AUTOMOBILE ELECTRICAL SYSTEMS 5. STARTING SYSTEM
Starting system is required because an internal combustion engine is not selfstarting; that is, it does not start running by itself when provided with fuel, which could be petrol or diesel, or a mixture of fuel and air. The engine is needed to be cranked by rotating the crankshaft at a specified minimum speed for the engine to start and run on its own power. Various methods are employed to start IC engines. Nowadays, electrical starting systems are used in automobiles, including some 2-wheelers.
5.0 i. ii. iii.
5.1
Objectives After going through this lesson, you will be able to: Understand the necessity of engine starting system. Know the methods by which engines can be started. Get an idea of the electric starting system.
Introduction
Some engines are self starting for e.g. steam engine, wind mill, steam turbine, and water turbine are self-starting. In these machines, as soon as the working fluid is supplied to them, the output shaft starts rotating and mechanical power is obtained. On the other hand, internal combustion engines used in automobiles are not self-starting. Hence every modern motor vehicle is provided with a starting system. We will study starting systems in this lesson. Internal combustion engines that are used in automobiles are not self starting. They do not start delivering power as soon as the working fluid is supplied to them. Instead, an internal combustion engine has to be cranked by rotating the crankshaft at a minimum specified speed so that the piston reciprocates, valves open and close and ignition of the charge takes place, thereby completing a cycle and then the engine starts to run on its own power. Hence a starting system is necessary in motor vehicles. We will study about IC engines in the second semester.
5.2
Methods of Engine Starting
Earlier, automobile engines were started by hand cranking. We have read in Lesson 1 of CAF 101 that the electric starter was available only in 1911. Until then cars were started by hand cranking. This was a major reason why cars were not first popular
with ladies. In fact, until a few years ago our very own Ambassador was provided with a handle for cranking even though it was provided with the self starter. This was just in case the battery got drained, lest we should be left high and dry on the road. But we are talking ahead of topic. Several methods are employed for cranking internal combustion engines. Apart from hand cranking there is the pull start method employed in small portable generators. A rope is wound around the pulley on the crankshaft and then pulled quickly, making the crankshaft rotate. This method is shown in fig. 5.1. The kick start method is used in several two-wheelers and is shown in fig. 5.2. Presently, all four-wheelers and some modern two-wheelers use the electric start method. (See Fig. 5.3). You might have seen sometimes a car/scooter being pushed for starting. This method, shown in Fig. 5.4, is adopted when the battery of the vehicle is drained and it does not have sufficient power to crank the engine. So how can we start a vehicle by just pushing it? In this method, if it is a petrol engine vehicle, the ignition switch is put ON, the transmission (usually second gear) is engaged, the clutch pedal is pressed and while the vehicle is in motion we release the clutch pedal suddenly. Then motion from the road wheels is transmitted back to the engine crankshaft via the power train thus giving it an impulse to start the engine. Once the engine gets rotated and a cycle completes, all other conditions of fuel and spark being fulfilled, the engine starts running on its own power.
Fig 5.1: Pull starting a portable generator.
Fig. 5.3: Electric self-start method.
Fig. 5.2: Kick-start method.
Fig 5.4: Pushing the car to start.
Self-check Questions 1. 2. 3. 4. 5.
5.3
_______________ engines are not self starting. Give examples of some self starting engines. What are the different methods used for starting IC engines? What happens when the rope of a portable generator is pulled? The ____________ method is used in automobiles now.
Electric Starting System
In a modern motor vehicle, an electrical starting system is used. The starter motor, which is also called the self-starter, draws current from the battery of the car and converts it into mechanical energy which is then used to crank the engine. A pinion at the end of the shaft of the motor rotates with the shaft and meshes with the flywheel ring gear, thus making the crankshaft rotate. The four operations of suction, compression, power, and exhaust take place in sequence, thereby completing a cycle, and the engine starts running on its own power. The electrical starting system consists of a special DC motor, a light duty ignitioncum-starter switch which is also called the main switch, a heavy duty magnetic switch which is also known as the solenoid switch, a shift relay and a starter drive. The starter drive that is generally used in small cars is the overrunning clutch or the Bendix drive. A 12 volt electric starting system is generally used in cars, with the wattage of the motor being 0.6 KW or higher. The no-load starter currents being hundreds of amperes or more, the cables connecting the motor to battery are heavy duty cables that are capable of carrying heavy currents. The following table provides the specifications and characteristics of the starting system used in the Maruti 800 car. Table 5.1 Electric starting system specifications of Maruti 800 car System Voltage DC Motor Output Rating No-load Characteristics Full-load Characteristics
12 volts 600 watt 30 seconds Maximum 50 A at 11V Minimum 5,000 RPM Maximum 230 A at 9.5V and 0.5 Kg-m torque Minimum 1200 RPM
The earth-return method is used to complete the electrical circuit. The battery used in the car is a lead acid battery of the re-chargeable type. A cable is connected from the positive terminal of the battery to a terminal of the motor and the return path is provided through the body of the car or the frame of the chassis, which is known as earth-return. This method is economical as it saves copper conductor.
