July 16, 2000 Pelican's Perch #32:
Those Fire-Breathing Turbos (Part 2) |
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In this second in a series of columns on turbocharged piston engines, AVweb's John Deakin offers a detailed walk-through of a typical turbo system — from intercooler to wastegate and everything in between. He then explains how the various system components function during each phase of flight from engine start through runup, takeoff, climb, cruise, and shutdown.
July 16, 2000
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| About the Author ... |
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John Deakin is a 35,000-hour pilot who worked his way up the aviation food chain
via charter, corporate, and cargo flying; spent five years in Southeast Asia
with Air America; 33 years with Japan Airlines, mostly as a 747 captain; and
now flies the Gulfstream IV for a West Coast operator.
He also flies his own
V35 Bonanza (N1BE) and is very active in the warbird and vintage aircraft
scene, flying the C-46, M-404, DC-3, F8F Bearcat, Constellation, B-29, and
others. He is also a National Designated Pilot Examiner (NDPER), able to give
type ratings and check rides on 43 different aircraft types.
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| Starting Your Engine with Low RPMs |
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Let me digress here for a moment into technique.
You are not
one of those dreadful people who start an aircraft engine and let the RPM soar
to some high value, are you? No, I thought not, sorry for insulting you. My
loyal three readers are the type who will gently nudge an engine to life,
letting it barely idle for a few moments, gradually and gently bringing it up
to 1,000 RPM or so, minimizing the damage to all those surfaces that do not
yet have oil on them. Some will tell you that "splash lubrication"
is important, and maybe it is, but I think we ought to give the oil a few
seconds to come up to full pressure, and reach into all the nooks and crannies
of the pressure feed system. On the big radials, we absolutely won't exceed
1,000 RPM until the oil pressure is fully up, and the oil temperature is
moving. We won't exceed 1,200 RPM until the oil temperature is 40C (104F). On
some airplanes, 1,000 or 1,200 RPM isn't even enough to taxi, so you'll see
many of them just sit there after engine start, waiting for 40C on the oil
temperature gauge.
But there can be too much of a good thing. As the oil temperature rises
slowly, the cylinder head temperatures are coming up too, and many airplanes
do not have sufficient cooling on the ground to allow this sort of thing. It's
always a trade off, for it may be better to take off with slightly low oil
temperature, rather than high cylinder head temperatures. Again, on the big
radials, it is not unknown to hear one call while waiting for takeoff,
"Getting hot!" Then very soon, if clearance is not forthcoming,
"The B-29 is shutting down for cooling."
The principles are the same for a flat engine, although the tolerances may
be somewhat less limiting.
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 In
my previous column, we explored a bit of the history
of supercharging, some of the many variations, and some of the reasons for it.
We touched lightly upon the turbochargers, which make it possible for an
aircraft engine to produce a manifold pressure greater than ambient pressure,
and thus more power. We also mentioned the turbonormalizers, which make it
possible for an engine to maintain
sea-level horsepower to some much higher altitude, usually 20,000 feet or more.
In reality, of course, the turbo systems themselves are virtually identical,
with few differences between the "turbosupercharger" and the "turbonormalizer."
The only real differences are the wastegate controller (there are several basic
types, and many variations), and the settings for the pressures to be used. From
this point on, I'll just call them all "turbo systems" unless there is
something specific to one or the other.
In this column, I'd like to dissect this very neat (and sometimes much
maligned) system, component by component, and perhaps take a bit of the mystery
out of it for some. Why "much-maligned"? Because a lot of the early
ones were badly designed, some to the point of being dangerous. Some imposed
significant additional workloads on the pilot, and it was easy to use them
improperly.
Modern turbo systems have come a long way from those early days. If you were
turned off by some of the stories before, I think it's time to take another
look.
You will find some minor differences between the system I describe here and
some of the turbo systems "out there" but it should be pretty easy to
figure those out. The following discussion is loosely based on the
"Whirlwind" turbo installation as done by "Tornado Alley
Turbo" (hereafter TAT) in Ada, Oklahoma. (This is the system I had installed on
my own Beech Bonanza.) I'll try and identify the exceptions as we go along, but
this is really intended as a broad, general look at the mechanical aspects of
turbos.
Here is the schematic diagram that appeared in the previous
column, showing the full installation again:
Click on the graphic above for a higher-resolution version in a separate
browser window. You might find it helpful to keep it open for ready reference as
you read the remainder of this article.
