Today's Message Index:
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1. 12:51 AM - Mogas versus 100LL (DEAN PSIROPOULOS)
2. 06:33 AM - Re: T-3 FIREFLY (Gary Casey)
Message 1
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| Subject: | Mogas versus 100LL |
--> Engines-List message posted by: "DEAN PSIROPOULOS" <dean.psiropoulos@verizon.net>
>Time: 08:24:46 PM PST US
>Subject: Re: AeroElectric-List: Mogas versus 100LL
>From: <rparigor@SUFFOLK.LIB.NY.US>
>Octane rating is not the only thing one needs to be concerned with when it
>comes to fuel. I don't know the technocrat term for it, but the speed at
>which the fuel burns is a definite concern. On huge displacement engines
>turning low RPMs, if you were to use a racing fuel designed for high RPM
>engines of the same octane you would most like run into problems. From
>what the locals at the airport say, high octane mogas should not be used
>in a O-540s because it burns too fast, and since the cylinders are the
>same as a O-360??
I believe you are referring to something called the Cetane rating
Ron. The only place I've ever heard this term used is in association with
DIESEL engines. Since the fuel is injected directly into the highly
heated and compressed air in the diesel's cylinders, it must burn at a
certain rate so as not to spontaneously combust and cause detonation (yes
diesels can be damaged by detonation too). I've never heard it used
in association with gasoline engines but that doesn't mean it's not
important.
>On the other hand, using 100LL that is a slow burning fuel used in a 73CC
>22HP (well over 11K) Yamaha YZ 80 engine that is in my self retrieving
>balloon kills performance and probably raises EGT to disheartening levels.
I could be wrong but I don't think it's the high octane that is
causing your situation here. Tetraethel lead in the fuel effectively
increases the resistance of the mixture to spontaneously combusting in the
higher temperatures of aircooled engines and higher cylinder pressures of
the old 1960s era high performance auto engines (10/11/12 to 1
compression ratios with the old design wedge heads). What may be
causing the 100LL to burn slower is the particular mix of aromatic
hydrocarbons that make up the composition of av-gas. The av-gas
mixture has a much lower volatility than mogas and that may be related to
why it burns at a different speed (assuming you are correct here).
>On Rotax 4 strokes you can use 100LL, but it does raise the EGTs because
>it burns slower than high octane mogas, and some of that burn makes its
>way into the exhaust.
There may be something to this. The old Ford tractor we used to mow
the runway at the soaring club was run on 100LL (always available from our
tow plane supply) for many years and although it seemed to run just fine,
eventually it needed a valve job. Not sure if that was due to 100LL or
just wear and tear. Old piston engines used softer valves and seats
and needed the cushioning effect of the lead in the fuel to get
reasonable service life from these components (mfgrs went to much
harder valves and seats when the lead was phased out many years ago).
>A old timer said on old auto engines that didn't use aluminium pistons,
>used to loosen the distributor, and run up the engine and begin retarding
>the ignition, he said he would get the exhaust glowing, hence carbon would
>be burned off. Slow burning fuel does the same in a engine designed for
>fast burning fuel.
Not sure why you'd want to do that unless the carbon was causing
detonation or pre-ignition. But yes, on an engine with severely
retarded timing the fuel would still be burning when the exhaust valve
opened and EGT would be higher (ala your "slow burning" fuel).
>Using a fast burning fuel in a engine designed for a slow burning fuel can
>cause detonation. Detonation raises temperatures, let it go and God forbid
>pre-ignition begins to occur.
First time I've really heard this argument Ron, I don't think the
lead is the culprit though, racing engines use lead in the gasoline to
this day as far as I know (you just can't get octanes much above 95 with
unleaded gasoline so you have to use lead or go to alcohol or some
other fuel to get there without the lead). But...it may be the fuel
formulation differences, I'll refer this one to Archie (and anyone else
who wants to chime in about the subject) on the engines list. There are
some very knowledgeable folks over there.
>Ron Parigoris
Dean Psiropoulos
Do not archive on the aeroelectric list
Message 2
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Thanks very much! In my design:
1. The gascolator is mounted in a cooled box at the bottom center of
the firewall (not above the exhaust pipe as it is on some Lancairs)
2. 3/8 fuel line used (almost went to 1/2 inch, but didn't).
3. Electric pump is at the lowest point in the whole fuel system.
I was afraid that what they "overlooked" were esoteric details that I
might have missed. Looks like they ignored the obvious.
Gary Casey
> From: teamgrumman@aol.com
>
> --> Engines-List message posted by: teamgrumman@aol.com
>
> 1. don't put the gascolator above the exhaust pipe
> 2. use at least 3/8 inch fuel line (not the 5/16 inch stuff like the
> FireFly)
> 3. keep the boost pump as low as practical
> - - the tests we did at SAIC showed a lot of pump cavitation
> when the
> pressure at the carb was low
> - - - although the fuel in the float bowl of the carb was probably
> fine.
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