Boost Control: Wastegate Spring Selection
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Wastegate Spring Selection
03.58
| 00:00 | - Most wastegate manufacturers offer a range of wastegate springs to fit inside their wastegates, that allow you to choose a base spring pressure for the wastegate operation. |
| 00:11 | The aim of this is to select a spring for the wastegate that will provide somewhere near to your desired minimum boost pressure, before any electronic control is applied. |
| 00:22 | This is important for two reasons. |
| 00:25 | Firstly, we want to make sure that we can control the minimum amount of power the engine is capable of producing. |
| 00:32 | Both from a reliability as well as from a usability perspective. |
| 00:37 | For example, with a two-wheel drive car it may be beneficial to have the ability to reduce the boost and hence power, low enough to reduce wheel spin on a wet road. |
| 00:48 | The flip side of this however, is that we can't achieve infinite control over the boost pressure using electronic control techniques alone. |
| 00:58 | What I mean by this, is that if our minimum boost pressure is perhaps 7 PSI, we may find that the maximum boost pressure we can achieve is 30 PSI, even if we remove the pressure source from the wastegate altogether. |
| 01:13 | The reason for this is that there are two forces acting to open the wastegate. |
| 01:18 | Understandably, we have the pressure source plummed to the waste gate actuator, providing boost pressure to open the valve. |
| 01:26 | But less obvious is the back pressure in the exhaust manifold. |
| 01:30 | I'll explain the situation in a little more detail. |
| 01:35 | The turbocharger actually presents a significant restriction to exhaust flow and in turn, this results in back pressure in the exhaust system between the exhaust valves and the turbine housing. |
| 01:49 | The amount of pressure varies depending on your turbocharger's size and engine capacity. |
| 01:54 | But in a factory turbo installation it's not uncommon to see the turbine inlet pressure reach double the boost pressure. |
| 02:02 | This means if you're seeing 15 PSI of positive pressure in the intake manifold, you could have 30 PSI of back pressure in the exhaust manifold. |
| 02:13 | This back pressure is acting on the wastegate valve and it has the effect of pushing it open. |
| 02:20 | This is why even with no boost pressure source on the wastegate actuator, at some point the back pressure will reach a point where it forces the wastegate open and the boost pressure stops climbing. |
| 02:33 | This is usually only an issue if you're tuning engines where you're wanting to achieve an extremely wide range and boost level. |
| 02:42 | An example of this might be a front-wheel drive drag car where only 10 PSI is used to launch the car but perhaps 45 to 50 PSI might be needed in top gear. |
| 02:53 | Options do exist to extend the range of boost control for these sort of applications, such as separate compressed air systems. |
| 03:02 | These are however, beyond the scope of this course. |
| 03:05 | Even in a street application, the effective turbine inlet pressure is still important to understand, as we'll see. |
| 03:14 | When it comes to choosing a spring for your wastegate, you'll find that the springs are rated to a certain pressure, a 10 PSI spring for example. |
| 03:22 | This seems simple enough, if you want your minimum boost pressure to be 10 PSI for example, just choose a 10 PSI wastegate spring. |
| 03:32 | The problem however, is that the spring rate will assume a certain level of back pressure in the exhaust manifold. |
| 03:39 | If your turbine inlet pressure is very high or very low, this will effect the actual boost level that you'll achieve. |
| 03:47 | For this reason, the spring pressure rated by the wastegate manufacturer is not necessarily going to be 100% accurate. |
