pressure gases, which are produced by the combustion, directly applies

force to a movable component of the engine( pistons) by moving it over a distance, generate useful

mechanical energy.

All internal combustion engines depend on the exothermic chemical process of combustion . The

reaction of a fuel , typically with oxygen from the air. The combustion process typically results in

the production of a great quantity of heat, as well as the production of steam and carbon dioxide

and other chemicals at very high temperature, the temperature reached is determined by the

chemical make up of the fuel and oxidizers.

The principle behind any reciprocating internal combustion engine: If you put a tiny amount of

high-energy fuel (like gasoline) in a small, enclosed space and ignite it, an incredible amount of

energy is released in the form of expanding gas.

Classification of I.C. Engines:

Internal combustion engines may be classified as given below:

1. According to cycle of operation:

• Two-stroke cycle engines

• Four-stroke cycle engines

2. According to cycle of combustion:

• Otto cycle engine

• Diesel cycle engine

• Dual-combustion

3. According to the fuel employed and the method of fuel supply to the engine cylinder:

• Petrol engine

• Diesel engine

•Oil, Gas engine

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4. According to method of ignition:

• Spark ignition (S.L) engine

• Compression ignition (C.I.) engine

5. According to method of cooling the cylinder:

• Air-cooled engine

• Water-cooled engine

6. According to number of cylinders:

• Single cylinder engine

• Multi-cylinder engine

Different Parts of I.C. Engines :

1. Cylinder 2. Cylinder head

3. Piston 4. Piston rings

5. Gudgeon pin 6. Connecting rod

7. Crankshaft 8. Crank

9. Engine bearing 10. Crank case

11. Flywheel 12. Governor

13. Valves and valve operating mechanism

Parts for Petrol Engines Only:

1. Spark plugs

2. Carburetor

3. Fuel pump

Parts for Diesel Engine Only:

1. Fuel pump

2. Injector

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Figure 1

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2.) AIM : Study of working of four stroke petrol engine and four stroke diesel engine with the help of cut section models.

FOUR-STROKE CYCLE ENGINES

Four Stroke Petrol engine Four Stroke Diesel engine

Four Stroke Petrol engine :The four stroke-cycles refers to its use in petrol engines, gas engines, light, oil engine and heavy oil engines in which the mixture of air fuel are drawn in the engine cylinder. Since ignition in these engines is due to a spark, therefore they are also called spark ignition engines.

Figure 2

SUCTION STROKE: In this Stroke the inlet valve opens and proportionate fuel-air mixture is sucked in the engine cylinder. Thus the piston moves from top dead centre (T.D.C.) to bottom dead centre (B.D.C.). The exhaust valve remains closed through out the stroke.

COMPRESSION STROKE: In this stroke both the inlet and exhaust valves remain closed during the stroke. The piston moves towards (T.D.C.) and compresses the enclosed fuel-air mixture drawn. Just before the end of this stroke the operatingplug initiates a spark which ignites the mixture and combustion takes place at constant pressure.

POWER STROKE OR EXPANSION STROKE: In this stroke both the valves remain closed during the start of this stroke but when the piston just reaches the B.D.C. the exhaust valve opens. When the mixture is ignited by the spark plug the hot gases are produced which drive or throw the piston from T.D.C. to B.D.C. and thus the work is obtained in this stroke.

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EXHAUST STROKE: This is the last stroke of the cycle. Here the gases from which the work has been collected become useless after the completion of the expansion stroke and are made to escape through exhaust valve to the atmosphere. This removal of gas is accomplished during this stroke. The piston moves from B.D.C. to T.D.C. and the exhaust gases are driven out of the engine cylinder; this is also called SCAVENGING.

Figure 3 (Theoretical P-V diagram of a four-stroke engine)

Four Stroke Diesel engine:

Figure 4

SUCTION STROKE: With the movement of the piston from T.D.C. to B.D.C. during this stroke, the inlet valve opens and the air at atmospheric pressure is drawn inside the engine cylinder; the exhaust valve however remains closed. This operation is represented by the line 5-1

COMPRESSION STROKE: The air drawn at atmospheric pressure during the suction stroke is compressed to high pressure and temperature as the piston moves from B.D.C. to T.D.C. Both the inlet and exhaust valves do not open during any part of this stroke. This operation is represented by 1-2

POWER STROKE OR EXPANSION STROKE: As the piston starts moving from T.D.C to B.D.C, the quantity of fuel is injected into the hot compressed air in fine sprays by the fuel injector and it (fuel) starts burning at constant pressure shown by the line 2-3.

