Since complete combustion is never obtained under practical working conditions, the actual evolution of heat and the actual temperatures are always much lower than those indicated by the CALORIMETER or heat measuring device. Besides the loss of heat due to imperfect combustion, there are many other losses such as the loss by radiation, connection, and slow burning, the latter being the principal cause of low combustion temperatures. From the statements in the foregoing paragraphs it will be seen that the theoretical or absolute calorific value of a fuel is not always a true index to its efficiency in the engine.
Complete combustion results in the carbon of the fuel being reduced to carbon dioxide (CO2) and the hydrogen to water (H2O), with the liberation of atmospheric nitrogen that was previously combined with the fuel, and some oxygen. The reduction of the fuel to carbon dioxide and water produces every heat unit available since the latter compounds represent the lowest state to which the fuel can be burned. Carbon however may be burned to an intermediate state without the production of its entire calorific contents when there is not sufficient oxygen present to thoroughly consume the fuel. Incompletely consumed carbon produces a gas, carbon monoxide, as a product of combustion, and a quantity of solid carbon in a finely subdivided state known as “soot.” Unlike the products of complete combustion, both the carbon monoxide and soot may be burned to a lower state with a production of additional heat when furnished with sufficient oxygen, both the soot and the monoxide being reduced to carbon dioxide during the process.
Fig. F-2. Sunbeam Engine with Six Cylinders Cast “En Bloc” (in one piece). At the Right and Under the Exhaust Pipe is the Compressed Air Starting Motor that Starts the Motor Through the Gear Teeth Shown on Flywheel. From “Internal Combustion.”
As the soot and monoxide have a calorific value it is evident that much of the heat of the fuel is wasted if they are exhausted from the cylinder without further burning at the end of the stroke. To gain every possible heat unit it is necessary to furnish sufficient oxygen or air to reduce the fuel to its lowest state. As the free oxygen and nitrogen contained in the fuel are without fuel value, their rejection from the cylinder occasions no loss except for that heat which they take from the cylinder by virtue of their high temperature.
With complete combustion the TEMPERATURE attained increases with the rate of burning, while the number of heat units developed remain the same with any rate of combustion. Because of the conditions under which the fuel is burned in the gas engine the fuel is burned almost instantaneously with the result that high temperatures are reached with fuels of comparatively low calorific value. With a given gas the rate of combustion is increased with an increase in the temperature of the gas before ignition and remains constant for all mixtures of this gas in the same proportion when the initial temperature is the same. The rate of combustion also varies with the composition of the gas, hydrogen burning more rapidly than methane. As a rule it might be stated that the rate of burning decreases with the specific gravity of the gas, the light gases such as hydrogen burn with almost explosive rapidity, while the heavier gases such as carbon dioxide are incombustible or have a zero rate of combustion. In practice an increased rate of burning is obtained by heating the charge before ignition by a process that will be explained later.
Another factor governing the output of an engine with a given size cylinder is the amount of air required to burn the fuel. The quantity of air necessary for the combustion of the fuel determines the amount of fuel that can be drawn into a given cylinder volume, and as we are dependent upon the fuel for the expansion it is evident that with two fuels of the same calorific value, the one requiring the least air will develop the most power. Since the air required to burn hydrogen gas is only one fourth of that required to burn the same amount of methane it is clear that more hydrogen can be burned in the cylinder than methane. This great increase in output due to the hydrogen charge is however, considerably offset by the greater calorific value of the methane.
Should the air be in excess of that required for complete combustion, or should a great quantity of incombustible gas, such as nitrogen be present in the mixture, the fuel will be completely burned, but the speed of burning will be reduced owing to the dilution. As the air is increased beyond the proper proportions the explosions become weaker and weaker as the gas becomes leaner until the engine stops entirely. Because of the fact that it is impossible in practice to so thoroughly mix the gas and air that each particle of gas is in contact with a particle of air, the volume of air used for the combustion is much greater than that theoretically required. A SLIGHT excess of air, making a lean mixture, increases the efficiency of combustion although it reduces the temperature and pressure attained in the cylinder. This is due to the fact that while the temperature of the mixture is lower than with the theoretical mixture the temperature of the burning gas itself is much higher. A mixture that is too lean to burn at ordinary temperatures will respond readily to the ignition spark if the temperature or pressure is raised.
(4) Compression.
In the practical gas engine the gas is not ignited at the beginning of the suction stroke by which it is drawn into the cylinder, but is compressed in the front end of the cylinder by the return stroke of the piston, and then ignited. The process of compression adds greatly to the power output of a given sized cylinder and increases the efficiency of the fuel and expansion. In order to understand the relation that the compression bears to the expansion let us refer to Fig. 2 in which C is the working cylinder, P the piston and G the crank. While the piston is moving towards the crank in the direction of the arrow A it draws the mixture, indicated by the marks x x x x x, into the cylinder, the quantity being proportional to the position of the piston. In this particular case let us assume that the area of the piston is 50 square inches and that the entire stroke (B) of the piston is 12 inches. To prevent confusion due to considerations of heat loss we will further assume that the cylinder is constructed of non-conducting material.