Given its ability to be combined with the three-way catalyst, the stoichiometric operation is significantly more attractive than the lean-burn process, when considering the increasingly severe NOx limit for cogeneration gas engines. However, the high temperature of the stoichiometric combustion results in increased wall heat losses, restricted combustion phasings (owing to knock tendency) and thus efficiency penalties. To lower the temperature of the stoichiometric combustion and thus improve the engine efficiency, exhaust gas recirculation (EGR) is one of the most effective means. Nevertheless, the dilution with EGR has much lower tolerance level than with excess air, which leads to a consequent drop in the thermal efficiency. In this regard, reducing the water vapor concentration in the recirculated exhaust gas and increasing the EGR reactivity are two potential measures that may extend the mixture dilution limit and result in engine efficiency benefits. Here, the reactive exhaust gas originates from a sub-stoichiometrically operated cylinder (of a multi-cylinder engine). In this work, the sub-stoichiometric (dedicated cylinder) as well as the stoichiometric (EGR-receiving cylinders) combustion processes with various EGR strategies are deeply analyzed. First, reaction kinetics simulations and engine experiments with comprehensive metrology were carried out to provide an accurate understanding of the sub-stoichiometric engine combustion process, the formation of the reactive species and the reactive exhaust gas properties. Among others, it was found that the increase in exhaust gas reactivity during mixture enrichment is associated with an increase in its specific heat capacity and hence in its diluting effect. Loss analysis has shown that the real combustion losses for high fuel-to-air ratios (λ < 0.8) cause a trade-off between exhaust gas reactivity and engine thermal efficiency. Second, the stoichiometric combustion process with various EGR characteristics, based on the outputs of the previous sub-stoichiometric investigations, was inspected relying on 1D flame simulations and engine trials with “artificially” reactive and variably dry EGR. “Artificially” reactive means that the reactive portion is provided by an external H2 supply into the intake path, while the inert portion (H2O, CO2, N2) is obtained from the recirculated λ = 1 exhaust gas. Here, the externally fed H2 represents all reactive species that would occur in the exhaust gas of a real sub-stoichiometric combustion (H2, CO, unburned HC). The results show that compared to the conventional EGR, the reactive and partially dry EGR provides an improved dilution tolerance, simultaneous decrease of wall heat and combustion losses and an increase of the isentropic exponent of the stoichiometric mixture, resulting in thermal efficiency benefits. In a last step, the increase of the geometric compression ratio was investigated relying on 0D engine/reactor simulations. Here, the knock detection was performed by assessing the ignition delay time based on the mixture properties in the unburned zone.
Session:
COMBUSTION & EMISSIONS
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| 08:30 - 09:00