MAP behavior question

[Bob F.[_3_]]

Here's a question ...
When a normally aspirated engine quits while at cruise, how does the MAP
react with respect to the throttle?
Does it indicate zero?
Does it indicate barometric pressure?
Does it pin? (Go to max)?
Does it follow the Throttle (high MAP with a high throttle setting , low
when set to low throttle setting)?

....and why?

[Bob F.[_3_]]

"Bob F." > wrote in message
. ..
> Here's a question ...
> When a normally aspirated engine quits while at cruise, how does the MAP
> react with respect to the throttle?
> Does it indicate zero?
> Does it indicate barometric pressure?
> Does it pin? (Go to max)?
> Does it follow the Throttle (high MAP with a high throttle setting , low
> when set to low throttle setting)?
>
> ...and why?
>
>
Reading my own question, I guess I need to qualify it.
1. Prop is still rotating.
2. There is a MAP gauge in the aircraft (172rg, Arrow, etc.)

--
Regards, Bob F.

[Morgans[_2_]]

"Bob F." > wrote

> Here's a question ...
> When a normally aspirated engine quits while at cruise, how does the MAP
> react with respect to the throttle?
> Does it indicate zero?
> Does it indicate barometric pressure?
> Does it pin? (Go to max)?
> Does it follow the Throttle (high MAP with a high throttle setting , low
> when set to low throttle setting)?

From a theoretical point of view, I vote for the last choice. Why?

I would think because, if the prop is still turning, the engine is still an
air pump, just as much so as if it were still running. The "suck" part of
the suck, squeeze, bang and blow is still there. If the throttle is closed,
the throttle butterfly is closed so the suck is trying to pull a partial
vacuum.

I would think that because the engine is turning more slowly, the ability to
pull as low of a MAP will be lessened, (higher MAP-just to clarify) than the
pressure you would normally see at the same throttle setting with the engine
running.
--
Jim in NC

[Bob F.[_3_]]

"Morgans" > wrote in message
...
>
> "Bob F." > wrote
>
>> Here's a question ...
>> When a normally aspirated engine quits while at cruise, how does the MAP
>> react with respect to the throttle?
>> Does it indicate zero?
>> Does it indicate barometric pressure?
>> Does it pin? (Go to max)?
>> Does it follow the Throttle (high MAP with a high throttle setting , low
>> when set to low throttle setting)?
>
> From a theoretical point of view, I vote for the last choice. Why?
>
> I would think because, if the prop is still turning, the engine is still
> an air pump, just as much so as if it were still running. The "suck" part
> of the suck, squeeze, bang and blow is still there. If the throttle is
> closed, the throttle butterfly is closed so the suck is trying to pull a
> partial vacuum.
>
> I would think that because the engine is turning more slowly, the ability
> to pull as low of a MAP will be lessened, (higher MAP-just to clarify)
> than the pressure you would normally see at the same throttle setting with
> the engine running.
> --
> Jim in NC
>
Correct, at least your first paragraph. With respect, I don't understand
your second paragraph without reading into it.

I ask this Q. because I often ask a student to tell me how to recognize and
engine fuel starvation. I often get the incorrect answer that the MAP goes
to zero.

Years ago, before best practices frowned upon this, you could set up a glide
at about 400 fpm down, 80 kts, and 2200 RPM, pull the mixture and you would
not hear or feel any engine loss. And if you pushed the throttle in, MAP
would increase, just like when it's running ok. So this is not a good
indication of engine fuel starvation.
You could set up the same scenario on a ME and find the right speeds and
settings to demo this.
--
Regards, Bob F.

[Morgans[_2_]]

"Bob F." > wrote

> Correct, at least your first paragraph. With respect, I don't understand
> your second paragraph without reading into it.

All I was getting at (perhaps incorrectly) that the engine would be running
slightly faster if it were producing power compared to the speed of the
engine at the same throttle setting if the engine was dead. That lower
windmilling RPM should therefore produce a slightly higher MAP.

I would expect that the difference would be nearly unobservable, but
theoretically should be present.

Certainly not enough difference for the student to be able to troubleshoot
whether the engine is still running or not, now that I know what situation
you were getting at.
--
Jim in NC

[Capt. Geoffrey Thorpe]

"Bob F." > wrote in message
...
>
>
> "Morgans" > wrote in message
> ...
>>
>> "Bob F." > wrote
>>
>>> Here's a question ...
>>> When a normally aspirated engine quits while at cruise, how does the
>>> MAP react with respect to the throttle?
>>> Does it indicate zero?
>>> Does it indicate barometric pressure?
>>> Does it pin? (Go to max)?
>>> Does it follow the Throttle (high MAP with a high throttle setting ,
>>> low when set to low throttle setting)?
>>
>> From a theoretical point of view, I vote for the last choice. Why?
>>
>> I would think because, if the prop is still turning, the engine is still
>> an air pump, just as much so as if it were still running. The "suck"
>> part of the suck, squeeze, bang and blow is still there. If the
>> throttle is closed, the throttle butterfly is closed so the suck is
>> trying to pull a partial vacuum.
>>
>> I would think that because the engine is turning more slowly, the
>> ability to pull as low of a MAP will be lessened, (higher MAP-just to
>> clarify) than the pressure you would normally see at the same throttle
>> setting with the engine running.
>> --
>> Jim in NC
>>
> Correct, at least your first paragraph. With respect, I don't understand
> your second paragraph without reading into it.
>

The windmilling prop will turn the engine slower than it was turning under
power. As a result the rate at which air is pumped out of the intake
manifold is reduced, which allows the air flow through the throttle body to
fill the manifold up to a somewhat higher pressure.

