The growing need for a sustainable worldwide mobility is leading towards a paradigm shift in the automotive industry. The increasingly restrictive regulations on vehicle emissions are indeed driving all of the world-leading road vehicles manufacturers to redesign the concept of transportation by developing new propulsion solutions. To this aim, a gradual electrification strategy is being adopted, and several hybrid electric solutions, such as extended-range electric vehicles with reciprocating engines or fuel cells, already represent a valid alternative to conventional vehicles powered by fossil fuels. Despite their appealing features, these hybrid propulsion systems present some drawbacks, mainly related to their complex architecture, causing high overall dimensions, weight and costs, which pose some limitation in their use for small-size vehicles. In this context, the Wankel engine may bring significant advantages, since it is characterized by an extremely compact and light design, it has excellent noise and vibration features, and it is potentially cheap to manufacture. As a consequence, the use of a rotary engine as range extender in hybrid propulsion systems represents a very attractive option, especially for small-size vehicles. In addition, the Wankel engine is particularly well suited to be powered by hydrogen fuel. In fact, hydrogen fuel, besides bringing clear advantages on the overall vehicle emissions, may diminish the inherent combustion difficulties that are caused by the shape of the combustion chamber of a rotary engine. Thus, in this work, we model a hydrogen-fueled rotary engine configuration to evaluate its potential as auxiliary power unit in ultra-low emission small-size hybrid vehicles. Starting from a baseline series hybrid electric vehicle with reciprocating internal combustion engine, we replace the range extender to numerically investigate on the performance of the proposed solution, in terms of energy and fuel consumption. The weight saving due to the use of Wankel engine is compensated by introducing additional battery modules, in such a way to keep the original weight of the baseline vehicle as a fixed parameter. Different range extender options are also analyzed for comparison, including a reciprocating and a rotary engine both fueled by gasoline.