This was originally an old black-and-white drawing, which I cleaned up and
colorized. It is a good "generic" schematic, but does not accurately
reflect the exact layout of the TAT system. In the remainder of this article, I
will extract each component from this diagram, piece by piece, and go into some
detail. Finally, I'll explain what all these parts are doing during various
phases of flight.
These two pictures illustrate the normal induction and exhaust manifolds
found in most "flat six" engines. (There are variations, but they're
not important here.) Teledyne Continental Motors (TCM) numbers the cylinders
starting with #1 from the right rear as shown here, while Lycoming starts at the
left front, so I have not numbered them at all.
The induction manifold is very simple ...at least that part of it between the
throttle and the cylinders. Whether the engine is carbureted or fuel-injected,
turbo'd or non-turbo'd, intake air passes through the throttle body and then it
splits into two equal-sized "log runner" assemblies, with a
"riser" coming off the main runner to each cylinder. On most of the
Continentals, this is an excellent intake system, which provides almost
perfectly equal air to both sides of the engine, and to each cylinder. (Yes,
this is somewhat contrary to popular belief!)
There is some mystery over the induction crossover tube at the front of the
engine, with no one quite knowing what it does, or why it's there. I think of it
as the mechanical equivalent of the human appendix. Some say it adds a little
bit of power at some settings, and takes it away at others, some say it has
something to do with resonance. If the real experts don't know, who am I to
guess? In any case, it comes with the engine, turbo or not, at least on the
Continentals. Perhaps some astute reader will have more on this?
On the non-turbo'd engine, the exhaust system is really simple, one pipe
comes out of each cylinder and flows into a main pipe on each side, which
conducts the hot gasses overboard. In some engines, there are "heat
muffs" around one or both exhaust pipes to extract some heat for cabin
heating and/or for carburetor heat on those engines so-equipped. These are
another form of heat exchanger, designed to transfer heat from one gas stream to
another, without actually mixing them. Fuel-injected engines do not normally
have carburetor heat, as they are far less susceptible to induction system
icing.
In order to spin a turbocharger, all the exhaust piping comes together in a
"wye" (clever technical term for a Y-shaped fitting), and the
resulting single pipe goes right back into another wye that splits it back into
two flows again. One of these pipes will go to the wastegate, the other to the
turbo. If the wastegate door blocks its branch of the system, then all the
exhaust gas must go through the turbo. On the other hand, if the wastegate door
is open, most of the exhaust is simply dumped overboard (taking the path of
least resistance), with only a small amount going through the turbo.
In the TAT system, the exhaust from the right side of the engine simply wraps
around the back of the engine to join with the left side exhaust. This neatly
leaves room on the right side for the intercooler. Both exhaust streams "wye"
apart again, one pipe to the wastegate, one to the turbo. In some systems, the
exhaust streams will remain separate, each leaving through its own exhaust pipe,
in others it will "wye" back together again, to leave through one
large exhaust pipe. Still other systems employ two small turbochargers instead
of one big one, each driven by the exhaust from one bank of cylinders.
This mysterious term is simply a nickname for that part of the induction
system that is between the turbocharger's compressor and the throttle plate.
(The portion of the induction system downstream of the throttle is cleverly
known as the "lower deck.")
The pressure in the upper deck area is usually regulated by the controller,
and can be quite independent of the manifold pressure in the lower-deck portion
of the induction manifold. In the TAT system, upper-deck pressure will always be
regulated to about 33 inches, provided there is enough energy from the exhaust
to drive the turbine. Because the intercooler creates a little resistance to the
flow of air, the air pressure after the intercooler will be as much as two
inches less than the compressor discharge pressure.
It is very important that you mentally separate the lower deck (downstream
from the throttle) from the upper deck (upstream from the throttle). Completely
different things take place in these two areas of the induction system at
anything less than full throttle.
Here is a rough schematic of the turbocharger itself, and a photo of a
typical turbo:
As you might suspect from the picture on the right, the actual air passages
and air flows are very curved and smooth, and the tolerances very tight in this
most critical component, which is capable of turning in excess of 100,000 RPM.
The blades in each part of this unit are shaped differently, with the turbine
looking something like an old-fashioned waterwheel. The hot exhaust gas strikes
those blades, and drives the turbine. On the other hand, ram air enters at the
center of the compressor, and is flung outwards by centrifugal force.
