Deleted member 68059
Staff Sergeant
- 1,058
- Dec 28, 2015
Just for general interest there are three principal limitations to "adding" gears to a supercharger, or indeed changing the ratios.
1) If the SC is mech. driven, sea-level throttling is a severe restriction because it impacts take-off. Thus one cannot simply gear a supercharger
to 50,000ft then take off with the throttle almost closed because the losses will be so high that the engine will (if it has been sized
with any sort of reasonable proportion) be unable to supply enough power for a realistic take-off run with full equiptment (obviously
the more gears the less this is a problem).
2) More seriously, all compressors are limited by reaching supersonic "choke", usually this will happen first at the impeller eye (less often in the diffuser)
As altitude rises, density drops, therefore to maintain MASS flow (upon which engine power relies) velocity into the supercharger must increase. Therefore one can
determine for ANY compressor system what the limiting rated altitude is, if one knows the boost pressure is, simply by calculating what the velocity into the
compressor is at that altitude, and mass flow rate - and seeing when it goes supersonic. (this is basically saying that at high altitudes you need a supercharger with a very big inlet
eye to avoid choke, BUT.... the ratio of inlet eye to impeller OD has established limits... therefore to increase the eye dia. you can end up with
an impeller which is too big in external diameter to package... this is what led to the development of the centripetal compressor which inducts air
at its periphery, not its ID... which is a more efficient use of area, hence retarding the onset of choke at high altitude).
3) Shaft speed. The shaft will have limits on speed based on the bearings and their lubrication, and also the critical speed (i.e. flexural resonance)
of the shaft. One, or both of these may be the limit. It is worth noting that unlike torsional resonances, flexural resonance in shafts is usually
regarded as a CEILING, not to be exceeded, whereas torsionals can be reached, managed then exceeded providing they are damped, or
if they are passed through speedily and not lingered in. Flexural
resonance is so dangerous it is generally regarded as fatal and therefore 1st order flexural resonance is the upper ceiling. This can only be
corrected by stiffening the supercharger shaft, or shortening it.
Turbochargers are somewhat more flexible in as far as they can have their speed more easily modulated and therfore get around #1 more easily, but they are
just as hamstrung by points 2 and 3 as a mechanical supercharger, and in some cases even more so as the turbine blades are very hot and therefore
the stress problem is potentially even more hazardous than for the compressor impeller. There will be a max turbine speed which could possibly
be capped by the turbine before it reaches the limit of the bearings or shaft flex.
1) If the SC is mech. driven, sea-level throttling is a severe restriction because it impacts take-off. Thus one cannot simply gear a supercharger
to 50,000ft then take off with the throttle almost closed because the losses will be so high that the engine will (if it has been sized
with any sort of reasonable proportion) be unable to supply enough power for a realistic take-off run with full equiptment (obviously
the more gears the less this is a problem).
2) More seriously, all compressors are limited by reaching supersonic "choke", usually this will happen first at the impeller eye (less often in the diffuser)
As altitude rises, density drops, therefore to maintain MASS flow (upon which engine power relies) velocity into the supercharger must increase. Therefore one can
determine for ANY compressor system what the limiting rated altitude is, if one knows the boost pressure is, simply by calculating what the velocity into the
compressor is at that altitude, and mass flow rate - and seeing when it goes supersonic. (this is basically saying that at high altitudes you need a supercharger with a very big inlet
eye to avoid choke, BUT.... the ratio of inlet eye to impeller OD has established limits... therefore to increase the eye dia. you can end up with
an impeller which is too big in external diameter to package... this is what led to the development of the centripetal compressor which inducts air
at its periphery, not its ID... which is a more efficient use of area, hence retarding the onset of choke at high altitude).
3) Shaft speed. The shaft will have limits on speed based on the bearings and their lubrication, and also the critical speed (i.e. flexural resonance)
of the shaft. One, or both of these may be the limit. It is worth noting that unlike torsional resonances, flexural resonance in shafts is usually
regarded as a CEILING, not to be exceeded, whereas torsionals can be reached, managed then exceeded providing they are damped, or
if they are passed through speedily and not lingered in. Flexural
resonance is so dangerous it is generally regarded as fatal and therefore 1st order flexural resonance is the upper ceiling. This can only be
corrected by stiffening the supercharger shaft, or shortening it.
Turbochargers are somewhat more flexible in as far as they can have their speed more easily modulated and therfore get around #1 more easily, but they are
just as hamstrung by points 2 and 3 as a mechanical supercharger, and in some cases even more so as the turbine blades are very hot and therefore
the stress problem is potentially even more hazardous than for the compressor impeller. There will be a max turbine speed which could possibly
be capped by the turbine before it reaches the limit of the bearings or shaft flex.
Last edited:
