In an effort to meet stringent federal government emissions standards, engine systems may be configured with exhaust gas recirculation (EGR) systems wherein at least a portion of the exhaust gas is recirculated to the engine intake. Such EGR systems enable reduction in exhaust emissions while also improving fuel economy, especially at higher levels of engine boost.
One example of such an EGR system is illustrated by Duret in U.S. Pat. No. 6,135,088. Therein, a first inlet port of the engine cylinder is configured to deliver EGR while a second inlet port is configured to deliver fresh air, boosted by a compressor, to the cylinder. In this way, charge stratification can be achieved in the cylinder to improve self-ignition.
However, the inventors herein have recognized potential issues with such a system. As one example, during some conditions, charge stratification may not be desired. Rather, charge homogenization may be desired to increase engine performance and improve EGR benefits. As another example, it may be difficult to maintain the charge stratification since both inlet ports discharge exhaust gas through a common exhaust port. As still another example, the desired effects taught by Duret may be erased if modified to use an exhaust turbine to drive the compressor.
Thus in one example, some of these issues may be at least partly addressed by a method of operating a boosted engine comprising, drawing at least some recirculated exhaust gas at or below barometric pressure from one of two exhaust passages into an engine cylinder through a first intake passage, and drawing at least some fresh air at compressor pressure into the cylinder through a second, separate intake passage coupled to the other of the two exhaust passages. In this way, fresh boosted air may be delivered separate from the recirculated exhaust gas. The aircharges may then be mixed with each other and with fuel in the cylinder. The combined aircharge-fuel mixture may then be combusted in the cylinder.
For example, an amount of exhaust gas (that is, low pressure EGR) may be drawn from a first exhaust passage into a first intake passage through a first EGR passage. The EGR may be naturally aspirated from the first exhaust passage via a first exhaust valve and delivered to an engine cylinder at or below barometric pressure through a first intake valve of the first intake passage at a first, earlier intake valve timing. For example, the EGR may be delivered at the onset of an intake stroke. At the same time, an amount of fresh intake air may be drawn through a turbocharger compressor included in a second intake passage. As such, the second intake passage may be separate from the first intake passage, and the turbocharger may be coupled only to the second intake passage and not the first intake passage. Also, the compressor may be driven by a turbine included in a second exhaust passage coupled to the second intake passage. For example, the compressed fresh intake air may be drawn into the engine cylinder through a second intake valve of the second intake passage at a second intake valve timing, later than the first intake valve timing (e.g., the boosted fresh air may be drawn in after the intake stroke has begun and after the first intake valve has already opened). The low pressure EGR (LP-EGR) and the boosted fresh intake air may be mixed in the cylinder. Further, the aircharge mixture may be mixed with fuel and combusted in the cylinder.
In this way, a stratified aircharge may be delivered to the cylinder but may be homogeneously mixed with fuel in the cylinder prior to combustion. By keeping the EGR out of the compressor, compressor fouling and contamination may be reduced. By not expending compressor work on delivering EGR, turbocharger efficiency can be improved. Further, by mixing the delivered LP-EGR with the delivered boosted fresh air in the cylinder, and not before, dilution of the boosted intake air with EGR in the intake passage may be reduced. By separating EGR delivery from boost delivery, delays in turbocharger control as well as EGR control, in particular during transients, can also be reduced. As such, the separate intake passages also enable the use of a smaller turbocharger to provide the desired boost without compromising boosting efficiency. Overall, engine efficiency and performance is improved.
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.