Abstract: A vaporizing nozzle which comprises a wall (16) of porous material through which a liquid to be evaporated percolates from one side thereof into a gas stream at the other side thereof and a thermoelectric device (30) located at or adjacent the wall of porous material whereby to regulate the temperature thereof. A computer is provided, with a read only memory (ROM) or equivalent containing a "map" of the air to fuel ratio required for given values of the various parameters and compares the sensed values with the map held in memory and sends an appropriate signal to thermoelectric device controller which responds by sending a current of the correct magnitude and polarity to the device such that the device, which is in close proximity to the wall heats or cools the surface of the wall to a desired level thus regulating the flow or fuel therethrough.