Theory on Vacuum?
06-03-2003, 08:19 AM
#1
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Theory on Vacuum?
Guys, I consider myself somewhat a pretty knowlegable person when it comes to the inner workings of a car.
I understand the physics that takes place in the internal combustion engine.
But I can't seem to grasp the concept of the vacuum system and its role in the Naturally Aspirated and Forced Induction world.
Can anyone provide some insights or point me in the right direction to better understand it.
Thank You,
Len
PS. I Know this is more of a tech thread, but I'm trying to figure out how vacuum works for my future boosting plans.
I understand the physics that takes place in the internal combustion engine.
But I can't seem to grasp the concept of the vacuum system and its role in the Naturally Aspirated and Forced Induction world.
Can anyone provide some insights or point me in the right direction to better understand it.
Thank You,
Len
PS. I Know this is more of a tech thread, but I'm trying to figure out how vacuum works for my future boosting plans.
06-03-2003, 08:58 AM
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Re: Theory on Vacuum? (len)
You need a vacuum source to have changes in pressure in the atmosphere of your BOV or Wastegate. If there's less pressure in those chambers, the BOV or Wastegate will open.
You can try it with a BOV. Try to move the valve with your hand. It's damn hard.
Now attach a Vac Line on the BOV, suck on it, and then try to push the valve open. Hey there ya go. It works.
You can try it with a BOV. Try to move the valve with your hand. It's damn hard.
Now attach a Vac Line on the BOV, suck on it, and then try to push the valve open. Hey there ya go. It works.
06-03-2003, 09:06 AM
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Re: Theory on Vacuum? (pub)
Having a vacuum/pressure source does various things for a car. Its can be a way of doing work (like your brake booster), it can be used for emissions (pvc system) it can be used to control fuel (fpr. the vac line makes sure the fuel flows at the same pressure no matter what the manifold pressure is), its needed for the ecu (map sensor) and its needed to open and close the bov and wastegate.
06-03-2003, 09:35 AM
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Re: Theory on Vacuum? (00SilverLS)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 00SilverLS »</TD></TR><TR><TD CLASS="quote">Having a vacuum/pressure source does various things for a car. Its can be a way of doing work (like your brake booster), it can be used for emissions (pvc system) it can be used to control fuel (fpr. the vac line makes sure the fuel flows at the same pressure no matter what the manifold pressure is), its needed for the ecu (map sensor) and its needed to open and close the bov and wastegate.
</TD></TR></TABLE>
on the fpr, doesnt more or less vacuum change the fuel pressure?
</TD></TR></TABLE>
on the fpr, doesnt more or less vacuum change the fuel pressure?
06-03-2003, 09:55 AM
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Re: Theory on Vacuum? (sparky69)
I take it you're asking why is manifold vacuum important, rather than how it's produced...
Vacuum and/or boost, taken with throttle angle and rpm, is used to determine the loading on the engine. Your Honda engine management uses that info to calculate the fuel quantity and spark timing based on a table programmed in the EPROM chip. Larger throttle angles for a given rpm will result in less vacuum, which the engine management understands to be higher load.
Vacuum and/or boost, taken with throttle angle and rpm, is used to determine the loading on the engine. Your Honda engine management uses that info to calculate the fuel quantity and spark timing based on a table programmed in the EPROM chip. Larger throttle angles for a given rpm will result in less vacuum, which the engine management understands to be higher load.
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06-03-2003, 10:20 AM
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Re: Theory on Vacuum? (roadrunner)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by roadrunner »</TD></TR><TR><TD CLASS="quote">I take it you're asking why is manifold vacuum important, rather than how it's produced...
Vacuum and/or boost, taken with throttle angle and rpm, is used to determine the loading on the engine. Your Honda engine management uses that info to calculate the fuel quantity and spark timing based on a table programmed in the EPROM chip. Larger throttle angles for a given rpm will result in less vacuum, which the engine management understands to be higher load.</TD></TR></TABLE>
Oh that makes alot of sense. One question though, in a boosted application, once the manifold sees a positive vacuum(oxymoron?) at full boost, how does the ECU react to this?
