conduit 43 the compressed air stream in conduit 43 is divided into two streams bearing a selected ratio volumetric or mass to each other the division is accomplished by providing mixing conduit 44 which is an extension of air conduit 43 and branch or auxiliary air conduit 45 conduits 44 and 45 are each connected to the precombustion chamber 50 air flow dividing orifice plates 46 and 47 are mounted in conduits 44 and 45 adjacent the branching or division point and the orifices in the plates are sized to bring about the desired division of the air flow preferably the volume of flow through auxiliary air conduit 45 amounts to about 8 to 10 percent of the air flow through mixing conduit 44 immediately downstream of orifice plate 46 in mixing conduit 44 there is provided a fuel inlet 48 flow in conduit 44 just downstream of the orifice plate 46 is quite turbulent and it is desirable to introduce the fuel at this point to initiate thorough and intimate mixing of the fuel and air furthermore it is preferred that mixing conduit 44 be fairly long in order to provide a full opportunity for thorough mixing of the air and fuel stream before it reaches the precombustion chamber mixing is also enhanced by the directional change in conduit 44 at bend or elbow 49 the diameter of mixing conduit.