Engine out cold-start hydrocarbon emissions generated before light-off of an exhaust system catalytic converter may contribute a large percentage of the total exhaust hydrocarbon (HC) emissions. Accordingly, engine exhaust systems may utilize hydrocarbon retaining devices, such as hydrocarbon traps, to retain cold start emissions for later reaction (HC storing), or for recirculation into the engine intake system (HC purging).
However, the inventors herein have recognized several issues with such systems. In one example, hydrocarbon traps may degrade when aged, for example beyond 150,000 miles. This is in part due to continuous exposure to hot vehicle exhaust which removes acidic adsorption sites on the HC trap. Further, due to the lack of suitable diagnostic routines in the exhaust systems, degradation of various components in an HC retaining system may not be detected in a timely manner, leading to increased HC emissions and accelerated degradation of an exhaust system.
In one example, the above issues may be addressed by a method of operating an engine having a hydrocarbon retaining system coupled to an engine exhaust and an engine intake, the method comprising, during an engine cold start, routing exhaust gas to the hydrocarbon retaining system to store hydrocarbons in the hydrocarbon retaining system. The method may further comprise, during a first purging condition where the exhaust gas is at a first temperature, purging the hydrocarbon retaining system with at least exhaust gas, while during a second purging condition where the exhaust gas is at a second temperature, said second temperature being higher than said first temperature, mixing the exhaust gas with an adjustable amount of fresh air to form a purging gas mixture, and purging the hydrocarbon retaining system with said purging gas mixture.
In this way, by adjusting the composition of a purging gas mixture to provide a temperature controlled gas for purging hydrocarbons (HCs) from a HC retaining system, it may be possible to improve the desorption of HCs from the HC retaining system while reducing the rate of degradation of various components in the HC retaining system. In one example, the amount of fresh air mixed with exhaust gas may be adjusted such that the temperature of the purging gas mixture may be raised high enough to allow improved desorption of stored HCs, in particular long HCs, without raising the temperature to levels that may cause aging of the HC retaining system. In another example, the composition and/or temperature of the purging gas mixture may be adjusted in consideration of the adsorbent and housing material used in the HC retaining system such that component degradation due to continuous exposure to hot vehicle exhaust may be reduced and replacement of components may be deferred to a later age. As such, the amount of fresh air mixed with the exhaust gas to form the purging mixture may be adjusted based on the temperature of the exhaust gas. In another example, the temperature of the purging gas mixture may be adjusted based on an estimated HC load of the HC retaining device such the purging gas mixture may enable appropriate heating of the HC retaining system and an increased desorption of HCs may ensue. By further including a diagnostics routine during HC storing and/or purging, the routine responsive to a temperature of exhaust gas estimated downstream of the HC retaining system, ineffective HC retention and/or release due to aging of HC traps and/or improper valve operations may be diagnosed and addressed in a timely manner, thereby averting degraded emissions quality.
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.