An investigation on the potential of dedicated exhaust gas recirculation for improving thermal efficiency of stoichiometric and lean spark ignition engine operation

Highlights: • Addition of H₂ and CO increases combustion rate as well as maximum flame temperature. • Thermal efficiency at D-EGR cylinder decreases with an increase of equivalence ratio. • Higher engine thermal efficiency is observed in D-EGR for higher equivalence ratio. • Lean combustion with D-EGR archives the highest engine thermal efficiency of 42.9%. To suppress knock for improving thermal efficiency (η_th) of spark ignition (SI) engines, external exhaust gas recirculation (EGR) has been used. However, the use of EGR reduces flame speed, which leads to observed increase of cycle-to-cycle variations of SI combustion. To overcome this problem by reforming the fuel to add reactive compounds such as H₂ and CO to the intake charge, dedicated EGR (D-EGR) concept is proposed, which uses a portion of the cylinders of a multi-cylinder engine to produce the entirety of the EGR consisted mainly of H₂ and CO. To maximize the potential of D-EGR and provide new insights for improving η_th of SI engines, this study computationally investigates the use of D-EGR over a wide range of fuel-air equivalence ratios in four-cylinder engine (one D-EGR cylinder and three normal cylinders) for stoichiometric and lean operation. For the computations of laminar burning velocity (S_L) and burned gas temperature (T_b), PREMIX in CHEMKIN-PRO were conducted with GRI-Mech 3.0 which is a detailed chemical-kinetic mechanism for methane (CH₄). The results show that for D-EGR cylinder, both S_L and T_b decrease with an increase of fuel/air equivalence ratios at D-EGR cylinder (ϕ_D-EGR) because the lower O₂ as the more fuel is supplied to D-EGR cylinder for higher ϕ_D-EGR. η_th at D-EGR cylinder (η_thD-EGR) decreases from 34.8% to 15.0% with an increase of ϕ_D-EGR from 1.0 and 3.0, regardless of changes in the amount of gross indicated work (W_gD-EGR) and heat transferred to the combustion chamber (Q_cD-EGR) at D-EGR cylinder. For stoichiometric combustion at normal cylinder (ϕ_Normal = 1.0), both S_L and T_b increase almost linearly with an increase of ϕ_D-EGR in the ϕ_D-EGR = 1.0–2.4 range. Furthermore, η_th at normal cylinder (η_thNormal) increases from 34.8% to 51.9% between ϕ_D-EGR = 1.0 and 3.0. As a result, η_th of four-cylinder engine (η_thEngine) with one D-EGR cylinder and three normal cylinders increases with an increase of ϕ_D-EGR, and the highest η_thEngine (39.4%) is achieved for ϕ_D-EGR = 1.9. For lean combustion at normal cylinder (ϕ_Normal = 0.9, 0.8, 0.7, 0.6, 0.5), the higher S_L and T_b are observed for the higher ϕ_D-EGR, similar to stoichiometric combustion (ϕ_Normal = 1.0). Eventually, by a combination of D-EGR addition and lean combustion, the highest η_thEngine of 42.9% is obtained for ϕ_Normal = 0.7 with ϕ_D-EGR of 2.0. This corresponds to a 16.0% increase of η_thEngine relative to the baseline which is a stoichiometric charge consisted of CH₄ and air without any EGR (η_thEngine = 26.9%).