Manifold Pressure/RPM
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Manifold Pressure/RPM
I am doing a bit of revision at the moment and I can't find the answer so could somebody please explain why when propellor RPM is reduced MP increases slightly.
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A very over simplified and basic answer, pistons were a long time ago!
When RPM is reduced for a constant throttle position, the rate of Inlet and Exhaust valve openings is reduced, thus, the engine cannot process the incoming Fuel/Air mix so quickly, nor can it dispose of the exhaust gases so quickly. Thus, the inlet pressure (Manifold Pressure) "backs up", leading to an increase in pressure.
Not so for mechanical superchargers however, lower engine RPM means lower supercharger RPM, which means less Fuel/Air mixture delivered to the cylinders. (Don't know about Turbo chargers, never had the privelage!
).
Pistons were 33 years ago, standing by for much more current operator's responses
Regards,
Old Smokey
When RPM is reduced for a constant throttle position, the rate of Inlet and Exhaust valve openings is reduced, thus, the engine cannot process the incoming Fuel/Air mix so quickly, nor can it dispose of the exhaust gases so quickly. Thus, the inlet pressure (Manifold Pressure) "backs up", leading to an increase in pressure.
Not so for mechanical superchargers however, lower engine RPM means lower supercharger RPM, which means less Fuel/Air mixture delivered to the cylinders. (Don't know about Turbo chargers, never had the privelage!
).Pistons were 33 years ago, standing by for much more current operator's responses
Regards,
Old Smokey
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When RPM comes down, so does the airflow through the carburetor.
With less airflow, there is less pressure drop across the venturi, assuming fixed throttle.
Manifold pressure (on an unsupercharged engine) is atmospheric (or ram) pressure minus the venturi pressure drop.
Yes, there are additional factors like the supercharger, air filter, intercooler etc. but the essentials of your answer are per the above.
With less airflow, there is less pressure drop across the venturi, assuming fixed throttle.
Manifold pressure (on an unsupercharged engine) is atmospheric (or ram) pressure minus the venturi pressure drop.
Yes, there are additional factors like the supercharger, air filter, intercooler etc. but the essentials of your answer are per the above.
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Here's another version of the same explanation
The engine sucks air through the induction system when pistons move down on the induction stroke.
To simplify the process, think of the engine as a vacuum cleaner motor. The vacuum cleaner hose represents the induction system.
1) When the vacuum cleaner is running, is the pressure in the hose more, less or the same as outside air pressure?
Generally get the answer - less.
So remember that a normally aspirated engine that is running, has a manifold pressure less than atmospheric pressure.
2) If I stick two fingers over the end of the hose (or close the throttle of the engine), will the pressure in the hose change? Remember that the motor is sucking just as hard, but there's less space for the air to get through.
Generally get the answer - the pressure gets less.
So remember that manifold pressure drops when the throttle is moved towards closed.
3) If I turn down the speed of the motor somehow, what will happen to the pressure in the hose? (Think carefully before answering.)
Hopefully, get the answer - it sucks less, so the pressure will increase.
If I don't get that answer, then I ask
3b) If I slow the motor all the way down until it is stopped, what will happen to the pressure in the hose? Refer to q 1.
Generally, get the answer - slower motor, less suction, higher pressure in the hose.
So remember that the faster the normally aspirated engine turns, the more it sucks, and the lower is the pressure in the induction manifold.
Hope this helps.
BTW the pressure drop is due to a lot more than the carburettor venturi. Possibly Barit1 you are confused between the pressure measured by the manifold pressure gauge, and the low pressure region generated in the carburettor to help meter the fuel. A well designed carburettor venturi will have exactly the same static pressure upstream as downstream (apart from minor changes caused by evaporation of fuel).
So what does cause the drop in pressure? Think what might cause a drop in pressure in the vacuum cleaner.
- a really long hose
- a narrow hose
- a kinked hose
- a dirty air filter
Which correspond to:
- a really long induction system (ideally, have one carburettor per cylinder and stick it right on top of the induction valve. Length = very short, efficiency = very high)
- a really wide induction system - bigger is better.
- no corners at all (ideally, mount the induction system on top of the engine like racing cars, so the air pipe goes straight into the induction valves)
- no filter at all (ok that's impossible, so use a new, clean, efficient filter)
Operation of valves as per Old Smokey's post? Yes, definitely vital for an efficient engine. But not really relevant for discussion of the manifold pressure, as the valves are at the downstream end of the manifold system and so don't affect the manifold pressure as recorded by the gauge.
As usual, I've started out writing a short note, and ended up with an essay. Sorry about that, and thanks for reading to the end!
O8
To simplify the process, think of the engine as a vacuum cleaner motor. The vacuum cleaner hose represents the induction system.
1) When the vacuum cleaner is running, is the pressure in the hose more, less or the same as outside air pressure?
Generally get the answer - less.
So remember that a normally aspirated engine that is running, has a manifold pressure less than atmospheric pressure.
