Turbocharger

[meritflyer]

Had a gentleman tell me recently that below 26" of MAP, the turbine of the turbocharger doesn't kick in and provide the compressed air to the cylinder. In fact, I've heard this a few times.

Anyone familiar with this?

 

[ppragman]

Had a gentleman tell me recently that below 26" of MAP, the turbine of the turbocharger doesn't kick in and provide the compressed air to the cylinder. In fact, I've heard this a few times.

Anyone familiar with this?

Depends greatly on the model.

 

[meritflyer]

Depends greatly on the model.

Of course. Generally speaking?

 

[USMCmech]

If the engine is running, the turbocharger is ALWAYS spinning and providing boost to the engine.

A device called the "wastegate" allows some exaust air to bypass the turbine wheel. effectively derating the compressor. On some aircraft engines the wastegate is fixed. The compressor is continually boosting the MAP above ambient. On some aircraft the wastegate is vairiable, these are commonaly called "turbonormalized" meaning they don't add much boost, but keep 30" MAP to a higher altitude.

 

[deadstick]

There are the Rayjay style, on-demand, turbos, and I think those are the variable wastegate types. The engine can be un-boosted until you move the wastegates. I saw this on a PA30 -- very nice.

Also, the auto-wastegates are use to dump the turbo in the event of an overboost. The fixed ones can get you in a jam ($$) pretty quickly.

 

[Barty]

If the engine is running, the turbocharger is ALWAYS spinning and providing boost to the engine.

That's not 100% correct. Below a certain speed, the turbo is, for practical purposes, windmilling. Air is passing through it, but it is in no greater volume than what the engine would normally receive if it were naturally aspirated. It depends on the design of the turbo at what engine speed that it will start producing any meaningful boost.

 

[unclenobby]

That's not 100% correct. Below a certain speed, the turbo is, for practical purposes, windmilling. Air is passing through it, but it is in no greater volume than what the engine would normally receive if it were naturally aspirated. It depends on the design of the turbo at what engine speed that it will start producing any meaningful boost.

Curious - what RPM would you consider windmilling for the turbine considering it can spin up to 80K RPM?

 

[CK]

Had a gentleman tell me recently that below 26" of MAP, the turbine of the turbocharger doesn't kick in and provide the compressed air to the cylinder. In fact, I've heard this a few times.

Anyone familiar with this?

It is more complex than that. In the majority of planes, the wastegate controller sets the wastegate to hold the MP you request. At sea level, 26" of MP is low enough that the engine does not need any compressed air. In this case the wastegate is fully open, routing the majority of the exhaust away from the turbine wheel. At 10,000 feet the engine does need compressed air to hold 26" of MP, so the wastegate is diverting air into the turbine section.

Also remember that even at a low power setting like 20" of MP at sea level, the induction air still passes over the compessor which spins the wheel, so the turbocharger is always spinning. But in this case the compessor wheel spins the turbine wheel and it is more of a hindrance on the flow of induction air than it is a benefit.

The Ray Jays are a manual turbocharger. I fly a Bonanza and a Twin Comanche that both have the Ray Jays, but they are very rare. In this case the pilot actually controls the wastegate manually through a throttle like control. Pushing the Ray Jay throttle all the way in closes the wastegate completely. The engines used on the Seneca II and Turbo Arrow have fixed wastegates that are always closed. If you bring the throttle all the way to the stop you will overboost the engine.

This is my basic understanding of turbochargers. I could be wrong, but it sounds right.

Alex.

 

[USMCmech]

Also remember that even at a low power setting like 20" of MP at sea level, the induction air still passes over the compessor which spins the wheel, so the turbocharger is always spinning. But in this case the compessor wheel spins the turbine wheel and it is more of a hindrance on the flow of induction air than it is a benefit.

This is my basic understanding of turbochargers. I could be wrong, but it sounds right.

I think you have misnamed the parts or are backwards in your understanding. Exaust gas turns the turbine wheel, which spins the shaft conected to the compressor. The compressor pushes extra air into the intake manifold.

At idle, especially with the wastegate open this boost is negligable, but it is still there.

 

[CK]

I think you have misnamed the parts or are backwards in your understanding. Exaust gas turns the turbine wheel, which spins the shaft conected to the compressor. The compressor pushes extra air into the intake manifold.

At idle, especially with the wastegate open this boost is negligable, but it is still there.

I didn't have the parts misnamed, I just didn't make my statement clear enough. My point was, the induction air that passes over the compressor will cause it to spin, even if the turbine wheel were to have no exhaust passing over it. I have no idea how much exhaust is diverted towards the turbine wheel with the wastegate fully open. I know in the Lycomings the exhaust would have to make a 90 degree turn to get to the turbine wheel, I do not know if the exhaust would make that turn with the wastegate fully open.

Alex.

