The flexible energy conversion in piston engines offers one possibility for storing energy from renewable sources. This work theoretically explores the engine-based dry methane reforming converting mechanical energy into chemical energy. The endothermic, endergonic reforming process is activated by the temperature increase during the compression stroke, assisted by a reduction of the heat capacity through dilution with argon. This leads to an increase in chemical exergy, as higher-exergy species are produced with small exergy losses while simultaneously consuming CO2. The engine-based homogenous dry reforming serves as a flexible power-to-gas process and energy storage solution, presenting an alternative to catalytic processes. The piston engine is simulated using a time-dependent single-zone model with detailed chemical kinetics, followed by an analysis of thermodynamics and kinetics. With inlet temperatures ranging from 423–473 K and argon dilutions of 91–94 mol%, CH4 and CO2 conversion are between 50%–90% and 30%–80%, respectively, resulting in synthesis gas yields of 45–55%. Additionally, higher hydrocarbons such as C2H2, C2H4, and C6H6 are produced with yields of up to 20%, 10%, and 10%. So, this power-to-gas process allows for exergy storage of up to 3.35 kW L−1 per cycle with an efficiency of up to 75%.