A vehicular propulsion system, a vehicular fuel system and a method of operating an internal combustion engine. A separation unit that makes up a part of the fuel system may selectively receive and separate at least a portion of onboard fuel and a flowable adsorbent in order to separate the fuel into octane-enhanced and cetane-enhanced fuel components. A controller may be used to determine a particular operating condition of the internal combustion engine such that the onboard fuel can be sent to one or more combustion chambers within the internal combustion engine without first passing through the separation unit during one operating condition, or instead to the separation unit in situations where the internal combustion engine may require an octane-rich or cetane-rich mixture in another operating condition.