Mogas: what happens to EGTs?

This was a little fun quiz that was circulated on a type specific
email list a few days ago. It was authored by George Braly of
Advanced Pilot Seminars, and principle of GAMI. If you're familiar
with George, you know he backs everything up with HARD DATA obtained
from his own research. In a couple of days I'll post the correct
answers (no, I didn't get them all) so that most can learn and a few
can refute. Have fun.

Ok... little MOGAS quiz for you.

You run the Left tank dry and fill it up with premium unleaded car
gas.

Right tank has 100LL.

Takeoff on the RH tank and climb, level off, and set up the mixture
for
cruise flight. Either ROP or LOP. Your choice.


THEN, you switch to the LEFT tank. You change nothing else.

Later you download your JPI and plot the data.

1 What is it going to show happened to the EGTs when you switched
tanks?

2 What is it going to show happened to the CHTS when you switched
tanks?

3 (not in the JPI, but: ) What is the airspeed going to do? Increase?
Decrease? Stay the same?

Why?

-
-1 What is it going to show happened to the EGTs when you switched
-tanks?

The EGT should show a very slight increase. The higher the octane, the smoother
and cooler the burn. You have gone from 100 octane to (arguably) 87 or 93
octane and the car gas should burn hotter.


-
-2 What is it going to show happened to the CHTS when you switched
-tanks?

If the EGT (internal temperature of the cylinders) goes up, the CHT must follow
suit, but at an even reduced value as regards the EGT rise.


-
-3 (not in the JPI, but: ) What is the airspeed going to do? Increase?
-Decrease? Stay the same?

Increase, ever so slightly. The efficiency (and therefore power output) of any
Carnot cycle engine is a function of the temperature difference between intake
and exhaust temperatures. If the intake remains constant (ambient) and the
exhaust rises, the horsepower produced goes up and the airspeed...EVER SO
SLIGHTLY...increases.

Then again, I'm a sparky. What the hell do I know about engines.


Jim



Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

On Sun, 07 Mar 2004 11:34:57 -0500, Jim Weir wrote:
-
-1 What is it going to show happened to the EGTs when you switched
-tanks?

The EGT should show a very slight increase. The higher the octane, the
smoother and cooler the burn. You have gone from 100 octane to
(arguably) 87 or 93 octane and the car gas should burn hotter.

I was under the (perhaps mistaken) understanding that mogas was measured
on a differing octane rating system, and that 93 mogas was ~100 octane
measured on the avgas system? (The question specified "premium" mogas).

Given that, I'd have thought everything would have been the same, across
the board.

Addison

Addison Laurent wrote:

I was under the (perhaps mistaken) understanding that mogas was measured
on a differing octane rating system, and that 93 mogas was ~100 octane
measured on the avgas system?

They use a different system, but it works the other way 'round. There is no mogas
that comes close to 100 octane avgas. Fortunately, most engines that are burning
100LL are actually certified for a lower octane.

According the the EAA fact sheet on autogas, you can get a good idea of the
octane equivalent by subtracting 5 from the mogas anti-knock index (the "octane"
rating on the pump). So 93 octane mogas would be about 88 octane on the avgas
scale.

http://www.eaa.org/education/fuel/detonation.html

George Patterson
Battle, n; A method of untying with the teeth a political knot that would
not yield to the tongue.

"G.R. Patterson III" wrote in message
...


Addison Laurent wrote:

I was under the (perhaps mistaken) understanding that mogas was measured
on a differing octane rating system, and that 93 mogas was ~100 octane
measured on the avgas system?

They use a different system, but it works the other way 'round. There is
no mogas
that comes close to 100 octane avgas. Fortunately, most engines that are
burning
100LL are actually certified for a lower octane.

According the the EAA fact sheet on autogas, you can get a good idea of
the
octane equivalent by subtracting 5 from the mogas anti-knock index (the
"octane"
rating on the pump). So 93 octane mogas would be about 88 octane on the
avgas
scale.

http://www.eaa.org/education/fuel/detonation.html


And this article as well:
http://www.avweb.com/news/columns/182132-1.html

May 27, 2001

Pelican's Perch #43:
Detonation Myths

We've all been taught about detonation in piston aircraft engines. It's what
occurs when combustion pressure and temperature get so high that the
fuel/air mixture to explodes violently instead of burning smoothly, and it
can destroy an engine in a matter of seconds. Right? Well, not exactly.
AVweb's John Deakin reviews the latest research, and demonstrates that
detonation occurs in various degrees - much like icing and turbulence - with
the milder forms not being particularly harmful. Heavy detonation is
definitely destructive, and the Pelican offers some concrete data on how to
avoid it.

Jim Weir wrote:

-
-1 What is it going to show happened to the EGTs when you switched
-tanks?

The EGT should show a very slight increase. The higher the octane, the smoother
and cooler the burn. You have gone from 100 octane to (arguably) 87 or 93
octane and the car gas should burn hotter.

Are not the auto and aviation octane scales different?

"Newps" wrote in message
news:1UQ2c.82422$PR3.1197113@attbi_s03...


Jim Weir wrote:

-
-1 What is it going to show happened to the EGTs when you switched
-tanks?

