NOVELTY - The method of operating a fuel processor e.g. a syngas generator to produce hydrogen-containing gas stream, comprises directing a reactant stream (35) through a venturi for downstream chemical conversion within the fuel processor. The reactant stream is choked as it passes through the venturi at 85% of a time during the operation of the fuel processor. The reactant stream is a mixture having a fuel stream and an oxidant stream (34), where the fuel stream is a liquid when at standard temperature and pressure. A speed of the reactant stream through the venturi is 300-700 m/s.    USE - The method of operating a fuel processor e.g. a syngas generator is useful to produce hydrogen-containing gas stream.    ADVANTAGE - The method operates a fuel processor in an economical manner without consuming much fuel.    DETAILED DESCRIPTION - The method of operating a fuel processor e.g. a syngas generator to produce hydrogen-containing gas stream, comprises directing a reactant stream (35) through a venturi for downstream chemical conversion within the fuel processor. The reactant stream is choked as it passes through the venturi at 85% of a time during the operation of the fuel processor. The reactant stream is a mixture having a fuel stream and an oxidant stream (34), where the fuel stream is a liquid when at standard temperature and pressure. A speed of the reactant stream through the venturi is 300-700 m/s, where the speed is greater than a flame speed of the mixture. The oxidant stream comprises engine exhaust gas from a combustion engine. The supply of the oxidant stream to the fuel processor is passively metered by directing it through the venturi. The reactant stream directed through the venturi comprises substantially all of the oxidant stream, which is supplied to the fuel processor during operation. INDEPENDENT CLAIMS are included for:    (1) a fuel processor for producing a hydrogen-containing gas stream; and    (2) an engine system.    DESCRIPTION OF DRAWING(S) - The diagram shows a critical flow venturi.    Critical flow venturi (31)    Fuel processor body (32)    Oxidant stream (34)    Reactant stream (35)    Throat. (36)