"It has before been proposed to heat compressed air or gas by means of the combustion in it of petroleum, alcohol or other suitable combustible liquid, thereby raising the temperature of the air and consequently increasing the volume which a given weight would occupy at a given pressure. The rise of temperature has other beneficial effects, notably that it is capable of preventing the formation of ice from any water which may have been carried forward by the compressed air. According to this invention the combustion takes place inside the pipe or passage through which the compressed air is supplied to the engine or pneumatic tool, which pipe or passage will usually be locally increased in diameter in order to provide a combustion-chamber of sufficient capacity and suitable shape, and the arrangement is such that the ratio of fuel to air can be kept practically constant or can be varied at pleasure."