What is the purposes of having a wastegate as I've not understood
the difference between the two? Thanks!
JP
Dump Valves and Wastegates - difference?
Dump Valves and Wastegates - difference?
I currently have a HKS DV on my Scoob. Is there a need for me to have a wastegate too?
What is the purposes of having a wastegate as I've not understood the difference between the two?
Thanks!
JP
What is the purposes of having a wastegate as I've not understood
the difference between the two? Thanks!
JP
Re: Dump Valves and Wastegates - difference?
Wastegate is used to regulate boost by varying the amount of exhaust flow that goes through the turbo.
96 MKIV Supra, S6GP, 591bhp & 523ft/lbs
Re: Dump Valves and Wastegates - difference?
When is the need to have a Wastegate? i.e. is it required at certain power levels and above? If so, what power are we talking about?
Re: Dump Valves and Wastegates - difference?
You have already got a wastegate, I imagine on your unit its integral to the turbo.
96 MKIV Supra, S6GP, 591bhp & 523ft/lbs
Re: Dump Valves and Wastegates - difference?
I didn't think I did to be honest hence why I thought best to put this up on here . I know I have a DV which is to atmosphere?
Is my wastegate built in separate or part of the DV? Sorry if this thread sounds annoying, but please bear in mind I am not too techy on this stuff so would appreciate any guidance.
Thanks!
Jag
. I know I have a DV which is to atmosphere?Is my wastegate built in separate or part of the DV? Sorry if this thread sounds annoying, but please bear in mind I am not too techy on this stuff so would appreciate any guidance.
Thanks!
Jag
Re: Dump Valves and Wastegates - difference?
Your wastegate has nothing to do with the DV
96 MKIV Supra, S6GP, 591bhp & 523ft/lbs
Re: Dump Valves and Wastegates - difference?
Is this Scoobynet ? 
-
pat
- Syvecs Staff - Cleaner
- Posts: 356
- Joined: Fri May 23, 2008 10:23 am
- Location: Out there... somewhere
- Contact:
Re: Dump Valves and Wastegates - difference?
Jag,
The wastegate on your car is what is known as an "internal wastegate", this means it is part of the turbocharger. The other type is known as, oddly enough, an "external wastegate" and is a separate unit from the turbo; turbos that are designed to work with external wastegates are incapable of controlling boost pressure themselves.
The function of the wastegate is to control the boost pressure by regulating the flow of exhaust gas through the turbine, which provides the power to the compressor. If the wastegate opens, it allows some of the exhaust gas to bypass the turbine, therefore the amount of power the turbine generates, and can provide to the compressor, falls. The compressor in turn is unable to compress as much air and so the boost pressure falls.... conversely if the wastegate closes, more exhaust gas flows through the turbine, it generates more power and so the compressor can raise the boost. If one were to simply weld the wastegate on your turbo shut, the midrange boost would be excessive, I would have thought in the region of 3 bar, which wouldn't do the engine any favours, so we keep the wastegate shut until we reach the desired boost, and then open it sufficiently to maintain that boost; ie not so little that the boost keeps going up, and not so much that it goes down, but just the right amount to keep it where it is.
This is one way of controlling boost pressure. It is not the best, and it is not the worst, but it is simple. Other ways are using blow off valves and variable geometry. With a blow off valve, the boost pressure is controlled on the cold side of the turbo, it permits the boost pressure to rise to a given value before it opens, in effect it bleeds away excess charge air, rather than excess exhaust gas... it is less efficient because the turbo is having to compress more air than necessary, some of which is just thrown away, but some tuners swear by this method, whereas I swear at it, LOL. Variable geometry is a better way than wastegate control but much more difficult to build. A wastegate is, as its name implies, wasteful because it discards exhaust gas that could do some useful work, if it were given the chance, but it is denied that chance because the result would be too much boost. With a variable geometry setup, all the exhaust gas always goes through the turbine, but the A/R ratio of the turbine housing is varied.... in a nutshell, if the A/R ratio is small, the turbo will spool like there is no tomorrow but it will soon choke up... you might get full boost at 2000 RPM but it may only go on to make 100BHP, whereas if the A/R is large it will spool late (or indeed never if it is too big period) but it will make good power... you might get full boost at 6000 RPM and it might make 500BHP, but the car will be horrible to drive. What variable geometry does is to change the A/R so that it is "just right" at any given point, ie at 4000 RPM it is small enough to make the boost you want, but too big to make any more; if the boost is too high then the A/R is increased and the boost comes back down, and vice versa, but all the time ALL of the exhaust gas does some useful work. Compared to a "conventional" exhaust housing, a variable geometry one will permit a greater overall airflow through the engine because it can operate with lower exhaust back pressure since it has more energy available to it, which results in an increase in volumetric efficiency.
