Complete combustion results in the carbon of the fuel being reduced to carbon dioxide (CO_{2}) and the hydrogen to water (H_{2}O), 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. [Illustration: 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.