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

Stu Gotts wrote:
1 What is it going to show happened to the EGTs when you switched
tanks?

I was taught that 87 and 100LL avgas burns at the same temperature. The
octane only allows the flame front to propagate smoothly through the
cylinder. Autogas is a different formulation, so it may burn at a
different temperature.
If autogas burns cooler, the EGTs will decrease. If autogas burns
hotter, the EGTs will increase.

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

CHTs will follow EGTs.


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

Depends on the energy content of the fuel, not the temperature. Pistons
work on pressure. The more energy released by the fuel, the more
pressure is generated to drive the piston. More energy means the engine
is turning faster (higher torque?). More torque transmitted to the prop
produces more airspeed.

That's my reasoning.

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.

"john smith" wrote in message
...
Stu Gotts wrote:
1 What is it going to show happened to the EGTs when you switched
tanks?

I was taught that 87 and 100LL avgas burns at the same temperature. The
octane only allows the flame front to propagate smoothly through the
cylinder. Autogas is a different formulation, so it may burn at a
different temperature.
If autogas burns cooler, the EGTs will decrease. If autogas burns
hotter, the EGTs will increase.

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

April 27, 2002

Pelican's Perch #55:
Lead in the Hogwash

Tetraethyl lead has been gone from automobile gasoline for two decades, and
it's only a matter of time before leaded avgas goes away as well. Despite a
huge amount of industry research, nobody yet has a suitable replacement
fuel, and nobody's yet quite sure what will happen to today's piston-powered
fleet when the supply of 100LL dries up. AVweb's John Deakin dispels a bunch
of myths about TEL, explains what it does and why it's so indispensable in
high-performance recips, and talks about one solution to the coming
unleaded-avgas crisis that actually works.

"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."

Before this morfs into how the innkeeper built his homemade fuel
pumper, I thought I would post the answers. I'm surprised many more
didn't take a shot at this but then again, it's not regular hangar
chat. How well did you do?

EGT = Exhaust Gas Temperature
CHT = Cylinder Head Temperature
ROP = Rich of Peak
LOP = Lean of Peak
(and I should have listed this earlier, sorry!)


The lower octane should cause a faster overall burn time and short
interval to peak pressure - - and that should result in lower EGTs.

For the same reasons, the peak pressure increase should cause higher
CHTs.

And the horsepower ?? It probably would not change much... 1-2 % .

The overall effect is the same as if you had improperly advanced the
timing 3-7 degrees.

Regards, George

Contact George at gami.com for any further explanations.






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?