A fuel reformer system that uses a steam reforming reaction in the exhaust gas recirculation (EGR) line with a catalyst was earlier proposed.(1) An analysis of engine test results revealed that not only hydrogen (H2) but also a H2 rich reformate additive in the air-fuel mixture was effective in suppressing knocking.To improve fuel economy via a high compression ratio, the knock limit is extended through the addition of H2 with its high octane number. In order to produce H2 on-board, we have proposed a fuel reformer for which the additions to the engine are an injector and a catalyst in the existing cooled EGR system. This method produces thicker H2 gas from gasoline by using heat and water vapor in the exhaust gas.The reformate mainly consists of H2, CO and CH4. The results of a previous study showed that hydrogen's high-speed flame had the effect of extending the EGR limit and that the reformate components besides hydrogen did not inhibit this effect.(1) Because the fuel has a higher octane number than ordinary gasoline, it is hypothesized that adding the reformate to the gasoline mixture might not only extend the EGR limit but also have an anti-knocking effect.In the present study, it was confirmed that RON of the total fuel in the engine was improved by around 2% by the reformate additive in this fuel reformer system. CO contained in the reformate gas had a negligible effect on eliminating H2's anti-knocking effect and the reformate gas additive can improve engine torque and fuel consumption like pure H2 under an operating condition with noticeable knocking. Finally, indicated specific fuel consumption was reduced by 1.6% as the anti-knocking effect of reformate H2 extended the knock limit by 6 degrees.