04. Ignition and Exhaust system

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1 New Polytechnic Kolhapur Page 1 of Ignition and Exhaust system 4.1 Introduction to Ignition System 4 Marks Requirements of ignition system. Magneto and Battery Ignition systems (Working only). Firing order used in 3,4 and 6 cylinder engines 4.2 Types of Exhaust system 4 Marks Function of Exhaust manifold. Construction, Working and types of silencer / Mufflers 4.1 INTRODUCTION TO IGNITION SYSTEM Requirement of ignition system. A reliable ignition system is required for proper working of engine. The requirement of such ignition system is as follows. 1. It should provide a good spark between the electrodes of the plugs at the correct timing 2. It should function efficiently over the entire range of engine speed. 3. It should be light in weight. 4. It should be reliable in service. 5. It should be compact. 6. It should be easy to maintain. 7. It should be cheap and convenient to handle. 8. It should not affect radio and TV frequency. Types of ignition system There are two types of ignition system 1. Battery ignition system (coil ignition system) 2. Magneto ignition system. Battery ignition system was generally used in 4-wheelers, but now-a-days it is more commonly used in 2-wheelers also (i.e. Button start, 2-wheelers like Pulsar, Kinetic Honda; Honda-Activa, Scooty, Fiero, etc.). In this case 6 V or 12 V batteries will supply necessary current in the primary winding. Magneto ignition system is mainly used in 2-wheelers, kick start engines. (Example, Bajaj Scooters, Boxer, Victor, Splendor, Passion, etc.). In this case magneto will produce and supply current to the primary winding. So in magneto ignition system magneto replaces the battery.

2 New Polytechnic Kolhapur Page 2 of 10 Battery or Coil Ignition System Figure shows line diagram of battery ignition system for a 4-cylinder petrol engine. It mainly consists of a 6 or 12 volt battery, ammeter, ignition switch, autotransformer (step up transformer), contact breaker, capacitor, distributor rotor, distributor contact points, spark plugs, etc. The ignition system is divided into 2-circuits : Primary Circuit : It consists of 6 or 12 V battery, ammeter, ignition switch, primary winding it has turns of 20 SWG (Sharps Wire Gauge) gauge wire, contact breaker, capacitor. Secondary Circuit: It consists of secondary winding. Secondary winding consists of about turns of 40 (S WG) gauge wire. Bottom end of which is connected to bottom end of primary and top end of secondary winding is connected to centre of distributor rotor. Distributor rotors rotate and make contacts with contact points and are connected to spark plugs which are fitted in cylinder heads (engine earth).

3 New Polytechnic Kolhapur Page 3 of 10 Working: When the ignition switch is closed and engine in cranked, as soon as the contact breaker closes, a low voltage current will flow through the primary winding. It is also to be noted that the contact beaker cam opens and closes the circuit 4- times (for 4 cylinders) in one revolution. When the contact breaker opens the contact, the magnetic field begins to collapse. Because of this collapsing magnetic field, current will be induced in the secondary winding. And because of more turns (@ turns) of secondary, voltage goes unto volts. This high voltage current is brought to centre of the distributor rotor. Distributorrotor rotates and supplies this high voltage current to proper stark plug depending upon the engine firing order. When the high voltage current jumps the spark plug gap, it produces the spark and the charge is ignited-combustion starts-products of combustion expand and produce power. Note : (a) The Function of the capacitor is to reduce arcing at the contact breaker (CB) points. Also when the CB opens the magnetic field in the primary winding begins to collapse. When the magnetic field is collapsing capacitor gets fully charged and then it starts discharging and helps in building up of voltage in secondary winding. (b) Contact breaker cam and distributor rotor are mounted on the same shaft.