2) If I stick two fingers over the end of the hose (or close the throttle of the engine), will the pressure in the hose change? Remember that the motor is sucking just as hard, but there's less space for the air to get through.
Generally get the answer - the pressure gets less.
So remember that manifold pressure drops when the throttle is moved towards closed.
3) If I turn down the speed of the motor somehow, what will happen to the pressure in the hose? (Think carefully before answering.)
Hopefully, get the answer - it sucks less, so the pressure will increase.
If I don't get that answer, then I ask
3b) If I slow the motor all the way down until it is stopped, what will happen to the pressure in the hose? Refer to q 1.
Generally, get the answer - slower motor, less suction, higher pressure in the hose.
So remember that the faster the normally aspirated engine turns, the more it sucks, and the lower is the pressure in the induction manifold.
Hope this helps.
BTW the pressure drop is due to a lot more than the carburettor venturi. Possibly Barit1 you are confused between the pressure measured by the manifold pressure gauge, and the low pressure region generated in the carburettor to help meter the fuel. A well designed carburettor venturi will have exactly the same static pressure upstream as downstream (apart from minor changes caused by evaporation of fuel).
So what does cause the drop in pressure? Think what might cause a drop in pressure in the vacuum cleaner.
- a really long hose
- a narrow hose
- a kinked hose
- a dirty air filter
Which correspond to:
- a really long induction system (ideally, have one carburettor per cylinder and stick it right on top of the induction valve. Length = very short, efficiency = very high)
- a really wide induction system - bigger is better.
- no corners at all (ideally, mount the induction system on top of the engine like racing cars, so the air pipe goes straight into the induction valves)
- no filter at all (ok that's impossible, so use a new, clean, efficient filter)
Operation of valves as per Old Smokey's post? Yes, definitely vital for an efficient engine. But not really relevant for discussion of the manifold pressure, as the valves are at the downstream end of the manifold system and so don't affect the manifold pressure as recorded by the gauge.
As usual, I've started out writing a short note, and ended up with an essay. Sorry about that, and thanks for reading to the end!
O8
Last edited by Oktas8; 3rd Jun 2006 at 01:25.
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MP increase.
Could you say that when the RPM is reduced there is less flow of the air/fuel mixture through the induction manifold resulting in less dynamic pressure which would mean an increase in static pressure hence an increase in MP.
Bernoulli’s theorem:
Static +Dynamic pressure is a constant.
Or have I got it wrong??
Bernoulli’s theorem:
Static +Dynamic pressure is a constant.
Or have I got it wrong??
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If by "dynamic pressure" you mean the difference between total (or ram) pressure and static pressure (in other words like IAS) then I guess you could look at it that way.
I look at it this way: if an unsupercharged four-stroke engine is 500 cubic inches displacement, then every two revolutions it pumps 500 cubic inches of mixture (mostly air). The pressure in the inlet manifold equals atmospheric pressure minus the drop across the venturi. The faster the engine turns, the more air volume is pumped per second, and thus the bigger the pressure drop.
So, at fixed throttle, more RPM >> lower MP, and less RPM >> higher MP.
I look at it this way: if an unsupercharged four-stroke engine is 500 cubic inches displacement, then every two revolutions it pumps 500 cubic inches of mixture (mostly air). The pressure in the inlet manifold equals atmospheric pressure minus the drop across the venturi. The faster the engine turns, the more air volume is pumped per second, and thus the bigger the pressure drop.
So, at fixed throttle, more RPM >> lower MP, and less RPM >> higher MP.
We 'wrinkly' motorists used to get a practical demonstration of this effect in the 30s/40s products from the Ford empire - Populars and such. The windscreen wipers were powered from the intake manifold. Trying to overtake in the rain became something of an act of faith as accelerating caused the wipers to slow down ... or stop!!
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MP/RPM.
Ok guys, I know because I am female I didn’t play around with engines when I was growing up, but I have done A level Physics. If you could use Physics concepts to explain the answer I would appreciate it. Could we leave the analogies out. Thanks.
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Sorry - didn't mean to be obscure -
But the vacuum-powered wipers were in effect a "manifold vacuum gage", driven by the pressure drop across the carb venturi. If the throttle was wide open, especially at low RPM, there is no pressure drop available to drive the wipers. (The MP almost equals atmospheric pressure)
But the vacuum-powered wipers were in effect a "manifold vacuum gage", driven by the pressure drop across the carb venturi. If the throttle was wide open, especially at low RPM, there is no pressure drop available to drive the wipers. (The MP almost equals atmospheric pressure)
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Hi again novicef
Could you say that when the RPM is reduced there is less flow of the air/fuel mixture through the induction manifold resulting in less dynamic pressure which would mean an increase in static pressure hence an increase in MP?
