The suction producer is limited to power application and cannot be used where the gas is to be used for heating, as in furnaces, ovens, etc., or where the engine is at a distance from the producer, unless pumped to its destination. The pressure producer does not yield a uniform quality of gas, hence requires a storage tank where low quality gas will blend with gas of higher calorific values and produce a gas of fairly uniform quality. The pressure producer is adapted to the use of all grades of fuels, such as bituminous coal and lignite. Anthracite coal contains little volatile matter and is an ideal fuel for the manufacture of producer gas, while bituminous coal with its high percentage of volatile matter and tar, requires more efficient scrubbing, as these substances must be removed from the gas. On starting the producer shown by Fig. 6, the producer is filled with the proper amount of kindling and coal, and a blast of air is sent into the ash pit by a small blower, the products of combustion being sent through the by-pass stack (K) until the escaping gas becomes of the quality required for the operation of the engine. The by-pass valve is now closed, and the gas is forced through the scrubber to the engine until the entire system is filled with gas. When good gas appears at the engine test cock the engine is started, and the blower stopped, the gas now being circulated by the engine piston. The volume of gas generated by the producer is always equal to that required by the engine so that no gas receiver or reservoir is required. Because of the friction of the gas in passing through the fuel, scrubber and piping, its pressure at the engine is always considerably below that of the atmosphere, which of course reduces the amount of charge taken into the cylinder. Because of the weak gas and the low pressure in the piping, it is necessary to carry a much higher compression with producer gas than with natural or illuminating gas.