All Broken In?..

[Finley Atherton]

Originally Posted by RV10inOz

*Table of fuel flows for different engines in the RV fleet. This is done beyond 40deg F LOP and around 75% power.

150HP 7.55GPH
160HP 8.05GPH
180HP 9.06GPH

Hi David,

I haven't had the benefits of your APS education so perhaps you could explain the following to me.

Your table above confirms my understanding that HP is directly related to FF when LOP. However I would have thought that rpm is also a factor as there are higher pumping and friction losses at higher rpm.

LOP and Peak EGT have almost the same BSFC and Lycomings Part Throttle Fuel Consumption graph shows different HP at different rpm for the same FF at Peak EGT.

For example for a 160 HP I0-320 operating at Peak EGT, a FF of 8.3 GHP will produce 120 HP (75%) at 2,200 rpm but the same 8.3 GPH also at peak will only produce about 108 HP (67/68%) at 2,700 rpm presumably due to pumping and friction losses. Why would the same relationship with rpm not apply when LOP

Fin
9A

[RV10inOz]

Finley,

Have a close look at this graph, courtesy of another thread and APS.



Not sure which graphs you are looking at but this one represents all engines, lawn mowers, P&W radials, and yours.

As for higher RPM losses, this is true, engine losses and prop efficiency all add up, but we are splitting hairs a bit. Pick a nice smooth RPM that works for you. Balance your prop too!

As for LOP, Fuel flow determines HP, there is more O2 than fuel, so hence Lean of peak, thus fuel is the key factor. No spare fuel particles. On the Rich side of peak there is no O2 left so to speak, but ample fuel, and detonation margin notwithstanding, varying fuel flow has very minimal impact on HP. So Mass Airflow is king.

At the end of the day, splitting hairs is not relevant as you can not fly that way nor set all 4/6 cylinders that way.

Basically you have it pretty much sorted By the way, LOP and at 75% power fuel flow should be about 8.05GPH, so 8.3 would in theory at least be ROP, but not a lot!

Cheers.

[hydroguy2]

Originally Posted by RV10inOz

....Hydro guy, if your CHT's are well under control, fine, if you want the extra drag, but running around for any period of time over 400 is not optimal so why do it. I assume you were doing something like 75% power with extra drag?

Cheers
David

My CHT's were low for my break-in. I did my phase one during the winter, so OAT's were 0*F or as high as 20*F. I should have taken the pants off. 25"2400rpm with a rich mixture and I'm 175kts+ TAS. To keep good MP, I had to stay in the valley coupled with wind coming over the ridges made me uncomfortable in the turbulence and high speed. I also was fighting a unstable roll at speed(aileron hinge high 1/16" left wing)

Probably my biggest issue, I was in a 200mph plane and only had a 100mph brain.

[Finley Atherton]

Originally Posted by RV10inOz

Not sure which graphs you are looking at

Lycoming 0-320 Operators Manual Fig 3-10 Page 3-22.

Originally Posted by RV10inOz

As for higher RPM losses, this is true, engine losses and prop efficiency all add up, but we are splitting hairs a bit.

I think you are missing the point I am making. You give a definite fixed figure for 75% LOP fuel flows. I am suggesting the figure is not fixed and will vary considerably depending on rpm. For example, using the above graph, 75% fuel flows at Best Economy (peak EGT) can range between about 8.9 GPH at 2,700 rpm down to about 8.3 GPH at 2,200 RPM. LOP and Peak have about the same BSFC so I assume there would be the same range in fuel flows for LOP. The range is even greater (7.1 to 8.0 GPH) at say, 65% power as rpm can be reduced to 2,000 rpm.

I have a carb engine with a CS prop and run at 2,200 rpm at peak (or LOP if possible) up to a max 65% power and I know from the above graph that 65% power will be at 7.3 GPH so I make sure I don't exceed this FF. However someone with a FP prop that may be running at say 2,500 rpm would know from the above graph that they could have fuel flows up to about 7.75 GPH to not exceed 65% power at peak or LOP at that rpm.

