Fuel Burn in combustion engines

[earl1937]

  I am not going to go into a lot of questions about this, but, in an engine, with dual mags, which mag fires spark plug nearest the exhaust valve?

Along that same line of questions, why is it normal for the L and R mags to have a difference in RPM drop?

When using an exhaust temperature gauge for maximum fuel range, how do you arrive at that peak performance?

Why is it important to know where your fuel pickups are located in your fuel cells?

Who or what organization, sets the fuel grade of different grades of gasoline for aircraft?

[Bino]

Hi Earl,

Really enjoy your posts, sir.  And this one is about one of my favorite topics: Internal Combustion!  


I am not going to go into a lot of questions about this, but, in an engine, with dual mags, which mag fires spark plug nearest the exhaust valve?
The twin-spark engines I've seen inside (old Maserati in-line six, modern Lycoming IO-360) have one intake and one exhaust valve, with the two spark plugs equidistant from the valves.  With two valves and two sparks, they're ranged around the periphery of the head at ninety-degree intervals:  valve-spark-valve-spark.

Along that same line of questions, why is it normal for the L and R mags to have a difference in RPM drop?
Depends on how the intake mix swirls into the cylinder while the piston descends, and how close the currently active spark plug is to that swirl.  In many engines with two-valve heads, the intake charge flows into the cylinder in a sort of corkscrew path.

When using an exhaust temperature gauge for maximum fuel range, how do you arrive at that peak performance?
Lean the mix slowly until you see the highest temperature (most complete combustion)... and then enrich the mix "a skosh" to keep the engine a little cooler.

Why is it important to know where your fuel pickups are located in your fuel cells?
So you can know what attitude and/or G-load might cause fuel starvation, I suppose?

Who or what organization, sets the fuel grade of different grades of gasoline for aircraft?
Depends on which country you fly within.  

[Oldman731]

When using an exhaust temperature gauge for maximum fuel range, how do you arrive at that peak performance?
Lean the mix slowly until you see the highest temperature (most complete combustion)... and then enrich the mix "a skosh" to keep the engine a little cooler.

Seems to me the modern trend is to go lean of peak, if the motor's suitable.

- oldman

[earl1937]

Seems to me the modern trend is to go lean of peak, if the motor's suitable.

- oldman

That seems to be the latest "guess-ta-mutt"! I generally let the total time on engine or since top overhaul dictate just how lean I would run the engine. The higher the engine time, the less I would push the peak lean!

[earl1937]

Hi Earl,

Really enjoy your posts, sir.  And this one is about one of my favorite topics: Internal Combustion!  


I am not going to go into a lot of questions about this, but, in an engine, with dual mags, which mag fires spark plug nearest the exhaust valve?
The twin-spark engines I've seen inside (old Maserati in-line six, modern Lycoming IO-360) have one intake and one exhaust valve, with the two spark plugs equidistant from the valves.  With two valves and two sparks, they're ranged around the periphery of the head at ninety-degree intervals:  valve-spark-valve-spark.

Along that same line of questions, why is it normal for the L and R mags to have a difference in RPM drop?
Depends on how the intake mix swirls into the cylinder while the piston descends, and how close the currently active spark plug is to that swirl.  In many engines with two-valve heads, the intake charge flows into the cylinder in a sort of corkscrew path.

When using an exhaust temperature gauge for maximum fuel range, how do you arrive at that peak performance?
Lean the mix slowly until you see the highest temperature (most complete combustion)... and then enrich the mix "a skosh" to keep the engine a little cooler.

Why is it important to know where your fuel pickups are located in your fuel cells?
So you can know what attitude and/or G-load might cause fuel starvation, I suppose?

Who or what organization, sets the fuel grade of different grades of gasoline for aircraft?
Depends on which country you fly within.  

YOu certainly know your air cooled combustion engines! You are correct, 90 degree placement is correct,  with one exception: the R-3350 compound engine on the B-29, 1049 Super G Connies and some others. The "compounding effect" of this engine required a more even burn at the exhaust valve area than normal, hence both plugs are closer to the exhaust port than normal.

In some aircraft, such as the B-25, the fuel p/u's were placed to forward in the fuel cells, hence about 60 total galleons were not usuable until the guys in the SWPA of combat discovered that by lowering 10 degrees of flaps, they could burn an additional 40 galleon of fuel, there fore increasing their range by about 35 to 40 min's.


ASTM D1405 / D1405M - 08(2013) Standard Test Method for Estimation of Net Heat of Combustion of Aviation Fuels. Many countries and companies used this method to determine the octance of fuel and is usually approved by the Commerce dept.

[Wolfala]

Seems to me the modern trend is to go lean of peak, if the motor's suitable.

- oldman

That is correct if you have the instrumentation and hardware set up for it it is the way to run. I just retired my engine on my Cirrus six months ago with 3200 hours on it and it was run lean of peak 99% of the time except for takeoff.

That is a $45,000 data point

[earl1937]

That is correct if you have the instrumentation and hardware set up for it it is the way to run. I just retired my engine on my Cirrus six months ago with 3200 hours on it and it was run lean of peak 99% of the time except for takeoff.

That is a $45,000 data point

just goes to show what can be done when the operator is competent in managing his power plants! <S> to you sir!

[Golfer]

That is correct if you have the instrumentation and hardware set up for it it is the way to run. I just retired my engine on my Cirrus six months ago with 3200 hours on it and it was run lean of peak 99% of the time except for takeoff.

That is a $45,000 data point

How many degrees LoP did you run?  There's about 125 degrees between peak and rough running due to lack of fuel in the Bonanza and the IO-550 in it.

I'm a little light on documentation but the Auricle makes life pretty easy with best power and best economy bugs.

[Puma44]

That is correct if you have the instrumentation and hardware set up for it it is the way to run. I just retired my engine on my Cirrus six months ago with 3200 hours on it and it was run lean of peak 99% of the time except for takeoff.

That is a $45,000 data point

How high above recommended TBO is 3200 hours?

[Oldman731]

How high above recommended TBO is 3200 hours?

I'm going to guess 1200 hours.

Do I win a prize?  (*tm Dipsy)

- oldman

[Puma44]

I'm going to guess 1200 hours.

Do I win a prize?  (*tm Dipsy)

- oldman

That would be some seriously nice engine management/care. 

[colmbo]

I got 2700 hours out of my first O-470 in the 182, TBO of 1200.  It was strong, running fine.  I changed it out when I found the cam lobes/lifters flaking metal off.   It wasn't really all that bad, you couldn't read part numbers or anything,  but it was making metal.

[Puma44]

Not to shabby! 

[Wolfala]

How many degrees LoP did you run?  There's about 125 degrees between peak and rough running due to lack of fuel in the Bonanza and the IO-550 in it.

I'm a little light on documentation but the Auricle makes life pretty easy with best power and best economy bugs.

It depends on the day what you can get, but generally high power LOP around 75% and at least 100 LOP.

[Golfer]

Can you help me a little more with how to use this chart?