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I know engines have a range of RPM's they work at and I know mechanically driven superchargers are geared to the shaft and spin at a given gear ratio, ratios, or range of ratios; I also know that superchargers produce disproportional compression when their RPM is ramped up.
The thing is how do you end up producing full RPM without over-boosting the hell out of the engine? I know the throttle isn't all the way forward when you're taking off and is run progressively further forward until critical altitude is reached () and that sort of thing.
How does boost get varied independent of engine RPM. It's probably something I should have asked right away when joining, but it somehow never popped into my head.
That all said, pushing the gas-pedal produces an increase in RPM: You'd figure pushing the throttle forward would increase boost and RPM.
Don't they have a waste gate?
How did they set and main maximum boost, on say a Merlin?Turbochargers use a wastegate to adjust the amount of exhaust going through the turbine, which controls the speed and boost.
Mechanical superchargers don't have that.
Modern turbos can also have a blow-off valve, which releases excess boost after the compressor. They could do that with a mechanical supercharger, but I don't believe they did in WW2.
The thing is how do you end up producing full RPM without over-boosting the hell out of the engine?
So basically, to summarize...
- Superchargers designed for altitude are, effectively oversized
- This takes more horsepower off the shaft to drive it
- For them to break even (the pressure produced adds enough HP to compensate for the HP taken off to drive it), you'd over-boost the engine at sea-level
- The engine runs innately with a horsepower reduction at low-altitude for the given manifold pressure
- That said, as the altitude goes up, they can keep producing full manifold pressure well above what a naturally aspirated engine would do (or an engine with a sea-level supercharger).
- All altitude superchargers are sea-level superchargers if the maximum manifold pressure could be raised high enough... (yeah, I'm taking that to the nth degree...)
So basically, to summarize...
- Airflow is generally reduced via a butterfly-valve
- The movement of the throttle adjusts the butterfly-valve
- This gives enough air to the engine to maintain the right RPM, without over-boosting
- The butterfly-valve effectively imposes additional losses because of the fact that it forces the supercharger to run at a higher pressure ratio (unsure I grasp)
For WW2 engine superchargers it can only have reduced teh pressure ratio, otherwise boost would have been higher.
Hmmm try not to confuse the pressure RATIO of the compressor with the subsequent ABSOLUTE pressure available to the engine, a slightly higher pressure ratio
acting on a considerably lower ABSOLUTE inlet pressure (caused by inlet throttling at constant compressor speed) can provide lower ABSOLUTE boost pressure.
UnderstoodYes. If the supercharger was designed to give the rated boost at sea level, then the engine would perform like a non-supercharged engine, but with more power. The exception is if the engine was used in conjunction with a turbo designed to provide sea level conditions up to its critical altitude.
If I had a sea-level supercharger, it would take off a certain amount of horsepower to drive it, but it would, presumably boost the pressure enough to raise horsepower beyond the amount that the driving of the impeller took off.Not sure what you are trying to say.
It's a joke. It was just taking the idea of boost to it's logical conclusion.Um, what?
A sea-level supercharger is one where the critical altitude or full throttle height is sea-level. An altitude supercharger is where the critical altitude or full throttle height is above seas level, at some nominal altitude.
What I find strange is that many engine charts often seem to list BHP as a straight line right on up to the critical altitude rather than revealing the pumping loss. I'm not sure why airplane performance graphs don't show that kind of thing -- you'd think it'd be important.This is a supercharger map for a modern centrifugal supercharger (like those used in most supercharge WW2 engines)
I was working along what Calum posted, he said something about raising the pressure ratio.Depending where you start on the map, decreasing mass flow can increase or decrease the pressure ratio.
I have a hunch I'm probably totally misinterpreting this (and I haven't really slept in almost 24 hours), but the butterfly valve closing, does that produce an area change like a bell-mouth? If so I could imagine that an undersized bell mouth would produce a pressure gain but not enough air...Hmmm try not to confuse the pressure RATIO of the compressor with the subsequent ABSOLUTE pressure available to the engine, a slightly higher pressure ratio acting on a considerably lower ABSOLUTE inlet pressure (caused by inlet throttling at constant compressor speed) can provide lower ABSOLUTE boost pressure.
I can understand you just fineSadly the audio and video quality is rubbish
What I find strange is that many engine charts often seem to list BHP as a straight line right on up to the critical altitude rather than revealing the pumping loss. I'm not sure why airplane performance graphs don't show that kind of thing -- you'd think it'd be important.
If you create enough boost then combustion occurs as soon as fuel is introduced. You can then dispense with the cylinders and other stuff and concentrate on refining the development of your jet engine.That said, if the engine could be boosted hard enough, eventually you would reach some comically absurd manifold pressure that would be impossible to achieve at any altitude but sea level because the air is too thin at any other altitude (Yeah, I know, lame).