Described here are systems and methods for reducing emissions of IC engines using a fuel processor bypass. In general, the systems described here include an exhaust pipe, a bypass pipe, a valve, a fuel processor, a fuel injector, and a NOx trap. When the valve is in the open position, the entire exhaust passes through the bypass pipe. When the valve is in a closed position, the entire exhaust passes through the exhaust pipe. In some variations, the systems described here also comprise a pre-combustor, a thermal mass, a mixer, and/or a DPF. Methods for regenerating or desulfating a NOx trap are also described. Typically these methods include introducing exhaust into an exhaust pipe, opening a valve located at the inlet of a bypass pipe, injecting fuel upstream of a fuel processor, and introducing a reducing mixture into a NOx trap. The injection of fuel may be pulsed or continuous.