STTeve R

My earlier post got some good answers. But, I forgot to mention that my main concern is with the turbo lag I experience when the A/C is activated vs. when it is not activated. Yes, the average altitude where I am driving is about 5600 + - 200 feet (within the city).

Any other thoughts or comments would be appreciated. Thanks to those who did respond.

-STTeve

Marius B

Now IF the A/C had drawn much power, it would make you apply more throttle and/or drive at higher rpm's, both of which bringing up the turbo's speed and boost, REDUCING the lag.

Hope you actually appreciate this, like you promised ;-)

Marius

STTeve R

This information was extrapolated (sic?) from a web page dealing with this issue and I read it after your response. Does this make sense? And, if it does, then is the size of the turbocharger in the TT sufficient? Thanks for non-technical American English responses:


How the turbo Anti-Lag System works

Bang-bang (also known as ALS which stands for Anti-Lag
System) is an engine management technique that allows to minimize the
turbo lag time.

As you might be aware of, turbochargers display what is
known as lag time which is the time needed for the turbine to reach its
full throttle from an intermediate rotational speed state. The
duration of a turbocharger's lag depends on many factors among which its
inertia, airflow efficiency, back pressure, etc. The
problem is partly dealt with by fitting a turbo dump valve, which acts
each time the driver lifts his foot from the throttle. The dump valve
will evacuate the pressurized air coming out of the turbocharger while
the inlet manifold is closed thus allowing not to stall the
turbine and avoiding possible damage to its bearings. In racecars it is
very common to fit oversized turbochargers in order to be able
to produce enough boost pressure and assure a sufficient engine output.

Big turbochargers display significant amounts of lag due to
their increased inertia. In these cases the dump valve is insufficient
to allow the turbocharger not to loose too much speed when the
driver lifts off. Additionally rally cars hold a turbo restrictor, which
is regulated by the FIA. One of the restrictor' effects is to
increase lag time. This is why in racing cars, and most specifically in
rally cars where torque and engine availability are critical
factors, most applications use anti-lag systems.

During lag time the engine is much less responsive and its
output well below nominal. To counter the effect of the turbo's lag time
drivers used to anticipate the engine's reactions by
accelerating well before they would have in a non-turbo car. Others have
used a technique, introduced by the German driver Walter Röhrl,
known as "left foot braking" where the driver uses his left foot to
brake the car while his right foot accelerates to keep the
turbocharger in optimal load. ALS was a simple idea but relatively
difficult to implement. To the best of my knowledge Toyota Team Europe
were the first to use it in racing (Toyota's implementation is known
as Toyota Combustion Control System while Mitsubishi call
the system Post Combustion Control System).

How ALS works

While the ignition timing is altered with 40° or more of
delay the intake air and fuel supply mixture is made richer when the
driver lifts his foot from the gas pedal. This results in air/fuel
mixture that keeps getting in the combustion chambers when the driver no
longer accelerates. The ignition being delayed, the
air/fuel mixture reaches the exhaust tubes mostly unburned. The exhaust
temperature being extremely high the unburned fuel explodes
at the contact of the exhaust tubes. Luckily the turbo sits right there
and the explosion keeps it turning (otherwise it would slow
down since its intake, the exhaust gases, is cut-off). The effect is
vastly lower response times with some downsides:

A quick rise of the turbo's temperature (which jumps
from ~800°C to the 1100°C+ region) whenever the system is
activated. A huge stress on the exhaust manifold and pipes
(mounted on a street car a bang-bang system would destroy the exhaust
system within 50-100 km)

The explosions which occur in the exhaust tubes
generate important flames which, sometimes, can be seen at the end of
the exhaust tube

The bang-bang system got its name from the loud explosion
noises one hears whenever the driver lifts his right foot. Depending on
the implementation, ALS systems are more or less
aggressive. While the systems mounted in Toyota and Mitsubishi racing
cars are relatively smooth and noiseless those fitted in Ford and
Subaru cars are much more noisy and aggressive. Most racing
implementations have user selectable anti-lag settings
depending on the terrain, usually three settings can be selected by the
driver going from moderate to very aggressive.

Note also that some regional or national European events
prohibit the use of ALS systems.

Marius B

My point was: If you're in «tactical situations» and want max responsiveness from the engine, just shift down a notch or two, which brings you into a rev range where the turbo has better speed and the engine will deliver full power on much shorter notice.

Marius

Marius B

My point was: If you're in «tactical situations» and want max responsiveness from the engine, just shift down a notch or two, which brings you into a rev range where the turbo has better speed and the engine will deliver full power on much shorter notice.

Marius

Marius B

nt

Ray

Does the Audi automatically turn off the A/C under full throttle or boost? If so, then the sensation of turbo lag might be accentuated when the engine transitions from A/C load to no load.