MACHINES positive displacement rotary or oscillating piston liquid and elastic fluid or elastic fluid liquid only reciprocating piston or other liquid and elastic fluid or elastic fluid liquid only non-positive displacement liquid and elastic fluid or elastic fluid liquid only ENGINES positive displacement rotary or oscillating piston liquid and elastic fluid or elastic fluid liquid only reciprocating piston or other liquid and elastic fluid or elastic fluid liquid only non-positive displacement liquid and elastic fluid or elastic fluid liquid only PUMPS positive displacement rotary or oscillating piston reciprocating piston or other non-positive displacement In this class, the following terms or expressions are used with the meanings indicated: "combustion" means the direct combination of oxygen gas, e.g. in air, and a burnable substance. Any other heat-producing combination of chemical substances, e.g. hydrogen peroxide and methane, iron oxide and aluminium, is covered by section C or by subclass ; "combustion chamber" means a chamber in which fuel is burned to establish a self-supporting fire or flame and which surrounds that fire or flame; "burner" means a device by which fluent fuel is passed to a combustion space where it burns to produce a self-supporting flame; "air" means a mixture of gases containing free oxygen and able to promote or support combustion. In this subclass, methods are classified in the groups that cover the apparatus used. COMBUSTION APPARATUS SPECIALLY ADAPTED FOR COMBUSTION OF TWO OR MORE TYPES OF FUEL COMBINATIONS OF TWO OR MORE COMBUSTION CHAMBERS FUNCTIONAL TYPES OF COMBUSTION APPARATUS Fluidised bed combustion Catalytic combustion Resonant combustion COMBUSTION APPARATUS CHARACTERISED BY SUBSYSTEMS Combustion chambers Arrangement or mounting of burners Air supply Arrangements for returning flue gases or combustion products SUBJECT MATTER NOT PROVIDED FOR IN OTHER GROUPS OF THIS SUBCLASS In this group, it is desirable to add the indexing code of group .