Measurement of Knock and Ion Current in a Spark Ignition Engine with and without NO Addition to the Intake Air

Several studies have shown that NO has a strong influence on engine knock. This paper reports an experimental study that addresses the connection between SI engine knock and the level of nitric oxide, NO, in the intake manifold gas under various conditions of engine operation.

Some theories explain the second ion-current peak as thermal ionisation of NO. Both temperature and NO concentration is of importance. By advancing the ignition angle the NO concentration can be increased, but the temperature is also increased. Addition of NO in the inlet manifold increases the NO concentration but has less effect on the temperature.

SI engine experiments were conducted over a number of different ignition timings, air/fuel ratios, engine speeds and intake manifold pressures. The NO level in the intake manifold was altered from 100 to 1600 ppm, increasing the amount by doubling. The study confirms that there is an increasing tendency of early knock when the NO amount increases. A slight increase in the second ion-current peak could be seen with addition of NO in the inlet manifold, especially with early ignition timing. This increase in peak amplitude may be a result of heavy knock, due to the addition of NO. Advancing the ignition angle 2 crank angle degrees has a stronger influence on the second ion-current peak than addition of 1600 ppm NO in the inlet manifold.