Publications Details

Influence of Nitric Oxide and Other Factors on Acoustic Knock Onset for Lean DISI Engine Operation

Sjoberg, Carl M.; Kim, Namho K.; Matsubara, Naoyoshi; Yokoo, Nozomi; Nakata, Koichi

Spark-ignition (SI) engine efficiency can be increased by operating lean and with increased compression ratio (CR), but both of these measures tend to increase the propensity for undesirable acoustic knock generation. It is well known that increased CR makes the engine more prone to knock due to increased combustion pressures and temperatures, but it may be less well understood why lean operation would exacerbate knock generation. For typical gasoline-range fuels, the laminar flame speed becomes very low (roughly only 20% compared to stoichiometric conditions) for an air-excess ratio (λ) of 2. Indirectly, this exacerbates the knock challenge in two ways; a) it may necessitate operation with a combustion phasing near Top Dead Center (TDC) to complete the combustion before expansion cooling occurs, b) it increases cycle-to-cycle variations, making it more challenging to operate near the knock limits. In addition, the high intake pressure required for lean operation (nearly a factor of two higher for λ = 2 compared to λ = 1) increases the oxygen concentration which promotes end-gas autoignition and knock generation. Towards overcoming these challenges of lean combustion, this study aims to provide a better understanding of fuel autoignition under various conditions. First, to reveal the octane appetite under lean conditions, this experimental work utilized fuels of varying Research Octane Number (RON) and octane sensitivity (S). It was found that lean operation favored fuels that have high RON and high S since those were less knock limited. However, two compositionally different fuels with similarly high RON and S exhibited notable difference in knock limits under lean operation, indicating that RON and S may fail to accurately rank order fuels' knock propensity. Second, the experiments show that under boosted conditions end-gas autoignition becomes sensitive to the level of trapped residual nitric oxide (NO), which in turn is very sensitive to variations of both actual λ and combustion phasing, among other factors. The results suggest that strong knock-suppression benefits could be realized if single-ppm NO mole fraction can be consistently maintained in the reactants. Finally, it is noted that maintaining knock-free operation is particularly important for lean operation because the lower peak combustion temperatures lower the speed of sound, which in turn shifts the frequency content of the in-cylinder knock to a lower frequency range. Lower knock frequencies can increase the transmission efficiency from the combustion chamber to the outer surfaces of the engine, potentially increasing engine noise levels if knock occurs.