Re: Turbos at Altitude (long)

[tdonohue@xnet.com (Terry Donohue)]

Several of you had some useful comments on this problem, but there were
also some misconceptions.  First, yes, a turbo can operate very well (but
working harder) at high altitude (lower air pressure), but the intake
manifold pressure still has to be controlled correctly.  Second, pressure
is not just pressure.  Most pressure gauges measure what is called "gauge"
pressure.  In particular, you normally find "psig" gauges; this refers to a
pounds per square inch gauge.  The most common type of pressure gauge used
for the past 100 years is the Bourdon tube, which is a bent hollow, closed
tube which flexes as pressure is applied to the inner portion of the tube
and typically drives a needle by a gear system.  You find this type on all
types of gas pressure gauges as well as oil pressure gauges.  This is a
relative method of measurement-it measures the difference between the
outside (which is typically at atmospheric pressure, and that varies, of
course) and the inside.  A more modern method uses a piezoelectric
transducer, where the flex of a semiconductor produces a voltage
proportional to the pressure difference.  Once again, a relative
measurement of pressure.  These methods can be made absolute by including
an absolute reference, usually a good vacuum.  Surround a bourdon tube by a
vacuum (must be good-can't have any leaks), and you have a psia gauge,
where "a" refers to absolute.  The same thing can be done with a piezo
pressure transducer, which is presumably what some modern turbo
manufacturers use.

Road & Track had some useful material on this issue in consecutive issues
in 1988.  April, p. 191 and May, p. 166.  Included is a graph from Porsche
showing how the power of a conventional turbocharged engine, with
differential pressure control, drops with altitude (at a rate of about
2%/1000 ft), and the 944 Turbo, which has absolute control, and maintains
its sea level power to 6560 ft.  Also noted is that Ford and Pontiac use
absolute boost control; no mention of Audi (this in 1988).  So the question
now is when (if) did Audi start using absolute pressure measurement for
boost control?

The final piece is how the wastegate controls boost pressure, which is
intake manifold pressure.  The pressure across the wastegate diaphragm
(which is the difference between the intake manifold pressure and whatever
is feeding the upper connection, either atmospheric or the frequency valve)
in the wastegate causes it to open, and this is resisted by the spring.
More pressure in the intake causes the valve to open, but a stiffer spring
delays this, allowing higher pressure.  The wastegate frequency valve is
controlled by the ECU, which can use an absolute pressure sensor.

Terry Donohue