Frank Zegler of ULA provides some background here, and with a follow-up here, on the development of their Integrated Vehicle Fluids (aka, internal combustion engine (ICE)) for the Centaur upper stage. A question and answer at Stack Exchange explains the practical benefit.
This is really great stuff, and may be what keeps ULA "in the space business game" once the expendable Atlas rockets are retired. No one else has technology like this that I'm aware of, and ULA's in-space "rocket trucks" could become the deuce-and-a-half of the coming space age. The increased mission duration and number of burns is particularly relevant for anything beyond LEO - such as missions to the Moon or an asteroid.
The shape of the business in the 2020-2030 range seems to be: SpaceX launch, ULA cargo movers throughout Cis-Lunar space, and Bigelow-leased campers and destinations (passenger cars, space stations, and lunar/asteroid habitats). Even companies developing their own in-space technology (such as Planetary Resources) might decide to not duplicate R&D efforts and rely on ULA for moving things around.
Also, take a moment to appreciate the irony of the fact that Elon Musk (by making ULA's Atlas rockets uncompetitive in the launch market) has forced them to create a new market for internal combustion engines.