Self-check Questions 6. What are the names of the starter motor? 7. The motor takes current from the _________ and converts it into ____________ energy. 8. Name the type of motor used in electrical starting system. 9. How many switches are used in the starting system? What are they called? 10. Name the starter drives used in small engines. 11. Name the components of electrical starting system. 12. What is the system voltage used in cars? 13. What is the wattage of Maruti 800 starter motor? 14. What is the value of current flowing through the motor at full load? 15. Where is the positive terminal of the lead acid battery connected? 16. How is the electric circuit completed?
5.4
Assignments
5.4.1 Class Assignment 1. Compare the various methods of engine starting and discuss the advantages and disadvantages of each. 5.4.2 Home Assignment 1. Make a list of two-wheelers in the market which use self start method for engine cranking.
5.5
Summing Up
In this lesson we studied why it is necessary to have a starting system for an IC engine and the various methods by which an IC engine can be started. We then studied in some detail the electric start method used in all four-wheelers and some modern twowheelers. Now in the next lesson we will study the working of the electric starting system in greater detail with the help of a circuit diagram of the system.
5.6 1. 2. 3. 4.
Possible Answers to Self-check Questions
Internal combustion steam engine, wind mill, steam turbine, and water turbine are self-starting Pull start, kick start, electric start, push start the pulley on the engine crankshaft rotates, which further rotates the engine crankshaft to crank the engine 5. electric start
6. Self starter, self, cranking motor 7. battery, mechanical 8. DC motor 9. two; main switch, solenoid switch 10. overrunning clutch, Bendix drive 11. DC motor, the main switch, a solenoid switch, shift relay and a starter drive 12. 12 V 13. 600 watt 14. 230 amp 15. motor terminal 16. through earth return
5.7
Terminal Questions
1. What is the necessity of the starting system? What are the various methods by which an IC engine can be started? 2. Describe the electric start method used in four-wheelers. 3. Explain the mechanism of starting a car by given it a push when it fails to start by the self start method.
5.8
References
1. www.howstuffworks.com
5.9
Suggested Further Reading
1. Kohli, P.L., 1987. Automotive Electrical Equipment. First Reprint. Tata McGrawHill Publishing Co. Ltd, New Delhi.
5.10 Glossary Earth-return
Cable which is connected from the positive terminal of the battery to a terminal of the motor and the return path is provided through the body of the car or the frame of the chassis, which is known as earth-return.
Engine cranking
To turn the engine in its direction of rotation in an effort to start the engine by external power, usually a self starter.
Hand cranking
To turn the engine in its direction of rotation externally by manual force using a tool.
Lead acid battery
A direct current electrical storage unit, consisting of the
basic active materials of lead and sulphuric acid, which converts chemical energy into electrical energy. Used to provide current for the operation of the starter as well as other equipment, such as the radio, lighting, etc. Pull start
A rope is wound around the pulley on the crankshaft and then pulled quickly, making the crankshaft rotate.
Push start
In this method, if it is a petrol engine vehicle, the ignition switch is put ON, the transmission (usually second gear) is engaged, the clutch pedal is pressed and while the vehicle is in motion we release the clutch pedal suddenly to start the engine.
Self starter
A high-torque electric motor used for the purpose of starting the engine, typically through a high ratio geared drive connected to the flywheel ring gear.
Starter drive
Mechanism to prevent the starter motor pinion from driving the motor shaft, when the pinion is engaged with the flywheel ring gear because this will cause the shaft to rotate at high speed due to gear ratio increase and destroy the armature.