Engine Rating and Altitude
When we think about engine power, we typically think in terms of horsepower. When we say that an aircraft engine has 300 horsepower, that gives us an idea of how quickly the aircraft can lift off, how efficiently it can climb, how fast it can fly, etc. However, an engine’s horsepower rating is measured by its performance at a set of agreed upon conditions known as ‘standard day.’
The conditions of standard day are recognized to be a barometric pressure of 1013.2 millibars of mercury, a PSI of 14.7 lbs/square inch, and an air temperature of 15 degrees centigrade at sea level. These are considered to be typical atmospheric conditions. To say that an engine is rated at 300 horsepower is to say that it will perform at 300 horsepower under these conditions; it does not necessarily account for deviations from standard day.
When flying at high altitude, those deviations can be extreme. At 18,000 feet, the air density is about half what it is at sea level. That means that a normally aspirated engine (an engine that simply takes in uncompressed air from the atmosphere) will perform at roughly 50% of its rating. The benefit of a turbocharger is that it can replicate sea-level air pressure at much higher altitudes, and likewise allow an engine to perform as if it were operating under much more preferable conditions.
How a Turbocharger Works
Most aircraft piston engines are manufactured by Lycoming or Continental Motors. These engines consume gases that are drawn into the engine by the downward stroke of the piston (producing a low-pressure area). The amount of air consumed, compared to the theoretical volume of air consumed if the engine could sustain atmospheric pressure, is called volumetric efficiency. The main function of a turbocharger is to increase the engine’s volumetric efficiency; this may be accomplished by increasing the intake air density.
The compressor of a turbocharger draws in ambient air. The air is then compressed before entering the intake manifold at a greater pressure. This results in a greater mass of air entering the cylinders with each intake stroke. The kinetic energy derived from the engine’s exhaust gases then spins the centrifugal compressor
A turbocharger utilizes an array of controllers to regulate air flow. This system has become very complex and evolved considerably over the last 100 years. Modern turbochargers may use a combination of the following: variable absolute pressure controllers (VAPC), absolute pressure controllers (APC), density controllers, differential pressure controllers, sloped controllers, rate controllers, pressure ratio controllers, wastegates, and pressure relief valves.
Breakdown of a Turbocharger
Turbocharged induction system mechanisms are similar to a normally aspirated system but with the addition of a turbocharger and turbocharger controllers. The location of the turbocharger itself is between the air intake and the fuel metering unit. A typical turbocharger consists of a solitary rotating shaft with a centrifugal compressor impeller mounted on one side and a small radial turbine fixed to the other side. Both the impeller (cold section) and turbine (hot section) are individual housings joined by a common bearing housing which contains two aluminum bearings that support the center shaft. In this configuration, the exhaust gas spins the turbine which translates the centripetal energy through the common shaft to the impeller. The impeller then draws in air and compresses it.
On a standard single engine aircraft the air comes in through an air intake located below the propeller. From there air is ducted to the turbocharger at the back of the engine. The turbocharger compresses the intake air and sends the newly compressed air to the air metering section of the fuel metering device. Once the air is metered it is ducted to the intake manifold through the cylinder intake valves where the air is then mixed with a metered amount of fuel.
In addition to the friction produced by high gyratory speeds, the exhaust gases ducted through the turbine heat the turbocharger. The turbine inlet temperature may get as high as 1,600⁰F, because of this there needs to be a large flow of oil keeping the bearings within a safe operating temperature. Therefore, a constant flow of engine oil, approximately four to five gallons of oil per minute, must be pumped through the bearing housing to cool and lubricate the bearings.
Wastegates function to regulate the power output of the turbocharger system. By controlling exhaust airflow, aircraft wastegates manage the turbine speed, and therefore the compressor housing intake. A series of aircraft controllers function as the brain behind the wastegate. At varying speeds, altitude, engine power, and air pressure, a variety of controllers keep the turbocharger system at equilibrium.
Preventative Maintenance on a Turbocharger
Maintenance on a turbocharger is critical to obtain a long and hassle free service life. A turbocharger must regularly withstand extreme operating conditions –with exhaust inlet temperatures exceeding 1600° F and the turbine wheel rotating at over 90,000 RPM. Turbine and compression wheel blades must be carefully inspected for any cracks or damage caused by foreign object debris. It is also important to turn the wheels by hand and inspect for any drag or rubbing against the housing. One of the most important processes in the turbocharger is the lubrication system, which is provided by the aircraft engine oil. Oil contamination, foreign object debris, and oil supply problems are the most frequent reasons for premature turbocharger failure and can lead to overheating of the bearings and the center housing.
Aircraft Turbo System FAQs
Below, you will find a list of common questions asked by pilots, mechanics, and enthusiasts about aircraft turbochargers and turbo systems.
What's the difference between a turbocharger and a supercharger?
Superchargers and turbochargers both do the same job—that is, provide consistent air pressure to the engine, but the work somewhat differently. Whereas a turbo system is powered by the engine’s exhaust gases, a supercharger is powered by the engine itself, usually with a belt or chain connected to the crankshaft.
Each system has its pros and cons. Because a supercharger is driven by the engine, it tends to take power away from the engine, thus reducing performance and fuel efficiency. Turbochargers, which operate independent of the engine, are more efficient in this regard. Moreover, because they feature a simpler design, turbochargers tend to be lighter than superchargers.
On the other hand, superchargers provide much more benefit under idle and low-speed conditions, and turbochargers sometimes experience power surge, whereby air pressure builds up in the intake manifold and cause a brief reversal of air flow, resulting in vibration. Most modern turbochargers are designed to mitigate this phenomenon.
Can you repair Turbo systems or are they overhaul only?
Turbochargers are overhaul only, but Quality Aircraft Accessories can overhaul and repair all wastegates, pressure relief valves, and controllers associated with your turbo system.
What is the turnaround time on Turbos/Wastegates/Controllers?
Quality Aircraft Accessories routinely repairs and overhauls all turbo system components in 3-5 days, but upon customer request Quality Aircraft Accessories can accomplish these tasks next day.
My turbocharger is leaking oil. Does it need to be replaced?
No, when a turbocharger is leaking oil the cause is almost always something else. The main causes are a bad outlet check valve, a kink in an oil line, or the scavenge pump. Checking other valves, oil lines, and pumps will almost always find the cause. Outlet check valves are generally responsible for oil leaking from the turbocharger.
I’m getting low fuel pressure from my fuel pump. What in the turbo system can cause this?
Nothing really. Low fuel pressure is an issue almost always caused by something on the fuel injection side, not your turbo system.
What is the recommended service following a prop strike?
Following a prop strike, Quality Aircraft Accessories recommends checking for metal contamination in the engine oil passed through the turbocharging system.
Looking for more information on aircraft turbochargers and turbocharger systems? Check out some of the helpful links below.
For more information on how a turbocharger functions: https://www.shorelineaviation.net/news—events/bid/57041/Aircraft-Engine-Turbochargers-Explained
For basic and operating information: https://flighttraining.aopa.org/students/solo/special/turbo.html
For information on operating and troubleshooting: https://www.avweb.com/news/maint/182847-1.html?redirected=1
For benefits, costs, and disadvantages of choosing a turbocharged engine: https://www.avweb.com/news/maint/182808-1.html?redirected=1