The oil system associated with the turbo system is a very simple lubrication
and control system.
The bearings in the center section of the turbocharger are quite critical,
and not only need an uninterrupted supply of full engine oil pressure for
lubrication and cooling, but a scavenge pump is also needed to literally suck
the oil out ("negative pressure" for the purists) as any back pressure
would reduce the efficiency of the oil flow, and might also allow oil to leak
through the bearings to either the compressor or the turbine. This scavenge pump
is a very important component, a significant part of the turbo conversion. It is
cleverly gear-driven by the engine off the starter gearing in the TAT
turbonormalizer. In some of the Lycoming systems, it's a separate gear-driven
pump mounted on a spare pad in the accessory section. In most TCM
factory-turbocharged engines, the scavenge pump is piggy-backed onto the
engine's regular oil pump and shares a common drive shaft.
Sometimes you will see a turbo system emit a small puff of oil smoke from the
exhaust on starting, and this is probably where it comes from. Any oil left in
the turbo at shutdown may pool and leak slowly past the seals into either side
of the turbo unit, to be forcibly ejected at the next engine start.
It is important for the engine oil to be properly warmed prior to pulling
significant power from any engine, and even more important for the turbo
bearings in a turbo'd engine. I've always been just a little uncomfortable with
Lycoming's advice to take off as soon as the engine will accept full throttle
without faltering, or "as soon as the oil temperature needle starts
moving." I sure didn't push things quite that much in my engine before the
turbo installation, and I sure won't now, thank you very much. But that's a
personal opinion.
The intercooler is the simplest device in the system, with no moving parts:
It is simply a radiator (or heat exchanger) designed to transfer heat from
one airstream to another. The two streams are kept separate from each other, but
pass inside and outside a common system of fins and tubes. Hot compressor
discharge air, heated by compression, passes through this device, and is cooled
a bit. Ambient ram air comes in, passes through the intercooler, picks up some
of the heat from the induction air, and is promptly dumped overboard.
The biggest differences you'll see between one turbo system and another is
the controller. There are a bewildering variety of them, each designed to
control the upper deck pressure in the way the designer felt was best.
A few installations have no controller at all, and the wastegate is either
fixed at some setting to serve the purpose, or is manually controlled by the
pilot (using an additional throttle-like cockpit control, or occasionally a
complex linkage to the main throttle control). As usual in aviation, all are
compromises in some way, and all have their advantages and disadvantages. But a
turbo system without an automatic wastegate controller adds greatly to pilot
workload, and seems like a really bad idea to me.
TAT has wisely chosen to use the very simple "Absolute Pressure
Controller" (APC), which strives to maintain a constant manifold pressure
of about 31 inches. In order to do this, the controller is set to maintain an
upper-deck pressure of about 33 inches to overcome the pressure drop across the
intercooler and throttle body. The controller "setpoint" is adjusted
by simply changing the mechanical compression on the spring load in the
controller with a screwdriver. The APC does an excellent job when the throttle
is wide open, but less so at partial throttle because the turbo may still be
working hard to produce an upper deck pressure that is not needed. For example,
if you set 15 inches MP for descent, the APC still commands the turbo to produce
33 inches in the upper deck. Many of us believe that full throttle operation is
the only way to fly this setup anyway (at least until it comes time to land), so
this is not a problem. On the other hand, the turbonormalized engine is not the
best choice for a pressurized aircraft, as the higher the upper deck pressure,
the more pressure available to the cabin, which will maintain a lower cabin
altitude.
The Variable Absolute Pressure Controller (VAPC) is very similar to the APC,
but instead of a screwdriver setting (or in addition to it) there is a
mechanical linkage to the throttle. If you reduce the throttle to some low MP
setting, the linkage will back off on the setpoint, effectively telling the
turbo to quit working so hard when it's not needed. This type will often be
found on pressurized aircraft with turbos. These engines typically run a much
higher MP, so "giving the turbo a break" is more important.
There are other variations, such as a "slope controllers" and
"pressure ratio controllers" (PRC) that are used in some installations
to vary the upper deck setpoint with altitude in some fancier way. For instance,
Mike Busch writes:
My T310R uses a combination of APC and PRC. Up to critical altitude
(16,000 feet MSL in my airplane), the APC maintains constant upper-deck
pressure (UDP) of about 34 inches. Above 16,000 feet MSL, the PRC takes over
and reduces UDP as the airplane climbs to maintain a constant 2.2-to-1 ratio
of UDP to ambient pressure. The purpose of the PRC in this installation is to
prevent turbocharger overspeed at altitudes above critical altitude by
limiting the compressor discharge-to-inlet pressure ratio to the value it has
at critical altitude.