 

[USMCmech]

I didn't have the parts misnamed, I just didn't make my statement clear enough. My point was, the induction air that passes over the compressor will cause it to spin, even if the turbine wheel were to have no exhaust passing over it. I have no idea how much exhaust is diverted towards the turbine wheel with the wastegate fully open. I know in the Lycomings the exhaust would have to make a 90 degree turn to get to the turbine wheel, I do not know if the exhaust would make that turn with the wastegate fully open.

Ah, I see. That does make more sense.

The wastegate only allows some of the exhaust gas to bypass the turbine wheel. So, even if it's wide open, it's still providing some boost.

 

[Barty]

Curious - what RPM would you consider windmilling for the turbine considering it can spin up to 80K RPM?

Probably below 20-30k for a typical turbo. Yeah, still fast as hell but the air being discharged from it is still no higher than atmospheric minus whatever restrictions are in the intake system. You can look at the compressor map for a turbo to determine what minimum speed is required to start producing useful pressures above atmospheric.

 

[Barty]

Ah, I see. That does make more sense.

The wastegate only allows some of the exhaust gas to bypass the turbine wheel. So, even if it's wide open, it's still providing some boost.

Yep. In fact, with most waste gates, they are all or nothing. They are designed to open rather rapidly when the desired amount of boost is attained. Most are set to about 6-8 psi by a spring in the waste gate actuator itself. Further adjustment is made possible by using solenoids or adjustable valves that delay part (by bleeding off part of the signal pressure) or all of the boost signal from reaching the waste gate until the desired pressure is reached. These are more fail-safe because even if the adjustable control fails, in most OEM setups, the waste gate just opens at its default pressure.

Provided that the turbine section has been properly sized to the compressor and to the engine, when the waste gate opens, it should hold the turbo at a fairly constant speed at that point, provided it can allow enough exhaust gas to bypass the turbine wheel. If an internal waste gate is not up to task, it will usually cause an external waste gate to be fitted to the exhaust system in order to prevent boost creep.

In addition, you can have blow off valves to prevent turbo damage when the throttle is rapidly closed, and/or as a safety mechanism should the waste gate fail to prevent overboost.

 

[norcalpilot25]

In addition, you can have blow off valves to prevent turbo damage when the throttle is rapidly closed, and/or as a safety mechanism should the waste gate fail to prevent overboost.

I hear the "pop off" valve all the time on modified WRX's and the like. People usually mod those things for that classic "hiss" sound anyway (just to let people know you have a turbo). I wouldn't think you could even hear the pressure relief valve on an aviation engine like you do on a car. I think its much less common for that Valve to even be used in an Aviation application anyway with our smooth throttle movements, but I dont know for sure.

Anyone know if you would notice a change in MP with the opening of the PRV?

 

[Barty]

I hear the "pop off" valve all the time on modified WRX's and the like. People usually mod those things for that classic "hiss" sound anyway (just to let people know you have a turbo). I wouldn't think you could even hear the pressure relief valve on an aviation engine like you do on a car. I think its much less common for that Valve to even be used in an Aviation application anyway with our smooth throttle movements, but I dont know for sure.

Anyone know if you would notice a change in MP with the opening of the PRV?

*Most* factory turbocharged vehicles use bypass valves that simply re-direct the boost back into the intake system ahead of the turbocharger. It accomplishes the same thing, without the cool sound. Also, most factory turbocharged vehicles use a mass airflow system for fuel metering, so dumping that air will cause a brief rich condition, which can lead to a damaging backfire if one were to step on the throttle quickly thereafter.

In aviation applications, I believe they are used more for overboost protection than for compressor surge. So it is unlikely you would hear one unless there is a problem or the throttle was advanced too quickly as to briefly overwhelm the waste gate. Particularly in an overboost situation, there would be a change in MP when the blow off valve opens.

 

[Medpilot]

Had a gentleman tell me recently that below 26" of MAP, the turbine of the turbocharger doesn't kick in and provide the compressed air to the cylinder. In fact, I've heard this a few times.

Anyone familiar with this?

So to answer the OP question.

Like mentioned above, even at idle the compressor/turbine shaft are always spinning. It depends on how much load you place on the engine that determines how much boost the turbo will produce.

Think about a normally aspirated engine. At sea level, normal cruse, the engine can produce approximately 30in/hg of manifold pressure (WOT). To get a higher power setting (> 30in/hg) you need a turbocharger.

Now at 5000ft, a normally aspirated engine can only produce 25in/hg. You lose 1" of manafold pressure for every 1000' of altitude gain. If you are cruising at 5000ft in a turbocharged aircraft and you are producing 26in/hg pressure, the turbo is producing 1in/hg of boost over standard pressure. So in other words, the turbo is "kicking in" if you are indicating a higer manafold pressure than if you were flying a normaly asperated engine at the same altitude.