1. INTERNAL COMBUSTION ENGINES
Engine is the heart of an automobile. It converts energy of fuel into mechanical energy so that the vehicle may move on the road. The aim of this unit is to introduce you to various types of engines, how an engine works, main parts of an engine, and various parameters related to engines. You will also study the advantages and disadvantages of the different types of engines. It is very important to understand the basic functioning of Internal Combustion (IC) engines so that troubleshooting can become easier. After we have learnt the basic construction and functioning of the engine, we would then explain the various sub systems associated with the engine which enable the engine to work efficiently for a reasonable period of time. These include the engine fuel and exhaust systems, cooling and lubrication systems, and the ignition system. It is important to understand the functioning of these sub-systems as the performance of the engine is dependent on proper functioning of these sub systems and their components. Without the basic understanding of these sub-systems, it will be very difficult to diagnose and repair a fault.
1.0 i. ii. iii.
1.1
Objectives After going through this lesson, you will be able to: Define an engine and classify into various types of engines. Understand the basic construction of an engine. List and describe the basic parameters of an engine.
Introduction
The aim of this course is to train you to repair automobiles and their engines. Towards that goal, we should first get to know the construction and working of an engine and that is the purpose of this lesson. In an earlier lesson, we gave an example of energy conversion referring to cooking gas in our homes. There, we also defined an engine as a device that converts heat energy into mechanical energy. In the same lesson, while describing the chassis we saw that the first unit on a chassis is the power unit. We also said that the power unit is an internal combustion engine. In this lesson, we shall understand the definition of an engine, know about the types of engine and the main parts that constitute the engine. In the end, the basic parameters on which an engine is designed will be introduced. A good understanding of the basic principles and working of an engine would make your subsequent lessons on repair and maintenance fun to learn. You would go about doing this by understanding the why and how of each and every task rather than doing it mechanically.
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Self-check Questions 1. What is the function of an engine in an automobile? 2. What are the engines that you have heard of?
1.2
Definition
An engine is any device used for converting one form of energy into another form, especially for converting other forms of energy into mechanical or kinetic energy. A coal engine converts heat energy (obtained by burning coal) to mechanical energy used for movement of the train. Similarly, an electric engine converts the electrical energy (electricity) to mechanical energy. In automobiles we convert energy of fuel which is either petrol or diesel or CNG etc. to mechanical energy.
1.3
Types of Engines
Engines can be classified into two types depending on whether the fuel is burnt outside the engine cylinder or inside the engine cylinder. These two types are called the external combustion engine and the internal combustion engine, respectively. We know that the earliest automobiles were driven by steam. A steam Fig. 1.1: Types of engines. engine is an external combustion engine. In a steam engine, fuel, which is coal, is burnt outside the engine cylinder and steam is generated by boiling water with the heat released by coal. This steam is then sent to the engine cylinder as working fluid. In 1860, the first internal combustion engine was devised and after that internal combustion engines came to be used in automobiles. In an internal combustion engine, fuel, which could be petrol, diesel, CNG or LPG, is burnt inside the engine cylinder. The method of igniting the fuel is different for petrol and diesel engines. CNG or LPG can be used in petrol engines as fuel with slight modifications. An internal combustion engine has the advantage that it is compact in design and light weight than an external combustion engine thus enhancing mobility. Also an internal combustion engine is more efficient i.e. it converts a greater part of the energy of fuel into mechanical energy. Hence, we use only internal combustion engines in automobiles today.
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Self-check Questions 3. In which form is energy obtained from an engine? 4. What are the two major classes of engines?
1.4
Parts of IC Engines
Let us take a look under the bonnet of a car and have a look at the main parts of an internal combustion engine and some of its various accessories.
Fig 1.2: Maruti 800 engine (a) Side view. (b) Rear view.
In the above photographs, Fig. 1.2, we see two views of a three-cylinder, fourstroke, water cooled over-head camshaft, carbureted engine. Though the registration of cars with carbureted engine has been discontinued in the major metros, it is one of the most widely used power packs on Indian roads. In Fig. 1.2 (a), we see various components of the engine such as the cylinder head cover, high-tension leads, alternator, oil filter, exhaust manifold, oil pan etc. In the other view of the engine from the rear (Fig. 1.2 (b)) we see the distributor, distributor gear case, carburetor, clutch cover etc. We can know how power from the engine is obtained, by understanding the four main parts of an internal combustion engine. These are the cylinder, piston, connecting rod, and crankshaft. The piston reciprocates within the cylinder and transmits the force of the expanding gases via the connecting rod to the crankshaft. But these four parts alone do not make an engine. An engine is a complicated aggregate of numerous parts assembled together to make a working system.
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Fig. 1.3: Parts of an IC engine.
