Bollinger

Okay, this probably seems dumb at first glance. It might seem dumb at second glance. But it is a serious question.

BPVs/BOVs/DVs work to dump the excess boost you get during a shift so it doesn't back up and stall the turbo (compressor).

This boost becomes excessive because the turbo is running at high speed ramming air in there and when you let off the gas to shift the throttle plate closes and blocks the airflow. Once you get in gear you hammer the throttle again and the throttle plate opens and everything returns to normal. It takes some time for the turbo to get working again though since it's feed of energy (exhaust) has been greatly diminished for a period and it will have slowed down at least a little.

So my question is, on a TBW (throttle-by-wire) system, why close the throttle plate during shifts? You can just shut off the injectors so that you don't produce any power, but the throttle plate is still wide open to allow air into the the cylinders and out the exhaust. This will provide more energy to the turbo than it would get otherwise (although not much since the air didn't expand in the cylinder), prevent a backup of air at the throttle plate, and finally make it so the turbo doesn't have to spend time repressurizing the intake plenum which would normally go to a strong vacuum behind the throttle plate.

So, what am I missing here?

I only see two issues.
1. How do you detect a shift, as you can't just do this all the time. I could imagine delaying all throttle closings when the clutch is put in for a short period perhaps to have the desired effect.
2. Will the revs fail to fall fast enough without the vacuum normally created in the intake plenum to rob it of energy? Could you get around this somehow?

Audiboy

pedal? Maybe different with auto tranny as computers control the shifting process. If you shut off the injectors (and ignition I hope) during a shift, how will exhaust energy be produced? Revs are maintained by flywheel inertia. By the way, on the TT, clutch in, half of the injectors go off. But I can still blip blip the throttle between shifts. It sucks on AutoX though.

Bollinger

As it doesn't have one.

I only mentioned the clutch once, and that was where I speculate as to how to detect a shift. You see, you can't just do this all the time, as when a person lets the gas off in gear it has a totally different meaning than letting the gas off to shift. That's why I mention the clutch.

Ignore the clutch stuff. Either I wrote it wrong or you are easily confused. I am fully aware that the clutch doesn't control the gas, so we don't need to speculate on that further.

Audiboy

My statement about it is different with Auto tranny's has nothing to do with a clutch, but the throttle algorithm itself because the computers can communicate with each other when the shifting.

But I think I see what you are trying to get at now or maybe one aspect of it. Like some system that have a continous flow of gases from intake to exhaust when there are no power demands (like when letting go of throttle that will affect shifting smoothness into the next gear). I try to picture this, but I see some issues here. Like the valves would have to float, the MAF calibrated air has to be ignored (or use a MAP system) and the residual intake pressure which will last in milliseconds is about a third of the exhaust energy driving the turbine so I can't picture see this system having significant returns for engineering invested.

But the point you brought up about vacuum in plenum is interesting specially on a turbo car. Hmmmm

BTW, I brought up clutch because you mentioned Why does Audi close the throttle plate. Since we do not Audi

ryoung

... but we'll probably skip over this step. The "cure" will be a paddle shifted sequencial gearbox, with <100 msec shifts and everything controlled by the computer once you tug on a paddle.

Mike O.

...all gas engines are like this because of this silly 14.7:1 ratio that must be maintained to keep the engine from melting.

Diesel engines do exactly what you propose because there is NO throttle body. The only thing that controls how much energy the engine produces is the injector pump. No throttle, no fuel, no power. Full throttle, full injector cycle, max power.

Theoretically, it would be possible for the TB to switch to Wide Open when the throttle is fully released, but you would have an AWFUL time trying to deal with "tip in". At tip in, you are trying to get back to a low airflow and low fuel flow (to maintain that 14.7:1 ratio). The only way to get a low airflow is to close the TB and slowly reopen it while adding the requisite amount of fuel. With all that extra air in there, the tip in would result in a lean-out, power surge, extremely high EGTs and then would settle back.

Not a fun experience driving a car like this or trying to be the engineer to tune to this environment.

Mike O.

john s

At WOT it would not be nearly enough for an ignitable mixture (say for example just proceeding the exhaust stroke for the benefit of the turbo) but it would measurably increase exhaust emissions. You could however still fire the injectors & delay the spark. Big help to keeping turbo running but murder on cylinder wall wear due to washing effect.

Bollinger

No fuel means no energy released, which means no overheating.

I do understand you can't start out this way or run under actual power this way. But you can just run the engine with no fuel. My question is why not do it only for shifts?

Even if it didn't melt, I don't think a gas engine would idle well with a wide open throttle plate.

Bollinger

For example on the Nissan Skyline, they use a throttle plate at each cylinder. When you cose the throttle on that car, you do pressurize the plenum (although it does have DVs) and about half of each (short ) intake runner.

Does it matter? I have no real idea. It would seem like it couldn't respond any slower than a throttle before the plenum.

Paul H. (near Annapolis, MD)

The auto trans sends a signal to the ECU when it is getting ready to shift which causes the ECU to reduce power below a certain level. Many modern cars do this. Helps make for smoother shifts, reduces wear, etc. On non-DBW cars it can be done with timing and fuel. On the DBW cars obviously there can be throttle plate control (whether Audi uses it or not I don't know)...

Paul