To a pilot, does turbocharging differ from supercharging?

egid
  • To a pilot, does turbocharging differ from supercharging? egid

    When it comes to operating an aircraft, what are the practical differences between a turbocharged engine and a supercharged engine? I'm aware of the mechanics - turbochargers being exhaust-driven, while superchargers are mechanically driven - so I'm looking for the differences in how pilots need to treat the systems:

    • Do power settings need to be set differently?
    • Are temperatures more of a concern with superchargers?
    • Is it easier or harder to overboost a supercharger? If you do, are there waste gates?
    • Do engines with superchargers need a cool-down period before being shut off?
    • Any other operational differences that would be meaningful for pilots to know.

    Similarly, what are the ownership implications of any differences? Does one cost more to maintain than the other due to the way they are constructed? Will a supercharged engine have a lower TBO because of the mechanical aspects of the system, or will it be the same as a turbocharged engine?

  • It's more or less the same as in a car. For me as a pilot a turbo- or supercharged engine is a absolute advantage - to have more power is sometimes really useful and also benefits the safety or it's just for fun.

    For everyone whos not sure about the working principle of these systems or the difference between a turbocharger and a supercharger I will give a short description:

    Both are compressors installed in the manifold wich are used to get more air or air-fuel mixture into the engine or simply raise the density, which especially at higher altitudes provides some extra power. The difference between both is the way the compressors are driven.

    A Turbocharger is driven by a turbine in the exhaust. That's very efficient because it uses some of the ~ 70% wasted energy in a piston engine but also creates some back pressure on the exhaust.

    A supercharger is like other auxiliaries, e.g. the engine fuel pump, driven by the engine it self wich of cause costs a bit of engine efficiency but all in all the engine benefits from the installation. The great advantage compared to a turbocharger is that a supercharger works fine at all RPMs while a turbocharger needs some higher exhaust pressure to start working properly which makes it effective only at high power settings.

    As a pilot you don't need to take care of some additional switches but must of cause treat your engine a little bit different (higher risk to overpower). Do not use intermittent throttle position during take off or climb - allways go full forward. The use of intermittent throttle positions only causes your engine to work at higher load and non optimal cooling for longer time. If RPM exceedslimits reduce the Prop pitch carefully - rapidly lowering the RPM can cause severe damage to the engine! If you are used to lean mixture on the ground, stick to it your engine (the spark plugs) will be thankful but go full mixture for take off, even at some higher altitudes to achive maximum internal cooling of the cylinders - monitor the temperatures. These are some general suggestions based on my experience and knowledge, please refer always to the manual!

    Another risk I can thin of is, that you might be not used to flying at higher altitudes or VFR 'over the tops'. Try both with an instructor or experienced pilot first before doing it solo. You might be healthy and never had problems with your medical, anyway you should be able to lear how your body behaves. Also you might not be used to flying without the normal horizon - this might be lost when descending or climbing in VMC but in close vicinity of clouds. It's not my intention to consider you as unexperienced but I like to write this answer in a way, that it's helpful for everyone whos interested in.

    Operator or owner of an aircraft should not only consider the advantages of forced ignition. Before I've edited this answer I wrote, that piston engines are nlt developed to be turbo/supercharged. This way to say it was obviously not really helpfull for you, sorry. What I want you to think of are the additional loads acting on the engine. You have high pressure within the cylinders, even some back pressure on the exhaust (turbocharger only) or additional friction to drive the supercharger. The engines whitch are equipped with forced ignition or those where turbochargers are available fore of cause are built to handel these forces but I still consider them to effect the life of those engines. So there are definitely more extra costs then the purchase of the equipment.

  • I have some of the answers:

    •Is it easier or harder to overboost a supercharger? If you do, are there waste gates?

    This is an important point. Waste gates, when opened, allow exhaust to bypass the turbocharger, reducing manifold pressure by slowing down the turbo. Older turbos had manually adjusted waste gates, and even fixed waste gates. The pilot would have to monitor boost pressure and adjust the waste gate to avoid overboosting the engine. Newer turbos have automatic waste gates. Superchargers, being belt or gear driven, do not have waste gates as the supercharger is not dependent on exhaust gas flow. I do not know if they have manifold pressure releif valves in case of overpressure but I doubt they would.

    So a key difference in a pilot's operation of the engine is that in a manual waste gate engine, the manifold pressure gauge needs to be scanned frequently and the waste gate adjusted accordingly. Less important in an automatic waste gate or supercharged engine.

    •Do engines with superchargers need a cool-down period before being shut off?

    No. The cooldown period in turbochargers is necessary to cool down the bearings in the turbocharger after high cruise power settings. Idling on the ground allows cooler oil to flow through the bearings, removing excess heat. Look at your exhaust gas temperature gauge - this is the temperature of the gasses flowing through your turbocharger. The oil present in the turbocharger bearings will "coke", or turn into a solid at that temperature. Upon landing, a slow taxi to the ramp will usually provide sufficient cool down time. Usually! Time this and don't forget to cool down as recommended.

    Superchargers are geared to the engine (or belted) and do not use the hot exhaust as their source of power. So even though they can get quite hot, they don't approach the heat of the exhaust gasses.

    •Any other operational differences that would be meaningful for pilots to know.

    This is not really an operational difference, but in addition to turbocharged and supercharged engines, there are also turbonormalized engines. TN engines are turbocharged engines with their boost limited to atmospheric pressure at sea level. Say 30 inches, not the 40 inches you might expect from a turbocharged engine. The TN engine permits sea level performance up to a critical altitude, above which the TN engine cannot maintain sea level manifold pressure. In other words, performance at a sea level takeoff will not be materially different from that of a naturally aspirated engine, but as you climb, the power will not decline as you climb. So your Leadville CO takeoff will be the envy of the stock C-172 crowd.

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