Author Topic: Altitude Compensating Fuel pump (Read 382 times)

Charles Browning · « **on:** December 17, 2011, 10:54:25 AM »

I have been hearing a good bit of talk about fuel flow on climb out. I have an altitude compensating pump, will not excess fuel simply be returned to the left tank no matter how high I set fuel flow? If cylinder temps are remaining in reasonable parameters...say 370 or less...why adjust max fuel flow? Are there other considerations that I am overlooking. I just dont understand why Im being adviced to set fuel flow at 30 gallons on my IO550-B. Is it possible to have too much fuel flow? Last question...at what point in the fuel system is excess fuel returned to the left tank?

Don Lawton · « **Reply #1 on:** December 17, 2011, 02:09:49 PM »

Either live or online, you need this.

Robert Siegfried · « **Reply #2 on:** December 18, 2011, 12:21:17 AM »

Hi Charles-

There are a number of considerations that make a sea-level full power fuel flow for an IO55B of 29-30 gph reasonable. Even though your CHTs may be acceptable at a lower flow, the timing of the peak pressure pulse within the cylinder is advanced at a lower fuel flow, leading to higher stresses and reduced detonation margin. The APS course provides a much more detailed and complete explanation, http://www.advancedpilot.com/.

As far as return fuel flow to the left tank, check the POH for your 36. I believe you will find that all return fuel goes to the tank selected.

Regards,

Bob

Stuart Spindel · « **Reply #3 on:** December 20, 2011, 08:12:07 PM »

1.Q. "will not excess fuel simply be returned to the left tank no matter how high I set fuel flow?"

A. No, each injector nozzle has a fixed orifice. Fuel flow is what goes to the fuel distribution valve, then to the six cylinder injector nozzles. All of the displayed fuel flow is consumed by the engine.

2.Q. "why adjust max fuel flow? Are there other considerations that I am overlooking"

A. Yes, the mixture is directly affected. For a given throttle setting and RPM, the volume of air is defined. The fuel flow then, determines the mixture. Too lean, reduced power. Too rich, reduced power as well as combustion deposits that mess up the innards of the engine. Perfect mixture for maximum power just gets the CHT too high and shortens engine life quite drastically. It seems that for higher powered engines, about 10 horsepower per GPH is about right.

3.Q. "I just don't understand why I'm being advised to set fuel flow at 30 gallons on my IO550-B"

A. For a 300 hp engine, 30 GPH is going to give the best compromise of engine life and takeoff power.

4.Q. "Is it possible to have too much fuel flow?"

A. Yes. Too high a SLTOFF (SeaLevelTakeOffFuelFlow) the power will fall off, the intake valves and top piston ring grooves will cake with carbon deposits, oil will get dirtier, greater amounts of lead will accumulate in the crankshaft and other internal engine parts. I am not even going to mention the cost of fuel!

5.Q. "Last question...at what point in the fuel system is excess fuel returned to the left tank?"

A. The excess fuel is taken from the top of the swirl chamber of the engine driven mechanical fuel pump. From the cockpit selector valve fuel, goes to the fuel pump inlet. The fuel is then metered, at the fuel metering unit, by the sum of the position of the throttle and mixture controls. The fuel seperates and some goes directly to the fuel distribution valve to be burned. The fuel not passed by the fuel metering unit goes back to the fuel pump. The fuel flow that you see in the cockpit gauge is the fuel that is actually sent to the cylinders. It is adjusted by a needle valve that restricts the fuel returned to the tank.

Hope that helps.

Tom Pelz · « **Reply #4 on:** December 20, 2011, 11:13:29 PM »

Charles,

What has been said above is accurate. I would like to add a bit more.

I have the 550B with the altitude compensating fuel pump.

While idling on the ground, I have the mixture leaned. I know it is leaned adequately when the engine runs smoothly.

On take off, I have between 29 and 30.5 GPH at 2700 and full throttle (depending upon density altitude).

After take off, I bring the gear up and then reduce the RPM to 2500 when I have cleared obstacles.   I then begin a cruise climb what ever altitude I planned to fly at.

After achieving my cruising altitude ( 6K - 12K depending upon what ever), I select my desired RPM (usually 2300, but sometimes 2400). I then leave the throttle alone (unless I am less than 5K, then I reduce to 21 - 22 inches).   I then quickly reduce the mixture to between 11.0 - 12.5 GPH. This gives my EGT a peak of a bit less than 1400 degrees. I am also LOP.

My CHT is about 320 peak (at cruise).

As Don Lawton said, Take the advance pilot seminar. The fellows make sense. Their engine care works.

