Firing

FIRING ORDER Every engine cylinder must fire once in every cycle. This requires that for afour stroke ,four -cylinder engine.The ignition system must fire for every 180 of crank rotation for a six-cylinder engine the time avialibei only 120 of crank rotation The order in which various cylinders of a multicylinder engine fire is called the FIRING ORDER.The the numbers of possibilites of firing order depends upon the numbers of cylinder and throws of crankshaft .It is derinable to have power impulses equally spaced and from the point of view of balincing .this has led to certain conventional arrengements of crankshaft throws.Further, There are three factors which must be considered before deciding the optimum firing order of an engine ,They are (i) ENGINE VIBRATIONS (ii) ENGINE COLLING (iii) DEVELOPMENT OF BACK PRESSURE consider that the first cylinder of the four-cylinder engine show in fig is fixed first.a pressure p is generated in the cylinder no 1 giving rise to load equal to pxa and pxb on the two bearings Aand B respectively. The load on baring A is lower than that on B. Now,if the next cylinder fired is number 2, this imbalance in laod on the two bearings would be further increased which would result in severe engine vibrations.If we fire the third cylinder after first, the load is eqally distributed. Further consider the effect of firing osequence on COLLING.when the first cylinder is fired its temprature is increased .If the next cylinder that fire in number 2,the pistion of the engine between the cylinder no 1&2 gets over heated.If then the 3rd cylinder is fired ,over heating is shifted to piston between the cylinder 2&4.Thus we see that the task of colling system becomes very DIFFICULT because it is then requires to cool more one place than the other place and this impliese great strain on the cooling system .If the 3rd cylinder is fired after the first the over heating problem can be controlled to great extent. Next,consider the flow of exhaust gases in the exhaut pipe .after firing the first cylinder, exhaut pipe.If the next cylinder fired in cylinder2 .We find that before the gases exhaut by the 1st cylinder go out of the exhaut pipe the gases exhaut froe the 2nd cylindertry to overtake them. This would require that the exhaut pipe be made bigger .Otherwise BACK PRESSURE IN IT WOULD BE INCREASE . and the POSSIBILITY OF BACKFLOW would arise . Instead of firing cyliner no2 ,cyliner3 is fired then by the time gases exhausted by the cyliner3 came into exhaut pipe ,the gases from the cylinerwould have SUFFICIENT TIME to travel the distance between the cyliner1& cyliner3 and thus ,the development of a HIGH BACK PRESSURE IS AVOIDED . It should be noted that to some extent all the above thesr requirements are conflicting and therefore a TRADE-OFF is necessary. For FOUR-CYLINDER ENGINES.the possible firing orders are: 1-3-4-2 or 1-2-4-3 For a SIX-CYLINDER ENGINES.the possible firing orders are: 1-5-3-6-2-4 or 1-5-4-6-2-3 or 1-2-4-6-5-3 or 1-2-3-6-5-4