Emissions regulations are requiring reduced emissions from vehicles, particularly the Euro 5, Bin 5 and US 06 regulations. To reduce harmful emissions, such as NOx, it is known to recirculate exhaust gas through the engine. Under normal conditions the exhaust gas is cooled before recirculation and it is known to pass the exhaust gas through an exhaust gas recirculation cooler. However, under “cold start” or low operating conditions, the gas can be over-cooled resulting in increased hydrocarbon emission and CO2 production.
It is known from various publications to provide a valve and a bypass to bypass the cooler or alternatively direct the exhaust through a portion of the heat exchanger in which the heat transfer is minimal.
This avoids cooling the exhaust gas during such low operating conditions.
British Patent No 2,303,177 discloses an EGR system in which a portion of the exhaust gases produced by an engine are recirculated from an exhaust line of the engine into an intake line of the engine. In this system a cooler is arranged to cool the recirculated portion of the exhaust gases, and a bypass line is arranged to bypass the cooler. U.S. Pat. No. 6,141,961 discloses a further EGR loop with bypass.
In these systems, a valve selectively directs the flow of exhaust to either the exhaust gas recirculation cooler or to the bypass around the exhaust gas recirculation cooler. The valve requires good high-temperature compatibility, corrosion resistance, and resistance to plugging or sticking due to soot and other contaminants in the exhaust gas; all whilst remaining cost effective.
Whilst generally satisfactory, the prior art suffers from problems associated with plugging and sticking of the valves and the high specification required for the valves increases their cost.
An object of the present invention is to eliminate or mitigate any of the problems associated with the prior art.