New-age magneto replacements, coming to an engine near you
Turbocharging invokes emotional discussion seemingly out of proportion to its stature in life, right up there with the division of church and state and the veracity of late-night talk show hosts. On one side, many pilots seem to feel that unnatural aspiration does nothing more than shorten the life of an engine, consume mass quantities of fuel, and deplete bank accounts with reckless abandon. There are those, contrarily, who couldn't imagine flying a light airplane without the benefits of turbocharging, costs and complexity be damned. They cite the ability to fly over weather, depart from mountain airports without a bead of sweat appearing at the brow, and cruise at higher true airspeeds once at altitude.
Pilots in the first camp point to the high costs of turbocharging as a main shortcoming. To a point, this is more than sheer sophistry. Turbocharged engines work harder for more of their overall life than do normally aspirated engines. They burn more fuel, despite the often destructive directives of airframe manufacturers with regard to powerplant management — aggressive leaning in particular. And by their having more parts, they will cost more to maintain and overhaul than a simpler powerplant. They have lower recommended times between overhaul, and, as a rule, they are less likely to make that number than are similar non-turbo models. Owners can mitigate those costs with careful operation and scrupulous maintenance.
Heated debates notwithstanding, turbocharging remains a simple beast. If you remember that an internal combustion engine is basically an air pump, it's easy to understand how power normally tails off with altitude. The less-dense air at altitude supports less fuel for combustion, and so the engine will be capable of less total power output. Turbocharging seeks to reverse the effects of thin air by pumping up the intake tract to pressures often far in excess of ambient. A good turbo system can maintain full power to 14,000 feet or more, and 75-percent power well into the mid-20s.
All turbo systems share some basic components — the turbocharger itself, of course, as well as the necessary plumbing to connect the exhaust system to the turbine side of the unit, and similar tubing to route compressor output to the induction system. Turbos also require a supply of lubrication, taken from the engine's main oiling system.
Turbo systems need a variety of safety valves, some that are controlled by the pilot and others that are automatic. Certified systems contain pop-off valves that prevent catastrophic over-boosting in the event of hardware failure or pilot brain-fade. They work by passing induction air out of the otherwise sealed system when the boost exceeds a preset level. Other methods of controlling manifold-pressure involve what's called a wastegate. It routes exhaust gas past the turbine section of the turbo, reducing its speed. Here are some of the most common types:
Manual wastegate. Among the simplest, the manual wastegate allocates boost authority directly to the pilot via a cockpit control. Closing the wastegate gradually directs more exhaust toward the turbo and results in increased manifold pressure; these systems operate just like normally aspirated models up to some intermediate density altitude, say, 5,000 feet. Simple and often inexpensive, these systems nonetheless can be abused; forget to open the wastegate during a go-around, and you'll destroy an engine in short order.
Fixed wastegate. This style of system is found on the Piper Turbo Arrow and Seneca, and Mooney 231. The ground-adjustable wastegate is set to obtain a balance between high-altitude boost production and excessive turbine speeds at low altitude. Usually, this compromise still leaves the system working too hard down low and offering limited boost up high. Boost control comes via the throttle; there are no other cockpit controls.
The fixed-wastegate system is jokingly said to be in "bootstrap" mode all the time. (In variable wastegate setups, when the turbo has reached its maximum output and the wastegate is fully closed, the system is said to be bootstrapping. Any change in airspeed, fuel flow, altitude, or temperature will alter manifold pressure.)
Throttle-linked wastegate. Combine a manual wastegate with a throttle interconnect, and you'll have setups like those found on the turbocharged Cessna Skylanes, Commander 112TCs, and Piper Turbo Saratogas. In this arrangement, advancing the throttle first opens the carburetor or fuel-injection butterfly and then gradually closes the wastegate to maintain a desired manifold pressure. In the Cessna installation, the carb is rigged to open fully before the wastegate begins to close, while in the others there is some overlap of functions. Of the simpler systems, these offer the best mix of reliability and efficiency.
Automatic wastegate. Although complicated, these are the best systems. And where you'll find different subspecies, all seek to do two things — limit full-throttle boost to the certified maximum, and maintain a certain set manifold pressure at other power settings.
Cessna frequently uses a device called an absolute pressure controller (APC), which manipulates the wastegate to maintain a set turbo output. Turbo Centurions and some twins use this arrangement. The wastegate is moved so that the turbo is always putting out a set amount of boost, usually a few inches of manifold pressure above redline. Throttle position controls the boost fed to the engine.
A variation on this system is the variable absolute pressure controller (VAPC); it utilizes a throttle interconnect to trim this normally fixed upper-deck value. This allows the turbo to loaf a bit during low-power operation.
Finally, there are the differential-pressure systems, frequently used in high-end piston aircraft and pressurized models. Two different wastegate controllers manage the power. One does nothing more than limit maximum boost during full-throttle operation. The other seeks to maintain a given differential in manifold pressure between the turbo output and the induction system downstream of the throttle. Differential systems offer the best of both worlds in that their operation is transparent to the pilot, plus they allow the turbocharger to work only as hard as necessary for the required boost.
So much for the hardware — you can find diagrams in your airplane's operating handbook, as well as in the engine operating handbook. But keeping a turbo system happy often requires going beyond the often cursory explanations offered in the POH.
Setting power. For many pilots, the most difficult exercise in the transition to turbocharged aircraft is setting power. Thanks to the turbo there are far more combinations of manifold pressure and rpm for any given percentage of power. There are relatively few hard-and-fast settings that work all the time, every time.
