The invention provides a vehicle engine system comprising: a fuel pump for selectively delivering fuel under high pressure; an accumulator having a first chamber for receiving an output from the fuel pump and a second chamber for receiving an oil feed, wherein as one chamber is filled up the other chamber is compressed; wherein on vehicle acceleration the fuel pump delivers fuel to a common rail fuel injection system, and on vehicle braking the fuel pump delivers fuel to the first chamber of the accumulator to thereby put the oil in the second chamber under pressure, and wherein on subsequent acceleration the oil chamber delivers an output under pressure.