Engine Vacuum System
#1
01-13-2008, 11:38 PM
Engine Vacuum System
I'm trying to get a grip on the engine vacuum system in my 1.8t A4. I'm not having any problems, I just want to understand it. I was leisurely browsing the Bently manual (as I often have a tendency to do) and ran across the Vacuum Diagram for my engine. Wow!
So, here is what I understand so far: The vacuum system is a collection of tubing throughout the engine used for brake boosting and emissions control. It is generated by the vacuum effect the engine has on the intake manifold.
1) Is my understanding correct? What else is it used for beyond brake boost and emissions? I saw something on the diagram about crankcase breather. What is that?
2) I understand that the secondary air pump is used to inject air into the exhaust system to speed up post-combustion burning of engine waste (i.e. -- help heat up the catalytic convertor). Is that correct?
3) What is the purpose of the vacuum reservoir?
4) Is my understanding right that the EVAP canister is how fumes from the gas tank are injected into the engine for combustion.
5) The wastegate is a valve that relieves pressure in the system from when the turbo is spinning faster than the engine can consume the pressure (i.e. -- when decelerating rapidly). Is that right?
Thanks for all your input. This is fascinating.
-Chris
So, here is what I understand so far: The vacuum system is a collection of tubing throughout the engine used for brake boosting and emissions control. It is generated by the vacuum effect the engine has on the intake manifold.
1) Is my understanding correct? What else is it used for beyond brake boost and emissions? I saw something on the diagram about crankcase breather. What is that?
2) I understand that the secondary air pump is used to inject air into the exhaust system to speed up post-combustion burning of engine waste (i.e. -- help heat up the catalytic convertor). Is that correct?
3) What is the purpose of the vacuum reservoir?
4) Is my understanding right that the EVAP canister is how fumes from the gas tank are injected into the engine for combustion.
5) The wastegate is a valve that relieves pressure in the system from when the turbo is spinning faster than the engine can consume the pressure (i.e. -- when decelerating rapidly). Is that right?
Thanks for all your input. This is fascinating.
-Chris
#2
01-14-2008, 01:01 AM
RE: Engine Vacuum System
ORIGINAL: ee99ee
So, here is what I understand so far: The vacuum system is a collection of tubing throughout the engine used for brake boosting and emissions control. It is generated by the vacuum effect the engine has on the intake manifold.
1) Is my understanding correct? Yes
What else is it used for beyond brake boost and emissions? operating the egr (exhaust gas recirculation) and opening the dv. But on a turbo car, the manifold is a great source for tapping into a boost gauge (shows psi and vac), and can be used for the boost source for the wastegate and vac source for the dv.
I saw something on the diagram about crankcase breather. What is that? It is that hose the comes out the back of the valve cover and to the intake. It relieves pressure from the head. The down fail of that is that oil vapors can/will get into the intake and then will go through the turbo, ic pipes, ic, and to the manifold. The oil will mainly pool in the ic and crust around the TB (throttle body) so those will need to be cleaned. A catch can can prevent this from happening.
2) I understand that the secondary air pump is used to inject air into the exhaust system to speed up post-combustion burning of engine waste (i.e. -- help heat up the catalytic convertor). Is that correct? It acually is there to warm up the engine faster along with the cat. You can see the combi valve (silver flat disk behind the engine under the above^ pcv) where the air is injected into the head.
5) The wastegate is a valve that relieves pressure in the system from when the turbo is spinning faster than the engine can consume the pressure (i.e. -- when decelerating rapidly). Is that right? Kind of. The turbo runs on exhaust gases. The wastegate lets some of that exhaust pass it when the turbo reaches the set psi for the wastegate. In this case, 5psi. However the n75 alters the boost reading to the wastegate so it doesnt see 5psi until the stock car sees its 7,9,11,14psi (depends on the car). Then it bleeds the 5psi to the wastegate and then the wastegate lets some exhaust by it. Until that point occurs the gate is closed.
Thanks for all your input. This is fascinating.
-Chris
wasnt 100% on 3 and 4 so didnt answer
So, here is what I understand so far: The vacuum system is a collection of tubing throughout the engine used for brake boosting and emissions control. It is generated by the vacuum effect the engine has on the intake manifold.
1) Is my understanding correct? Yes
What else is it used for beyond brake boost and emissions? operating the egr (exhaust gas recirculation) and opening the dv. But on a turbo car, the manifold is a great source for tapping into a boost gauge (shows psi and vac), and can be used for the boost source for the wastegate and vac source for the dv.
I saw something on the diagram about crankcase breather. What is that? It is that hose the comes out the back of the valve cover and to the intake. It relieves pressure from the head. The down fail of that is that oil vapors can/will get into the intake and then will go through the turbo, ic pipes, ic, and to the manifold. The oil will mainly pool in the ic and crust around the TB (throttle body) so those will need to be cleaned. A catch can can prevent this from happening.
2) I understand that the secondary air pump is used to inject air into the exhaust system to speed up post-combustion burning of engine waste (i.e. -- help heat up the catalytic convertor). Is that correct? It acually is there to warm up the engine faster along with the cat. You can see the combi valve (silver flat disk behind the engine under the above^ pcv) where the air is injected into the head.
5) The wastegate is a valve that relieves pressure in the system from when the turbo is spinning faster than the engine can consume the pressure (i.e. -- when decelerating rapidly). Is that right? Kind of. The turbo runs on exhaust gases. The wastegate lets some of that exhaust pass it when the turbo reaches the set psi for the wastegate. In this case, 5psi. However the n75 alters the boost reading to the wastegate so it doesnt see 5psi until the stock car sees its 7,9,11,14psi (depends on the car). Then it bleeds the 5psi to the wastegate and then the wastegate lets some exhaust by it. Until that point occurs the gate is closed.
Thanks for all your input. This is fascinating.
-Chris
wasnt 100% on 3 and 4 so didnt answer
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