Evaluation and optimization of H2 and C2H4 production in piston engines via an adjoint-based approach

In the context of the energy transition, efficient energy systems are desired. One promising approach is the use of piston engines for energy and species conversion. Before such processes are in fact realized, they are studied using simulations so that their capabilities can be identified. To optimize corresponding processes regarding polygeneration and exergy storage, it is necessary to analyze a large number of process parameters fast and reliably.In this work, we extend an adjoint-based approach for the sensitivity analysis of homogeneous reactors by a piston engine model and apply it for the optimization of H2 and C2H4 production by modification of initial conditions and additives. The approach reliably delivers good results for representative configurations. It was found that the optimum equivalence ratio for H2 production in piston engines ranges from 2.33-2.86, depending on which additives may be added, leading to H2 yields up to 80%. For C2H4 optimization, pyrolysis reactions get more important, leading to very fuel-rich conditions with an equivalence ratio of 6.2 to 7.2. C2H4 yields up to 12% are achieved. To evaluate these conditions thermodynamically, an exergetic analysis was carried out, revealing exergetic efficiencies up to 88% and 95%, respectively, if the unconverted fuel is recirculated. In addition, the approach provides time-dependent information on all process parameters, including mixture and engine parameters, which can be used to develop alternative processes and performance strategies.