An injection/mixing device (2) for improving the mixing of fuel and air in a petrol driven internal combustion engine. The device (2) is fitted in the engine inlet manifold (3) close to the fuel inlet from a carburettor or fuel injector (6) and uses a vortex generator. The vortex generator is in the form of an array of lenticular, conical or deltoid disks (22) located in the fuel/air flow and which serve to disperse the impinging fuel droplets and mix the fuel and air. The effect of the device is preferably enhanced by allowing the vortex generator to rotate or spin about its axis. This rotation is achieved by means of a turbine (40) associated with the disks or by means of an external motor. The device may also be connected to a vacuum relief valve (1) to attenuate the level of the depression in the intake manifold under certain conditions. The vacuum relief valve may be integral or separate from the device.