Knock and Pre-Ignition Limits on Utilization of Ethanol in Octane-on-Demand Concept 2019-24-0108

Octane-on-Demand (OoD) is a promising technology for reducing greenhouse emissions from automobiles. The concept utilizes a low-octane fuel for low and mid load operating conditions, and a high-octane additive is added at high load operating conditions. Researchers have focused on the minimum ethanol content required for operating at high load conditions when the low-octane fuel becomes knock limited. However, it is also widely known that ethanol has a high tendency to pre-ignite, which has been linked with its high laminar flame speed and surface ignition tendency. Moreover, ethanol has a lower stoichiometric air-fuel ratio, requiring a larger injected fuel mass per cycle. A larger fuel mass increases the potential for oil dilution by the liquid fuel, creating precursors for pre-ignition. Hence, the limits on ethanol addition owing to pre-ignition also need consideration before the technology can be implemented. In this regard, experiments were performed using light naphtha (RON 68) and ethanol in direct and port-fuel injection configuration, respectively. The engine load was parametrically swept by simultaneously increasing the intake air and fuel quantity until the pre-ignition limited load was reached. Three different engine speeds, namely 1500, 2000 and 2500 rpm were tested. In general, it can be said that OoD concept helps suppress pre-ignition intrinsically by splitting the amount of direct-injected fuel in the engine. Light naphtha was found to be the limiting fuel, more often than ethanol, due to the larger fuel fraction injected via the direct injector. However, at large ethanol fractions, pre-ignition re-emerged, albeit with low knock intensity.