Turbo vs Supercharged-Please Help
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Turbo vs Supercharged-Please Help
Hello everyone-I could really use your expert help with some explanations here
While going thru the study phase to take my ATPL theory exams, apparently the chapter on power augmentation from the OAA manual really kicked my butt yesterday. It's either been really long since studying this material since my MEP training or it's being put in a different way making it slightly more difficult to grasp.
To begin with, the biggest confusion deals with super vs turbochargers. Now I know both of them share a common purpose (to increase weight of cylinder charge), only differing in the way they are driven. The amount of increase in weight of charge can either be so much as to increase sea level power (ground boosting) or to only maintain sea level power up to a certain altitude (altitude boosting aka turbo normalizing). The altitude at which both of these methods stop being effective is critical altitude or full throttle height.
Onwards, the chapter continued with an explanation of the turbo charger, after which it showed a graph comparing the performance of a turbo charged engine to a normally aspirated one. The graph showed that both of these engines start losing power from the moment they start climbing-I understand aspirated ones do, but I always remembered being shown comparison graphs btwn these 2 engines as one where the turbo charged one maintains a straight horizontal line of constant power until Crit. Alt. while the normally aspirated one has its normal downward slope.
When talking about superchargers (gear driven) the comparison graph shown by the book stated that again between two engines that are otherwise identical in power output, a supercharged one has less power at sea level due to loss of some BHP to drive the supercharger gearing-this makes sense. However, as the engine climbs, it actually develops MORE power with altitude, because according the book, the reduced back pressure and lower temp with altitude provide more efficient cylinder scavenging and better air density. I always knew that at altitude, decreased pressure has the predominated effect on lessening air density.
Now it seems, this train of thought is being inverted to say that these minor changes which in reality might lessen the rate of change of density drop, now provide the major effect for more engine horse power with altitude-theoretically making it possible to maintain altitude the higher you go, with no throttle at all. I have included both graphs for you to consult.
My questions are, does this make sense? Or, is this graph making certain assumptions that I haven't picked up on, but still carries the same meaning I am familiar with, put in a different way?
Also, with regards to altitude vs ground boosting, is there one type of equipment that is generally only associated with a certain type of boosting (supercharger for altitude, turbo for ground) or it is common to see either done by either. Finally, if an altitude boosted engine's role is to only maintain SL rated perf to a higher alt, does that mean it only has a max MAP reading of 30 inches on the gauge?
Any attempts to unconfuse me will be well appreciated; regards
While going thru the study phase to take my ATPL theory exams, apparently the chapter on power augmentation from the OAA manual really kicked my butt yesterday. It's either been really long since studying this material since my MEP training or it's being put in a different way making it slightly more difficult to grasp.
To begin with, the biggest confusion deals with super vs turbochargers. Now I know both of them share a common purpose (to increase weight of cylinder charge), only differing in the way they are driven. The amount of increase in weight of charge can either be so much as to increase sea level power (ground boosting) or to only maintain sea level power up to a certain altitude (altitude boosting aka turbo normalizing). The altitude at which both of these methods stop being effective is critical altitude or full throttle height.
Onwards, the chapter continued with an explanation of the turbo charger, after which it showed a graph comparing the performance of a turbo charged engine to a normally aspirated one. The graph showed that both of these engines start losing power from the moment they start climbing-I understand aspirated ones do, but I always remembered being shown comparison graphs btwn these 2 engines as one where the turbo charged one maintains a straight horizontal line of constant power until Crit. Alt. while the normally aspirated one has its normal downward slope.
When talking about superchargers (gear driven) the comparison graph shown by the book stated that again between two engines that are otherwise identical in power output, a supercharged one has less power at sea level due to loss of some BHP to drive the supercharger gearing-this makes sense. However, as the engine climbs, it actually develops MORE power with altitude, because according the book, the reduced back pressure and lower temp with altitude provide more efficient cylinder scavenging and better air density. I always knew that at altitude, decreased pressure has the predominated effect on lessening air density.
Now it seems, this train of thought is being inverted to say that these minor changes which in reality might lessen the rate of change of density drop, now provide the major effect for more engine horse power with altitude-theoretically making it possible to maintain altitude the higher you go, with no throttle at all. I have included both graphs for you to consult.
My questions are, does this make sense? Or, is this graph making certain assumptions that I haven't picked up on, but still carries the same meaning I am familiar with, put in a different way?
Also, with regards to altitude vs ground boosting, is there one type of equipment that is generally only associated with a certain type of boosting (supercharger for altitude, turbo for ground) or it is common to see either done by either. Finally, if an altitude boosted engine's role is to only maintain SL rated perf to a higher alt, does that mean it only has a max MAP reading of 30 inches on the gauge?
Any attempts to unconfuse me will be well appreciated; regards
Last edited by Skymaster15L; 16th Oct 2012 at 09:11.
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Maybe this will help:
The power available from a supercharged engine will fall with height as it would with a normally aspirated one, because what comes out depends on what goes in. It is more accurate to say that a supercharged engine, at whatever height, will give out as much power as an unsupercharged engine of similar capacity and RPM would give at ground level. The engine’s power output increases slightly in the climb, because exhaust back pressure and inlet temperature are reducing. This increases volumetric efficiency.
Because they are internal devices, and driven by the engine, superchargers can use up a lot of power (150 HP in a Merlin) and therefore fuel, increasing costs and reducing the range of the aircraft. They must also be controlled and continually adjusted by the pilot.
Turbochargers use the engine’s spent exhaust gases to operate an automatic wastegate, so turbochargers are external exhaust-driven superchargers. In fact, the word turbocharger is a contraction of turbosupercharger.
As exhaust back pressure increases when the turbocharger's wastegate is closed, engine power reduces slightly. Engine power remains much the same up to full throttle height for that power.
Phil
The power available from a supercharged engine will fall with height as it would with a normally aspirated one, because what comes out depends on what goes in. It is more accurate to say that a supercharged engine, at whatever height, will give out as much power as an unsupercharged engine of similar capacity and RPM would give at ground level. The engine’s power output increases slightly in the climb, because exhaust back pressure and inlet temperature are reducing. This increases volumetric efficiency.
Because they are internal devices, and driven by the engine, superchargers can use up a lot of power (150 HP in a Merlin) and therefore fuel, increasing costs and reducing the range of the aircraft. They must also be controlled and continually adjusted by the pilot.
Turbochargers use the engine’s spent exhaust gases to operate an automatic wastegate, so turbochargers are external exhaust-driven superchargers. In fact, the word turbocharger is a contraction of turbosupercharger.
As exhaust back pressure increases when the turbocharger's wastegate is closed, engine power reduces slightly. Engine power remains much the same up to full throttle height for that power.
Phil