The function of a dump valve is to protect the compressor wheel, intercooler pipework and intercooler, as well as possibly reduce lag time on gear changes or any other rapid throttle movement where the throttle closes and reopens. The compressor wheel is "dumb", it just moves air, and it is the restriction of this air going into the engine that generates boost pressure. Imagine for a moment blowing down a 4 inch drain pipe, your cheeks would never inflate because the drain pipe can flow more air than you can provide. Now imagine doing the same thing with a drinking straw; your lungs can supply air faster than the straw can flow it away, your cheeks inflate because pressure builds as the air cannot escape fast enough. The same thing happens with a turbo and an engine.... the turbo is trying to stuff air into the engine faster than the engine can consume it, so pressure is built which "forces" the air into the engine (hence the term forced induction). So far so good, but now you slam the throttle shut; the engine can consume even less air but the turbo is still trying to shove it in there, so the reaction is that the boost pressure post-turbo but pre-throttle rises rapidly... the compressor doesn't "know" you've slammed the throttle shut, it just tries to keep stuffing the air in there. This sudden increase in pressure creates a shock loading on the compressor and turbo shaft, and the resultant pressure spike stresses the intercooler and its pipework... the engine may never see more than 2 bar, but the intercooler may see 3.5 when you lift off. What the dump valve does it allow the airflow to continue when the throttle is closed; instead of the air going into the engine it rushes out of the dump valve. The pressure doesn't spike because the flow isn't restricted, it's going somewhere else. This has the effect of extending turbo life considerably, sometimes by a factor of two! But many people mis-apply dump valves and set them waaaay too soft. Remember they are only there to prevent excessive pressure, they should NOT allow all the air in the intercooler pipework to escape, only enough to prevent the pressure spike. Many a time I have seen dump valves set so they are "permanently open" when the engine is in vacuum, this is wrong, it should require some boost in the pipework as well as manifold vacuum.... indeed a blow off valve would be more effective than a dump valve. If you allow the pressure to fall, you have to build it back up again, so you have more lag. But on the flip side, in order to maintain the pressure requires the compressor to receive power, and because the engine isn't flowing air it isn't providing exhaust gas the turbine can derive power from, which means it cannot provide power to the compressor, but as luck would have it the rotating assembly has mass, and when mass moves it has inertia, there is some energy stored in the rotating assembly that can be depleted to maintain power delivery to the compressor. It is only a small amount of energy, perhaps sufficient to maintain the boost for 1/4 second or so, but if your gear changes are quick, it's enough. If your gear changes are slow then the boost will just collapse due to lack of compressor power. The amount of power required is a function of the airflow and also the boost pressure, if we allow the boost to drop from (say) 1.8 to 1 bar, then we might have 0.4 seconds worth of energy in the rotating assembly... this is why a dump valve is more useful than a blowoff valve, you can tweak it to regulate the boost pressure to a level that is less than full boost, but only when the throttle is closed, whereas a blowoff valve is not influenced by the throttle. If your vehicle has a sequential gearbox and gear changes are swift then a blowoff valve may be a better choice than a dump valve, but if you have a conventional H-pattern gearbox then a dump valve is most likely better.
And if that wasn't confusing enough, the Solaris ECUs have a full throttle shift feature, which allows you to change gear without ever lifting off the accelerator.... no closed throttle for pressure to build up against, no boost spikes, no interruption to exhaust gas flow, no real need for a dump valve for gear changes, but of course you have to lift off sooner or later, and at that point you'de still want to have a dump valve!
Hope this helps,
Pat.
The wastegate on your car is what is known as an "internal wastegate", this means it is part of the turbocharger. The other type is known as, oddly enough, an "external wastegate" and is a separate unit from the turbo; turbos that are designed to work with external wastegates are incapable of controlling boost pressure themselves.
The function of the wastegate is to control the boost pressure by regulating the flow of exhaust gas through the turbine, which provides the power to the compressor. If the wastegate opens, it allows some of the exhaust gas to bypass the turbine, therefore the amount of power the turbine generates, and can provide to the compressor, falls. The compressor in turn is unable to compress as much air and so the boost pressure falls.... conversely if the wastegate closes, more exhaust gas flows through the turbine, it generates more power and so the compressor can raise the boost. If one were to simply weld the wastegate on your turbo shut, the midrange boost would be excessive, I would have thought in the region of 3 bar, which wouldn't do the engine any favours, so we keep the wastegate shut until we reach the desired boost, and then open it sufficiently to maintain that boost; ie not so little that the boost keeps going up, and not so much that it goes down, but just the right amount to keep it where it is.