They will also tell you have to climb Lean of Peak. At this time, I don't fly LOP during climb. I have not seen the benefits. My airplane climbs quickly to my desired cruise altitude. I then fly lean of Peak. As a result, my usual power settings give me a 155 - 160 KTAS at 12 GPH. This means I have a fuel economy of about 15 - 16 MPH at 185 MPH.

With my tip tanks I have a 1000 mile range with favorable winds and I can land with 25 - 27 Gallons of fuel remaining.

In other words, fly LOP. It really makes a difference.

Tom

Robert Martin · « **Reply #5 on:** December 21, 2011, 09:09:36 PM »

Charles,

The advantage of having an altitude compensating fuel pump is you can go full rich on take off at any field elevation, reduce to climb RPM as desired and forget about adjusting mixture or fuel flow on cimb out. The ACFP will lean for you at your field elevation and contiue to lean on climb. During cruise set your mixture as disired. The fuel pump must be set up in accordance with the 39 page TCM Service Information Directive SID97-3E. With full open throttle and full rich mixture at sea level, assuming standard conditions your IO-550B should be 24.9 to 26.6 GPM. This data is provided on page 28 of the SID. Also a flight test must be accomplished to insure correct FF at each 1000 ft up to 12,000 to insure the flow is within the parameters specified on chart 7 page 28. Personally, I would set on the high side of the range. A higher fuel flow will defeat the purpose of the ACFP. Your 94 A36 has a digital fuel flow transducer just befor the FF divider that reads analog on your gauge, so the FF indication is what is being used by your engine. Any owner of an ACFP engine should read and understand SID-3E. TCM recommends complying with this SD each annual.

R. Martin

Tom Pelz · « **Reply #6 on:** December 22, 2011, 02:26:26 PM »

Charles,

Last summer I had my ACFP overhauled as part of my engine overhaul. Prior to the overhaul, my take off fuel flow (digital) was about 29.5 - 30.5 GPH. When the pump was returned to me, it was set as Bob Martin said. At full throttle, I could only get 26.5 GPH.

In my opinion, that fuel flow is insufficient for adequate engine cooling at full power.   Therefore, I have had my pump re-adjusted so that it is about 29.5 - 30.5 GPH at take off.

At that setting my highest EGT is about 1300 - 1400 F and my warmest cylinder is less than 380 degrees.

As Don Lawton said: Attend the Advanced Pilot Seminars.   They had a long time experience with obtaining the longest life possible from the Continental IO520 and IO550.

Following their directions, I got 2200 hours on a 520 (the case started to crack, so I overhauled it).

Tom

Robert Martin · « **Reply #7 on:** December 22, 2011, 07:29:04 PM »

Although I don't disagree with anyone here, I am inclined to rely on data provided by the engine and airframe manufacturer. The Beech Maintenance Manual specifies 24.9-26.6 GPM and TCM specifies 24.9-26.6 FF at full power sea level in standard conditions for the IO-550B. The POH for cylinder head operating tempature range is 93-238C or 199-464F. My IO-550 is set to 26.6 GPM at full power. CHT is never over 400F. The engine is a new IO-550B Platinum, BDS baffles and Super Semitar prop. I will discusse this matter with TCM technical and report back here. In the meantime, could anyone provide a technical reference for using 30 GPM other than what someone says or believes?

R. Martin

IA A&P

Gene Vertkin · « **Reply #8 on:** December 30, 2011, 06:01:36 PM »

I have Baron-58 with IO-550B's and POH recommends to Have Mixture Full- Rich for clime and descend and leaning for cruise only. Does everybody have the same POH recommendations? How do I know that altitude compensation fuel pumps work properly?

Don Lawton · « **Reply #9 on:** December 31, 2011, 06:10:42 AM »

Quote from: Gene Vertkin

I have Baron-58 with IO-550B's and POH recommends to Have Mixture Full- Rich for clime and descend and leaning for cruise only. Does everybody have the same POH recommendations?

Take care of those engines, and your wallet - get thee to one of the Advanced Pilot Seminars, either live or in-person.

Quote

How do I know that altitude compensation fuel pumps work properly?

Register at TCM (sorry, "Continental Motors"), Aviator Services. It's free. Then go to the Service Bulletin link and download SID97-3E. It's got the information to answer your questions.

Paul Gretschel · « **Reply #10 on:** December 31, 2011, 04:36:10 PM »

Tom:

On a slightly different point, I would suggest that you climb at Vy until at least 1000-1500' AGL. If you were to lose an engine down low, you would wish that you had that extra couple of hundred feet in order to maneuver to a safe landing spot.