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At the point 3 fuel supply is cut off. The fuel is injected at the end of compression stroke but in actual practice the ignition of the fuel starts before the end of the compression stroke. The hot gases of the cylinder expand adiabatically to point 4. Thus doing work on the piston.

EXHAUST STROKE: The piston moves from the B.D.C. to T.D.C. and the exhaust gases escape to the atmosphere through the exhaust valve. When the piston reaches the T.D.C. the exhaust valve closes and the cycle is completed. This stroke is represented by the line 1-5.

Figure 5 (Theoretical p- V diagram of a four-stroke Diesel Engine)

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3.)AIM : Study of working of two stroke petrol and two stroke diesel engine with the help of cut section models.

TWO-STROKE CYCLE ENGINES

Two Stroke Petrol engine Two Stroke Diesel engine

TWO STROKE ENGINES

In this engine suction and exhaust strokes are eliminated. Here instead of valves, ports are used. The exhaust gases are driven out from engine cylinder by the fresh charge of fuel entering the cylinder nearly at the end of the working stroke.A two-stroke petrol engine (used in scooters, motor cycles etc.).The cylinder L is connected to a closed crank chamber C.C. During the upward stroke of the piston M, the gases in L are compressed and at the same time fresh air and fuel (petrol) mixture enters the crank chamber through the valve V.

Figure 6

When the piston moves downwards, V closes and the mixture in the crank chamber is compressed the piston is moving upwards and is compressing an explosive change which has previously been supplied to L. Ignition takes place at the end of the stroke. The piston then travels downwards due to expansion of the gases and near the end of this stroke the piston uncovers the exhaust port (E.P.) and the burnt exhaust gases escape through this port.

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The transfer port (T.P.) then is uncovered immediately, and the compressed charge from the crank chamber flows into the cylinder and is deflected upwards by the hump provided on the head of the piston.

It may be noted that the incoming air-petrol mixture helps the removal of gases from the engine-cylinder; if, in case these exhaust gases do not leave the cylinder, the fresh charge gets diluted and efficiency of the engine will decrease.

The piston then again starts moving from B.D.C. to T.D.C. and the charge gets compressed when E.P. (exhaust port) and T.P. are covered by the piston; thus the cycle is repeated.

Figure 7

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(4.) AIM : Study of cooling systems of an IC Engine (air cooling and water cooling)

A cooling system in an internal combustion engine that is used to maintain the various engine components at temperatures conducive to long life and proper functioning.Gas temperatures in the cylinders may reach 4500°F (2500°C). This is well above the melting point of the engine parts in contact with the gases; therefore it is necessary to control the temperature of the parts, or they will become too weak to carry the stresses resulting from gas pressure.

The lubricating oil film on the cylinder wall can fail because of chemical changes at wall

temperatures above about 400°F (200°C). Complete loss of power may take place if some spot in

the combustion space becomes sufficiently heated to ignite the charge prematurely on the

compression stroke.

A thin protective boundary of relatively stagnant gas of poor heat conductivity exists on the inner surfaces of the combustion space. If the outer cylinder surface is placed in contact with a cool fluid such as air or water and there is sufficient contact area to cause a rapid heat flow, the resulting drop in temperature produced by the heat flow in the inside boundary layer keeps the temperature of the cylinder wall much closer to the temperature of the coolant than to the temperature of the combustion gas.

If the coolant is water, it is usually circulated by a pump through jackets surrounding the cylinders and cylinder heads. The water is circulated fast enough to remove steam bubbles that may form over local hot spots and to limit the water's temperature rise through the engine to about 15°F (8°C).In most engines in automotive and industrial service, the warmed coolant is piped to an air-cooled heat exchanger called a radiator (see figures below ). The airflow required to remove the heat from the radiator is supplied by an electric or engine- driven fan; in automotive applications the airflow is also supplied by the forward motion of the vehicle.The engine and radiator may be separated and each placed in the optimum location, being connected through piping. To prevent freezing, the water coolant is usually mixed with ethylene glycol.Engines are often cooled directly by a stream of air without the interposition of a liquid medium. The heat-transfer coefficient between the cylinder and air stream is much less than with a liquid coolant, so that the cylinder temperatures must be much greater than the air temperature to transfer to the cooling air the heat flowing from the cylinder gases.To remedy this situation and to reduce the cylinder wall temperature, the outside area of the cylinder, which is in contact with the cooling air, is increased by fanning. The heat flows easily from the cylinder metal into the base of the fins, and the great area of finned surface permits heat to be transferred to the cooling air.

