A nozzle is disclosed for the air injection of fuel into the combustion chamber of a diesel engine cylinder. The nozzle includes a piston reciprocably mounted in the nozzle body to define therein a premixing chamber to be placed in and out of communication with the combustion chamber via spray holes at the tip of the nozzle body. Slidably fitted at one end in an axial bore in the piston, a needle valve is sprung to normally close the spray holes with its other end. Toward the end of the compression stroke of the piston, the mixture of fuel and air trapped in the premixing chamber acts on the needle valve to cause same to open the spray holes against the bias of the spring and thus is injected into the combustion chamber. The spring serves to cause the needle valve to firmly close the spray holes immediately upon full injection of the fuel-air mixture.