A study by a team at the Propulsion Systems Research Labs, GM R&D has concluded that the combustion instability of homogeneous charge compression ignition (HCCI) at the low-load range is caused by lack of oxygen concentration rather than lower in-cylinder mixture temperature. Their paper is published in the International Journal of Engine Research.
While HCCI offers significant efficiency improvements compared to conventional gasoline engines, traditional homogenous charge compression ignition combustion can be realized only in a limited operating range. High-load range HCCI operation is limited by the trade-off between excessive combustion noise and deteriorated combustion stability; low-load operation is limited by combustion instability.
GM has spent a great deal of effort over the past several years in trying to expand the overall HCCI range. (Earlier post.)
To understand the auto-ignition characteristics at low-load operation, the GM team investigated the effect of negative valve overlap, injection timing and external exhaust gas recirculation. These parameters have a significant impact on temperature and oxygen concentration of the mixture and thus the auto-ignition characteristics.
They observed that at a given speed and load, there is an optimum combination of negative valve overlap and injection timing for stable auto-ignition in homogeneous charge compression ignition (HCCI) combustion.
Overall, they found that when the amount of reforming is low, the in-cylinder temperature is not high enough for auto-ignition. This causes delayed auto-ignition phasing (unstable combustion). When the amount of reforming is high, the cyclic variability of reforming increases, which leads to instability of the main combustion event. When exhaust gas recirculation is employed, the amount of reforming does not change, leading to the conclusion about the role of oxygen concentration.
Hanho Yun, Orgun Guralp, Ronald O Grover Jr, Paul M Najt (2013) “The effect of temperature and oxygen concentration on auto-ignition at low-load operating conditions in a gasoline homogeneous charge compression ignition engine,” International Journal of Engine Research vol. 14 no. 5 512-524 doi: 10.1177/1468087412463304