Abstract
Homogenous charge compression ignition combustion offers significant efficiency improvements compared to conventional gasoline engines. However, due to the nature of homogenous charge compression ignition, traditional homogenous charge compression ignition combustion can be realized only in a limited operating range. Homogenous charge compression ignition operation in the high-load range is limited by the trade-off between excessive combustion noise and deteriorated combustion stability. The low-load operation of homogenous charge compression ignition combustion is limited by combustion instability.
In order to understand the auto-ignition characteristics at low-load operation, the effect of negative valve overlap, injection timing and external exhaust gas recirculation was investigated. These parameters have a significant impact on temperature and oxygen concentration of the mixture and thus the auto-ignition characteristics. To help interpret the results, an auto-ignition integral calculation was developed and verified with experimental data. It is confirmed that the newly developed auto-ignition integral predicts well the ignition timing and can be used to interpret experimental observations.
It is found that at a given speed and load, there is an optimum combination of negative valve overlap and injection timing for stable auto-ignition. 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, thus the combustion instability is caused by lack of oxygen concentration rather than lower in-cylinder mixture temperature.
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