V8 as example
In 2008, BMW launched a new twin-turbo V8 with codename N62. The engine has an unusual intake and exhaust arrangement contrasting to conventional wisdom: the hot exhaust manifolds rest inside the V-valley, while the intake manifolds are located at either sides. This arrangement must have taken its engineers a lot of effort to solve the thermal insulation and cooling problems. It does make the engine more compact, at least in terms of width, if not height. However, the primary reason for the radical change is unlikely to be compactness, but the compatibility with its new cross-bank turbocharging technology. Here we are going to see how it works.
Now we can use the same principles learned from the previous section of Twin-scroll turbo to analyse the engine. If you have not read that section, please read it first.
The firing order of a typical V8 engine is 1 - 5 - 4 - 8 - 6 - 3 - 7 - 2. If we combine the exhaust gas of all cylinders together, we will get a pulse train as below. It shows a lot of interference. Each pulse is partly offset by the negative tails of the preceding pulses. This reduces the strength of the resultant pulse thus hampers turbo response.
In 2008, BMW launched a new twin-turbo V8 with codename N62. The engine has an unusual intake and exhaust arrangement contrasting to conventional wisdom: the hot exhaust manifolds rest inside the V-valley, while the intake manifolds are located at either sides. This arrangement must have taken its engineers a lot of effort to solve the thermal insulation and cooling problems. It does make the engine more compact, at least in terms of width, if not height. However, the primary reason for the radical change is unlikely to be compactness, but the compatibility with its new cross-bank turbocharging technology. Here we are going to see how it works.
 |
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| Unconventionally, the BMW
V8 has its exhaust manifolds and twin-turbo mounted inside the valley. |
The exhaust manifold is
compact yet sophisticated. Note the turbos are twin-scroll. |
 |
An illustration to the connection of
exhaust manifolds: Cylinder 1 and 6 are connected to one of the scrolls of the first twin-scroll turbocharger. Another scroll connects to Cylinder 4 and 7. Cylinder 2 and 8 are connected to one of the scrolls of the second twin-scroll turbocharger. Another scroll connects to Cylinder 3 and 5. As you can see, each turbo is supplied by both cylinder banks, unlike conventional twin-turbo. This explains why we call it cross-bank turbocharging. It is also why the exhaust manifolds and turbochargers have to be located inside the valley. If they were mounted outside the engine, the cross-bank exhaust manifolds would have to be too long and cumbersome, adding considerable weight and turbo lag. |
Now we can use the same principles learned from the previous section of Twin-scroll turbo to analyse the engine. If you have not read that section, please read it first.
The firing order of a typical V8 engine is 1 - 5 - 4 - 8 - 6 - 3 - 7 - 2. If we combine the exhaust gas of all cylinders together, we will get a pulse train as below. It shows a lot of interference. Each pulse is partly offset by the negative tails of the preceding pulses. This reduces the strength of the resultant pulse thus hampers turbo response.
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