The EGT should show a very slight increase. The higher the octane, the
smoother
and cooler the burn. You have gone from 100 octane to (arguably) 87 or
93
octane and the car gas should burn hotter.

Are not the auto and aviation octane scales different?


From http://www.avweb.com/news/columns/182149-1.html

April 27, 2002

Pelican's Perch #55:
Lead in the Hogwash

By John Deakin

"If the theory of "octane" is simple, the numbers are not. It is a very
common mistake in pilot lounges and coffee shops to talk about octane at the
gas pump where you fill your automobile, and the pump where you fill your
airplane. The calculations are VERY DIFFERENT, and they cannot be directly
compared! It's like talking about knots vs. miles per hour, or using
"gallons." Is that American gallons, or imperial gallons? There are several
entirely different ways of measuring "octane." There is "Research Octane
Number" (RON), "Motor Octane Number" (MON), "(R+M)/2" which is nothing more
than an average of the two, and "Observed Road Octane Number," (RdON).

Finally, there is the octane number we talk about in GA. It is close to
"Motor Octane Number" but not identical. So much for standards. Actually,
there are fairly good reasons for several different octane measurements, as
"octane" works differently in different situations (race engines, road
engines, aircraft engines, air cooled vs. water cooled, intake air
temperature, RPM, etc.). For more on this, there's a short but decent
explanation at http://www.osbornauto.com/octane.htm."

"Newps" wrote in message
news:1UQ2c.82422$PR3.1197113@attbi_s03...


Jim Weir wrote:

-
-1 What is it going to show happened to the EGTs when you switched
-tanks?

The EGT should show a very slight increase. The higher the octane, the
smoother
and cooler the burn.

I don't think I agree with this last sentence. First of all the burn may
be just as smooth with a lower octane with a low compression engine. High
octane fuel like 100LL has no benefits for an A-65 Continental engine with
6.3:1 compression ratio and loves the 80 octane gasoline it was designed
for.

And if the burn is cooler when 100LL (rather than 80) is run through the
carb, why is it the engine must have special 100LL exhaust valves, which are
designed with high-temperature alloys?


You have gone from 100 octane to (arguably) 87 or 93
octane and the car gas should burn hotter.

Are not the auto and aviation octane scales different?


I don't know the answer but I tell you what I suspect. I suspect the egt
of 100LL will be higher. Think about it and correct me if I'm wrong. The
low compression Continentals when they are rebuilt -- the A-65's, O-200's,
and O-300's --- have optional 100-octane exhaust valves available from the
parts supply houses like Fresno Airparts. Now WHY is that, other than
because the mix is still burning when the exhaust valve opens? My
suspicion. You (anyone) tell me, with an intelligent explanation, if I'm
wrong.

Higher octane gasoline like 100LL, furthermore, is a little less volatile
than mogas and burns more slowly because of the higher RON numbers, so that
when the exhaust valve opens combustion is less complete than with the same
charge of mogas -- in the low-compression O-300. So if you're running
100LL in an O-300-powered 172, why do you need 100-octane exhaust valves,
whose faces are something like inconel or another exotic alloy designed for
temperatures much higher than the stock exhaust valves?

" jls"
shared these priceless pearls of wisdom:

-
-And if the burn is cooler when 100LL (rather than 80) is run through the
-carb, why is it the engine must have special 100LL exhaust valves, which are
-designed with high-temperature alloys?

The alloy of a valve in a low-compression Continental engine is precisely the
same alloy as the 100LL exhaust valve. The 100LL valve has had some machine
work and clearance work (mainly in the stem area and in the angle of the valve
seat area) done to make it compatible with the excess 100LL lead that jams
valves.



-I don't know the answer but I tell you what I suspect. I suspect the egt
-of 100LL will be higher.

You suspect wrong. I have made this exact same measurement on several
low-compression Continentals before I started using mogas (primarily the O-300D,
but a couple of more engines to a lesser extent. Mogas burns hotter according
to the EGT.


-Higher octane gasoline like 100LL, furthermore, is a little less volatile
-than mogas and burns more slowly because of the higher RON numbers, so that
-when the exhaust valve opens combustion is less complete than with the same
-charge of mogas -- in the low-compression O-300. So if you're running
-100LL in an O-300-powered 172, why do you need 100-octane exhaust valves,
-whose faces are something like inconel or another exotic alloy designed for
-temperatures much higher than the stock exhaust valves?

You predicate your entire argument on the fallacy of "exotic alloy" valves, when
in fact, the only difference is in the settings of the lathe that cuts them.

Jim

Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

Before I would buy into any of this (this runs hotter/cooler than that) I would
like to see some kind of quantitative study, i.e. a quantity of different size
engines from the same manufacturer and then different manufacturers. Other
things also like what seasonal additives the mogas may or may not have. After
13 years of dynomometer testing on air cooled VW engines I have seen the same
motors react ever so slightly to things like different fuel, etc. due to things
like, for example, timing (ignition, valve, whatever) being a tick different.
What I'm saying is that one engine may seem to run hotter with a different fuel
while another may seem to run cooler with the same switch.

Jim