The internal construction of an engine and some of its major components are shown in Fig. 1.3. In Fig. 1.4 we see two sectional views of a 4 cylinder water cooled petrol engine. In the view on the left, which is a view from the front of the engine, apart from the components and accessories seen on the outside of the engine, we see the piston which is internal and to which is connected the connecting rod, and the crank shaft. We also see the camshaft which operates the valves through the push rod and the rocker. Lower down we see the oil strainer and the dip stick in the oil pan. The dipstick is a measuring device and helps us get an idea about the quantity of lubricating oil in the pan. On the outside of the oil pan we see the oil filter.
Fig. 1.4: Sectional views of a 4-cylinder water-cooled IC engine.
In the view on the right, we see four pistons in cylinders, all connected to the same crank shaft through their respective connecting rods. You will notice that the first and the fourth pistons are at one level and the second and the third pistons are at another level. We shall study later why this is so. At the front end, we also see the water pump and the cooling fan driven by V-belt from the crank shaft. At the rear of the engine we see the flywheel which has gear teeth on its periphery. Toward the upper side of the engine we see the valves, 2 for each cylinder, cylinder head cover, and the thermostat.
Self-check Questions 5. List the main parts of an Internal Combustion Engine. 6. From which part do we get power output from the engine?
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1.5
Important Parameters of Engines Let us now define some important parameters of an engine.
1.5.1 Bore and Stroke The bore of the cylinder is the inner diameter of the cylinder. In this cylinder, the piston reciprocates i.e. it moves up down. The top position of its travel is called the Top Dead Centre and the bottom position is Fig. 1.5: (a) Bore. called the Bottom Dead Centre. (b) Stroke and stroke length. The movement between the top dead centre and the bottom dead centre is called stroke and the distance between the top dead centre and the bottom dead centre is called stroke length. Bore and stroke are the two major dimensions of any engine. For example the Maruti 800 car engine that we have seen has a bore of 68.5 mm and stroke 72 mm. During 1 stroke of the piston, the crank shaft to which the piston is connected moves through half rotation i.e. by 180 deg. Hence 2 strokes of the piston constitute 1 full rotation of the crank shaft. 1.5.2 Compression Ratio When the piston moves from top dead centre to bottom dead centre the air present in the cylinder expands and occupies the whole cylinder volume. Next, when the piston moves upward from bottom dead centre to top dead centre, the volume of air in the cylinder decreases and its pressure increases. The ratio of volume at bottom dead centre to volume at top dead centre is called the compression ratio. The higher the compression ratio, the higher is the thermal efficiency of the engine.
Fig. 1.6: Compression ratio.
1.5.3 Displacement This is the volume displaced by the movement of piston in one stroke from TDC to BDC. It is generally given in cc (cm3) for small engines and in litres for large displacement engines. It measures the maximum amount of charge that can inducted into the engine cylinder in one stroke. Greater the displacement, greater is the power of the engine. Maruti 800 engine has a displacement of 796cc. This value is the sum of the displacement of all the 3 cylinders.
1.5.4 Mean Effective Pressure The pressure in engine cylinder varies through out the cycle. MEP can be understood as an imaginary pressure that would act on the piston and remain constant during the entire stroke, so that work-done at MEP in 1 stroke equals the work developed in the cylinder during the complete cycle. Engines of similar design working under similar conditions develop same MEP irrespective of cylinder size. Hence, MEP is the true measure of the performance of engine design. MEP is used to calculate IHP. IHP =
MEP × L × A × N 4500
where, L=stroke-Iength, m
π
× D 2 =cylinder area, cm2 (Note- D=cylinder bore, cm) 4 N=no. of working strokes per min. = rpm, for 2-stroke engines rpm =( ) for 4-stroke engines 2
A=
1.5.5 Horsepower Power produced by the engine is generally given in Metric Horsepower though now units of watt and PS are also used. The power output depends on the speed of the engine and therefore any value of power output is accompanied by the rpm at which it is available. Indicated horsepower It is the power produced by the engine in its cylinders due to combustion of fuel. Power available at the crankshaft for moving the car is much less than IHP. Brake horsepower This is the power available at the crankshaft to move the car. It is called brake horsepower because we will have to apply as much power at the crankshaft from outside to stop the engine. Frictional horsepower This is the amount of power spent in overcoming friction between piston & rings and cylinder, shaft and bearings etc. in the engine. BHP is less by this amount from IHP. BHP = IHP − FHP 1.5.6 Torque This is the turning force produced at the crankshaft. It also depends on the rpm of the engine. 2πNT BHP = 4500
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where N=rpm BHP depends on the Torque (T) of the engine. 1.5.7 Mechanical Efficiency Ratio of BHP and IHP is called Mechanical Efficiency. BHP ηm = IHP 1.5.8 Thermal Efficiency This is the ratio of work output and heat input. BHP × 4500 η th = F × Cv × J where, F = fuel consumed in kg per minute Cv = higher calorific value of fuel in kcal/kg J = Mechanical equivalent of heat 1.5.9 Specific Fuel Consumption This is the fuel consumed for producing one BHP per hour. F SFC = BHP where, F=fuel consumption per hour. The lower the SFC, the higher will be the fuel average given by the car.