Vacuum and/or boost, taken with throttle angle and rpm, is used to determine the loading on the engine. Your Honda engine management uses that info to calculate the fuel quantity and spark timing based on a table programmed in the EPROM chip. Larger throttle angles for a given rpm will result in less vacuum, which the engine management understands to be higher load.</TD></TR></TABLE>
Oh that makes alot of sense. One question though, in a boosted application, once the manifold sees a positive vacuum(oxymoron?) at full boost, how does the ECU react to this?
06-03-2003, 10:22 AM
#9
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Re: Theory on Vacuum? (00SilverLS)
Regarding the Fuel Pressure Regulator, where does it get it's vacuum source from? In fact where are most of the vacuum source created to supply the BOV or Wastegate or Fuel Pressure Regulator? The Power Brake Booster?
06-03-2003, 10:33 AM
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Re: Theory on Vacuum? (len)
All those the vacuum that is "created" accesssories tap into this source through the intake manifold (behind the throttle body, and in front of the intake valves). what creates pressure, is the engines rotating assembly moving, and the fact that the throttle plate is almost completely closed, with an idle air control valve letting only enough air into the engine to idle. (at idle)
Once an engine see's pressure, if it is designed for it, it just goes to the next pressure range for the rev range you're currently at (say 1 psi at 4000 RPMS) and it will add in the fuel necessary to keep the engine from running lean.
Vacuum powers many things. an FPR will decrease fuel pressure, or increase fuel pressure based on vaccum and or boost (sucks on the diagphram, or pushes on it, because as there are pressure variances in the engine, the injectors must be compensated for theese pressure/vacuum changes)
Also, on a lot of cars, EGR is mostly vacuum operated, it opens/closes the solenoids.
Also, one other important feature that uses vacuum is your brake booster. you see, there is a check valve in the tube going to, or in the brake booster itself. Which only lets air OUT of the booster. What happens, is there is a large diaphram there, and when there is absense of air, it boost the brakes considerably by creating a vacuum in the rear chamber of the booster, so when you press on the bedal, it more or less sucks it.
Also, most crankcase vent systems are vacuum powered, by which vacuum created by the engine, evacuates the vapors from the crankcase (thus sucked down the engine through the intake manifold) this is known as a closed system.
Also things like the Fuel/charcoal canister are vacuum powered as well. Or how about vacuum advance on distributors of old?
And of course the all important MAP sensor...
My engine has 5 vac lines on it.
Wastegate
Bov
Brake Booster
Map
Fuel Pressure Regulator
this is on my geo. I've simplified the vac system considerably by removing things like the EGR, canister, and a whole bunch of other stuff.
if i've missed any ?'s reply.
Once an engine see's pressure, if it is designed for it, it just goes to the next pressure range for the rev range you're currently at (say 1 psi at 4000 RPMS) and it will add in the fuel necessary to keep the engine from running lean.
Vacuum powers many things. an FPR will decrease fuel pressure, or increase fuel pressure based on vaccum and or boost (sucks on the diagphram, or pushes on it, because as there are pressure variances in the engine, the injectors must be compensated for theese pressure/vacuum changes)
Also, on a lot of cars, EGR is mostly vacuum operated, it opens/closes the solenoids.
Also, one other important feature that uses vacuum is your brake booster. you see, there is a check valve in the tube going to, or in the brake booster itself. Which only lets air OUT of the booster. What happens, is there is a large diaphram there, and when there is absense of air, it boost the brakes considerably by creating a vacuum in the rear chamber of the booster, so when you press on the bedal, it more or less sucks it.
Also, most crankcase vent systems are vacuum powered, by which vacuum created by the engine, evacuates the vapors from the crankcase (thus sucked down the engine through the intake manifold) this is known as a closed system.
Also things like the Fuel/charcoal canister are vacuum powered as well. Or how about vacuum advance on distributors of old?
And of course the all important MAP sensor...
My engine has 5 vac lines on it.
Wastegate
Bov
Brake Booster
Map
Fuel Pressure Regulator
this is on my geo. I've simplified the vac system considerably by removing things like the EGR, canister, and a whole bunch of other stuff.
if i've missed any ?'s reply.
06-03-2003, 10:54 AM
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It's also important to note that the location of the port where the vacuum/boost signal is taken will determine it's vacuum/boost to rpm to throttle angle profile. Ports in the venturi just after the throttle plate do not produce the same profile as a port in the intake plenum.
