This paper presents a theoretical and experimental study of direct fuel injection at conditions relevant to spark ignition (SI) and compression ignition (CI) engines. The focus of this work is to identify the conditions under which fuel droplet formation should occur or be suppressed An experimental investigation of the injection of sub- and supercritical propane into gaseous nitrogen is first discussed: This includes study of one case in which the fuel remained supercritical with respect to temperature and pressure throughout the injection event, and which appears to, be the first time, that truly supercritical, hydrocarbon fuel injection is examined experimentally. A nondimensional parameter tau representing the ratio of the time scales of droplet formation and droplet evaporation is also proposed and used to explain the observed occurrence or suppression of fuel droplets at different conditions. While instants of tau < 1 suggest that droplets should always be observed in any plausible SI or CI engine design, tau > 1 also occurs during the bulk delivery of heavier hydrocarbons in CI engines. In such cases, this should justify simplified modeling of the spray as a dense fluid that mixes with its surroundings, ignoring droplet transport during the less important parts of the injection event.