The TAT turbonormalizer system doesn’t have this limitation. The turbo
speed was checked during the STC altitude flight testing, and will not overspeed
under any circumstances.
Whichever controller variation is employed, the basic construction is
essentially the same.The wastegate controller consists of a spring-loaded poppet
valve, similar to those in traditional hydraulic control valve systems. It has a
vacuum-sealed reference aneroid (chamber) hooked to a spring with adjustable
tension. Air pressure from the upper deck works against the aneroid and the
spring, which moves the hydraulic poppet valve, porting engine oil as needed to
control the oil PRESSURE to the wastegate actuator.
That oil pressure, in turn, moves a small hydraulic piston in the wastegate
actuator over a distance of about two inches, and this movement controls the position
of the wastegate door, which diverts the flow of exhaust gas to the turbo or
away from it, driving the turbo faster or slower as needed to raise or lower the
upper-deck pressure.
If all this sounds a bit Rube Goldbergish to you, you're not alone. But there
are good reasons for all this monkey motion, and the system works remarkably
well.
Notice the engine oil pressure first flows through the wastegate actuator,
then on to the controller. Seems backwards, somehow, right? In this case, the
controller acts as a dam, creating or relieving backpressure in the line. In
case you're wondering, it doesn't affect the main engine oil pressure very much,
because these control lines are much smaller, and the tiny bit of
"leakage" used for control will not affect the main engine oil system.
Just in case the control system malfunctions, there is an additional device
to provide protection from overboost.
This is a simple mechanical relief valve, totally stand-alone, not dependent
on any other system. It's just a tin can with a spring-loaded valve inside, with
an adjustment to pop open if the actual upper-deck pressure rises to some limit,
generally several inches above the setpoint at the controller. Normally, this
"popoff" valve remains closed at all times, only opening (as shown in
the inset) for overpressure. To my knowledge, all turbo'd engines have this
safety feature to protect from a gross overpressure due to failure of the
control system.
Look at this photo of a turbocharged engine:
Note the air line from the upper deck to the injectors. Why send air to fuel
injectors?
It turns out that just injecting a stream of liquid fuel into the intake port
is not a very good way to do it, because there isn't time for the fuel to
atomize. On a non-turbo'd engine, each injector will have tiny holes to the
ambient air at the point where the fuel is moving the fastest. There are tiny
screens to keep trash out. The high-speed fuel creates a suction, which draws in
ambient air. That is mixed with the fuel right in the injector and the intake
port, producing a mist of fuel going into the combustion chamber, for better
combustion.
On the turbo'd engines, supplying ambient air to the injectors doesn’t work
very well, because ambient pressure can be so much lower than manifold pressure.
(If you tried using normally-aspirated injectors on a turbocharged engine, fuel
would squirt out the injector air holes instead of air being sucked in.)
Instead, we steal the pressurized air from the upper deck (which is guaranteed
to be greater than manifold pressure), and pipe it to the injectors. If you
examine any injected, turbocharged engine, you will see a fairly extensive
system of "upper-deck reference lines" for this.
Now that we've looked at the individual parts of the turbo system, I'd like
to run through the various phases of flight from the standpoint of what is
happening in the system. This is NOT intended as an operating manual! I'll get
into TECHNIQUE in the next column. This is simply a review of what all the parts
are doing during some sort of "normal operation."
With the engine at rest, the spring in the CONTROLLER drives its poppet valve
closed trying to increase the oil pressure in the actuator. Of course, with no
oil pressure at all, nothing happens. Since there is no oil pressure yet, the
spring in the wastegate ACTUATOR drives the wastegate door open in our drawing.
I know, who cares, right? Hey, we gotta start somewhere!
