## 1 Introduction  
Forecasts for the future of passenger car propulsion predict that battery electric vehicles will be dominant [1]. A valid alternative is to use hydrogen as an energy carrier [2], with significant benefits in terms of employing/adapting existing infrastructure. The properties of this fuel render it a zero-carbon energy conversion solution (i.e., it contains only hydrogen atoms in its molecules) with a high mass-based energy density of 120 MJ/kg, a wide flammability range of 4 to 76%vol, and a laminar flame speed around 10 times higher at ambient pressure temperature compared to other fuels such as gasoline [3]. Internal combustion engines (ICEs) fueled with hydrogen may play an important part in heavy-duty applications [4], given that range requires a high weight/volume of batteries. On the other hand, converting the legacy fleet to \(\mathrm{H_{2}}\) power is a valid approach for achieving zero-GHG emissions with minimum cost.  
The use of hydrogen in ICEs raises many issues that need to be covered, with combustion properties being one of the defining aspects [5, 6]. Improved efficiency is ensured by high laminar flame speed, along with better stability [7], whereas pressure rise rate limits constrain power density [8]. This parameter is also influenced by lower volumetric efficiency [9] compared to \"classic\" fuels, resulting in the need to apply super- or turbo-charging. Direct injection further improves performance [10] and reduces the risk of backfire [11]. Port fuel injection (PFI), on the other hand, maximizes fuel tank emptying and does not require a dedicated high-pressure pump; one important issue is that injector placement needs to be carefully assessed [12] in order to minimize the chances of causing abnormal combustion [13]. Even if all of these aspects highlight several difficulties of applying hydrogen fueling in ICEs, a demonstration of close-to-commercial H\({}_{2}\) vehicles [14] has shown that practically zero emissions is possible.  
One particular property of H\({}_{2}\) is that it has very good tolerance to dilution, be it with fresh air or exhaust gas recirculation (EGR) [15]. This is directly related to its high laminar flame speed [16], which allows the engine to be operated with relatively high concentrations of inert gas without significant effects on combustion stability. Even low concentrations of H\({}_{2}\) used as an additional fuel can bring significant benefits for gasoline- [17; 18] or biogas-based engines [19]. In addition to the benefits of ultra-lean operation [20] (well known for ensuring low NO\({}_{\text{x}}\) emissions [21]), EGR can improve combustion characteristics [22]; it also allows for the application of three-way catalytic converters, which is less complex than the after-treatment solutions required for lean fueling [23]. It has further beneficial effects with respect to margins for controlling abnormal combustion [24; 25; 26]. Recirculation can be employed as load control as well, with improved margins [27].  
Within this context, the present work studies the specifics of converting a small-size passenger car to H\({}_{2}\) operation. The approach is control oriented and based on 0D/1D simulations aimed at evaluating the benefits of applying EGR. Compared to previous versions of the model, the components related to gas recirculation were added and attention was paid to the residual gas effect, as well as thermal aspects directly linked with the application of EGR. Two recirculation configurations were under scrutiny in the form of high- and low-pressure recirculation. Apart from full-load characteristics, four conditions representative of urban and highway driving were considered. A wide range of EGR rates of up to 30% concentration in the intake manifold was investigated in light of the high tolerance of H\({}_{2}\) to diluted combustion. Boosting requirements and combustion characteristics were analyzed and compared to gasoline fueling, which was taken as the reference.