how does the motor pull vacuum??
10-26-2005, 06:57 AM
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how does the motor pull vacuum??
I'm assuming it pulls it on the intake stroke (duh), but how exactly? is there an instant when the piston is moving down and all the valves are closed, or does the downward motion create a vacuum up in the TB? Thanks, and I hope I'm not grossly misunderstanding something here.
10-26-2005, 07:24 AM
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Re: how does the motor pull vacuum?? (pbink)
The intake valve is open, & the piston is moving down. That sucks air out from the intake manifold. So if the throttle is closed, it makes vacuum in the intake manifold.
10-26-2005, 09:31 AM
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Re: how does the motor pull vacuum?? (JimBlake)
Ah okay. So at idle, the iacv is letting in just enough fuel mixture to keep the engine turning, but there is still a negative pressure in the manifold. What is the pressure in the intake manifold at WOT? In other words, is there a negative pressure in the manifold at WOT, and does that pressure change as the RPMs increase? Thanks for the post.
10-26-2005, 09:54 AM
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Re: how does the motor pull vacuum?? (pbink)
the iacv is not responsible for adding fuel. iacv makes the motor turn at higher speeds when the car is cold to warm it up. when the coolant becomes hot, a plastic pellet in the iacv expands and the engine speed reduces to its warm idle.
i dont know about your manifold pressure question. i would guess that it would become more negative pressure (vacuum) as the engine speeds up.
i dont know about your manifold pressure question. i would guess that it would become more negative pressure (vacuum) as the engine speeds up.
10-26-2005, 09:55 AM
#5
Re: how does the motor pull vacuum?? (pbink)
The throttlebody has a small passageway for air to go thru a needle valve. That's the idle adjusting screw above the bore. Air comes in thru that, & also thru the IACV.
Without a turbo- or supercharger the manifold pressure HAS to be negative (vacuum). But at WOT it's the flow resistance of the air filter, intake tube, & thru the (open) throttlebody.
So, WOT & low revs it's pretty close to atmospheric. WOT & high revs there's a lot more air being pulled in, so that flow resistance causes more pressure drop.
Without a turbo- or supercharger the manifold pressure HAS to be negative (vacuum). But at WOT it's the flow resistance of the air filter, intake tube, & thru the (open) throttlebody.
So, WOT & low revs it's pretty close to atmospheric. WOT & high revs there's a lot more air being pulled in, so that flow resistance causes more pressure drop.
10-26-2005, 10:44 AM
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Re: how does the motor pull vacuum?? (JimBlake)
Your question is kinda general, but since I'm bored, I'll try to answer some of it.
As you probably know our engines (ICE's) are 4 stroke engines. There is an intake stroke, a compression stroke, a power stroke, and an exhaust stroke. During the intake stroke, the the intake valve(s) will be open causing in the "suction" of air. At this time (depending on computer settings), fuel will also be added into the mixture. Simultaneously, the piston is travelling downward.
During the compression stroke, the piston is coming upward. The air and the fuel is being compressed. As the piston reaches TDC (top dead center), or at some point near it (timing also controlled by the computer), a spark is ignited. This causes the combustion to occur. Lots of thermodynamic theory can be gone into here, but we'll skip all that.
This is where the power stroke begins.
The power stroke is where you asked if a piston travelling downward has both sets of intake/exhaust valves closed at the same time. This is where it happens. If anything was open, you'd get loss of power/compression/etc.
During the exhaust stroke, the spent gasses from the resultant combustion are pushed out through the exhaust valve(s).
That being said, and there are 4 cylinders in most of our Hondas, there is always going to be at least one cylinder in the intake stroke. Thus, we always have some sort of vacuum in the system. Taking into account all the above posts, they are all correct. At WOT, the system is very close to atmospheric pressure since the throttle plate is pretty much open to the atmosphere. At idle or anything less, the pressure will still be a little negative. Naturally aspirated cars like ours never have positive pressure in the system.
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by pbink »</TD></TR><TR><TD CLASS="quote">Ah okay. So at idle, the iacv is letting in just enough fuel mixture to keep the engine turning, but there is still a negative pressure in the manifold. What is the pressure in the intake manifold at WOT? In other words, is there a negative pressure in the manifold at WOT, and does that pressure change as the RPMs increase? Thanks for the post.</TD></TR></TABLE>
There will always be negative pressure in the intake manifold in naturally aspirated conditions. Even under WOT, the computer is taking into account readings from the throttle position sensor, the MAP sensor, timing, fuel delivery, etc. So even under WOT, everything is just basically moving "faster." More air is going into the system so that you can create more power.
Yes, there is always a pressure change anytime something changes in the system. I'm not an engineer, but my gf tried to explain some of this to me. What is it again? The Ideal Gas Law? PV = nRT. According to that pressure is dependent on many things.
As you probably know our engines (ICE's) are 4 stroke engines. There is an intake stroke, a compression stroke, a power stroke, and an exhaust stroke. During the intake stroke, the the intake valve(s) will be open causing in the "suction" of air. At this time (depending on computer settings), fuel will also be added into the mixture. Simultaneously, the piston is travelling downward.
During the compression stroke, the piston is coming upward. The air and the fuel is being compressed. As the piston reaches TDC (top dead center), or at some point near it (timing also controlled by the computer), a spark is ignited. This causes the combustion to occur. Lots of thermodynamic theory can be gone into here, but we'll skip all that.
This is where the power stroke begins.The power stroke is where you asked if a piston travelling downward has both sets of intake/exhaust valves closed at the same time. This is where it happens. If anything was open, you'd get loss of power/compression/etc.
During the exhaust stroke, the spent gasses from the resultant combustion are pushed out through the exhaust valve(s).
That being said, and there are 4 cylinders in most of our Hondas, there is always going to be at least one cylinder in the intake stroke. Thus, we always have some sort of vacuum in the system. Taking into account all the above posts, they are all correct. At WOT, the system is very close to atmospheric pressure since the throttle plate is pretty much open to the atmosphere. At idle or anything less, the pressure will still be a little negative. Naturally aspirated cars like ours never have positive pressure in the system.
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by pbink »</TD></TR><TR><TD CLASS="quote">Ah okay. So at idle, the iacv is letting in just enough fuel mixture to keep the engine turning, but there is still a negative pressure in the manifold. What is the pressure in the intake manifold at WOT? In other words, is there a negative pressure in the manifold at WOT, and does that pressure change as the RPMs increase? Thanks for the post.</TD></TR></TABLE>
There will always be negative pressure in the intake manifold in naturally aspirated conditions. Even under WOT, the computer is taking into account readings from the throttle position sensor, the MAP sensor, timing, fuel delivery, etc. So even under WOT, everything is just basically moving "faster." More air is going into the system so that you can create more power.
Yes, there is always a pressure change anytime something changes in the system. I'm not an engineer, but my gf tried to explain some of this to me. What is it again? The Ideal Gas Law? PV = nRT. According to that pressure is dependent on many things.
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