An Experimental Investigation of the Effect of Nitric Oxide (NO) on Knock Onset in a Spark-Ignition Engine

This paper studies the effect of nitric oxide (NO) on the onset of knock in a cooperative fuel research (CFR) engine. The experiments are conducted at both the standard knock intensity (SKI) condition for research octane number (RON) tests, as well as at a richer operating condition of a similar knock intensity but a higher compression ratio. Iso-octane is the fuel. The experiments show clearly that knock onset is consistently suppressed with increasing NO additions at the SKI condition. However, different trends are seen at the richer condition, with increased NO addition first advancing knock onset and then retarding it. Using established chemical models of the interaction between NO and hydrocarbons from the literature, these differing results are argued as most likely due to the different concentrations of residual NO at these different engine operating conditions. The authors’ planned kinetic modelling at these conditions will confirm whether these arguments are correct.