), but it is not the main effect. You are confusing two concepts. This is all about total energy loss due to friction, so Bernoulli's theorem does not apply. Remember the very important part of Bernoulli's theorem that mentions "a streamlined ideal fluid" - airflow in the induction manifold is not streamlined, and therefore suffers from real energy loss. Dynamic + static is not constant.The harder the engine sucks, the faster the air will flow, and the more q (dynamic pressure) you will get inside the manifold. But friction causes you to lose static pressure - the more q you have, the more pressure is lost to friction, and the greater the energy loss. When bringing the air to rest again inside the cylinder head, you will end up with a much lower static pressure because so much energy has been lost due to friction.
You asked for an explanation with physical concepts. I wonder if an example might clarify?
Atmospheric pressure (static) 100
Air accelerated into an induction system running at high rpm: static 50, dynamic 50.
Downstream of this point, past some kinks and corners in the tube: static 25, dynamic still 50 (air is still moving just as fast). We've lost half our static pressure due to friction.
In the cylinder head (air brought to rest): dynamic converted to static pressure, total now 75. We've lost 25% of our original pressure.
Atmospheric pressure (static) 100
Air accelerated into an induction system running at low rpm: static 70, dynamic 30.
Downstream of this point: static 60, dynamic 30. Because the air is flowing more slowly, it can get past the obstructions with less turbulence and less loss in static pressure.
In the cylinder head (air brought to rest): dynamic converted to static pressure, total now 90. We've only lost 10% of our original pressure, and you the pilot will see a higher manifold pressure.
Barit1
But the vacuum-powered wipers were in effect a "manifold vacuum gage", driven by the pressure drop across the carb venturi. If the throttle was wide open, especially at low RPM, there is no pressure drop available to drive the wipers.
Older light aircraft sometimes have a little double venturi tube beside the pilot's window; the static pressure drop is used to drive the vacuum instruments. However, the carburettor venturi is used to supply fuel to the induction airflow, and no-one is going to mess with that very critical part by putting other services in the way!
Last edited by Oktas8; 16th Jun 2006 at 06:08.
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Originally Posted by Oktas8
Barit1
Again, and hopefully without causing offence, no, no, no. Manifold pressure (even that which drives windscreen wipers) is not related to the carburettor venturi. A fuel injected engine, which has no venturi, still has a variable manifold pressure as a function of throttle and rpm. Windscreen wipers etc might be driven by the pressure difference between the atmosphere and the induction manifold. Carburettor doesn't come into it...
Again, and hopefully without causing offence, no, no, no. Manifold pressure (even that which drives windscreen wipers) is not related to the carburettor venturi. A fuel injected engine, which has no venturi, still has a variable manifold pressure as a function of throttle and rpm. Windscreen wipers etc might be driven by the pressure difference between the atmosphere and the induction manifold. Carburettor doesn't come into it...
the pressure difference between the atmosphere and the induction manifold.
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Hi barit1,
I suppose you're right there. But isn't the pressure drop across the venturi minor compared to the factors you're neglecting? The venturi is designed to be as close to lossless as possible. You can see that by the fact that every surface is smooth and rounded, with no sharp corners.
I suppose if I were to rank causes of loss of manifold pressure in a typical aero engine, I would say throttle (except when open!), then air filter, then ducting & corners, then carburettor venturi. As I'm not an engineer I could stand to be corrected though.
The main problem with talking about the pressure drop across the venturi from my point of view, is that it tends to confuse the inexperienced. There is a big difference between the pressure drop within a venturi and across a venturi - one is Bernoulli, which is where novicef got confused, and the other is a loss of energy caused by turbulence and the airflow being choked, which is definitely not Bernoulli.
Hope this clarifies my posts,
O8
I suppose you're right there. But isn't the pressure drop across the venturi minor compared to the factors you're neglecting? The venturi is designed to be as close to lossless as possible. You can see that by the fact that every surface is smooth and rounded, with no sharp corners.
I suppose if I were to rank causes of loss of manifold pressure in a typical aero engine, I would say throttle (except when open!), then air filter, then ducting & corners, then carburettor venturi. As I'm not an engineer I could stand to be corrected though.
The main problem with talking about the pressure drop across the venturi from my point of view, is that it tends to confuse the inexperienced. There is a big difference between the pressure drop within a venturi and across a venturi - one is Bernoulli, which is where novicef got confused, and the other is a loss of energy caused by turbulence and the airflow being choked, which is definitely not Bernoulli.
Hope this clarifies my posts,
O8
Grandpa Aerotart
novicef go here and all your questions will be answered. Scroll down and on the right hand side you'll see 'Engine Related Columns'. You're looking for one called 'Manifold Pressure Sucks'. Read also the one on Props and especially 'Mixture Magic'.
http://www.avweb.com/news/columns/182146-1.html
I used to correspond a lot with Deacon and I operate my Bonanza this way.
LOTS of other entertaining reading there too...he's an interesting fella.
Chuck.
http://www.avweb.com/news/columns/182146-1.html
I used to correspond a lot with Deacon and I operate my Bonanza this way.
LOTS of other entertaining reading there too...he's an interesting fella.
Chuck.
