Engines may be configured with an exhaust heat recovery system for recovering heat of exhaust gas generated at an internal combustion engine. The heat is transferred from the hot exhaust gas to a coolant through an exhaust gas heat exchanger system. The heat from the coolant, circulated through the an exhaust gas heat exchanger, may be utilized for functions such as heating the cylinder head, and warming the passenger cabin, thereby improving engine efficiency and passenger comfort. In hybrid electric vehicles, the recovery of exhaust heat improves fuel economy by enabling engine temperatures to be maintained longer, thereby allowing for a faster engine shut-off and extended use of the vehicle in an electric mode.
Exhaust heat may also be retrieved at an exhaust gas recirculation (EGR) cooler. An EGR cooler may be coupled to an EGR delivery system to bring down the temperature of recirculated exhaust gas before it is delivered to the intake manifold. EGR may be used to reduce exhaust NOx emissions. Further, EGR may be used to assist in the reduction of throttling losses at low loads, and to improve knock tolerance.
Various approaches are provided for exhaust heat recovery and EGR cooling. In one example, a diverter valve is provided at a location upstream of an exhaust catalyst in an exhaust passage. During cold start, hot exhaust gases are diverted at the diverter valve via an EGR passage to an EGR cooler, where heat is transferred to the engine coolant. Subsequently, exhaust gases, after passing through the EGR cooler, are returned back to the exhaust passage at a location downstream of the diverter valve but upstream of the exhaust catalyst in the direction of exhaust flow.
However, the inventors herein have recognized potential disadvantages with the above approaches. As one example, it may be difficult to package the EGR passage, the diverter valve and the return tube prior to the exhaust catalyst. Further, during cold start condition, diverting exhaust to the EGR cooler may delay exhaust catalyst warm-up and hence, light-off. Still further, the diverter valve operates in an uncooled environment, which may cause durability issues.
The inventors herein have identified an approach by which the issues described above may be at least partly addressed. In one example, a method for an engine, comprises: during a cold start condition, before light-off of an exhaust catalyst, flowing exhaust drawn from a first exhaust valve of each engine cylinder to the exhaust catalyst while bypassing a heat exchanger; and after light-off of the exhaust catalyst, flowing exhaust drawn from a second exhaust valve of each engine cylinder to the heat exchanger, and transferring exhaust heat to an engine coolant at the heat exchanger. In this way, coolant warm-up may be achieved without delaying exhaust catalyst light-off. Thus, engine efficiency may be increased while engine emissions are reduced.
As one example, an engine system may be configured with a first exhaust manifold receiving exhaust from a first exhaust valve of each engine cylinder and a second exhaust manifold receiving exhaust from a second exhaust valve of each cylinder. Further, a heat exchanger may be included within the second exhaust manifold, and an EGR valve may be located downstream of the heat exchanger for regulating exhaust gas recirculation from downstream of the heat exchanger into an intake manifold. When an exhaust catalyst temperature is below a light-off temperature, exhaust may be drawn from the first exhaust valve and delivered to the exhaust catalyst via the first exhaust manifold bypassing the heat exchanger. After light-off, if an engine coolant temperature is below a threshold, exhaust may be drawn from the second exhaust valve and delivered to the heat exchanger, where exhaust heat is transferred to the engine coolant for warm-up. Further, when the engine coolant is below the threshold, in order to reduce exhaust gas condensation that may occur when exhaust gas hits cooler engine parts, EGR may not be provided. Therefore, during cold-start conditions, EGR valve may be closed and exhaust from downstream of the heat exchanger may be returned to the exhaust catalyst via a return passage.
In this way, by bypassing heat exchanger and delivering exhaust directly to the exhaust catalyst, catalyst light-off may be expedited. After catalyst light-off, by channeling exhaust gases directly to the heat exchanger, engine warm-up may also be expedited. Further, by not providing EGR until engine warm-up is achieved, condensation of exhaust gas may be reduced.
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.