Now, the moment we start the engine, and run it at low RPM (see sidebar
please) the oil pressure builds up in the engine oil system and in the turbo
bearings, then against the wastegate actuator, driving it fully closed. Why
fully closed? Look at the diagram of the entire system again. The controller's
poppet valve is still closed, because the upper-deck pressure is not yet high
enough to open it. The controller always WANTS a full 33 inches of pressure in
the upper deck at all times, and will stubbornly remain closed (trying to raise
the control oil pressure) until it gets 33 inches. That closed control valve
traps the engine oil pressure in the control line, forcing it to rise to the
full engine oil pressure, which drives the wastegate actuator closed against the
spring. When the wastegate closes, it backs up the exhaust gas in the pipe where
the wastegate is located, which diverts all the exhaust through the turbine.
That spins the turbocharger up, but since there is so little exhaust gas at idle
RPM, it's probably not fast enough to produce full pressure in the "upper
deck." On the other hand, some systems may do so, for only about 3 inches
additional pressure is needed. Remember, the turbo is sucking in ambient air at
29 or 30 inches (at sea level), and only has to work a little.
Now, if you've read the column on MP, you'll understand that at idle, the
pressure downstream of the throttle (on the engine side) is very low (high
suction), because the throttle plate at idle is fully closed, and the pistons
are desperately trying to suck air past it. The air in the "upper
deck" at idle is probably pretty close to ambient, plus or minus whatever
effect the compressor is having.
To beat on this horse a little more, it may be helpful to remember that we
are dealing with four different air pressure areas, and three different
temperatures in the TAT turbo system.
First, and most obvious, ambient pressure and temperature (OAT).
Second, a slightly different pressure occurs after the air filter, before the
compressor. This pressure is usually less than ambient, because the entire
system is "sucking" air in, and the filter is a slight impediment to
the free flow of air. At high indicated airspeed, the "ram effect"
will overcome this to some degree, and on very well designed systems, ram air
will more than compensate for the filter loss. There may be a tiny difference in
air temperature due to the pressure change, but this can be overlooked.
Third, the "upper deck pressure" and "compressor discharge
temperature" (CDT) occurs after the compressor, but before the throttle
plate. This is the air pressure regulated by the controller. This hot upper deck
air is cooled by the intercooler, giving us our third temperature,
"Induction Air Temperature" (IAT).
Finally, there is the manifold pressure the pilot sees on the MP indicator.
This is the pressure after the throttle plate but before the intake valves
(i.e., the "lower deck"). On those aircraft equipped with Carburetor
Air Temperature (CAT) gauges, it will be measured here.
At some point, as we increase the throttle setting (allowing the engine to
suck more air in, develop greater power (increased RPM), increase exhaust flow,
drive the turbine faster) the air pressure in the upper deck will hit the
setting that triggers the wastegate controller to say "okay, that's
enough." That upper-deck pressure will drive the poppet valve open just
enough to let a little oil trickle out of the control line, which drops the oil
pressure at the wastegate actuator. That allows the wastegate to open a bit,
allowing some exhaust gas to escape through the other pipe (bypassing the
turbo), thereby depriving the turbo of some of the driving force. The turbine
(and the compressor) slow a bit, and the upper deck pressure drops back a tiny
bit. In stable conditions, the system will quickly stabilize at a point where
just the right amount of oil is bleeding by the controller, just enough exhaust
is driving the turbo, and just enough upper deck pressure is maintained.
With any partial throttle setting, the manifold pressure in the engine
(downstream of the throttle) will always be lower than the upper deck pressure.
Even with a fully open throttle, it will be very slightly lower, due to the
resistance of the intercooler and a tiny bit of blockage by the throttle plate
thickness. The manifold pressure you read in the cockpit is, of course, sensed
downstream from the throttle. You cannot measure the upper deck pressure without
special test instruments. However, at full throttle, you can assume it's about 2
to 3 inches above the manifold pressure you see.
One of the many nice things about a turbo is that the spring tension
adjustment in the wastegate controller can be tweaked a little, just enough to
bring the MP up enough to make up for the loss of pressure due to the
intercooler, the filter, and the kinks and bends in the system, and return the
engine to the full power the manufacturer intended and specified. For this
reason, a properly set controller and proper use of the mixture control means
that you take little or no "hit" in the horsepower department at sea
level, contrary to popular belief.
Okay, runup complete, we take the runway, and open the throttle for takeoff.
We don't worry about overboosting with the turbonormalizer, because the system
will maintain about 33 inches of pressure in the upper deck, and the manifold
pressure will be about two inches less due to the intercooler. Under no
circumstances can the manifold pressure exceed the upper deck pressure, of
course.