Self-check Questions 7. How much is the cylinder inner diameter and distance between TDC and BDC in case of a Maruti 800 car engine? 8. Give the relation between stroke of piston and rotation of crankshaft. 9. How does compression ratio affect efficiency? 10. MEP is always more for bigger engines. State whether True or False. 11. What is meant by IHP? 12. What are the other terms for HP? What do they mean? 13. Higher SFC implies better fuel average. State whether True of False?
1.6
Assignments
1.6.1 Class Assignment 1. Discuss the various types of engines used for transport by road and rail. 2. Discuss the interconnection of the main parts of an IC engine and how power is transmitted to the crankshaft.
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1.6.2 Home Assignment 1. Check out at least 5 different vehicles and categorize their engines. Also write down their basic parameters (engine specification) that you have studied in this lesson.
1.7
Summing Up
In this lesson we have had an overview of the types of engines and the main components of the engine. All of them, besides so many others, shall be studied in detail at a later stage. We shall also look into the various systems and sub systems that make the engine perform efficiently. We also studied some parameters by which an engine is specified. These parameters- bore, stroke, compression ratio, mean effective pressure, IHP etc. will be different for different engines depending on their category and application. For e.g. a diesel engine will have compression ratio which is much greater than that of a petrol engine. Also its displacement and power output will be greater, hence it is used in heavy vehicles.
1.8
Possible Answers to Self-check Questions
1. To convert energy of fuel into mechanical energy. 2. Petrol and Diesel engines. 3. Mechanical energy 4. External and Internal combustion engines. 5. Cylinder, piston, connecting rod, crankshaft. 6. Crankshaft 7. 68.5 mm and 72 mm 8. 2 strokes of piston constitute 1 rotation of crankshaft 9. Efficiency increases with increase in compression ratio 10. False 11. IHP is the power produced in an engine cylinder due to combustion of fuel 12. BHP is the power available at the crankshaft for doing external work such as moving the car and FHP is the power consumed in overcoming friction between engine parts 13. False
1.9
Terminal Questions
1. Define an engine. Also give the classification of engines and state where each type is used. 2. Name the main parts of an engine and state how they are connected. What is the function of each part? 3. List the important parameters of an engine by which it can be specified and give the relation for each.
1.10 References 1. AA Book of the Car. 2nd Ed. Drive Publications Ltd, London. 8
1.11 Suggested Further Reading 1. Gupta, K. M., 2002. Automobile Engineering, Vol. 1. Umesh Publications, Delhi. 2. Nakra, C. P., 2007. Basic Automobile Engineering (English). Reprint. Dhanpat Rai Publishing Co., New Delhi. 3. Chhikara, A., 1999. Automobile Engineering, Vol 1: Engine Systems. Satya Prakashan, New Delhi.
1.12 Glossary BDC
Bottom Dead Centre, lowermost position to which piston will come down in down-stroke
BHP
Brake Horse Power. Power available at the crankshaft after frictional losses
Bore
Cylinder inner diameter
CNG
Compressed Natural Gas, used as fuel. It is clean burning.
Compression ratio
Ratio by which charge in cylinder is compressed when piston is moving from BDC to TDC.
Displacement
Volume displaced by piston when it moves from TDC to BDC.
Efficiency
Ratio of work output to energy input.
Engine
Machine which converts energy of fuel into mechanical energy
External combustion engine
Engine in which combustion of fuel takes place outside the engine cylinder
IHP
Indicated Horse Power. Power produced in the cylinder due to combustion of fuel
Internal combustion engine
Engine in which combustion of fuel takes place in the engine cylinder
LPG
Cooking gas, mainly propane and butane. Also used as fuel in IC engines.
MEP
Mean Effective Pressure, an imaginary average pressure in the cylinder at which work done in one stroke would be equal to the work
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done in the entire cycle
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SFC
Specific Fuel Consumption. Fuel consumption per hour for producing 1 HP.
Stroke
Movement between TDC and BDC
TDC
Top Dead Centre, topmost position to which piston will go up in up-stroke
Torque
Turning effect available at the crankshaft. It is the product of force of expanding gases and the perpendicular distance between the axis of piston and the crankpin