Lazerus:
You got a garage for that gang of cars? =Þ
Lazerus:
You got a garage for that gang of cars? =Þ
06-03-2003, 11:19 AM
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Re: Theory on Vacuum? (roadrunner)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by roadrunner »</TD></TR><TR><TD CLASS="quote">
Vacuum and/or boost, taken with throttle angle and rpm, is used to determine the loading on the engine. Your Honda engine management uses that info to calculate the fuel quantity and spark timing based on a table programmed in the EPROM chip. Larger throttle angles for a given rpm will result in less vacuum, which the engine management understands to be higher load.</TD></TR></TABLE>
As an expansion of this:
If the ECU knows the air pressure, temperature, and volume of air going in (engine size times volumetric efficiency) it can exactly calculate how much (how many molecules) of air is entering the engine and insert exactly the right amount of fuel.
this all comes from the ideal gas law: PV=nRT
Vacuum and/or boost, taken with throttle angle and rpm, is used to determine the loading on the engine. Your Honda engine management uses that info to calculate the fuel quantity and spark timing based on a table programmed in the EPROM chip. Larger throttle angles for a given rpm will result in less vacuum, which the engine management understands to be higher load.</TD></TR></TABLE>
As an expansion of this:
If the ECU knows the air pressure, temperature, and volume of air going in (engine size times volumetric efficiency) it can exactly calculate how much (how many molecules) of air is entering the engine and insert exactly the right amount of fuel.
this all comes from the ideal gas law: PV=nRT
06-04-2003, 04:16 PM
#13
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Re: Theory on Vacuum? (00SilverLS)
Len,
I think the folks did a good job explaing vacuum. Let me just try to add a little bit on how a vacuum actuator works.
One key thing is not to think of two different states, vacuum and pressure. Think of only pressure. A little pressure and a lot of pressure. For vacuum is really only an indication of absolute pressure less than standard atmospheric pressure or about 14.7 pounds per square inch absolute (psia).
Outer space would have zero pressure absolute (0 psia), if you came down from there to Mt. Evans in Colorado (Worlds highest paved motor highway at about 14,100 feet) the pressure would be about 8.5 psia ... no wonder I had a splitting headache up there .... then when we get to sea level we have 14.7 psia.
Now lets make an actuator with these pressures out of a coffee can ... say a three pounder with a diameter of say 6 inches. ... this would be like a waste gate, or a power brake booster application. The area of coffee can lid would be pi times the diameter squared divided by 4 or PI*6*6/4 or 28.3 square inches....lets call it 30 square inches.
Now if I drill a hole thru that coffe can and insert a rod up to the "diaphram" of the lid (and have a perfect seal on the rod) I have a vacuum actuator. Lets connect the inside of the can to outer space (0 psia) and have the other side exposed to atmosphere at sea level. The pressure on the can side is zero so 0 lbs/sq in*30 sq in = 0 lbs. (Note how the units of sq inches cancel so the result is in lbs force) Now on the atmospheric side of the lid we have 14.7 lbs * 30 sq in = 441 lbs. So that rod is being pushed on with 441 lbs of force.
Now if we vent the inside of the can to Mt. Evans altitude we have 8.5 lbs/sqin*30sqin pushing from the inside = 255 lbs. This force reacts against the same atmospheric force we had above so that rod now only has 441 - 255 lbs of force on it or 186 lbs of force.
This is how we modulate these actuators by varying the pressure on one (or both) sides of a diaphram to get various forces. You can dither things by putting springs on the inside or outside of that coffee can lid or actuator diaphram to get various "set points" or in the case of the waste gate, various boost pressures.
A power brake booster has that diaphram in the middle, with the same vacuum on both sides -> hence there is a zero net force. When you stab the brake, you bleed off vacuum (or raise the pressure) on the side of the booster near the brake pedal, therefore that rod we talked about gets jammed harder into the master cylinder. More pressure on the pedal side, more force on the master cylinder. More vacuum on the engine side more force on the master cylinder. Note that we usually hit the brakes when we have our foot off the gas and the throttle plate is closed, hence there is a good vacuum there. Also they put a check valve in there so it "stores" the good high vacuum or low pressure.
Hope this helps some. Just remember 'vacuum' is just another name for reduced pressure.
If you have any more questions, just ask. I'll be glad to try to explain more.
Regards,
BigMoose
I think the folks did a good job explaing vacuum. Let me just try to add a little bit on how a vacuum actuator works.
