Engines may be configured with exhaust gas recirculation (EGR) systems to divert at least some exhaust gas from an engine exhaust manifold to an engine intake manifold. By providing a desired engine dilution, such systems reduce engine knock, throttling losses, as well as NOx emissions. Still other diluents may be used. For example, engine dilution may be achieved by direct injecting water or performing a lean combustion.
One example approach for coordinating engine dilution control via the use of different diluents is shown by Surnilla et al. in US 2011/0174267. Therein, a direct injection of water is coordinated with EGR flow to provide a desired engine dilution while addressing EGR transients. In particular, in response to a sudden request for engine dilution (such as during a tip-in), an EGR flow is increased while water is direct injected to substantially immediately provide the desired dilution. Then, the water injection is decreased as the EGR flow increases to the desired dilution level. In addition, load leveling may be performed with the different diluents in anticipation of load changes.
However, the inventors herein have identified a potential issue with such a system. Specifically, engine dilution may degrade engine combustion stability wherein the combustion stability limit is different for each diluent used. That is, the engine may have a combustion tolerance limit for EGR, an equivalent tolerance limit for lean combustion, and likewise an equivalent combustion limit for water vapor. Therefore, when a combination of diluents are used, the combustion stability limit of each diluent may be affected by the amount of dilution already in place, as well as the nature of the diluent already in use. In other words, an amount of water that can be direct injected to provide engine dilution may be affected not only by the combustion stability limits of direct injected water, but also by the combustion stability limits of the amount of EGR in place, the amount of lean combustion in place, etc.
Thus in one example, some of the above issues may be at least partly addressed by a method of operating an engine comprising, providing a desired engine dilution by combining a plurality of engine diluents, the diluents selected based on respective combustion stability limits. In this way, engine dilution may be better controlled by taking into consideration the combustion stability limits and interactions of each diluent used.
For example, an engine controller may translate each available diluent into elemental constituents, and determine a combustion stability limit for that diluent based on the concentration of each elemental constituent. Other combustion related parameters, such as ignition timing, exhaust temperature, feed-gas emissions, etc., may also be determined based on the elemental composition of the diluents. As an example, a combustion stability limit for EGR-based dilution may be determined as a function of the carbon dioxide concentration, the nitrogen concentration, and the water concentration of the available EGR. As another example, a combustion stability limit for lean combustion-based dilution may be determined as a function of the oxygen concentration and the nitrogen concentration of the available lean combustion. The controller may then select one or more engine diluents from the available diluents based on the respective combustion stability limits. For example, a first diluent may be selected having the highest combustion stability limit to provide at least some of the desired dilution. After the first diluent reaches its combustion stability limit, a remainder of the desired dilution may then be provided by a second diluent having the next highest stability limit, and so on.
In this way, a controller may select one or more engine diluents to provide an engine dilution to reduce a likelihood of engine knock. By mapping the engine based on the combustion stability limits of the available diluents, engine dilution control may be improved. Specifically, engine performance and diluent usage may be improved, while providing all the knock-addressing benefits of the engine dilution, and without degrading combustion stability.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.