On the Role of Nitric Oxide for the Knock-Mitigation Effectiveness of EGR in a DISI Engine Operated with Various Gasoline Fuels

Abstract

<div class="section abstract"><div class="htmlview paragraph">The knock-suppression effectiveness of exhaust-gas recirculation (EGR) can vary between implementations that take EGR gases after the three-way catalyst and those that use pre-catalyst EGR gases. A main difference between pre-and post-catalyst EGR gases is the level of trace species like NO, UHC, CO and H<sub>2</sub>. To quantify the role of NO, this experiment-based study employs NO-seeding in the intake tract for select combinations of fuel types and compression ratios, using simulated post-catalyst EGR gases as the diluent. The four investigated gasoline fuels share a common RON of 98, but vary in octane sensitivity and composition. To enable probing effects of near-zero NO levels, a skip-firing operating strategy is developed whereby the residual gases, which contain trace species like NO, are purged from the combustion chamber.</div><div class="htmlview paragraph">Overall, the effects of NO-seeding on knock are consistent with the differences in knock limits for preand post-catalyst EGR gases. This suggests that for a majority of the conditions studied here, variations in NO concentration dominate the autoignition-influencing role of trace species that are present in pre-and post-catalyst EGR gases. The specific response of the knock limits to NO vary greatly between fuels, and the response also varies with compression ratio for a given fuel. Generally, the greatest sensitivity to NO exists in the 0 - 50 ppm range for all fuels, with increasing [NO] strongly promoting end-gas autoignition. For higher levels of NO, the autoignition-enhancing effect of NO saturates. This is particularly true for the Alkylate fuel with low octane sensitivity, for which addition of NO beyond 100 ppm reverses the trends, with further NO addition actually suppressing knock, although not beyond the level achieved without NO present.</div></div>