Forced induction and wastegates

[easyrider]

Ive got a few questions here, first of all do superchargers have wastegates like turbos?
And why is there a waste gate if certain size turbos can only make so much boost? I know itll blow up but like say you have an engine that can handle very high power and boost and it has a very small turbo wheres the need for a waste gate? The only reason I could come up with was no matter what size the turbo the boost just keeps increasing constantly until engine failure.

Is that right?

 

[stewartcs]

do superchargers have wastegates like turbos?

No. They're driven by the engine instead of a turbine so there is no need for one.

And why is there a waste gate if certain size turbos can only make so much boost? I know itll blow up but like say you have an engine that can handle very high power and boost and it has a very small turbo wheres the need for a waste gate?

Wastegates are not necessarily required; however, they are generally used. It really depends on the whole system design. Wastegates just stabilize the boost pressure in the engine to a designed pressure based on the engine capacity.

The only reason I could come up with was no matter what size the turbo the boost just keeps increasing constantly until engine failure.

Is that right?

The blow-off valve on the intake manifold would lift before you destroyed the engine.

 

[easyrider]

So you don't have to run a wastegate? Is a blow off valve required if you don't have one?

Because someone told me without a wastegate or something to control boost the turbo would just keep increasing boost until destruction.

 

[stewartcs]

So you don't have to run a wastegate? Is a blow off valve required if you don't have one?

Because someone told me without a wastegate or something to control boost the turbo would just keep increasing boost until destruction.

Superchargers are driven by the engine via a belt so it is limited by the RPM's of the engine. This also results in some power loss as it must be taken from the engine in order to drive the supercharger.

A turbo is driven by a turbine which is driven by exhaust gases which can cause the turbine to over-run which can lead to higher pressure in the engine since the turbo also drives the compressor. If the pressure exceeds the design it will cause damage to the engine.

Wastegates are not necessarily required on turbos, however, on most gasoline powered cars, they are used. The ones that don't require them (usually commercial diesel types) are generally referred to as free-floating turbochargers. If the turbo doesn't have one (i.e. doesn't have an internal one) then it will require an external one to be installed if you need high performance from it (i.e in gasoline powered engines).

Blow-off valves should be used to prevent the compressor from surging.

 

[brewnog]

Most cars generally employ small turbos to provide extra power at low engine speed. At higher engine speeds a small turbo would tend to overspeed (and potentially cause engine and turbo damage) if it weren't for the wastegate. (The wastegate allows exhaust gas to bypass the turbine).

Of course, you can use a larger turbo to provide boost with less of a need for a wastegate, but these take longer to respond to transient engine load/speed conditions and promote turbo lag.

Engines in non-automotive applications are frequently used without wastegates since the turbos are matched to well-defined (narrower) operating speed ranges. You can run an automotive turbocharged engine without a wastegate, but you must make the tradeoff between power at your rated speed, and turbocharger durability.

 

[Mech_Engineer]

For superchargers, they actually DO have an element similar to a turbocharger's blowoff valve, usually called a bypass valve.

The basic reasoning behind this device is that you don't want the output pressure of the supercharger to be determined solely by engine speed, it should also determined by throttle input. Usually, the bypass valve will be set so that the max available pressure for a specific RPM range is achieved at close to 100% throttle input.

I have a supercharged Toyota 4Runner and the effect of the bypass valve is clearly distinguishable, as manifold pressure (vac/boost) will generally follow throttle input. So, even if my engine is running at 5000 rpm, if I am giving 0% throttle input the manifold pressure is in vacuum. it is true however that the maximum boost available at 5000rpm is significantly higher than at 2000rpm.


...and, on the topic of turbocharger blowoff valves-

Wikipedia Article

Anti-Surge/Dump/Blow Off Valves

Turbo charged engines operating at wide open throttle and high rpm require a large volume of air to flow between the turbo and the inlet of the engine. When the throttle is closed compressed air will flow to the throttle valve without an exit (i.e. the air has nowhere to go).

This causes a surge which can raise the pressure of the air to a level which can be destructive to the engine e.g. damage may occur to the throttle plate, induction pipes may burst...

The primary use of this valve is to prevent damage to the engine by a surge of compressed air and to maintain the turbo spinning at a high speed.

 

[brewnog]

For superchargers, they actually DO have an element similar to a turbocharger's blowoff valve, usually called a bypass valve.

The basic reasoning behind this device is that you don't want the output pressure of the supercharger to be determined solely by engine speed, it should also determined by throttle input. Usually, the bypass valve will be set so that the max available pressure for a specific RPM range is achieved at close to 100% throttle input.

I have a supercharged Toyota 4Runner and the effect of the bypass valve is clearly distinguishable, as manifold pressure (vac/boost) will generally follow throttle input. So, even if my engine is running at 5000 rpm, if I am giving 0% throttle input the manifold pressure is in vacuum. it is true however that the maximum boost available at 5000rpm is significantly higher than at 2000rpm.

It's worth bearing in mind here that a supercharger bypass isn't too similar to a turbo's wastegate, and isn't fulfilling the same function (though it is similar to a turbocharger's compressor bypass, where fitted). Also, on a gasoline engine, the manifold pressure will still be under vacuum at zero throttle with a turbocharged engine; this isn't a feature restricted to supercharged or nat asp engines. (Obviously diesel engines differ in this respect.)

 

[Old Pokey]

Hi all. New guy here with his first post. I hope I do this right and make friends, not enemies.

If I may post this link as a little help with this subject. http://www.streetracersonline.com/articles/turbo/wastegate.php
It should also be noted IMO, that there are many different types of forced induction systems. For instance a "supercharger" can also be a passive system with a centrifigal impeller as well as a roots type or other positive displacement compressors. Each with its own needs for relieving excess boost.

Thanks.