Let's talk about this act of opening the throttle. Many pilots will do that
very gingerly, afraid of damaging the turbo, or the engine. While I don't like
"yanking" any control around in an airplane, this can be carried too
far, in my opinion. While some may say, "well, it doesn't hurt," that
may not be true. The longer you sit there at high power with no cooling airflow,
the hotter that engine is getting. I have no beef with taking the engine up to
some fairly decent power setting, doing a final quick check of the engine
instruments, then getting the rest of the power applied in a fairly expeditious
fashion. Some will hold the brakes during this "engine check," but I'd
just as soon let the airplane start its roll while this is going on, as that
will attain a cooling airflow sooner (and get you out of the way of the next
airplane). Bluntly, once you are comfortable in any airplane, if you're not
capable of controlling the airplane during the early takeoff roll while doing a
quick check of the engine instruments, you probably ought to find another line
of work, or another hobby. I'm not referring to beginners here, or to those
flying a strange airplane for the first time, or other special conditions like a
short runway.
But let's get back to what the turbo system is doing. On the turbonormalized
engine, as you bring the power up, remember that wastegate is initially closed,
as the controller wants about 33 inches. That is BARELY above ambient, at sea
level, IN THE UPPER DECK. As the turbo speeds up, it will provide that 33 inches
of upper deck pressure very early, well before you see significant MP on the
cockpit MP gauge. That means that the moment you hit full pressure in the upper
deck, the system will not allow the turbo to run any faster, no matter how much
throttle you give it. As you further open the throttle, the wastegate will open
up to keep that constant 33 inches. It is important to realize that the throttle
is NOT directly controlling the turbo (unless, of course, your system uses a
VAPC controller with its throttle linkage).
There may be a slight lead and lag in the system as the various devices move
in response to each other, but for all practical purposes, the wastegate simply
moves to keep the same upper deck pressure.
In summary, as you apply takeoff power with the throttle, the following
sequence of events occurs:
- The throttle plate moves away from nearly closed, allowing more air to get
sucked into the cylinders by the pumping action of the pistons,
- The fuel control unit responds by pouring more fuel into the
system, producing more and more power,
- The additional exhaust (wastegate still closed, remember) spins the turbo,
which spins the compressor, which pumps up the pressure in the upper deck.
- As soon as the pressure in the upper deck hits the preset value (just
barely above sea level pressure if we're talking about a turbonormalized
engine), the controller starts letting oil pass, dropping the oil pressure
in the actuator line,
- With the reduced oil pressure at the actuator, the spring in the actuator
drives it towards the open position, dumping more exhaust overboard,
dropping the exhaust pressure in the pipe to the turbo, maintaining the same
turbo speed (roughly).
Now, let us suppose we don't touch a thing after takeoff. Just let the engine
run "wide open," throttle, prop, and mixture all the way in.
Think for a moment, and see if you can predict what all those little thingies
in that engine compartment are doing as you climb.
First and foremost, of course, ambient pressure slowly drops with altitude.
At 5,000 feet MSL, ambient pressure will be about 24 inches, at 10,000 feet,
about 19 inches, and at 18,000 feet, somewhere around 15 inches (the classic
"one inch per thousand feet" is only approximately correct below
10,000 feet, and it's less than that above 10,000).
But that upper deck controller still wants 33 inches! What does it do? As the
ambient pressure drops, the engine wants to make less power, and less exhaust,
which would, without intervention, reduce the upper deck pressure. But our
little controller senses that drop, blocks the flow of engine oil through it,
which raises the oil pressure in the oil line to the actuator. That drives the
wastegate a bit more closed, which forces more exhaust through the turbo,
speeding it up, bringing our upper deck pressure back up to 33 inches. Since the
throttle is still wide open and there is no barrier (except the intercooler)
between the upper deck and the intake manifold, the actual MP reading will show
about 31 inches, and will stay there to the critical altitude, somewhere above
20,000 feet msl.
At the higher altitudes, the turbo must work harder to bring that low ambient
pressure up to the full desired upper-deck pressure. To work harder, it must
take more and more of the exhaust gasses, and guess what that does to the
temperature of the exhaust gas driving the turbine? Yup, you got it, it gets
hotter as you climb. How hot? Well, the turbonormalizer TIT (Turbine Inlet
Temperature) may show around 1,200F or less during a sea level full rich
takeoff. In the flight levels, at very high power, with the mixture leaned to
peak TIT, it may approach or even exceed the maximum for the turbo (1,650F on my
system).
