Re: Turbo Theories

[Frank Deutschmann <fhd@interport.net>]

Robert Phillips sez:
> 
> On Fri, 17 Feb 1995, Steven Buchholz wrote:
> 
> > This might actually be the source of the flames that someone spoke about.  
> > The fuel was probably injected into the exhaust.  I don't think that the 
> 
> The way I understood it, with my only basic knowledge of turbos, was the 
> flames on the race cars were a result of "dumping" the extra fuel/air 
> mixture directly into the atmosphere, rather than putting it back into 
> the system somewhere.  I don't think that the EPA Nazis would be very 
> pleased about this on a "street" car.

The flames from turbocharged race exhaust systems are the result of an overly
rich condition as the throttle is abruptly closed.

To understand what is really happening beyond this simple explanation, you need
to look a little closer at a fuel injected motor.  First, the
injectors will deliver a fixed rate of fuel while they are open; the
FI computer will adjust the quantity of fuel delivered by varying the
duration of the injection period.  When choosing injectors for a
fuel-injected motor, we would like to choose the smallest injectors
possible, as small injectors give finer control over mixture -- the
time/volume granularity is improved.  However, we also need to size
the injectors large enough that they can deliver all of the fuel which
the motor needs at max RPM -- and they only have 720 crankshaft
degrees max to deliver that fuel load.

In an ideal world, we would use a large number of very small
injectors, and choose the sizes so that the max fuel requirement at
max RPM could all be delivered during the (relatively short) intake
valve open interval.  But this is the real world, and we are generally
limited to just a few (typically max 2) injectors, which are forced to
operate at 75-95% duty cycles (injectors open 540 or more crankshaft
degrees at max RPM full load).  We choose this situation as a ballance
between better fuel control at max RPM/power (really huge injectors
open only during intake valve open) versus better fuel control at low
RPM/power (tiny injectors open for long duty cycles).  To offset this
unfair choice somewhat, we use strategies such as dual injectors which
are sequenced, etc.  But we still end up with all injectors running
high duty cycles at full load/max RPM situations; such is life!

Whenever our injectors are not operating perfectly "sequential" --
inkjectors open only when the intake valve is open -- we are spraying
fuel onto a closed intake valve, and this fuel will enter the cylinder
at the next intake opening.  This is where the flames come from: the
motor is operating at a near max-RPM/full load wide open throttle, and
the injectors are running at long duty cycles, spraying fuel onto
closed intake valves in anticipation of the fuel needs for the next
induction cycle.  But then the driver sunndenly lifts of the throttle
-- the throttle plates bang closed, and the computer suddenly decides
to cut back the fuel delivery for the upcoming induction cycle, but it
is already too late, as there is already a significant amount of
accumulated fuel in the intake tract.  On the next induction cycle,
less air is inducted (the air response is immediate), but too much
fuel is inducted, resulting in an overly rich situation.  The excess
fuel is dumped in the exhaust, which is routed through the wastegate
(as the wastegate opens on the intake overpressure/throttle closed
condition).  The excess fuel is ignited by the extreme temperatures of
the wastegate valve, and shoots out of the straight exhaust as flames.

So why don't consumer turbo cars light up the night on every shift?
First, the consumer exhaust has all sorts of lenght and obstructions
to it, preventing the high-O2 conditions necessary for ignition.
Second, the consumer wastegate is not nearly as hot as the race wastegate,
making ignition unlikely.  Third, and most important, the consumer
injectors are never run with such a broad range of fuel demands as the
race injectors -- even when the throttle is abruptly closed from a WOT/max
RPM/full load situation, not nearly as much fuel accumulates.

-frank
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