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Figure 8

Figure 9

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KNOCKING (DETONATION)

Knocking (also knock, detonation, spark knock, pinging or pinking) in spark ignition internal combustion engines occurs when combustion of some of the air/fuel mixture in the cylinder does not result from propagation of the flame front ignited by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. The fuel-air charge is meant to be ignited by the spark plug only, and at a precise point in the piston's stroke. Knock occurs when the peak of the combustion process no longer occurs at the optimum moment for the four-stroke cycle. The shock wave creates the characteristic metallic "pinging" sound, and cylinder pressure increases dramatically. Effects of engine knocking range from inconsequential to completely destructive.

Knocking should not be confused with pre-ignition—they are two separate events. However, pre-ignition can be followed by knocking.

Normal and Abnormal CombustionUsually, normal combustion should begin with the ideal blend of fuel and air. The ignition process of the mixture should begin effortlessly and progressively so that the utmost quantity of pressure can be produced just as the piston travels downward after getting to the top dead center (TDC). This leads to the production of the most effective engine performance.

Alternatively, abnormal combustion upsets the effortless process through the creation of a second ignition cycle which may be created before or after the regular cycle that a spark plug produced. You will hear a knock or pink sound as the second cycle expresses itself. Abnormal combustion has the capacity of blowing the head gasket, damaging a valve, cracking the combustion chamber or cylinder or blowing a hole in the pistol.

Detonation can be prevented by any or all of the following techniques:

the use of a fuel with high octane rating, which increases the combustion temperature of the fuel and reduces the proclivity to detonate

enriching the air–fuel ratio which alters the chemical reactions during combustion, reduces the combustion temperature and increases the margin above detonation

reducing peak cylinder pressure decreasing the manifold pressure by reducing the throttle opening or boost

pressure reducing the load on the engine retarding (reduce) ignition timing

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Knocking is more or less unavoidable in diesel engines, where fuel is injected into highly compressed air towards the end of the compression stroke. There is a short lag between the fuel being injected and combustion starting. By this time there is already a quantity of fuel in the combustion chamber which will ignite first in areas of greater oxygen density prior to the combustion of the complete charge. This sudden increase in pressure and temperature causes the distinctive diesel 'knock' or 'clatter', some of which must be allowed for in the engine design. Careful design of the injector pump, fuel injector, combustion chamber, piston crown and cylinder head can reduce knocking greatly, and modern engines using electronic common rail injection have very low levels of knock. Engines using indirect injection generally have lower levels of knock than direct injection engine, due to the greater dispersal of oxygen in the combustion chamber and lower injection pressures providing a more complete mixing of fuel and air. Diesels actually do not suffer exactly the same "knock" as gasoline engines since the cause is known to be only the very fast rate of pressure rise, not unstable combustion. Diesel fuels are actually very prone to knock in gasoline engines but in the diesel engine there is no time for knock to occur because the fuel is only oxidized during the expansion cycle. In the gasoline engine the fuel is slowly oxidizing all the time while it is being compressed before the spark. This allows for changes to occur in the structure/makeup of the molecules before the very critical period of high temp/pressure

What causes engine knock1- Engine Overheating

Engine overheating can lead to abnormal combustion and it is also possible for overheating to be triggered in so many ways. You should inspect the coolant level as well as radiator for clogging.The operation of the fan cooling the engine should be verified and antifreeze should be replaced in accordance to the manufacturer’s schedule. You can use the repair manual of your vehicle to properly check the system if any overheating is noticed.

2- Ignition Timing Too AdvancedYou should inspect the ignition timing of your vehicle in accordance with its repair manual. If your manual is lost or you don’t have at all, you can purchase a cheap aftermarket manual.If the engine comes with a distributor, you can try to adjust its timing by disengaging and turning the distributor around. On state-of-the-art vehicle engines with OBD II systems, you should investigate if the sensors controlling ignition timing require replacements.

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3- Carbon BuildupCarbon deposit can be caused by the use of engine oil containing a greater weight than the one suggested by the manufacturer. It can also be brought about when you use fuel of low quality or drive the engine lower than normal operating temperature or you are used to intermittent short trips.You should inspect the spark plug one by one and if you discover dirty deposits around their electrodes, it may be an indication that there is a carbon build up on the valves and pistons. You can use a flashlight to peek into the spark hole, even though this is very hard to do if you don’t have an endoscope camera.If you are interested in decarburizing the chamber, you can use Sea foam or homogenous products. This will help you to clean inner part of the chamber, cylinder, fuel injectors and intake manifold.

