Turbocharger/supercharger Compressor Type

[Z]

Ian,

Thanks for the link. The PDF file shows it as a two stage radial (centrifugal) compressor, followed by two (???) axial stages for the turbines (I guess one to drive the compressors and the other to drive the shaft, but can't see too clearly).

Interesting is the quoted specific fuel consumption, at about 280-300 g/kWh. Aside from the usual Engineering industry's abomination of units ("kilowatt-hour" for energy!!!, with "hours" being the standard units from bus-timetables!!!) this fuel efficiency is quite poor. Translated into more sensible units of "percentage of fuel-energy converted into useful shaft-energy" it comes out at about 25-30% (depending on energy content of the fuel). This is mediocre by petrol engine standards, and piss-poor compared with diesels.

Of course, 1,000+ kW from <180 kg does sound good. But when you add the mass of fuel required for longer journeys, then fuel efficiency quickly becomes more important (ie. consider the extra tonnage of fuel to lug around). Essentially, the gas-turbine is a simple, cheap way of getting a lot of power from a reasonably small and reliable bit of machinery (because low parts count), albeit at rather poor fuel efficiency.

As a warning to you students, jet-engine spin-doctors often quote the thermal efficiencies of their aeroplane engines as being better than diesels (ie. typically >45%). However, the jet-engine calcs are done at ambient (= exhaust) temperatures of something like -50 C (it's cold up there above the clouds!). Run a diesel up there and it will be even more efficient....
~~~~~o0o~~~~~

Because it is bucketing down rain outside ..... here is one sexy-arsed bit of machinery for you all to consider ...



This ~1934 Auburn 851 Speedster has an all-alloy, flat-head (= side-valve), straight-eight engine, with a radial-flow (centrifugal) supercharger driven mechanically from the engine. You can see the SC just under the carburettor. (SC has vertical shaft, so horizontal shaft driven by crank nose runs next to engine, then 90 degree bevel-gearbox, and shaft up to SC?)



A centrifugal-blower like this is a good match for a side-valve engine. At low revs the lazy, large capacity engine gives easy, tractable, and quiet cruising (side-valves are very quiet). At higher revs the small valves start to restrict airflow, but that is just when the blower kicks in. Hence a steady increase in torque and power...

Z

[d.haddon]

Z,

In addition to the reasons you quoted above, radial compressors have seen little use in aircraft engines due to very high weights and gyroscopic forces. As engines were upscaled for more power, these rotors became much more heavy, (partly to cope with the very large pressure ratio, orders of magnitude higher than seen by a single stage of an axial compressor) and the very high rpm of jet engines led to much greater gyroscopic forces forces being experienced.

The smaller radial compressors don't have this problem, and when scaled down, the final stages of axial compressors have VERY thin blades, so moving to radial compressors (and turbines) can increase simplicity and reliability. When considering the size of an automotive application, it seems like there is much less that can go wrong with a single-stage radial blower versus a multi-stage axial compressor.

Regards, Damon.

[VFR750R]

Don't forget about inertia. Multi-stage axial compressors work fine at steady state, but have much higher inertia than a single stage radial compressor working at higher pressure ratios.

I also seem to remember that axial compressors are only more efficient than radials beyond a certain size. At really low airflows which necessitate a very small inlet size and wheel size, an axial loses a ton of efficiency out at the tips of the blades (sealing) and the fact that there is such a surface speed gradient from the base to tips of the blades.

Small compressors of both types lose efficiency due to a number of effects such as higher operating speeds, and higher surface area to flow rate, and higher leak percentages.

[MCoach]

I currently have nothing to contribute to this thread (and hope to not derail it!), but Z, I would like to congratulate you on your 1000th post on this forum. That's a lot of words! I ..err... have no sort of prize, but maybe a trip to the local ice cream shop could be good enough.

[Big Bird]

Wow, Z, congrats
(celebrating the occasion with maybe my shortest post ever....)