Originally Posted by RV10inOz

By the way, LOP and at 75% power fuel flow should be about 8.05GPH, so 8.3 would in theory at least be ROP, but not a lot!

Got my 8.3 figure from the above graph and if my thinking is correct then it could even be up to 8.9 GPH if at 2,700 rpm.

I am happy to be corrected on all this.

Fin
9A

[RV10inOz]

So long as your instruments are known to be accurate, go fly and test it for yourself.

Email me the data and lets talk on the phone.

[jetdriven]

Our factory overhaul 200 HP IO-360 in our Mooney 201 is showing symptoms much like Paul's. Our CHTs were in the 370 range full rich (cowl flaps 1/2 open) for the first couple hours. Anythine less than full rich would go over 400 quick.

Then, they gradually trended downwards over the next 4 hours to around 350 half open and 390 closed. With 20 hours on it now, we were starting to get worried. LOP 50 degrees (10 GPH) we still could not close the cowl flaps and keep a <380 CHT. Old engine was 330 with cowl flaps closed easily and it had 25 degrees timing.

So, at about hour 20, cowl flaps closed, 75% power, 370 CHT, they dropped in the span of a few minutes to 330. Woohoo. Finally. It still creeps up to 380 at times but not as easily.

The engine has used perhaps 1/2 quart in the first 20 hours. It must be tight.

[zav6a]

I am about to contradict Paul ? something along the lines of proceed at your own peril has entered into mind.

I have been reading accounts of CHT drops being indicative of break in. I have a tough time believing it. I don?t think that the slight additional friction in the cylinders associated with unseated rings would be reflected as such a significant increase in temp at the CHT probe locations.

I think what we are seeing is formation of insulating deposits on the piston crown and combustion chamber of the head that occurs in the first few hours of operation. And the richer we run, the faster it builds up.

Many of us run lean and burn mogas. I may be wrong, but both of those operating conditions typically lead to reduced deposits, perhaps leading to increased heat transfer to the piston and head ? relative to what we might have seen running rich with LL. Our engines evolved running rich on heavily leaded gas. And the metal in our heads is already critically hot due to air cooling.

So, where I am headed with this? Perhaps ceramic coatings of the piston crown and combustion chamber would be beneficial in this day and age. Anybody with experience using them. Reno racers?

[aerhed]

It's not just the rings, I don't think. There's lapping going on in the rocker box. Valve tip/rocker arm, push rod/rocker arm. Also wear-in friction on rocker shaft and valve guides. Wouldn't those account for a little break-in heat? Or would that be negligible?

[Ironflight]

Originally Posted by zav6a

I am about to contradict Paul ? something along the lines of proceed at your own peril has entered into mind.

Hey, no problem here - I am always learning more about aircraft engines from the folks that have been building them for decades. CHT drops have always been an indicator of break-in according to them, and it has worked well for me in the past. I can't tell you the why and the mechanism for sure, so won't debate it.

Paul

[rv8ch]

Originally Posted by Ironflight

Hey, no problem here - I am always learning more about aircraft engines from the folks that have been building them for decades. CHT drops have always been an indicator of break-in according to them, and it has worked well for me in the past. I can't tell you the why and the mechanism for sure, so won't debate it.

Paul

I'm starting to plan for my first engine start and hopefully soon after break in, and as usual, overthinking everything. I just found this thread, and it did trigger a question - why would CHTs drop when the rings seat?

In the many years that have passed since this discussion, has anyone come up with some ideas?

The only factors I can think of are:

1) when rings are not seated, some combustion gasses escape down the side of the piston/cylinder wall, perhaps causing more heat to get conducted to the head;
2) combustion energy is "wasted" when the gasses leak past the rings, causing less efficient power generation for a particular fuel flow/RPM;
3) the thermal insulation due to deposit buildup from carbon and lead on the head/piston theory mentioned above.

Anyone have any other ideas?