Why does manifold pressure drop below ambient on start up?
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Why does manifold pressure drop below ambient on start up?
Before you start the engine the gague reads the outside air pressure. Correct?
When you start the engine the manifold pressure is seen to drop. Correct?
I was wondering why the new pressure is actually lower than outside pressure. My brain figures that an engine compresses air so I now need a kind soul to explain.
Many thanks.
When you start the engine the manifold pressure is seen to drop. Correct?
I was wondering why the new pressure is actually lower than outside pressure. My brain figures that an engine compresses air so I now need a kind soul to explain.
Many thanks.
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The inlet manifold is outside the cylinder. With the engine not running then pressure, both in the manifold & the cylinder, is equalised with ambient.
When the engine is running it 'sucks' air or air/fuel mixture out of the inlet manifold, past the inlet valve & into the cylinder. Then the inlet valve closes & the mixture that is now in the cylinder is compressed, not the air or air/fuel mix that remains in the inlet manifold.
Meanwhile replacement air or air/fuel is trying to equalise the (now reduced) pressure in the manifold - except that the cycle is repeated.
The air has inertia & cannot move to refill the volume instantaneously so the overall effect of the engine sucking air out of the manifold whilst at the other end air is trying to refill the manifold, is a reduction in manifold pressure.
All the above for a normally aspirated engine of course.
Hope this helps.
When the engine is running it 'sucks' air or air/fuel mixture out of the inlet manifold, past the inlet valve & into the cylinder. Then the inlet valve closes & the mixture that is now in the cylinder is compressed, not the air or air/fuel mix that remains in the inlet manifold.
Meanwhile replacement air or air/fuel is trying to equalise the (now reduced) pressure in the manifold - except that the cycle is repeated.
The air has inertia & cannot move to refill the volume instantaneously so the overall effect of the engine sucking air out of the manifold whilst at the other end air is trying to refill the manifold, is a reduction in manifold pressure.
All the above for a normally aspirated engine of course.
Hope this helps.
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Hi Homepage,
You're right, an engine does compress, however in normally-aspirated engines that only happens in the cylinders once the inlet valves are closed during the compression stroke.
Manifold pressure is measured in the engine inlet manifold after the throttle butterfly valve but before the inlet valve, so never gets to measure the compression in the cylinders.
The pressure drop that you see after starting is due to the throttle valve acting as a choke when partially closed. Chokes cause a pressure drop when the air is moving (ie. engine running) but don't when it isn't moving (ie. engine stopped). This is because when the air is stationary, the pressure can equalise around the throttle valve, however if the air is moving in one direction there is a pressure drop caused by the restriction and turbulence behind the valve which can't be relieved (because the air is flowing one way).
So, the manifold pressure you see when the engine is running is roughly proportional to the size of the opening at the butterfly valve and therefore the throttle position.
Hope this explains it.
cheers!
foggy.
<stuff below added later>
What I should have added is that the reason the air is moving past the throttle valve is because the cylinder is sucking it in during the induction stroke, as Tinstaafl correctly says.
Tinstaafl - looks like we posted together!
[This message has been edited by foghorn (edited 04 April 2001).]
You're right, an engine does compress, however in normally-aspirated engines that only happens in the cylinders once the inlet valves are closed during the compression stroke.
Manifold pressure is measured in the engine inlet manifold after the throttle butterfly valve but before the inlet valve, so never gets to measure the compression in the cylinders.
The pressure drop that you see after starting is due to the throttle valve acting as a choke when partially closed. Chokes cause a pressure drop when the air is moving (ie. engine running) but don't when it isn't moving (ie. engine stopped). This is because when the air is stationary, the pressure can equalise around the throttle valve, however if the air is moving in one direction there is a pressure drop caused by the restriction and turbulence behind the valve which can't be relieved (because the air is flowing one way).
So, the manifold pressure you see when the engine is running is roughly proportional to the size of the opening at the butterfly valve and therefore the throttle position.
Hope this explains it.
cheers!
foggy.
<stuff below added later>
What I should have added is that the reason the air is moving past the throttle valve is because the cylinder is sucking it in during the induction stroke, as Tinstaafl correctly says.
Tinstaafl - looks like we posted together!
[This message has been edited by foghorn (edited 04 April 2001).]
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The simplest way I have ever heard an internal combustion (reciprocating engine) described is as being basically a simple air pump.
That is imagine you have an infinite amount of power available from say an electric motor attached to the crankshaft of your engine spark off, fuel off.
The effort required to turn the engine will be a result of the amount of throttle applied.
That is close the throttle/butterfly and restrict the air flow when the engine is turning (that electric motor) and it sucks, big time, (reduction in MP), fully open the throttle, then the MP should now approximate ambient pressure except the mechanical restriction of the gas path going round corners, friction and inefficiencies effectively reduces the presssure to around 2" less than ambient.
See where the term throttle comes from. Throttling the engine is like choking someone by you know how!
Therefore when the engine is at rest, throttle (butterfly) open or closed (engine not turning) the MP should approximate ambient pressure.
Take off power not more than around 2" less than ambient.
A good way to check whether the correct power is being produced. More than 2" then there is an industion problem and whilst you may be getting RPM you will NOT be getting max available power.
See the fence/trees coming.
Ok so our electric power bill is getting a bit high now and we have run out of coins to feed the meter.
So we add fuel and a spark every now and then to make it self sustaining.
Same principle applies we control the amount of air available to pass through the engine and therefore the power.
Think it through.
Capiche.
That is imagine you have an infinite amount of power available from say an electric motor attached to the crankshaft of your engine spark off, fuel off.
The effort required to turn the engine will be a result of the amount of throttle applied.
That is close the throttle/butterfly and restrict the air flow when the engine is turning (that electric motor) and it sucks, big time, (reduction in MP), fully open the throttle, then the MP should now approximate ambient pressure except the mechanical restriction of the gas path going round corners, friction and inefficiencies effectively reduces the presssure to around 2" less than ambient.
See where the term throttle comes from. Throttling the engine is like choking someone by you know how!
Therefore when the engine is at rest, throttle (butterfly) open or closed (engine not turning) the MP should approximate ambient pressure.
Take off power not more than around 2" less than ambient.
A good way to check whether the correct power is being produced. More than 2" then there is an industion problem and whilst you may be getting RPM you will NOT be getting max available power.
See the fence/trees coming.
Ok so our electric power bill is getting a bit high now and we have run out of coins to feed the meter.
So we add fuel and a spark every now and then to make it self sustaining.
Same principle applies we control the amount of air available to pass through the engine and therefore the power.
Think it through.
Capiche.
