heat is needed to raise the temperature of the excess of air, that the temperature of the gases immediately after complete ignition never approaches the theoretical.
If the actual temperature of combustion is known, it is possible to determine the increase of pressure that a given quantity of gas will undergo. As the process of combustion is practically the conversion of one mixture of gases into another, it follows that for equal temperatures the volume of the products of combustion will not necessarily be greater than that of the mixture before combustion. In fact, it has been found that, for the ordinary gas-engine fuels, the volume of the products of combustion is slightly less, and, when the water vapor in them condenses, the volume is very much smaller. However, the condensation of this vapor does not need to be considered here, since the temperatures from combustion, until the gases are discharged, are far above the boiling point of water.
DISSOCIATION
28. An important cause of the lowering of the temperature is the phenomenon known as dissociation. It may be defined as the decomposition due to the high temperatures generated in the gas-engine cylinder. Water in the form of steam, for instance, commences to decompose into hydrogen and oxygen at a temperature between 1,660° and 1,830° F. At about 2,200°, dissociation is increased; but at this temperature, the limit of dissociation appears to be reached. Since a temperature of 2,800° is quite frequently produced in the gas engine, it follows that dissociation becomes an important factor in the determination of what is taking place during the cycle.
Besides reducing the temperature from what might be obtained, dissociation prevents the complete combustion of the fuel at the beginning of the stroke. Hence, heat is constantly being added to the contents of the cylinder as the piston proceeds and the temperature of the mixture falls, allowing the dissociated gases to combine.
