Thermodynamic Cycles

Abstract

Though both Otto cycle and Diesel cycle works on engine , so the question raises why the diesel cycle was invented as there already exists an Otto cycle . In Otto cycle air-fuel mixture enters into the cylinder . As the fuel is also there in the cylinder before it is ignited by the spark plug , compression is limited i.e. the compression of the mixture is limited by the fact is that the temperature rise due to compression should be less than the ignition temperature of the fuel in the mixture. So the mixture cannot be compressed so that it is ignited only by compression before the sparking. So the compression ratio cannot be increased beyond certain limit . As the compression ratio is not very large so the large work output cannot be attained . Hence such cycle cannot operate engine for heavy vehicles where greater work output is needed .If Otto cycle would be used for heavy vehicle engine the engine size would be very large . Moreover an engine operating on Otto cycle with high compression ratio would cause noise and engine problem called detonation . So to overcome these limitations the Diesel cycle was employed. There is no spark ignition in the engine operating on diesel cycle. First only air is compressed and at end of compression , fuel is injected . So it is ignited only by compression . As only air is compressed , it can be compressed higher than in a Otto cycle as the chance of spontaneous ignition before fuel injection is less . A higher compression ratio can be attained and greater work output is achieved. Hence it can be used for heavy vehicle also . No trouble as detonation in Otto cycle operated engine is occurred.