Knock and Pre-Ignition Limits on Utilization of Ethanol in Octane–on–Demand Concept
To be published on September 9, 2019 by SAE International in United States
Octane-on-Demand (OoD) is a viable technology for reducing global greenhouse emissions from automobiles. The concept utilizes a low-octane fuel for most operating conditions. Previous research has focused on the minimum ethanol content required for achieving a specific load at a given speed as the low-octane fuel becomes knock limited as the load increases. However, it is also widely known that ethanol has a high tendency to pre-ignite, attributed by few to its high laminar flame speed and surface ignition tendency. Moreover, ethanol has a lower calorific value, requiring a larger fuel mass to be injected to achieve similar power. A larger fuel mass increases the oil dilution by the liquid fuel, creating precursors for pre-ignition. Hence the limits on ethanol addition owing to pre-ignition also needs consideration before the technology can be implemented. In this regard, experiments were performed using light naphtha RON 68 gasoline and ethanol, in direct and port injection configuration respectively. Load was parametrically swept by increasing the intake air and fuel quantity, until pre-ignition limited IMEP was reached. Three different engine speeds; 1500, 2000 and 2500 rpm were tested. In general, it can be said that OoD concept helps suppress pre-ignition innately, by merely splitting the fuel amount going to direct injection. RON 68 light naphtha was found to be the limiting fuel, more often than ethanol, due to larger fuel fraction injected via direct injector. The large fuel mass at knock limited conditions led to higher fuel-wall impingement and hence, higher tendency to pre-ignite.