The secret weapon of the modern automobile engine is a tiny nozzle with an opening less than one millimeter across: the fuel injector. Since fuel injections systems replaced carburetors in the 80’s and 90’s, they have helped automakers build cars with higher fuel efficiency, lower emissions, and higher performance. Because of this important role, manufacturers are in fierce competition to find better designs or systems that can utilize a new generation of biofuels. But the complex physics and chemistry happening at the very tiny tip of a fuel nozzle are extremely difficult to study with standard experimental methods. Engineers at the CI and Argonne increasingly overcome these obstacles -- and other challenges in combustion engine design -- through the use of detailed simulations performed on powerful supercomputers and computing clusters.