Vacuum/Boost Gauge Interpretation

[Mustang Convert]

This is my first turbo engine and I'd like a little bit of help in understanding when exactly the turbocharger is doing its job. I usually have the vacuum/boost gauge displayed and don't see the needle going into boost all that often during my daily drives. Does that mean the turbocharger isn't doing anything when the needle is pointing toward the vacuum side of the gauge? In other words, is the turbocharger really adding any power when the needle is on the vacuum side of the gauge?

My friend (who is familiar with turbos) drove it and said the turbocharger was spooling up even when it's not showing boost on the gauge. I'm kind of confused as to when the turbocharger is actually pumping air into the engine and how to tell.

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[Alex]

The turbo is always "pumping" air into the engine as long as the it is spinning. And it is always going to be spinning as long as the engine is running and exhaust gases are going through the turbo tubine.

Positive boost pressure requires the turbo to be spinning quite fast to pump enough air into the motor. During low load and idle conditions the turbo is just not spinning fast enough to create positive pressure in your intake manifold. It is still pushing air into the manifold but the engine is consuming that air and still able to generate vacuum That's perfectly normal.

Lots of great turbo educational articles and videos on youtube. I also like Garrett's information pages:

http://www.turbobygarrett.com/turbobygarrett/basic

 

[Juben]

There's some good info in that link. Thank you kindly, sir!

 

[Mustang Convert]

The turbo is always "pumping" air into the engine as long as the it is spinning. And it is always going to be spinning as long as the engine is running and exhaust gases are going through the turbo tubine.

Positive boost pressure requires the turbo to be spinning quite fast to pump enough air into the motor. During low load and idle conditions the turbo is just not spinning fast enough to create positive pressure in your intake manifold. It is still pushing air into the manifold but the engine is consuming that air and still able to generate vacuum That's perfectly normal.

Lots of great turbo educational articles and videos on youtube. I also like Garrett's information pages:

http://www.turbobygarrett.com/turbobygarrett/basic

Thank you. So is fair to say that under a vacuum reading, this engine is still putting out more power than the same engine wothout a turbo would put out?

 

[flomoloko]

If you ever have a line pop off (a vacuum line) you will quickly realize what the turbo is and is not doing for you. This is true even under normal driving. In other words, a 2.5 liter IL4 naturally aspirated engine in our car is not so awesome, needs a turbo for sure.

 

[arghx7]

The turbo is always "pumping" air into the engine as long as the it is spinning. And it is always going to be spinning as long as the engine is running and exhaust gases are going through the turbo tubine.

Positive boost pressure requires the turbo to be spinning quite fast to pump enough air into the motor. During low load and idle conditions the turbo is just not spinning fast enough to create positive pressure in your intake manifold. It is still pushing air into the manifold but the engine is consuming that air and still able to generate vacuum That's perfectly normal.

While that especially applies to old turbo engines with cable operated throttle, the Ecoboost and other modern turbo DI engines are more complicated than that. I'll simplify it though:

1. When you push the gas pedal down, you are telling the ECU you want a certain amount of torque

2. The ECU figures out how much air and spark it needs to get the torque

3. To get the airflow, First the ECU moves the cams to change valve timing, which it coordinates with the throttle to try and run as little vacuum as possible.

4. Finally, it calculates how much airflow it needs to get from the turbo, and controls the turbocharger speed to get the right boost pressure for the given valve timing.


Go drive your car and pop it into 6th gear at about 1200-1500 rpm. Give it a bit of throttle. You'll see it go to about 0psi boost quickly. The ECU is deciding to make the engine breathe its own exhaust gases by changing valve timing. This is good for fuel economy but bad for torque. So to make the torque, it opens the throttle (which is why you make hardly any vacuum) and uses the turbo to help supply airflow.

This is called "dethrottling." It's a feature of modern engine software, and it relies on models of the engine inside the computer to coordinate fuel, spark, throttle, turbo, and cams. It's more fuel efficient to lug the engine and open the throttle while using a turbo than it is to rev higher and choke the engine down with a closed throttle. This is why Ecoboost engines exist in fact.

 

[Herr Squid]

argh7, I think you're saying the ECU tries to minimize boost pressure and still hit the target torque? I've noticed that following the MT shift recommendations causes the engine to lug way more than I'm used to but apparently the engine is more efficient down in those low RPM ranges.

I think you're saying it controls turbocharger speed via valve timing? This is making my brain hurt a bit, but I'll take another look when I have a clearer head.

 

[arghx7]

In the old days, you pushed on the gas pedal and it pulled the cable to open the throttle. The ECU just reacted to that. These modern ECUs are all based on models of the engine in the software. They use models and a bunch of logic to figure out the most fuel efficient and cleanest way to achieve the torque requested while still having the engine drive smoothly.

One of the most important ways to be more fuel efficient is to open the throttle more so that the engine doesn't waste energy pumping air through a restriction. That means the ECU wants to run with less vacuum (as in, run closer to psi boost rather than say 15 in Hg), or even go into boost, as long as it doesn't need to run a rich mixture to prevent knock or cool down the catalytic converter. Open throttles and boost (or very little vacuum) are good for fuel economy in certain areas of engine operation.

The other way to be more fuel efficient is to make the engine breathe its own exhaust. This is called exhaust gas recirculation, or EGR, which can be achieved by changing valve timing. The ECU needs to coordinate all these approaches.

So to put it another way:

1) first it figures out how much air it needs to make the torque, based on the torque model
2) then it figures out how change the valve timing for the most fuel efficient breathing (gasflow model)
3) because the most fuel efficient breathing & valve timing isn't good for torque, it most open the throttle to allow air into the engine (actuator coordination)
4) with the throttle open more, less energy is wasted ("reduction of pumping losses")
4) the turbocharger supplies additional air to make the requested torque. (turbocharger model)

the turbocharger wastegate controls have models and look-up tables of the turbocharger . Using those models it figures out how to control the wastegate to control the turbocharger speed, which controls the pressure and airflow coming from the turbo.

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