A vehicular fuel system with onboard fuel characterization including an onboard combustion modifier source capable of supplying a combustion modifier agent, such as hydrogen, and a fuel tank capable of storing a fuel. An injector-igniter is operative to direct inject the fuel and a proportionate amount of the modifier agent into a cylinder of an internal combustion engine. An engine control unit is operatively connectable to the combustion modifier source and the injector. The system may further comprise a mixing valve operative to proportionately mix the fuel and modifier agent. The combustion modifier source may be a tank containing hydrogen or a thermo-chemical reactor, for example.