Mechanism Triggering Pre-ignition in a Turbo-charged Engine. 2019-01-0255
Pre-ignition in modern engines is attributed to mobile deposits and oil-fuel droplets igniting before the spark timing. Previous researchers have found pre-ignition events triggered by high hydrocarbon emissions as well as late spark timing. Recently, an ideally scavenged engine was found to be not limited by pre-ignition. These observations point to a critical role of residual in triggering pre-ignition event. In this regard, this work studies the effect of residuals by varying the exhaust back pressure. Experiments were performed with fixed mass flow rate of air and the exhaust back pressure was varied. However increasing exhaust back pressure led to increase in intake pressure for fixed air flow rate. In next set of experiments, exhaust back pressure was reduced from a maximum value (2.6 bar in our case) and intake pressure was kept at 1.9 bar, chosen to have statistically significant pre-ignition count. As the back pressure is reduced, the intake temperature is increased to fix the intake pressure value. The spark timing was knock limited. This strategy allowed isolating the effect of increasing residual while keeping other parameters fixed. It was found that pre-ignition tendency increased with increasing exhaust back pressure. However, as the temperature was increased for the cases with lower back pressure, the auto-ignition tendency, and hence knock resistance is lowered. Hence, although lower in number, pre-ignition events were followed by very high pressure oscillations. This study shows the effect of residuals as well as the difference in the two phenomenon: pre-ignition and superknock.
Eshan Singh, Robert Dibble
King Abdullah Univ. of Science & Tech.