10/30/2003

TC2 and boost control

Putting a buchi exhaust on the in line ford 6 to drive the tiny turbo has just
about eliminated the use of a waste gate.  Another means of boost control is
needed.

Will be using a bosch gasoline pumped injection with six individual pumps that
control the fuel flow to each cylinder.  The throttle will directly connect to
the pump.

The air valve will be computer operated to provide the measured correct
mixture using an EGO as part of the loop.  For those of use who have actually
worked on control systems, part of the philosophy is fail-safe and limited
control authority. But will not waste your time with further insights.  The
burnt out bulb knows so much more about everything.  Let him harangue you
about how it won't work on his time.

Fuel leads air basic structure, with EGO feedback and table modification.

Since the tiny turbo is off a Mack Truck, it probably was not designed to seal
against gasoline.  So the throttle blade is downstream of the turbo.  Normal
so far.

Now consider the TC-2 as described by MacGinnes.  Its a variable restriction
between the turbo and the intake parts.  Works very well with the major
problem being building up some additional heat over a normal throttle valve.
It has the major advantage of not bleed off exhaust gas or reducing the boost
under very close to the set point.

Consider now the effect of a computer controlled "TC-2".  A small side note on
boost pressure.  The only reason to restrict boost is to minimize knock.  Saab
follows this policy with the apc system.  

On a conventional system, the intake butterfly is normally fully open.  On a
fuel leads air system, the intake butterfly is varied to adjust the fuel air
mixture. As the air increases over optimum, the valve begins to close.  Close
- hmm that increases the restriction in the intake and limits the boost.  As
the mixture decreases below optimum, the valve begins to open.  Ah - that
decreases the restriction in the intake and air flow and boost pressure
increases.

Nah - couldn't work.  We'll just have to replace the buchi system with a POS 6
into 1 system so those that read the first section of Vizard on turbo cams and
completely skipped over the part that says a proper exhaust system like a
buchi could reduce the backpressure of the system until it was well below
intake pressure and behaves like a mechanical supercharged system.  Then we
can put a wastegate on it and a fart valve and fart pipes and be just like all
the rest of the dorkweeds.

And forget about using the valve as a spool up valve during deceleration by
killing the fuel and snapping the valve wide open and cooling off the engine
with blow thru air and using the air flow to keep the turbine spun up.
Probably would not work anyway.

---------------------------

TC-2 Brand Name for an inlet air pressure restrictor regulator used back when
waste gates were expensive and turbos were just getting popular.  See
MacGinnes on boost control.  Actual name was Impco TC2 Turbocharger Boost
Control.

Works by placing a restriction in the air pipe that varies by set point.  As
the boost increases above the pressure set point, it restricts flow of air
into engine and maintains the down stream pressure.  The increased upstream
pressure tends to slow the compressor down and requires more work from the
exhaust turbine to maintain speed.  This heats the air to a modest amount.

But, since the exhaust is never throttled, there is no exhaust restriction at
any time.  Waste gates do not open suddenly - they start slowly bleeding off
gas at about half the set point pressure and are hopefully about mid open at
the designed boost pressure.

Controlling boost pressure is the easy way to limit performance so that
detonation does not occur.  Saab, in there APC "Automata tic Pressure Control"
system does not limit boost - until a knock occurs.  Thus significantly higher
average boost pressures can be maintained with an APC than with an arbitrary
waste gate pressure.

The whole operation of a TC-2 is to restrict air flow to an appropriate level.
In the TC2 case, this is done by sensing and controlling downstream air
pressure and balancing that against a spring loaded sleeve valve.  When the
spring pressure is exceeded downstream the piston starts closing the sleeve
valve until the pressure is balanced.

Note there is no intake or exhaust restriction until the set point pressure is
reached.

With a computer generated butterfly valve, a variable resistance can be placed
into the intake air stream downwind of the turbo outlet.  Using boost as its
only input, any boost value can be used.  As the boost pressure closely
approaches set point value, the blade can be partially closed until it imposes
enough restriction to maintain the downstream pressure regardless of the
upstream pressure.  Again, this increase in pressure as seen by the
compressors slows the compressor, removing more work from the exhaust to
maintain the pressure etc.  

End result is that the pressure is maintained to the set point.

The Saab APC varies the set point not by boost pressure downstream, but by
knock.

In a Fuel Leads Air system, Fuel is fixed by driver input.  Air is modified to
provide the correct mixture.  Modifying the fuel input by boost pressure
defeats the entire concept as the level of power delivered by the engine is no
longer under driver control.

In an Air leads Fuel system, when the amount of power is controlled by the
amount of air with fuel modified ( in other words, fuel ( and power ) is
modified lagging ( well after ) an air change, varying fuel by boost pressure
keeps the fuel in close proximity to air fuel.  

In a Fuel leads Air or air changes lag fuel changes system, it is necessary to
control boost pressure to bring the mixture into the correct value.  Driver
foot feed controls the fuel - not any source of air flow.  The EGO will set
the restrictor position to the point needed for the amount of fuel commanded
by the driver and not by whatever amount of air is potentially available.

Note that EGO is metaphorical, as it will probably be set by a number of
inputs computed against TPS and RPM.  And corrected or tuned or compared
against a wide band EGO.  Boost pressure downstream will be used to CORRECT
THE AIR FLOW as needed and in conjection with the EGO values.

This eliminates the need to wazoo up some weird way off putting two waste
gates on the system - split scroll turbo with no built in waste gate.

There will be other issues with the temperature increase that I will need to
deal with, but that's down the road.