Engines may be configured with an exhaust heat recovery system for recovering heat from 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 may be utilized for functions such as heating the cylinder head and warming the passenger cabin. 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, as shown in US 20120017575, Sloss discloses an exhaust assembly with an exhaust bypass passage having a diverter valve for controlling the flow of exhaust gas into the main exhaust passage and/or the bypass passage. A heat exchanger may be coupled to the main exhaust passage. The diverter valve is utilized to selectively direct exhaust flow through the heat exchanger during conditions such as cold-start when exhaust heat recovery is desired. During conditions when exhaust heat recovery is not required and/or when heat exchanger working limit has been reached, the diverter valve may be positioned to route the exhaust via the bypass passage thereby bypassing the heat exchanger. In still other approaches, a heat exchanger may be positioned in the bypass passage and a diverter valve may be used to redirect exhaust through the heat exchanger in the bypass during cold-start conditions, the exhaust then redirected through the main exhaust passage once the engine is warm enough.
However, the inventors herein have recognized potential disadvantages with the above approaches. As one example, in approaches requiring diverter valves to route exhaust through the heat exchanger coupled to the exhaust passage, component costs are increased. As such, both electrically and pneumatically actuated valves add significant component costs. For electrically actuated valves, the valve may be more expensive. For the pneumatically actuated valves, component costs may be increased due to the associated hardware required for vacuum actuation of such valves. The valves may also be difficult to seal without leakage. Coordinating the operation of the diverter valves with other engine system components may also result in additional control complexity. Also, in the system of US 20120017575, even though heat is extracted from exhaust at a heat exchanger, the cooled exhaust is not recirculated, resulting in the need for an additional EGR cooler.
In still further approaches, during the cold-start condition, a wastegate valve coupled across an exhaust turbine is opened to enable heated exhaust to be directed to an exhaust catalyst (to expedite catalyst light-off) and a heat exchanger (for heat recovery). Herein by not directing the exhaust through the turbine, exhaust heat loss at the turbine is reduced, allowing for a larger portion of the exhaust heat to be recovered at the heat exchanger.
However the inventors have identified potential issues with such approaches too. As one example, opening the wastegate to enable increased exhaust heat recovery during a cold-start can result in delays in turbine spin-up (turbo lag), and thereby a delay in providing a demanded boost pressure. If the wastegate were closed to expedite turbine spooling, exhaust heat recovery would be limited, resulting in a cabin heating demand not being met. In addition, the exhaust reaching the catalyst post-turbine would be cooler, delaying catalyst light-off. As such, this could degrade cold-start emissions.
The inventors herein have identified an approach by which the issues described above may be at least partly addressed. One example method for a boosted engine comprises: during an engine cold-start condition, closing a wastegate coupled across an exhaust turbine while flowing air from downstream of an intake compressor to a tailpipe via an ejector; and drawing exhaust flow from downstream of an exhaust catalyst into a heat exchanger coupled in an exhaust bypass via ejector generated vacuum. In this way, during a cold-start, exhaust heat may be recovered for engine heating while a turbine spools up.
In one example, an engine system may be configured with a heat exchanger positioned downstream of a catalytic convertor in an exhaust bypass disposed parallel to a main exhaust passage. An ejector may be coupled to the exhaust bypass assembly, for example, downstream of the heat exchanger. As elaborated herein, motive flow through the ejector may be used to create ejector vacuum that is then used for drawing in exhaust from the main exhaust passage into the heat exchanger. As a result, the lower cost ejector is used to perform the function of the higher cost diverter valve. During cold-start conditions, a wastegate valve coupled to a wastegate passage of an exhaust turbine may be closed, allowing the turbine to spool up. At the same time, a portion of intake air may be routed from downstream of the compressor to an exhaust tailpipe via the ejector. By flowing air through the ejector, a low pressure area may be created across the heat exchanger which facilitates drawing in of hot exhaust into the heat exchanger, the exhaust then expelled to the atmosphere via the tailpipe. During the flow of exhaust through the heat exchanger, heat may be transferred to a coolant circulating through the heat exchanger, the hot coolant then used for functions such as cabin heating. Based on a heating demand at the cold-start (e.g., cabin heating demand), the routing of boosted air via the ejector may be adjusted. When exhaust heat recovery is no longer desired, such as when cabin heating demand is met, air flow through the ejector may be suspended. Due to the lack of motive flow through the ejector, exhaust may resume flowing directly to the tailpipe through the main exhaust passage without being diverted into the bypass with the heat exchanger. The heat exchanger may now be utilized as an EGR cooler by recirculating exhaust gas to the engine intake via the heat exchanger while bypassing the ejector. Optionally, the heat recovered at the heat exchanger during EGR flow may also be transferred to the circulating coolant. In an alternate embodiment, the ejector may be positioned upstream of the heat exchanger in the bypass assembly such that all flow through the heat exchanger (including flow of intake air and flow of EGR) is directed via the ejector.
In this way, by utilizing an ejector coupled to an exhaust bypass assembly, heating requirements of an engine system may be met during cold-start conditions, reducing the need for costly exhaust diverter valves. By coupling the ejector to a heat exchanger in the bypass assembly, motive flow through the ejector may be used to draw at least some exhaust from a main exhaust passage into the heat exchanger, allowing for increased exhaust heat recovery. By using the exhaust heat recovered during cold-start conditions for cabin heating, the electrical load of the engine may be reduced. The technical effect of closing the waste gate valve during cold-start conditions is that turbine spin-up can be expedited, and boosted intake air may be provided earlier during engine operation. By positioning the heat exchanger and ejector downstream of an exhaust catalyst, emissions quality may not be degraded even if there are minor leaks. In addition, by reducing the reliance on diverter valves, the occurrence of leaks is reduced. By operating the heat exchanger for exhaust heat recovery during some conditions and as an EGR cooler during other conditions, the need for a dedicated EGR cooler is reduced, providing component reduction benefits. Overall, by improving the amount of waste heat that can be recovered from exhaust using fewer components, engine fuel economy, and performance may be improved. In addition, by enabling exhaust heat to be recovered while a turbine is spooled up, boost control 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.