One key thing is not to think of two different states, vacuum and pressure. Think of only pressure. A little pressure and a lot of pressure. For vacuum is really only an indication of absolute pressure less than standard atmospheric pressure or about 14.7 pounds per square inch absolute (psia).
Outer space would have zero pressure absolute (0 psia), if you came down from there to Mt. Evans in Colorado (Worlds highest paved motor highway at about 14,100 feet) the pressure would be about 8.5 psia ... no wonder I had a splitting headache up there .... then when we get to sea level we have 14.7 psia.
Now lets make an actuator with these pressures out of a coffee can ... say a three pounder with a diameter of say 6 inches. ... this would be like a waste gate, or a power brake booster application. The area of coffee can lid would be pi times the diameter squared divided by 4 or PI*6*6/4 or 28.3 square inches....lets call it 30 square inches.
Now if I drill a hole thru that coffe can and insert a rod up to the "diaphram" of the lid (and have a perfect seal on the rod) I have a vacuum actuator. Lets connect the inside of the can to outer space (0 psia) and have the other side exposed to atmosphere at sea level. The pressure on the can side is zero so 0 lbs/sq in*30 sq in = 0 lbs. (Note how the units of sq inches cancel so the result is in lbs force) Now on the atmospheric side of the lid we have 14.7 lbs * 30 sq in = 441 lbs. So that rod is being pushed on with 441 lbs of force.
Now if we vent the inside of the can to Mt. Evans altitude we have 8.5 lbs/sqin*30sqin pushing from the inside = 255 lbs. This force reacts against the same atmospheric force we had above so that rod now only has 441 - 255 lbs of force on it or 186 lbs of force.
This is how we modulate these actuators by varying the pressure on one (or both) sides of a diaphram to get various forces. You can dither things by putting springs on the inside or outside of that coffee can lid or actuator diaphram to get various "set points" or in the case of the waste gate, various boost pressures.
A power brake booster has that diaphram in the middle, with the same vacuum on both sides -> hence there is a zero net force. When you stab the brake, you bleed off vacuum (or raise the pressure) on the side of the booster near the brake pedal, therefore that rod we talked about gets jammed harder into the master cylinder. More pressure on the pedal side, more force on the master cylinder. More vacuum on the engine side more force on the master cylinder. Note that we usually hit the brakes when we have our foot off the gas and the throttle plate is closed, hence there is a good vacuum there. Also they put a check valve in there so it "stores" the good high vacuum or low pressure.
Hope this helps some. Just remember 'vacuum' is just another name for reduced pressure.
If you have any more questions, just ask. I'll be glad to try to explain more.
Regards,
BigMoose
06-04-2003, 04:55 PM
#16
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Re: Theory on Vacuum? (BigMoose)
Oh my, my brain is hurting right now. Hehehe...It'll take me a bit to digest that information.
Anywho, it's probably safe to say that the term Vacuum is really misused in this instance. When speaking with a technician today at work he was trying to explain that the intake manifold pressure(which happens to be dubbed vacuum) was created by the downward motion of the piston. What he didn't care to indulge on was the fact that no matter how hard the pistons "sucked" intake manifold pressure would never reach zero pressure absolute like you explained. In fact on Naturally aspirated vehicles, the "negative" readings we get is really Atmoshperic(14.7) minus the "negative" pressure. So we're never talking about anything under zero pressure absolute? Am I doing good so far? I understand that the pressure differences between two objects(couldn't think of any other word to use)could be used as an actuator just like the wastegate, fuel pressure regulator, and blowoff valve.
Here's what confused me. When I was using the term "vacuum" loosely when talking discussing the intake manifold pressure, I got it totally wrong. We used negative vacuum to describe anything reading zero on the boost guage and positive vacuum to describe anything reading above zero on the boost guage. That's where we're confused. Like you said, the boost guage "0.0psi" reading is really "14.7psi."
And there is no such thing as positive or negative vacuum. It's all pressure. 
In the meantime I was busy contemplating the fact that how could the fuel pressure regulator control fuel pressure if it had no "vacuum" during boost. LoL
Man, Art told me you were the guy to ask. LoL...I'll be sure to hit you up again for future questions. Hey if you ever want to chat, hit me up at work, I love learning new stuff.