In general, for any given horsepower, you'll see much higher TITs (100 to 150
F, same HP) with the TSIO and TIO engines (7.5:1 compression ratios) than you
will with the turbonormalized engine (8.5:1). The reason for the lower exhaust
temperatures is that the higher compression ratio allows the exhaust gases to
expand more during the expansion stroke. This expansion will cool the exhaust
flow that leaves the combustion chamber.
In some of the TIO/TSIO engines, the turbocharger's turbine rotor is made of
a different alloy (Inconel), and this allows greater operating temperatures, up
to 1,750 F.
We said before that each system has its advantages and disadvantages, and in
the operating temperature department, the turbonormalized system has the clear
advantage. Only when there is an overriding need for higher manifold pressures
will the TIO/TSIO engines be "better," as in the need for cabin
pressurization or greater power.
The TIO/TSIO engines suffer here, because the higher temperatures over the
long term affect the integrity of the steel used in the turbo and in the exhaust
plumbing. At and above about 1,600F or 1,650F for very long periods of exposure,
these exhaust components may start to turn brittle, leading to early replacement
and higher costs. Not a good thing.
A word about the "altitude-compensating" fuel pumps. To my
knowledge, they are not used on any of the normally-aspirated engines except the
IO-550 series. This pump has a pressure-sensing device (an aneroid), which
measures ambient pressure and reduces the fuel output as altitude increases. In
theory, it is supposed to produce a full rich mixture from sea level to about
3,000 feet, and above that it is supposed to lean the mixture by about one gallon
per hour per thousand feet of altitude. It's fine in theory, this produces a
"better" mixture at the higher altitudes, and when properly set up,
little or no manual leaning is required for high altitude takeoffs. Somehow,
"it don't hardly seem natchural" to me, but I suppose I'm a bit of
dinosaur on this one. Many of them are not properly set up, so a bit of
experimentation during a normal full-power climb is in order, with adjustments
at the fuel pump by your mechanic as needed until it is right.
This pressure-sensing pump is used almost universally on the turbo'd engines.
However, since the turbo'd engine maintains sea level power to higher altitudes,
the air reference for the fuel pump is tied into our old friend, the upper deck,
giving a more accurate mixture setting.
In either case, the pump fuel pressure output responds to changes in the
reference pressure, decreasing fuel flow as the reference pressure drops.
The actual results from this aneroid-controlled fuel pump are a little crude.
It is not capable of precise control, so depending on just how well it is set
up, you may need to occasionally tweak the fuel flow a bit with the mixture
control, to maintain the desired temperatures (TIT and CHT). We'll get into that
more next time.
At cruise power, the upper-deck pressure should still be about 33 inches, and
the MP indicated in the cockpit will be whatever you set with the throttle.
There are exceptions to this, however. For example, if you're high enough, and
you pull the RPM or the mixture back too far, you'll reduce the mass airflow
from the combustion process, and there may not be enough to drive the turbo,
even with the wastegate fully closed. Even at full throttle, you'll see the MP
drop, along with the upper deck pressure. (This is known as
"bootstrapping.") In this case, you'll have to put another log on the
fire (enrich the mixture), or run the RPM up a bit, until you get the desired MP
again.
There are persistent rumors of a number of improvements in store. Chief among
them is an electronic ignition system that is far superior to anything in any
engine (car or aircraft) today. It will look directly at each combustion event,
and modify the spark timing as needed for the succeeding event. This will
produce the ideal pressure event for best power, regardless of power settings,
RPM, ambient pressure/temperature, or fuel octane. There is every reason to
believe this system will allow all our aircraft engines to run at full rated
power on 94- to 98-octane fuel, with NO lead. That will solve the aviation fuel
crisis overnight.
Another nice thing to have would be an electronic turbo controller. This
could be programmed for optimum operation in all conditions. There have been
several attempts at these, but the certification issues are a nightmare. Unison
is rumored to be working on one. Atlantic Aero had a prototype at AOPA a couple
of years ago. Apparently TCM is also working on one of these. GAMI has a
prototype electronic wastegate controller built and being tested. The details
are not public for any of these systems, but there should be some real benefits,
if the FAA doesn’t get in the way too much.
In my next column, I hope to get into real operations, and some new and more
scientific ways of operating these engines. Simpler ways, too!
Be careful up there!
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