Turbo vs NA

[Ernie Martin]

OK, I'm buying the argument that from an engineering viewpoint an engine monitor appears more useful at ROP than LOP, and the further argument that it may not have been included in legacy aircraft because they didn't exist. However, I think you would agree that proponents of LOP consistently indicate the need for an engine monitor and that many engine overhaulers blame the lack of monitors for damage caused by LOP (although, I won't argue with your earlier view that some of these overhaulers are clueless on operation and such damage may be due to improper operation by the pilot rather than not having an engie monitor).

Ernie

[ipasgas1]

Great discussion...I have never been instructed on the lean of peak method so I will need to learn them for my next aircraft. My friend has a Cirrus SR22 and now my wife wants one of them instead of a 337 which is a discussion I got into a couple years ago and ended up with a 337. Hopefully, I will get out of my partnership and into my own so need to make a decision on which way to go...again.

[Walter Atkinson]

Originally Posted by Ernie Martin

However, I think you would agree that proponents of LOP consistently indicate the need for an engine monitor and that many engine overhaulers blame the lack of monitors for damage caused by LOP

LOL! Ya got that right. But, why are you right?

From my experience teaching these topics for over a decade to thousands of pilots, I would offer the observation that those who have come to understand mixture management and routinely operate LOP are the same pilots who have gone to the trouble to become educated about the relationships between the EGT, CHT, ICP, HP and BSFC curves as well as the value of engine monitors and how to interpret the data they display.

I would also agree with your premise about builders placing blame. These are usually the same builders who've never operated an engine LOP or even looked at the 5 Landmarks to Engine Management, much less understand their relationships. If one cannot draw the EGT, CHT, ICP, HP and BSFC curves they have little chance of understanding the issues we are discussing so openly.

[n86121]

20 years ago, when I bought my turbo 337, I had not really seen the need for turbos, but this one was STOL, etc, so I bought it.

I transitioned in a non-turbo, and remembered an anemic rate of climb once above 8,000 or so, with three on board plus luggage.

20 years later, I can testify the best part of the turbo's is they allow you to maintain a high climb rate all the way, to as high as you want (the best thing should you find yourself in icing by the way). In the 10k++ ft haze around Washington DC summers, it's nice to climb FAST to get above it all.

Trubo's also give you the OPTION of going fast at altitude IF YOU WANT. As a helicopter pilot first, often going down the beach, I often like low and slow. Throttle back, and you've got the efficiency of a C182, with a LOT more payload, and total redundancy.

The 337 turbos have automatic wastegates, which means
a) You can almost set and forget them to a desired MP pressure (almost), and
b) If you want to loaf along at low power and fuel flow, then just throttle back, bring the turbos almost idle, and you have a regularly aspirated engine.

You can have your cake and eat it too.

Yes, IF you take advantage of their boost in power at altitude, they will use more fuel.
If you operate the like a non-turbo, I speculate the engines work just like a non-turbo. MP is MP.

In 20 years I've overhauled both turbos to new, at about $3.5k each.
In the overall scheme of things not a big deal.

D

[Walter Atkinson]

David:

You hit the nail pretty squarely on the head with minor exceptions.

Originally Posted by n86121

b) If you want to loaf along at low power and fuel flow, then just throttle back, bring the turbos almost idle, and you have a regularly aspirated engine.

....

If you operate the like a non-turbo, I speculate the engines work just like a non-turbo. MP is MP.

Well, the turbos don't really go to idle, but they do slow down, BUT you do not have a "regularly" aspirated engine. The low compression pistons are not nearly as efficient as the normally aspirated model's higher compression pistons. To make matters worse, the increased exhaust back pressure from throttling back reduces the volumetric efficiency of the cylinders so MP is not really MP where the number of air molecules are concerned (increased IAT, too). Turbocharged engines are more efficient at higher MPs than lower MPs.

However, your points are well taken and I agree with your basic premise that turbos are hard to beat for flexibility and capability.

[hharney]

When operating LOP are the exhaust valves in jeopardy because of the decreased amount of cooling and lubrication from using less fuel? It seems that I have heard that cylinders are changed out more often on Cont. 360's when operated at LOP. Is this a management issue? It is not true?

[Walter Atkinson]

Originally Posted by hharney

When operating LOP are the exhaust valves in jeopardy because of the decreased amount of cooling and lubrication from using less fuel? It seems that I have heard that cylinders are changed out more often on Cont. 360's when operated at LOP. Is this a management issue? It is not true?

Herb:

Exhaust valve temperature is most affected by CHT. The lower the CHT, the lower the exhaust valve temperature. EGT has no effect on exhaust valve temperature. As a matter of fact, if you are at 40dF ROP and lean the mixture to peak, the EGT will be going up but the exhaust valve temperature will be going down. 40-50dF ROP is the mixture where the exhaust valve runs the hottest.

There is no difference in valve lubrication across the mixture sweep. Valve guides are lubricated by oil. The notion that fuel acts as a lubricant is misplaced. Fuel is a solvent, not a lubricant. Some claim that the lead in the fuel acts as a lubricant, but this is also misplaced. Lead exists in during the combustion event as a salt of bromide--lead-oxy-bromide. Salts are abrasive. Abrasives make crummy lubricants!

So, if running the exhaust valves cooler is desirable (it is) and running the combustion chamber cleaner to keep the exhaust valve cleaner and a better valve-seat interface is desirable (it is), then LOP mixture management is less stressful or damaging for the exhaust valves than ROP mixtures.

There is no hard data to support the claim that more cylinders are changed when operated LOP. There is hard data to support that the converse is true. There are over 400 million flight hours of data supporting the reality that LOP is easier on cylinders than ROP operation. There is data to support the contention that not running the mixture rich enough when ROP does put increased stress in the form of heat and pressure on the exhaust valve. That's probably where the notion came from that if you run too lean (not rich enough on the rich side) you'll burn up the exhaust valves.

Does that help?

[rhurt]

Great thread!

I have some questions to add:

Walter stated: Turbocharged engines are more efficient at higher MPs than lower MPs.

Do turbocharged engines have lower BSFC than NA engines (LOP, 65-100% power)? What boost pressure is most efficient?

What is ICP?

Why not run LOP on climbout, after the initial power reduction?

In my NA, conforming (via GAMI) IO360 engines, how many degrees LOP should I run?

I usually fly WOT at 9-10k', running 2,500 rpm. Should I run 2,600 rpm or more when operating LOP to get my power back?

GAMI uses a cost effective method of tuning the F/A ratio by adjusting each cylinder's fuel flow to match the air flow. But the cylinders are not all producing the same power. Is it not most efficient to tune the intake airflow and exhaust gas flow so that all the cylinders will run at the same airflow, create the same power and also have the correct F/A ratio? Does the Cirrus SR22 accomplish this with the tuned induction IO550 combined with tuned exhaust?

How much does differing exhaust gas flow restriction affect the F/A ratio in each cylinder? If I tune the exhaust in my IO360 Skymaster by making some headers, how much will the tuned exhaust affect the F/A ratio for each cylinder? Will the 'GAMI spread' be enlarged? I wonder if I could tune the exhaust restrictions to make up for the intake flow inequalities and go back to stock injectors?

Is PRISM close to certification? How much will it reduce the BSFC?