Anywho, it's probably safe to say that the term Vacuum is really misused in this instance. When speaking with a technician today at work he was trying to explain that the intake manifold pressure(which happens to be dubbed vacuum) was created by the downward motion of the piston. What he didn't care to indulge on was the fact that no matter how hard the pistons "sucked" intake manifold pressure would never reach zero pressure absolute like you explained. In fact on Naturally aspirated vehicles, the "negative" readings we get is really Atmoshperic(14.7) minus the "negative" pressure. So we're never talking about anything under zero pressure absolute? Am I doing good so far? I understand that the pressure differences between two objects(couldn't think of any other word to use)could be used as an actuator just like the wastegate, fuel pressure regulator, and blowoff valve.
Here's what confused me. When I was using the term "vacuum" loosely when talking discussing the intake manifold pressure, I got it totally wrong. We used negative vacuum to describe anything reading zero on the boost guage and positive vacuum to describe anything reading above zero on the boost guage. That's where we're confused. Like you said, the boost guage "0.0psi" reading is really "14.7psi."
And there is no such thing as positive or negative vacuum. It's all pressure. In the meantime I was busy contemplating the fact that how could the fuel pressure regulator control fuel pressure if it had no "vacuum" during boost. LoL
Man, Art told me you were the guy to ask. LoL...I'll be sure to hit you up again for future questions. Hey if you ever want to chat, hit me up at work, I love learning new stuff.
06-04-2003, 05:06 PM
#17
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Re: Theory on Vacuum? (BigMoose)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by BigMoose »</TD></TR><TR><TD CLASS="quote">The area of coffee can lid would be pi times the diameter squared divided by 4 or PI*6*6/4 or 28.3 square inches....lets call it 30 square inches.</TD></TR></TABLE>
BigMoose,
Shouldn't the formula for an area of a circle be A = pi x r x r or A= pi x r^2 ?
If that's the case then the area of the coffee can lid would be A = pi x 3in x 3in . That would make A = 3.141... x 9in^2 or 28.27... or uh...wait...that's the same thing. LoL
Dood, What's that other formula you used? PI*6*6/4 ?
Ok...I hate to say it, but I have to. This thread sucks. lol
BigMoose,
Shouldn't the formula for an area of a circle be A = pi x r x r or A= pi x r^2 ?
If that's the case then the area of the coffee can lid would be A = pi x 3in x 3in . That would make A = 3.141... x 9in^2 or 28.27... or uh...wait...that's the same thing. LoL
Dood, What's that other formula you used? PI*6*6/4 ?
Ok...I hate to say it, but I have to. This thread sucks. lol
06-04-2003, 05:32 PM
#18
Honda-Tech Member
Re: Theory on Vacuum? (len)
len,
Your doing great! Learning about this world is one of the things I have enjoyed the most over my life and I am always ready to share what I learned .....and maybee make it a little easier for someone else.
Don't worry about how hard it is sometimes..... "the joy is in the journey!" (stole that from some commercial) .... and about the frustration.... I told Art about some "problems" I was having with some programming today...... spent 6 hours chasing a memory access error in some "C" code....... the problem? a darn ; thats it. 200,000 lines of code and I couldn't see one misplaced ; in the middle of a line........... What a hoot!
Take care. And you are right you cannot have an "absolute pressure" below 0, as zero pressure represents the absence of any molecular motion in the gas state... either no gas molecules to move around, or the substance is at absolute zero (temperataure) with no brownian (thermal) motion..........
Your equations look good too!
Regards,
BigMoose
Your doing great! Learning about this world is one of the things I have enjoyed the most over my life and I am always ready to share what I learned .....and maybee make it a little easier for someone else.
Don't worry about how hard it is sometimes..... "the joy is in the journey!" (stole that from some commercial) .... and about the frustration....
I told Art about some "problems" I was having with some programming today...... spent 6 hours chasing a memory access error in some "C" code....... the problem? a darn ; thats it. 200,000 lines of code and I couldn't see one misplaced ; in the middle of a line........... What a hoot!Take care. And you are right you cannot have an "absolute pressure" below 0, as zero pressure represents the absence of any molecular motion in the gas state... either no gas molecules to move around, or the substance is at absolute zero (temperataure) with no brownian (thermal) motion..........
Your equations look good too!
Regards,
BigMoose
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