4 New Polytechnic Kolhapur Page 44 of 10 Magneto Ignition System In this case magneto will produce and supply the required current to the primary winding. In this case as shown, we can have rotating magneto with fixed coil or rotating coil with fixed magneto for producing and supplying current to primary, remaining arrangement is same as that of a battery ignition system. Comparison between battery and magneto ignition system

5 New Polytechnic Kolhapur Page 55 of 10 FIRING ORDER The order or sequence in which the firing takes place, in different cylinders of a multicylinder engine is called Firing Order. In case of SI engines the distributor connects the spark plugs of different cylinders according to Engine Firing Order. Advantages (a) A proper firing order reduces engine vibrations. (b) Maintains engine balancing. (c) Secures an even flow of power. Firing order differs from engine-to-engine. Probable firing orders for different engines are : - 3 cylinder = cylinder engine (inline) = cylinder horizontal opposed engine = (Volkswagen engine) - 6-cylinder in line engine = (Cranks in 3 pairs) cylinder in line engine

6 New Polytechnic Kolhapur Page 66 of 10 8 cylinder V type Cylinder 1 is taken from front of inline and front right side in V engines.

7 New Polytechnic Kolhapur Page 77 of TYPES OF EXHAUST SYSTEM MANIFOLD: (W-11) There are separate sets of pipe attached to the cylinder head, which carries airfuel mixture & the exhaust gases. These are called as manifolds. There are 2 types of manifolds: A) Inlet Manifold. A) Inlet manifold: B) Exhaust manifold. 1) Its function is to carry air-fuel mixture from carburetor to the cylinder 2) It must be large enough to allow sufficient flow of the charge & small enough to maintain adequate velocity. 3) The various manifold passages are called as runners. 4) In inline engine usually, the inlet & exhaust manifolds are mounted on the same side. The heat from the exhaust manifold is absorbed by the inlet manifold, thus vaporization of air fuel mixture occurs effectively. B) Exhaust manifold: 1) Its function is to carry the exhaust gases from the cylinder head to the silencer. 2) It is made of cast iron, so that it withstands heat of exhaust gases.

8 New Polytechnic Kolhapur Page 88 of 10 3) Exhaust manifold are designed in such a way that minimum restriction is provided to the flow of exhaust gases. 4) A smoother flow of exhaust gas is achieved by 2 in 1 manifold as compared to the simple one. SILENCER: A) Function: When the exhaust valve opens, high pressure exhaust gas is released; this causes a pressure wave in the air, leading to an explosion. Since the high pressure gases are released rapidly & repeatedly it tends to form a uniform noise. This is combination of the low frequency &r high frequency noise. To reduce the noise, the engine is connected via an exhaust pipe to the silencer, which is called as muffler. Types of Mufflers: 1) Baffle type 2) Wave cancellation type 3) Resonance type 1) Baffle type muffler: 4) Absorber type 5) Combined Resonance and Absorber type.

9 New Polytechnic Kolhapur Page 99 of 10 It is generally cylindrical in shape with a number of baffles spot welded inside. There are many baffles type mufflers, but principle in all cases is the same, i.e. closing any direct passage for the gas. The major drawback of this type of muffler is their low efficiency. Because of the restriction provided to the flow by the baffles, the back pressure is increased, thus causing loss of engine power. 2) Wave cancellation type muffler: In this type, exhaust gases are divided into 2 parts. The length of these paths are so adjusted that after they come out of the muffler, the crests of one wave coincide with the troughs of the second wave, thus cancelling each other and reduce the noise to zero theoretically. In this muffler resistance to the main gas flow is very small compared to the baffle type. 3) Resonance type muffler: This type of mufflers consists of number of resonators in series, through which a pipe containing access ports passes. The exhaust gases flow through this type and thus experience no resistance. Series of resonators eliminate the fundamental and higher harmonics of the engine noise.

10 New Polytechnic Kolhapur 4) Absorber type muffler Page 1010 of 10 Straight type absorber muffler The sound absorbing material, usually fiber glass is placed in this case around the perforated tube through which the exhaust gases pass. During high pressure fluctuation the gases pass through the perforations to the sound absorbing material, when these fluctuations are reduced the intensity of noise is also reduced. This muffler may be straight type or reverse flow type. 5) Combined Resonance and Absorber type muffler: It is seen that the absorber type muffler has a drawback, that it is not efficient in reducing noise of low frequency. To obviate this defect, combined resonance and absorber type muffler is used. It is found that this type of muffler is more efficient than either the simple resonance or the absorber type muffler.