A fluid delivery apparatus with a flow sensing means for delivering a first fluid into a second fluid. In a fluid delivery apparatus using a common rail method, which produces a pulsated flow, the flow sensing means generates sensing signals indicative of the flow rate and the temperature of the second fluid, the delivery rate of the first fluid, and the evaporating rate of the first fluid, while in a fluid delivery apparatus using a pump metering method, the flow sensing means is able to provide a sensing signal indicative of the delivery rate of the first fluid. The sensing signals can be used in a feedback control for controlling delivery rate, in limiting delivery rate according to the evaporation capability of the first fluid, and in a diagnostic system detecting failures and abnormalities in the fluid delivery apparatus.