# Airflow to the engine

#### amare

What-will-happen-if-the-air-entering-to-the-intake-manifold-is-smaller-than-the-air-entering-the-cylinder-from-the-manifold?
the following expression is the rate of change of the manifold pressure interms of the mass flowing to the manifold and the mass of air flowing to the cylinder from the manifold. if the air flow to the manifold is zero the manifold pressure will be negative. what does it mean having a negative pressure?

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#### phinds

Gold Member
why-do-you-put-dashes-in-between-words???

#### jim hardy

Gold Member
2018 Award
Are you not familiar with 'absolute pressure' versus 'gage pressure' ?

what does it mean having a negative pressure?
All it means is 'absolute pressure' inside the manifold is less than outside it.
That's why most people call it 'manifold vacuum' instead.

Were there no vacuum what would make air flow into the manifold ?

#### amare

why-do-you-put-dashes-in-between-words???
you also put dashes between words.... why you did that ????

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#### phinds

Gold Member
you also put dashes between words.... why you did that ????
You might want to give that some thought

#### jim hardy

Gold Member
2018 Award
if the air flow to the manifold is zero the manifold pressure will be negative.

Actually your expression is telling you the rate of change of pressure is negative, not the pressure itself.

It's P with a dot above, not just P.
When there's no more air to pump out of the manifold pressure inside it will stop decreasing, of course.

There cannot be an absolute pressure less than zero .
That's why we have to be careful in using formulas , so we don't try to apply them beyond the natural limits imposed on them by Mother Nature..

old jim

#### amare

Actually your expression is telling you the rate of change of pressure is negative, not the pressure itself.

View attachment 243953
It's P with a dot above, not just P.
When there's no more air to pump out of the manifold pressure inside it will stop decreasing, of course.

There cannot be an absolute pressure less than zero .
That's why we have to be careful in using formulas , so we don't try to apply them beyond the natural limits imposed on them by Mother Nature..

old jim
oh i noticed it is the rate of pressure that is negative !!! thank you very much ... please help me !!! i am modeling and engine in simulink. under normal condintion every thing goes ok with the model on simulink. but I want to design a control valve to be installed between the throttle body and the intake manifold to restrict the flow of air when the vehicle is overloaded ( passenger over loaded for example). so if the vehicle is over loaded (than the required), the airflow to the engine will be blocked by the valve ( not by the butterfly valve in the throttle body) even if the driver press the gas pedal and open the butterfly valve in the throttle body. that means airflow to the intake manifold will be blockd and the engine is expected to stop !!! . but in the simulink model the engine speed is not zero. it is negative valve …… because the above equation has a negative valve since the rate of air flow to the manifold is zero ( blocked by the control valve). that is my problem .... the engine model is like in the link below . but i added a control valve model in the throttle and manifold submodel to control airflow. the simulation gives me negative engine speed

#### berkeman

Mentor
you also put dashes between words.... why you did that ????
He was gently telling you that it is obnoxious to put in dashes like that. Please do not do that -- it make is harder to read what you write, which means fewer people will bother to read your question and respond. Thanks for not doing it in the future.

#### jim hardy

Gold Member
2018 Award
Yes that's the trouble with linear equations they dont recognize natural limits that make physical processes nonlinear.

Were i doing this in Fortran i would place a lower limit on manifold pressure.
An engine (or any other single stage piston pump) cannot reduce manifold pressure to less than
(its discharge pressure) divided by (its compression ratio),
you should think for a while why that is so.

So that places a lower bound on your manifold absolute pressure and it is a positive value.

old jim

#### rbelli1

Gold Member
the engine is expected to stop
What about disabling the ignition or fuel injectors? That would be a simple electrical change rather than adding mechanical parts.

BoB

#### amare

What about disabling the ignition or fuel injectors? That would be a simple electrical change rather than adding mechanical parts.

BoB
How? do you have any method to do that please?

#### rbelli1

Gold Member
That depends on the engine. On small magneto type ignition systems there is often a lead that when connected to ground (the engine block for example) the ignition stops functioning. If not then you need to somehow electrically isolate it from the engine and your kill switch will re-make that contact to allow ignition.

If it uses an ignition coil you can disconnect one side of the coil and it will not work anymore.

For diesel engines you need to stop the flow of fuel. This is similar to the ignition coil disconnection method except you break the electrical continuity on the injectors. You may have to disable the injectors on an injected gasoline engine to avoid backfiring.

Except for the magneto kill wire these are all fairly high current devices so you will probably want to use a relay.

BoB

"Airflow to the engine"

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