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. This subclass only covers combustion wherein the main body of fuel is either essentially stationary during combustion or mechanically transported, as opposed to pneumatically transported or suspended in air, during combustion. In this subclass, the first place priority rule is applied, i.e. at each hierarchical level, classification is made in the first appropriate place. In this subclass, methods are classified in the groups that cover the apparatus used. Methods that are not related to a particular type of apparatus are classified in group . In this subclass, it is desirable to add the indexing codes of groups . COMBUSTION APPARATUS Combinations of two or more combustion chambers Specially adapted for portability or transportability Functional types Returning solid combustion residues to the combustion chamber Creating a distinct flow path for flue gases or for non-combusted gases given off by the fuel COMBUSTION METHODS NOT RELATED TO A PARTICULAR TYPE OF APPARATUS SUBJECT MATTER NOT PROVIDED FOR IN OTHER GROUPS OF THIS SUBCLASS