This is one way of controlling boost pressure. It is not the best, and it is not the worst, but it is simple. Other ways are using blow off valves and variable geometry. With a blow off valve, the boost pressure is controlled on the cold side of the turbo, it permits the boost pressure to rise to a given value before it opens, in effect it bleeds away excess charge air, rather than excess exhaust gas... it is less efficient because the turbo is having to compress more air than necessary, some of which is just thrown away, but some tuners swear by this method, whereas I swear at it, LOL. Variable geometry is a better way than wastegate control but much more difficult to build. A wastegate is, as its name implies, wasteful because it discards exhaust gas that could do some useful work, if it were given the chance, but it is denied that chance because the result would be too much boost. With a variable geometry setup, all the exhaust gas always goes through the turbine, but the A/R ratio of the turbine housing is varied.... in a nutshell, if the A/R ratio is small, the turbo will spool like there is no tomorrow but it will soon choke up... you might get full boost at 2000 RPM but it may only go on to make 100BHP, whereas if the A/R is large it will spool late (or indeed never if it is too big period) but it will make good power... you might get full boost at 6000 RPM and it might make 500BHP, but the car will be horrible to drive. What variable geometry does is to change the A/R so that it is "just right" at any given point, ie at 4000 RPM it is small enough to make the boost you want, but too big to make any more; if the boost is too high then the A/R is increased and the boost comes back down, and vice versa, but all the time ALL of the exhaust gas does some useful work. Compared to a "conventional" exhaust housing, a variable geometry one will permit a greater overall airflow through the engine because it can operate with lower exhaust back pressure since it has more energy available to it, which results in an increase in volumetric efficiency.
The function of a dump valve is to protect the compressor wheel, intercooler pipework and intercooler, as well as possibly reduce lag time on gear changes or any other rapid throttle movement where the throttle closes and reopens. The compressor wheel is "dumb", it just moves air, and it is the restriction of this air going into the engine that generates boost pressure. Imagine for a moment blowing down a 4 inch drain pipe, your cheeks would never inflate because the drain pipe can flow more air than you can provide. Now imagine doing the same thing with a drinking straw; your lungs can supply air faster than the straw can flow it away, your cheeks inflate because pressure builds as the air cannot escape fast enough. The same thing happens with a turbo and an engine.... the turbo is trying to stuff air into the engine faster than the engine can consume it, so pressure is built which "forces" the air into the engine (hence the term forced induction). So far so good, but now you slam the throttle shut; the engine can consume even less air but the turbo is still trying to shove it in there, so the reaction is that the boost pressure post-turbo but pre-throttle rises rapidly... the compressor doesn't "know" you've slammed the throttle shut, it just tries to keep stuffing the air in there. This sudden increase in pressure creates a shock loading on the compressor and turbo shaft, and the resultant pressure spike stresses the intercooler and its pipework... the engine may never see more than 2 bar, but the intercooler may see 3.5 when you lift off. What the dump valve does it allow the airflow to continue when the throttle is closed; instead of the air going into the engine it rushes out of the dump valve. The pressure doesn't spike because the flow isn't restricted, it's going somewhere else. This has the effect of extending turbo life considerably, sometimes by a factor of two! But many people mis-apply dump valves and set them waaaay too soft. Remember they are only there to prevent excessive pressure, they should NOT allow all the air in the intercooler pipework to escape, only enough to prevent the pressure spike. Many a time I have seen dump valves set so they are "permanently open" when the engine is in vacuum, this is wrong, it should require some boost in the pipework as well as manifold vacuum.... indeed a blow off valve would be more effective than a dump valve. If you allow the pressure to fall, you have to build it back up again, so you have more lag. But on the flip side, in order to maintain the pressure requires the compressor to receive power, and because the engine isn't flowing air it isn't providing exhaust gas the turbine can derive power from, which means it cannot provide power to the compressor, but as luck would have it the rotating assembly has mass, and when mass moves it has inertia, there is some energy stored in the rotating assembly that can be depleted to maintain power delivery to the compressor. It is only a small amount of energy, perhaps sufficient to maintain the boost for 1/4 second or so, but if your gear changes are quick, it's enough. If your gear changes are slow then the boost will just collapse due to lack of compressor power. The amount of power required is a function of the airflow and also the boost pressure, if we allow the boost to drop from (say) 1.8 to 1 bar, then we might have 0.4 seconds worth of energy in the rotating assembly... this is why a dump valve is more useful than a blowoff valve, you can tweak it to regulate the boost pressure to a level that is less than full boost, but only when the throttle is closed, whereas a blowoff valve is not influenced by the throttle. If your vehicle has a sequential gearbox and gear changes are swift then a blowoff valve may be a better choice than a dump valve, but if you have a conventional H-pattern gearbox then a dump valve is most likely better.
And if that wasn't confusing enough, the Solaris ECUs have a full throttle shift feature, which allows you to change gear without ever lifting off the accelerator.... no closed throttle for pressure to build up against, no boost spikes, no interruption to exhaust gas flow, no real need for a dump valve for gear changes, but of course you have to lift off sooner or later, and at that point you'de still want to have a dump valve!
Hope this helps,
Pat.