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

To suppress knock for improving thermal efficiency (eta(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 H-2 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 H-2 and CO. To maximize the potential of D-EGR and provide new insights for improving eta(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 (phi(D-EGR)) because the lower O-2 as the more fuel is supplied to D-EGR cylinder for higher phi(D-EGR) . eta(th) at D-EGR cylinder (eta(th_)phi(D-EGR)) decreases from 34.8% to 15.0% with an increase of phi(D-EGR) from 1.0 and 3.0, regardless of changes in the amount of gross indicated work (W-g,W-D-EGR) and heat transferred to the combustion chamber (Q(c_D-EGR)) at D-EGR cylinder. For stoichiometric combustion at normal cylinder (phi(Normal) = 1.0), both S-L and T-b increase almost linearly with an increase of phi(D-EGR) in the phi(D-EGR) = 1.0-2.4 range. Furthermore, eta(th) at normal cylinder (eta(th_Normal)) increases from 34.8% to 51.9% between phi(D-EGR) = 1.0 and 3.0. As a result, eta(th) of four-cylinder engine (eta(th_Engine)) with one D-EGR cylinder and three normal cylinders increases with an increase of (PD-EGR, and the highest eta(th_Engine) (39.4%) is achieved for OD-EGR = 1.9. For lean combustion at normal cylinder (phi(Normal) = 0.9, 0.8, 0.7, 0.6, 0.5), the higher S-L and T-b are observed for the higher phi(D-EGR), similar to stoichiometric combustion (phi(Normal) = 1.0). Eventually, by a combination of D-EGR addition and lean combustion, the highest eta(th_Engine) of 42.9% is obtained for phi(Normal) = 0.7 with phi(D-EGR) of 2.0. This corresponds to a 16.0% increase of eta(th_Engine) relative to the baseline which is a stoichiometric charge consisted of CH4 and air without any EGR (eta(th_Engine) = 26.9%).