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HeliTorque :: View topic - Automatic Boost Control
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HeliTorque Forum Index » Flight Dynamics

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Automatic Boost Control
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WindSwept
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PostPosted: Thu Apr 08, 2010 6:43 pm    Post subject: Automatic Boost Control Reply with quote

How does it work? Im looking at my CPL studies and it has not even got a diagram, nothing but a page of text.

What im stuck with is it talks about one system using an evacuated capsule.

It says the capsule is exposed to inlet pressure while being linked to the throttle via an oil operated servo piston. When the capsule is compressed the throttle is partly closed and vice versa.

When the engine starts the induction maniful pressure falls which is sensed by the capsule which then expands, to make oild flow below the servo piston, which is forced to the top of its stroke. If the throttle is opened anymore, the capsules compress and the oile supply is eventually cut off and redirected to the top of the piston making it go down and close the throttle.

Now i don't understand how the capsule expands when the engine is started yet when you open the throttle surely simulating the same thing as the engine starting that the capsule suddenly compresses. It doesn't make sense.

I don't even have an idea of what this setup looks like as there is no picture! Joy!
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veeany
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PostPosted: Thu Apr 08, 2010 7:59 pm    Post subject: Reply with quote

If I recall correctly ABC is used on turbo and possible supercharged engines.

I can't comment on your notes or ABC (because I know nothing about either) but I'll take a stab at one of your problems.

Quote:
Now i don't understand how the capsule expands when the engine is started yet when you open the throttle surely simulating the same thing as the engine starting that the capsule suddenly compresses. It doesn't make sense.


What you describe sounds much like what happens in the manifold of a normally apsirated piston engine aka R22, the pressure in the inlet manifold goes down on start up, then as you open the throtttle the pressure increases as you allow more air in per stroke of the engine.

It would make sense that a capsule sensitive to pressure would expand as the pressure goes down (engine start) and compress as the air pressure around it increases (as the throttle opens).
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flip2
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PostPosted: Thu Apr 08, 2010 9:19 pm    Post subject: Reply with quote

Here is my understanding of it – it will be interesting to see what everybody else thinks Smile

The purpose of an automatic boost control:
To maintain a constant manifold pressure, typically during changes in altitude.

The “mechanics” of the system:
When the capsule expands, the servo piston will partially open the throttle valve.
When the capsule gets compressed, the servo piston will partially close the throttle valve.

The system in practice:
As an aircraft climbs (for example), the air pressure decreases. Without any intervention, there will be a corresponding decrease in manifold pressure.

However, with an automatic boost control fitted, the lower pressure allows the capsule to expand which then partially opens the throttle valve.

The result of the throttle valve opening is that instead of the manifold pressure dropping, it is maintained.

In Summary:
Aircraft Climbs – Pressure Decreases – Capsule Expands – Opens Throttle – Manifold Pressure maintained.

Aircraft Descends – Pressure Increases – Capsule Contracts – Closes Throttle – Manifold Pressure maintained

Your notes:
I agree with veeany. If you watch the manifold pressure gauge when you start the engine you can clearly see it drop (in fact, if you think about your manifold pressure settings they are always below typical ambient pressures).

Now think back to the effects of controls – when the throttle is opened the manifold pressure increases.

So, I believe your notes are trying to explain how it is self-regulating:

Your notes say that when the engine is started the capsule expands – that makes sense as we know the pressure decreases on start.

The effect of the capsule expanding is for the throttle to open.

The result of the throttle opening is to increase manifold pressure... which we know will compress the capsule causing the throttle to close.

Hopefully you can see that an equilibrium will be reached, whereby manifold pressure remains constant.
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PostPosted: Fri Apr 09, 2010 10:17 am    Post subject: Reply with quote

Ahhh ok! That makes perfect sense now, i think the problem was there wasn't enough detail (it also doesn't really explain how this servo piston works to change the throttle - but i doubt i really need to know how its linkages work)

It never mentioned in the text that it was at all relying on atmospheric pressure, it mentioned monitoring inlet pressure from the induction manifold which lead me to believe that it was measuring the suction of the engine, and thus when the throttle was opened the only thing that could happen would be for the pressure to decrease and the capsule expand no matter what you did. Which is why it was confusing that it said if you open the throttle more it would suddenly start compressing instead. I think what you've written makes more sense logically to me and il stick with that as my notes.
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PostPosted: Fri Apr 09, 2010 10:40 am    Post subject: Reply with quote

WS

This may help (or confuse the issue even further)

When the engine turns it can only draw in a constant VOLUME of air per revolution.

The mass of this parcel (volume) of air can only be efficiently mixed with a corresponding mass of fuel.

The amout (mass) of air in the volume drawn in allows us to calculate the density of the air (or mixture) in the cylinder.

Opening the throttle allows more mass of air per unit volume and as the manifold pressure goes up, so does the denisty of the fuel air mixture but the mixing ratio remains approximately constant (the throttle is acting as a mass or density control).

Opening the throttle makes manifold pressure increase, puts more fuel and air in and if under contant load makes the RPM increase; or if under increasing load may cause to it remain the same (or go up or down, depends on actual load and power output at that precise moment).

GS
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flip2
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PostPosted: Sat Apr 10, 2010 2:51 am    Post subject: Reply with quote

I'm glad it helped Smile I hope I haven't created any ambiguity through oversimplification...

The capsule will only respond to changes in manifold pressure, although changes in ambient pressure will affect that.

As you've hinted at, you can think of an engine as an air pump generating suction. The manifold pressure gauge simply measures the absolute pressure of the air in the manifold before it enters the cylinder, making it indirectly a measure of suction.

Think of an engine intake system running without an air filter, throttle butterfly or any other "stuff" that is going to restrict the flow of air (you'll still need the venturi to get any fuel!). The manifold pressure will be the same as ambient air pressure as there is nothing to obstruct it, and the engine will run at maximum output.

Now if you start to restrict the flow of air, most obviously by adding a throttle butterfly, suction will start to increase as the engine tries to suck that impeded air in. The result is a drop in manifold air pressure.

As veeany so eloquently explained, it's all about the mass of air and fuel that can get into the fixed volume of the cylinder. Lots of mass = lots of air and fuel = lots of power.

If there isn't much air pressure in the manifold, this will limit the mass of air and fuel that can charge the cylinder when the piston moves down sucking it in.

So what causes changes in manifold pressure (in general terms)?
The first cause is a change in ambient air pressure. With the throttle position remaining fixed, any changes in ambient air pressure will be reflected by a similar change in the manifold pressure
The second cause would be a restriction to the flow of air i.e. the throttle butterfly closing.

My understanding of automatic boost control (ABC) is to deal with the first (a change in ambient air pressure, typically through changes in height):

Lets say you're flying straight and level and you have your throttle control set to give the RPM you want.
You now decide to climb (but you're not going to touch the throttle).
As the ambient pressure drops, the manifold pressure drops. We know that means there is less charge getting into the cylinders, which is going to cause the RPM to drop.
However, with ABC fitted, the drop in manifold pressure will cause the capsule to expand. This will open the throttle butterfly, and the RPM will remain constant... Happy days!
And because you've left your throttle control where it was, when you descend back to your original height the ABC removes it's correction and the RPM is still constant Very Happy

My knowledge of ABC is very limited, but that's basically what I understand of it - if it's purpose is something else then disregard everything I've said Laughing
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