Effectiveness of Fuel Enrichment on Knock Suppression in a Gasoline Spark-Ignited Engine 2018-01-1665
Knock, and more recently, super-knock, have been limiting factors on improving engine efficiency. As a result, engines often operate rich at high loads to avoid damage resulting from knock and protect the after-treatment system from excessive thermal stress. In this work, port-fuel injection and direct injection of excess fuel is explored as a mechanism to suppress knock and super-knock. Under naturally aspirated conditions, increasing the fuel enrichment initially increases knock intensity. However, further increasing fuel enrichment subsequently decreases knock intensity. The competing mechanism from calorific value and latent heat of vaporization can be used to explain the phenomenon. However, when directly injecting the excess fuel after the spark plug has been fired, knock intensity monotonically decreases with increasing fuel quantity. This decrease is shown to be due to fuel quenching the flame that is propagating from spark location. Under boosted conditions, the amount of fuel injected is of critical importance in avoiding super-knock. A lower fuel quantity leads to knock suppression. But beyond a critical value, higher quantities of fuel result in more interaction with the oil film on the cylinder liner, leading to a greater number of pre-ignition precursors (fuel + oil droplets) and a higher number of pre-ignition events. These spontaneous pre-ignition events arising from fuel enrichment are further advanced and do not lead to super-knock behavior due to high amounts of charge cooling from evaporation of the excess fuel. Furthermore, such spontaneous pre-ignition events are characterized by higher pressure in the intake stroke and dominance of higher frequency oscillations in the cylinder.