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

[Dave S]

Bob F. wrote:
> Here's a question ...
> When a normally aspirated engine quits while at cruise, how does the MAP
> react with respect to the throttle?
> Does it indicate zero?

Only in outer space. Or if the gauge is broken.

> Does it indicate barometric pressure?

With an open throttle and stopped prop.

> Does it pin? (Go to max)?

Why would it do that? If you are 'cruising' on the taxiway, you might
get the 29.9-30 inches reading, if it dies there.

> Does it follow the Throttle (high MAP with a high throttle setting , low
> when set to low throttle setting)?

Yes. except as noted above.

>
> ...and why?
>

Because an engine is an air pump. If the pistons are moving air, then
the air pressure can be measured by the MP gauge. The throttle will
manipulate the air pressure as the engine pumps air past it. Closed
throttle, low pressure. Open throttle, high pressure.

[Dave S]

Bob F. wrote:

>>
> Reading my own question, I guess I need to qualify it.
> 1. Prop is still rotating.
> 2. There is a MAP gauge in the aircraft (172rg, Arrow, etc.)
>

Item two is purely for fluff. Manifold pressure has no way of knowing
its being measured. You can discuss manifold pressure, and its effect,
without a gauge installed.

But I'm guessing you knew that.

[Dave S]

Capt. Geoffrey Thorpe wrote:
As a result the rate at which air is pumped out of the
> intake manifold is reduced, which allows the air flow through the
> throttle body to fill the manifold up to a somewhat higher pressure.

Careful.. there is very little "filling" going on. Unless you are in a
Mooney, at wide open throttle, with the ram air door open.

Manifold pressure is almost solely the effect of the engine sucking air
in by piston action past the air filter, and ram air difference is
negligible in the typical trainer.

Respectfully, you guys are trying to measure something with a micrometer
(ram air contribution), that was cut with an axe (throttle) (to quote
Jim Weir)..

I think some of the best reading on MP was in John Deakin's series of
articles on Fire Breathing Turbos. The first two I think were dedicated
to fundamentals...with and without turbo charging.

Anyone interested, go to www.avweb.com and search for his material.
Dave

[Thomas Borchert]

Bob,

Same indication as with engine running.

The important thing to always remember is that "Manifold pressure
sucks", which is also the title of John Deakin's brilliant Avweb column
on the topic - absolutely required reading.

--
Thomas Borchert (EDDH)

[Andrew Sarangan]

On Aug 24, 6:22 am, Thomas Borchert >
wrote:
> Bob,
>
> Same indication as with engine running.
>
> The important thing to always remember is that "Manifold pressure
> sucks", which is also the title of John Deakin's brilliant Avweb column
> on the topic - absolutely required reading.
>
> --
> Thomas Borchert (EDDH)

I absolutely second that recommendation. I learned a lot from reading
John Deakin's columns. But you have to read past his writing style. He
tends to get wordy, and after a long introduction, he quickly glosses
over the final point. Its like a novel that ends rather abruptly. I
had to read them several times to get all the subtleties. But it is an
absolutely required reading for anyone wanting a deeper understanding
of aircraft piston engines.

[Capt. Geoffrey Thorpe]

"Dave S" > wrote in message
...
> Capt. Geoffrey Thorpe wrote:
> As a result the rate at which air is pumped out of the
>> intake manifold is reduced, which allows the air flow through the
>> throttle body to fill the manifold up to a somewhat higher pressure.
>
> Careful.. there is very little "filling" going on. Unless you are in a
> Mooney, at wide open throttle, with the ram air door open.

Of course there is filling. Where do you think the air that mixes with
gasoline in the engine comes from? Open the throttle wide, lots comes in,
manifold pressure goes up. Close the throttle, and only a little comes in,
manifold pressure goes down (manifold "emptying").

>
> Manifold pressure is almost solely the effect of the engine sucking air
> in by piston action past the air filter, and ram air difference is
> negligible in the typical trainer.

Ram air and filter pressure drops are pretty small compared to the pressure
drop across the throttle body except at wide open throttle. It's the
throttle that pretty much controls manifold pressure at any given engine
speed. (ignoring boost from turbo's, and effects of altitude for the
moment).

>
> Respectfully, you guys are trying to measure something with a micrometer
> (ram air contribution), that was cut with an axe (throttle) (to quote Jim
> Weir)..
>

Who said anything about ram air?

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

[Tman]

Nomen Nescio wrote:

>
> Quite simply, a piston on the down stroke (suck) has no idea why it's
> on the downstroke. Is it being droven by a windmilling prop, or the other
> cylinders firing? It doesn't know............it doesn't care.
>
> For a given RPM and throttle setting, MAP will be a constant.
>


No, cause engines don't have 100% volumetric efficiency. On the exhaust
stroke, with combustion happening, there will be a higher volume of
exhaust, which is discharged less completely, resulting in less
intake... and a higher MAP when the engine is running compared to not
running (firing); RPM and throttle remaining the same.
:)

>