Unexpected Behavior with Audi A3 e-tron Turbocharger boost

[Vanadium 50]

TL;DR Summary

What is the normal behavior of turbocharger boost?

So the other day I plugged in one of those cheap-o OBD-II bluetooth adapters and looked at my car's boost pressure on my phone. Moat of the behaviors look reasonable. Others, not so much.

Car is an Audi A3 etron. Has a 1.4T engine putting out 150 hp and an electric motor putting out 100 hp. This sums to 204 hp. Normally, 90% of my driving is on electricity, but of course when it is very cold I use gas - the battery range suffers, and if the engine is going to go on at all during the trip, better to use it at the beginning to heat the cabin.

If the engine is off, the boost is zero. This makes sense.
If the engine is running at idle, there's about 12 psi of vacuum. I'm OK with this too.
If I am on the highway and passing (accelerating but not racing), there's 5-6 psi of boost. So far so good.

The weird thing is that when I am at a stop with the engine off, but in hybrid mode, and start to move, if the engine comes on, I am immediately at around 4 psi of boost, and that's at maybe 2000-2500 rpm. The first non-zero number is positive, and it happens under a second after the car starts to move.

My question is how the turbo spins up that fast. When I am stopped, there's no exhaust gas to move it. Shouldn't I see vacuum and then boost? And doesn't 2000-2500 sound like a low boost threshold?

 

[Baluncore]

Maybe that is the turbo boost numerical target.
It will be reached once started and running.

 

[Tom.G]

Or maybe the display value is 'smoothed'; as in showing a 1 or 2 second average. A warmed engine can easily start in under a second, often only a couple compression strokes.

 

[tech99]

Maybe it is induction tract inertia in response to throttle movement.

 

[Vanadium 50]

Maybe that is the turbo boost numerical target.

I'm having a hard time seeing a) how this would work, and b) how this would be reported with OBD-II.

a) If the turbocharger was above or below target, what could the car's computer do about it?
b) OBD-II has a sensor for manifold absolute pressure and another for absolute barometric pressure, so calculating boost pressure is simple. I don't see anything on a PID list that looks like a target.

 

[Vanadium 50]

Or maybe the display value is 'smoothed'; as in showing a 1 or 2 second average.

Then wouldn't it show boost after it occurred, not before?

 

[Vanadium 50]

induction tract inertia

I don't know what that is.

 

[Baluncore]

If the boost takes a positive step at the instant before starting, then there must be energy stored somewhere that is released on starting to provide the boost.
1. Compressed air start? Clever, but where is the start reservoir? charged when stopping?
2. Super-charger, driven by what?
3. Motor spun up fast by electric motor? Maybe with valves initially opened.

 

[Vanadium 50]

I don't see anything like this in the car. #3 could be done through the drivetrain, I suppose, but that doesn't sound like a very good idea - and what would be the point?

 

[tech99]

I meant that if the throttle is opened for starting, and suddenly closes when the engine fires, then the air in the intake pipes is moving and has inertia, so when the throtle closes it gives a pressure peak due to ram effect.

 

[Vanadium 50]

so when the throtle closes it gives a pressure peak due to ram effect.

But why wouldn't that happen all the time? If that were the explanation, the engine would never be at vacuum.

 

[essenmein]

To me the 4psi boost at 2000-2500rpm does not sound odd.

From audi:
"The 1.4 TFSI produces 110 kW (150 hp), and its peak torque of 250 Nm (184.4 lb-ft) is available continuously between 1,600 and 3,500 rpm to harmonize perfectly with the electric motor. "

So 2500rpm is right in the center of its peak torque band, seems to me the turbo is likely sized to push air from about 1200-1500rpm, and if its anything like the Q3 rental I drove, it pulls hard upto about 4krpm and then falls on its face. Clear sign of a small turbo that has very little lag, but not near enough volume capacity as the rpm climbs. Here you need a bigger snail and all the negative (laggy) effects that come with it.

Then:
"When the combustion engine starts, it is tow-started by the electric motor via the clutch. Once its reaches the same speed as the electric motor, the clutch closes. This happens smoothly within a few tenths of a second. "

So the combustion engine is started by the electric main motor, and in well less than a second. I would say that given a hot restart and then immediately delivering torque is likely why you immediately see positive boost. That is what the hybrid system is doing, as quick as possible getting the ICE to start either generating or adding torque "with no detectable interruption of propulsive power ".

https://www.audi-mediacenter.com/en...audi-a3-sportback-e-tron-2624/drivetrain-2635

You have to remember this is no longer a throttle cable to a butterfly on the intake, there is a bunch of computer wizardry happening between your foot and the engine. Driver pedal position is no longer a real indication what the computer is making the engine actually do.

Then you have to also consider the update rate of OBD2, maybe its updating every second, so you could easily miss the start up pressures since they are very short.

 

[Vanadium 50]

To me the 4psi boost at 2000-2500rpm does not sound odd.

OK, I'll take your word for it. Tiny turbo means boost sooner than I am used to. It is certainly true I experience very little turbo lag, but I have always attributed this to using the electric motor to provide power while the turbo spools up. One feature of this car is near-immediate acceleration. And you're right - the gas pedal is but one input to the car. The days of a physical cable are over.

A while back I got to drive the car on just the engine - a kind of limp-home mode. It is very under-powered and sluggish. It's the same engine as in the Jetta (the Skoda 1.4T) but the Jetta is 700 pounds lighter.

 

[Dr_Nate]

Are the zero values identically zero (i.e. without any noise)? If so, then you're likely not getting fed the sensor reading during that time.

 

[Rive]

So the combustion engine is started by the electric main motor, and in well less than a second. I would say that given a hot restart and then immediately delivering torque is likely why you immediately see positive boost.

Likely right. To spin up the engine that fast I guess requires a decent flush of fuel => plenty of gas to drive the turbo.
I don't think this could work without a computer overseeing the process...

By the way, I wonder if EGR works during this phase?

 

[russ_watters]

My question is how the turbo spins up that fast. When I am stopped, there's no exhaust gas to move it. Shouldn't I see vacuum and then boost? And doesn't 2000-2500 sound like a low boost threshold?

I can only answer the second; boost is based on throttle position only, not RPM:

I drive a Kia Stinger 2.0T (non-hybrid). The motor stops at stoplights though, so I'll see if I can capture a launch. The above screenshot was taken coasting into, then going up a hill and stomping on the gas. There's no shifting during the hill climb and the RPM stays pretty low.

I'll take a guess about the fast spinup: Turbo rpm depends on the needed airflow and pressure. With the engine rpm low and throttle open, the airflow is still relatively low. That means relatively low turbo rpm can give high pressure.

 

[russ_watters]

#3 could be done through the drivetrain, I suppose, but that doesn't sound like a very good idea - and what would be the point?

Well, a fast spin up would be a benefit and there are others, but I think you'd know if it had one:
https://en.m.wikipedia.org/wiki/Hybrid_turbocharger