An internal combustion engine system for a vehicle includes an internal combustion engine, ICE, operable on a low cetane fuel and having a cylinder at least partly defining a combustion chamber and an ignition source for the low cetane fuel; a fuel injector for injecting the low cetane fuel into the combustion chamber; an ignition improver device in fluid communication with the fuel injector and further configured to supply an ignition improver fluid to the low cetane fuel; a control unit configured to selectively operate the ICE in a spark ignition, SI, mode and a compression ignition, CI, mode. The control unit determines an ICE operating condition and controls the ignition improver device to supply a given amount of ignition improver fluid to the low cetane fuel on the basis of said determined ICE operating condition.