4- Low-Octane FuelIf you’re using low-octane fuel, you will be giving problems to your engine. A lot of manufacturers produce fuel that have been blended with anti knock additives so that gasoline’s combustion properties will be lowered in order to guard against engine knock.Octane rating refers to the ability of a particular brand of gasoline to prevent knocking. The higher the rating, the better resistance will be provided against knocking. For example, fuel rated as 87 will not produce better knocking resistance as one rated as 91.It does not mean that if you get high octane fuel, you are very special. It is advisable to use high-octane fuel for high-compression engines and the lower version for low-compression engines.In many cases, you can go for a higher octane fuel than the one suggested by the manufacturer and this will serve as a preventive action for your engine. But it is not advisable to use a lower octane fuel than the one recommended by your manufacturer so that you don’t knock your engine.

5- Incorrect Spark PlugsSpark plugs come in two ratings, they either be rated as hot or cold. The ability of the plug to transfer heat from the combustion chamber to the cooling system remains their vital classification criterion.The insulator of a hot plug has a larger and longer diameter. Its rate of heat transfer is not as fast as compared to a code plug and this helps in clearing away deposits that may want to build up on the plug.A cold spark plug will ensure that heat is transferred from the engine into the cooling system faster so that pre-ignition and overheating will be prevented. It is very important for you to use the spark plug suggested by your manufacturer in order to prevent engine knock.

6- Exhaust BackpressureOne of the common problems with exhaust systems is high back pressure. This is caused due to clogged catalytic converter, exhaust pipe or muffler. Exhaust back pressure formation is usually caused by a clogged converter. This will hinder engine airflow making the engine to work at a high temperature and be deprived of power which will eventually lead to knocking.

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In order to inspect the exhaust system so that you will be able to discover any high pressure, you can lift the front of your car and place it on your jack stand. The converter should then be tapped with the aid of a rubber mallet and if it jiggles, it is an indication that the catalyst material is reducing to pieces. You can also use a vacuum gauge to check high back pressure.

7- Vacuum LeaksSolenoids, actuator and switches in the engine are operated by vehicle emission systems through vacuum. For instance, the manifold absolute pressure (MAP), EGR valve, sensor, purge valve, positive crankcase ventilation (PCV) valve as well as other components may be operated by vacuum.When these components experience vacuum leaks, it can lead to spark pinging or knocking. You should inspect these systems’ vacuum hoses for loose connections as well as damage.

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BASIC TERMINOLOGY:

The following terms and abbreviations are commonly used in engine technology, they should be

learned to assure maximum understanding in the laboratory:

Spark Ignition (SI): An engine in which the combustion process in each cycle is started by

use of a spark plug.

Compression Ignition (CI): An engine in which the combustion process starts when the air-

fuel mixture self-ignites due to high temperature in the combustion chamber caused by the

high compression. CI engines are often called Diesel Engines, especially in the non-technical

community.

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Top-Dead-Center (TDC): Position of the piston when it stops at the furthest point away

from the crankshaft. Top because this position is at the top of the most engines (not always),

and dead because the piston stops at this point.

Bottom-Dead-Center (BDC): Position of the piston when it stops at the point closest to the

crankshaft.

Bore: Diameter of the cylinder or the diameter of the piston face, which is the same minus a

very small clearance.

Stroke: Movement distance of the piston from one extreme position to other: TDC to BDC

or BDC to TDC.

Clearance Volume: Minimum volume in the combustion chamber with piston at TDC.

Displacement or Displacement Volume: Volume displaced by the piston as it travels

through one stroke. Displacement can be given for one cylinder or for the entire engine.

Some times it is called Swept Volume.

Ignition Delay (ID): Time interval between ignition initiation and the actual start of

combustion. Some times it is called Ignition Timing.

Air-Fuel Ratio (AF): Ratio of mass of air to mass of fuel input into engine.

Fuel- Air Ratio (FA): Ratio of mass of fuel to mass of air input into engine.

Brake Maximum Torque (BMT): Speed at which maximum torque occurs.

Specific Fuel Consumption (SFC): Amount of energy observed from the fuel being

combusted in the engine.

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Documents

Normal and Abnormal Combustion · Web viewThe principle behind any reciprocating internal combustion engine: If you put a tiny amount of high-energy fuel (like gasoline) in a small,