Experiment. Using the engine-maker's power charts (don't always believe that the airframe power charts are the best), try different combinations. Don't be afraid of running well "oversquare," either. (That is, with the manifold pressure in inches greater than the rpm in hundreds.) Turbo powerplants almost always have lower compression ratios than similar nonturbo installations, so the increased manifold pressure is necessary to replace the power lost through the less efficient combustion. You won't be doing the engine any favors by running a low manifold- pressure/high-rpm setting. You'll likely find a boost/rpm combination that results in the least amount of noise and vibration, while still providing the desired amount of power for the altitude — use it. And don't be surprised to find that different combinations will work best for different altitudes.
Consult your power charts for the correct temperature variables — most turbo systems call for juggling manifold pressure based on differences from standard-day conditions. Also notice if the manufacturer offers a skew factor for using best-power or best-economy settings; you'll usually have to add a bit of boost if you plan to lean aggressively.
Boost production at altitude is determined by many things, including air flow through the engine, turbo size, wastegate type, and induction-system efficiency. You may find at high altitude that the turbo system cannot maintain boost for, say, 65-percent power at less than 2,300 rpm; then use whatever is necessary to get the power you want.
There are some compelling reasons to use lower rpm settings, especially at low altitude and for fixed-wastegate systems. Lower engine speed means there are fewer exhaust pulses over time, which result in slower turbine speeds. At low altitude, where the turbo does not need to compress the ambient air as much is as required up high, a low-rpm setting will allow the turbo to spin more slowly; this releases less heat into the induction stream and relieves stress on virtually every component of the system.
Finally, consider using a more sedate power setting. Some turbo installations list maximum cruise at or above 75-percent power. If you look carefully at a flight profile, you'll find that the extra fuel burned offers relatively little payback as shorter trips. These settings do, however, stress the engine — especially at high altitude, where the thin air is less effective at wicking heat out the engine.
Managing fuel. Turbocharged engines have the capability to turn themselves into molten metal if mismanaged. Fuel is an important part of power management. It helps to know the rough fuel flow for a variety of power settings, especially takeoff and cruise-climb. Most turbo installations prohibit leaning for takeoff or climb settings in excess of 75-percent power. That's easy, but you need to know what the engine requires to remain cool during these high-stress times; the maintenance manuals will spell out minimum and maximum fuel flow for various power settings, and these should be adhered to. Also, you should ascertain the accuracy of the fuel-flow gauge in your airplane; the purely pressure- operated instruments are subject to inaccuracies stemming from clogged fuel injection nozzles.
A good rule of thumb says to keep the mixture full rich for takeoff and climb — but don't forget to lean on the ground, since most turbo systems are set up quite rich and the engine is essentially unturbocharged during taxi. If there's any doubt about the power setting (and with practice there seldom will be), err on the side of caution. Fuel flow is critical to cooling and the prevention of detonation in turbo engines, and it's widely regarded as cheap insurance.
When you can set power at a value below the manufacturer's threshold for leaning, by all means do so. Some installations can be leaned to peak exhaust-gas temperature (EGT) or turbine-inlet temperature (TIT) at 75-percent power or less; others set that limit at 65 percent and permit leaning only to best-power settings between 65 percent and 75 percent.
The trick to leaning accurately in a turbocharged engine is patience. Once leveled off in cruise, the engine may take several minutes to settle down temperature-wise. Multiprobe EGT and cylinder-head temperature instruments ought to be considered mandatory for a turbo installation.
When you begin to lean, pick a benchmark fuel flow, such as the predicted best-power value. Let the engine run that way for at least five minutes. (This is an arbitrary number, granted, but it seems to work quite well in a variety of installations.) Then gradually reduce fuel flow until the first cylinder reaches peak EGT. In fuel-injected systems, this will usually happen well in advance of the peak TIT.
Engines with good fuel distribution will typically run quite well at the first cylinder's peak EGT, with the other jugs running in much the same relationship to peak. Some of the carbureted turbo systems, however, have miserable mixture distribution, so don't be surprised to see one cylinder peaking well before the next, and well ahead of peak TIT. For the life of that cylinder, it's best to stop leaning there, or even enrich slightly to prevent lean misfire. (Such intercylinder discrepancies are why a multiprobe EGT gauge is invaluable. Leaning by a single-point probe or TIT instrument means that you could be running one or more cylinders lean of peak. Few engines tolerate this for the duration of the TBO.)
So what's best — best economy or best power mixture? Depends. At best economy, the exhaust system will see the greatest temperatures, as will the turbo; depending upon the peak temperature and the quality of the components, this may well be a comfortable setting. For the smaller, less heat-tolerant turbos (like the Rajays in the Piper Turbo Arrow and Seneca, and the Mooney 231) these temperatures can get too close to the design limit of the turbo.
Experience has shown that moderate TITs will prolong turbo and exhaust-system life. This would seem to indicate that leaning to best- power would be ideal — but hold on. It stands to reason that at a setting producing the most horsepower there will be the most heat being shed by the cylinders; you'll usually see CHT peak at the best-power setting.
So that leaves a compromise, the setting used successfully by most turbo drivers — leaning to 50- or 75-degrees rich of peak on the first cylinder to peak. Generally, this offers the best combination of EGT/TIT and CHT, as well as moderate fuel flow.
A steady hand. Beyond minding the numbers, pilots of turbocharged airplanes ought to be aware that gentle power changes, gradual heating and cooling-off periods, and general tenderness will go a long way toward improving the life of a turbo installation. Shock cooling — still called a red herring by many knowledgeable engine experts — can be avoided by making small power reductions at the top of the descent, and by managing the mixture to keep TIT and CHT as steady as possible.
These are, of course, only the highlights of advanced turbo management — but they are the important ones. Try new power settings, know the accuracy of your instruments, and understand how much fuel the engine needs for any power setting. These guidelines will help point the way for a longer-lived and more reliable turbo airplane, and may also quell the haranguing from the anti-turbo set.
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