Mimicking physical odor sensations virtually can present users with a real - time odor synthesis that approximates what users would smell in a virtual environment, e.g., as they walk around in virtual reality. To this end, we devise a Smell Engine that includes: (i) a Smell Composer framework that allows developers to configure odor sources in virtual space, (ii) a Smell Mixer that dynamically estimates the odor mix that the user would smell, based on diffusion models and relative odor source distances, and (iii) a Smell Controller that coordinates an olfactometer to physically present an approximation of the odor mix to the user’s mask from a set of odorants channeled through controllable flow valves. Through a three - part user study, we found that the Smell Engine can help measure a subject’s olfactory detection threshold and improve their ability to precisely localize odors in the virtual environment, as compared to existing trigger - based solutions.