of movement in the direction of arrow b all the compressor air is routed through the burner passage 7 for primary combustion in this position the burner passage 7 has maximum cross sectional area with the minimum restriction to air flow see cross hatched area of fig2 the air bypass valve arrangement 6 being fully closed so that no air can pass through it this configuration corresponds with the engine maximum load condition conversely when the actuator rods 5 move the combustor wall portion 3 to an extreme limit of movement in the direction of arrow c the cross sectional area of the burner passage 7 is reduced to a minimum see cross hatched area of fig2 a so that the primary air flow passing through the burner passage 7 is limited the remaining air passing through the fully open ports of the air bypass valve arrangement 6 this configuration relates to engine low load condition it will be appreciated that by controlling the actuator rods 5 the combustion chamber 3 4 may be set to any position between those illustrated in fig2 and 2 a so that it is possible to maintain the correct primary to secondary air ratio to ensure acceptable exhaust pollution and engine efficiency standards for various load conditions it will be understood that by this simple and convenient arrangement the primary and secondary air flows are varied in inverse proportion to each other fig3 and 3 a illustrate the manner in which a port defined through a wall of the downstream combustion chamber portion 4 can be closed by the so.