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From: "George Lendich" <lendich@aanet.com.au>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
References: <list-4388200@logan.com>
Subject: Re: [FlyRotary] Re: Dyno
Date: Sun, 11 Jul 2010 11:27:55 +1000
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Charlie,
Running the engine hotter produces more negatives ( I believe ) than 
positives. Hotter engine = thinner oil and more wear; hotter coolant = more 
expansion = more wear; greater tolerances= more blow-by = less compression 
.
My understanding is that hotter engines produce better BSFC, because of more 
complete burn of fuel. If we could produce fuel in a gas (vapour) form we 
would achieve the same outcome, also the hotter fuel would not suck as much 
heat from the large surface area. I believe this has been done before, 
however the hotter gaseous fuel might reduce volumetric efficiency = less 
power. If you had a hot gas and cold air inlet, the cold air might condense 
the fuel back into larger fuel droplets and we are back to where we started.
It's all a bit of a trade-off I guess.
George (down under)

>> eanderson@carolina.rr.com writes:
>>
>>     Don't know the answer - however, my view is at 5500 rpm even with
>>     100% Ve
>>     the engine is ingesting only so much air mass - Which means you
>>     can only
>>     throw in so much fuel before you are burning all the ox in the
>>     air.  I don't
>>     think that any WOT operation at/near full power is going to vary
>>     the BSFC
>>     much from the 0.55 - but, ignore my response lets see what Lynn says.
>>
>>     Ed
>>
>> As so many quickly point out to us, the rotary uses more fuel than a 
>> piston engine. So you need a snappy comeback to counter that attack. What 
>> could that be?
>> Oh yeah, well I can run way lean of peak EGT and you can't. .........And 
>> so you can.
>> Piston engines collect the fuel / air charge and mashes it into a tiny 
>> space that contains the heat of compression and the hot exhaust valve 
>> face and the hot spark plug tip. This provides a charge temperature not 
>> far from detonation. This allows the piston engine to operate close to 
>> the edge of doom for its whole life. The only way to get closer to doom 
>> would be to add a turbo-charger, and they do that too. Detonation is 
>> charge temperature dependant.
>> But having that mixture in a small hot package allows for some very 
>> complete burns with only small amounts of spark advance. The closer you 
>> get to using all of the fuel in the burn the more efficient the engine 
>> will be. The piston engine has a slight advantage in this area.
>> In the rotary the charge is squeezed gently into a large cold combustion 
>> chamber that has no squish areas to drive mixture to the plugs. Near the 
>> apex seals the rotor face and housing are close enough to stop the flame 
>> front. So mixture in this area does not burn.
>> Much of the heat of compression is lost to the chamber before ignition. 
>> The fuel is trying to condense into droplets for lack of heat. For the 
>> most part, not a good show.
>> The problem seems less so when we see that the poor chamber heating makes 
>> any fuel appear to have a higher octane rating than it does. The mixture 
>> contains less energy, and the flame front speed is low. Pressure build up 
>> is slow and constant. The better dwell holds the chamber closed for a 
>> long time, and this improves the burn. The exhaust gas temperature
>> is high compared to a piston engine, until you remember the piston engine 
>> is heating an exhaust valve to orange on each cycle, while the rotary is 
>> dumping gas at the speed of sound right onto the EGT probe.
>> If you mix enough fuel to reduce the available oxygen in the chamber 
>> during the burn, the unburned fuel cannot combust on top of the EGT 
>> probe. So as you lean the rotary, less fuel burning in the chamber makes 
>> additional oxygen available to burn escaping fuel on top of the probe, 
>> and we see the high EGT. This is happening at just lean of peak power.
>> See Sky Ranch page 143.
>> So leaning to peak EGT is already past best power. Our concern here is 
>> the apex seals passing over the peripheral exhaust port where it is 
>> exposed to the exhaust gasses leaving the chamber. Renesis owners my 
>> leave the room now. Over heating the apex seal is very bad Mojo. So we 
>> stay well rich of peak EGT until a few minutes into cruise, and lean very 
>> quickly past Peak, and into lean of peak EGT. (Or, switch to the "B" 
>> controller that you have set up for lean cruise. The engine will smooth 
>> further. The burn rate slows, because the clumps of fuel and air are 
>> further apart. (Acts like higher octane fuel) The EGT going down saves 
>> the muffler. Reduces noise. Extends range. Reduces cooling load.
>> How so? Less fuel=less energy=Less heat. But also less power. (So the 
>> plane slows down)
>> The BSFC in Pounds (Of fuel) per Horse Power Hour goes down. Could get 
>> very close to piston engine numbers.
>> The difference is that BSFC is mostly a function of surface area exposed 
>> to the flame. The piston engine has very little, and the rotary has a 
>> bunch. So the piston engine looses less HP as you reduce BSFC. The rotary 
>> looses more HP as you reduce BSFC.
>> So you compare BSFC at the RPM you plan to use. So if you have one built 
>> you want a copy of the dyno sheet, or look at a sheet from a similar 
>> engine. This will give you BSFC for best power at whatever RPM.
>> A number of gags are used to reduce the poor burn. Like a very high 
>> energy  multi strike ignition system.  Using more ignition advance. In 
>> the 16X dream engine you see fuel injected late, and directly into the 
>> chamber. Unless a turbo is involved, high octane fuel is less effective 
>> than low octane fuel. Plus low octane fuel generally has more BTUs per 
>> pound.
>> Leaning past best power (Rich of peak EGT) takes you to peak EGT, and if 
>> you stay right there for a while, this is where the Lycoming swallows an 
>> orange exhaust valve head, and blows a rod out through the case, and the 
>> rotary begins to overheat the apex seals in non Renesis engines.
>> Power can be (and often is)  controlled by leaning the mixture, with the 
>> throttle left wide open.
>> In the olden days, all of the big radial engines were run well lean of 
>> peak EGT. The only way to get to England from the Azores, was lean of 
>> peak. They carried drums of engine oil in case of a problem. But never 
>> had extra fuel.
>> Lynn E. Hanover
> The points above make me ask a question I've wondered about for years. 
> I've seen it mentioned several times that best power in a rotary is with 
> relatively low water (or was it oil?) temps; around 160 degrees. I assume 
> that's empirical data, & don't question it. But when the discussion turns 
> to efficiency, I wonder if that principle (yes, I had to look up which way 
> to spell it...) still applies.
>
> Decades ago, I read of Japanese car manufacturers experimenting with 
> all-ceramic engines without any cooling systems to get efficiency up by 
> keeping the engine's heat in the combustion chamber. If we make an 
> assumption that with synthetic oil & better quality 'soft' seal materials 
> the rotary can be safely run at higher temps, say in the 220 degree range, 
> would that be enough to improve BSFC? Obviously nowhere near the temps 
> where a ceramic engine could operate, but would it be any help at all in 
> compensating for the large swept area of the rotary combustion chamber?
>
> Charlie
>
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