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Ed Anderson wrote:
Depending on the compressor type used on the supercharger your efficiency can drop down to below 50% (some of the older roots type expellers) , the centrifugal compressors (which can be on super or turbochargers) are generally higher efficiency (can approach 75%) in converting input energy into high pressure air. So a bit more efficient.
I'm going to sidestep Ed's argument about using energy before or after it has been converted to mechanical energy, because just thinking about it is making my head hurt. Instead, I'll move on to the type of blower and it's mounting. I spoke about this at the Rotary Roundup a couple of years ago. Tom is just about through with machining the parts, and I'm about through building my fuselage, so I should have real hardware to test with "Real Soon Now"(TM).
SNIP
It won't deliver a LOT of
boost, but neither will it require pop-off valves, waste-gates, intercoolers or a heavy mounting system. I will consider it a win, if I can get 200Hp at 7000RPM.
Well, if you can get 7000 rpm at a 12.56 Air/Fuel Ratio then you could get around 197 HP at sea level without any blower added. I assumed 29.92 " Hg with a loss of 0.5" Hg in the intake which indicates a very good - intake.
600 cubic Feet minute from your blower is approx twice of the 307 CFM flow requirement of the engine would be sucking in on its own to produce 197 HP. So having that air flow already moving could probably free up some engine energy to turn faster and pull in more air - but, of course, you have to substract the energy required to turn the blower.
It gets complicated here.... the blower may be capable of putting out 600 CFM with no restriction - but the engine is not going to have any more mass flow than its displacement * the air density gives. Since its displacement is fixed that means basically you have to increase the air density to get more air into the engine per revolution (yes, you can always get more air by mathematically increasing the rpm - but in reality, you may not have the power to increase the rpm against the prop load).
Now, I am going to try to present the best case possible case I can think of using the arrangement you described.
IF your 600 CFM could be made to flow through the engine you would have to double the air density - to get it through (fixed displacement remember). IF you could do that then your engine would produce close to 400 HP, but that will not happen because your blower basically increases the velocity of air not the density.
However, even though your blower will not increase the density anywhere near that magnitude, you still might get some benefit. If the additional airflow can not pass through the engine, then it will at least convert to static pressure increase. you get 600 cubic feet/min at 150 mph the dynamic pressure component would be approx 0.5 psi. Since the engine intake is flowing in the same direction, not certain whether you would get all of the 0.5 psi, but lets assume you do.
IF so, you may succeeded in overcoming the losses in your induction system - typically 1" - 1/2" So I subtracted 1/2" from 29.92 to give a manifold pressure of 29.42 (assuming 0.5 " of losses) to get the 197 HP. So if we recover that 0.5" Hg loss you could get a manifold pressure of around get 29.92 " Hg at sea level. That would give 205 HP at 7000 rpm instead of 197 HP for a 8 HP gain. BUT, you then would have to subtract the amount of power it would take to turn the blower. That's about the best you could get - the way I see it. So whether that gain is worth the pain is something only you can decide.
The reason the turbo/supercharger gets so much power out of the engine is not because it increases the pressure of the intake air or the flow volume - its because the increases the air density of the intake charge providing more oxygen per cubic unit of air flow. The rest is just "side effects". The rotary like any positive displacement air pump will displace the same volume for each rotation regardless of the air density. The displacement is a constant - so only the changing density of the air affects the power (well, the amount of fuel as well, of course) produced per revolution.
But, I have been wrong before and will be again. Certainly, I am not trying to discourage your experiment, I am eagerly looking forward to the results. But, felt I had to give you my viewpoint of the best to expect - the worst is you have a very noisy fan up front {:>)
Ed
Any ideas on what I should do, not do, or do differently? I'm still technically in the planning stages, so I would appreciate any and all input.
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