Since the hydrocarbons are easily condensed on coming into contact with the coal walls of the piping, to form trouble making tars and oils, they must either be washed out of the gas in the purifier or passed again through the high temperature zone to convert them into permanent gases. In the usual producer, the hydrocarbons are reheated, as they form a considerable percentage of the heat of the gas. After the volatile constituents are reheated, the gases pass through the boiler (B), which absorbs the heat of the gas in generating steam, and from this point the gases enter the scrubber where the dust and the residual tars are removed. The scrubber, which is a sort of filter, is an important factor in the generating plant, for if the dust and dirt were allowed to pass into the cylinder of the engine it would only be a question of a short time until the valves and cylinder would be ground to pieces. When the steam from the boiler is allowed to flow into the ash pit of the producer and up through the incandescent fuel, the heat separates the water vapor into its two elements, oxygen and hydrogen. The oxygen set free combines with the carbon in the coal forming more carbon monoxide, while the hydrogen which is unaffected by the combustion adds to the heat value of the gas. The last additions to the combustion due to the disassociation of the steam are really what is known as “water gas.” A limited amount of steam may be admitted continuously in this manner without lowering the temperature of the fuel below the gasifying point, and its presence is beneficial for it not only provides more CO and hydrogen but produces it without introducing atmospheric nitrogen. The steam is also a great aid in preventing the formation of clinkers on the grate bars. Since the air used in burning the fuel in the first reaction contains about 79 per cent of nitrogen, which is an inert gas, the producer gas is greatly diluted by this unavoidable admixture, which accounts for its low calorific value. [Illustration: Fig. F-6. Diagram of Suction Gas Producer Showing the Generator, Boiler and Washer. ] While the air required for the combustion of the fuel is drawn through the producer by the suction of the engine in the example shown (=SUCTION PRODUCER=), there is a type in common use called a =PRESSURE PRODUCER= in which the air is supplied under pressure to the ash pit by a small blower, which causes a continuous flow of gas above atmospheric pressure. Gas producers are divided into two classes: suction producers and pressure producers. The suction producer presents the following advantages: 1. The pipe line is always less than atmospheric pressure, hence no leaks of gas to the air are possible. 2. The regulation of the gas supply is automatic. 3. No gas storage tank is required. 4. The production of gas begins and stops with the engine. 5. Uniform quality of gas.