I was traveling behind a BMW SUV yesterday, and I noticed a flap valve opening and closing periodically in one of two exhaust pipes emanating from the single muffler. This seemed odd to me, where it would open at idle and during acceleration, and snap closed the rest of the time. It didn't make sense to me to add back-pressure to the exhaust, after all, since a dual exhaust is marketed as something that adds horsepower.
I looked it up, and apparently closing the valve helps lower emissions and bring the catalytic converter to operating temperature more quickly. It also provides a better exhaust note, an important thing to some people, and quieter exhaust.
"Exhaust gas flaps can fulfil a variety of functions in the exhaust system. They are used for a wide variety of functions, both with spark-ignition engines as well as with diesel engines.
"In acoustical optimisation, exhaust noises are influenced by adapting the volumes and pipe lengths of silencers.
"In the new BMW 7 series, one of the two tailpipes on the rear silencer is equipped with an exhaust gas flap and, as a result, always provides the right "sound" of the twelve-cylinder engine, throughout the entire speed range. Below: BMW 7 series.
"Skilful positioning of the exhaust gas flaps in the exhaust system, can achieve a considerable reduction of the required silencer volume. As a result, the weight and the space required for the entire system can be downsized.
"In emission control, exhaust gas flaps are used to divert the exhaust gas flow. In this way, they trigger the HC (Hydrocarbon) absorbers or bring the starter catalyst to operating temperature more quickly.
"A more recent method of emission control is the use of DeNOx catalytic converters. These cause a part of the exhaust gas flow to be guided through a cooling zone by an exhaust gas flap , thus ensuring an optimum effect and a long service life of this catalyst.
"By varying the exhaust gas back pressure, the efficiency can be improved in certain characteristic areas. This is also done via an exhaust gas flap in the exhaust system.
"Exhaust gas flaps are specifically dimensioned to meet the high demands in the exhaust gas system: they must outlast gas temperatures of up to 950°C, whilst closing correctly and remaining moveable during their entire service life."
No comments:
Post a Comment