Turbo vs NA

[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?

[Walter Atkinson]

WOW, you have a bushel basket of questions! I'll try to answer them succinctly, but most of these are answered in detail in the APS class.

Originally Posted by rhurt

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

TC'd engines with 7.5:1 CR are generally in the .42 - .43 range on BSFC(min). The NA engines with the 8.5:1 CR generally have BSFC(min) numbers of about .385 -.39. The higher CR ratio engines are more efficient.

Typically, the more boost, the more efficient because reducing the boost (MP) creates greater exhaust back pressure which reduces the volumetric efficiency of the cylinder.

Originally Posted by rhurt

What is ICP?

Internal cylinder pressure. Generally referencing the peak ICP.

Originally Posted by rhurt

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

No reason engine-wise, however the pilot workload *could be* a bit higher. I generally will do a LOP climb only when it's a max range trip and the couple of gallons saved might make the difference in meeting MY personal reserve of one hour remaining. As a general rule I climb both NA and TC'd engines ROP and cruise LOP.

Originally Posted by rhurt

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

That depends entirely on the power being produced. More power needs to be further LOP. At 65%, I like 20dF LOP with a nearly straight line change to 80dF LOP at 90% power.

Originally Posted by rhurt

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?

I only do that when the OAT is quite high and the extra RPM increases mass airflow while holding the FF the same. This makes one further LOP and the CHTs will be lower.

Originally Posted by rhurt

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?

The ideal would be mass air flow tuned cylinders and exhaust, but the gain is hardly worth it in most cases. The air flows are usually close enough that the GAMIjectors create a set of cylinders producing very, very close to the same HP. So close that it runs nice and smooth LOP.
The IO-550 induction in the CIrrus is efficient because each cylinder has it's own induction and is not a log-runner. BUT, many Cirrus engine benefit by having GAMIjectors. Some are pretty good as delivered.

Originally Posted by rhurt

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?

IN most of our engines it's not worth the trouble unless you're racing at Reno and a quarter knot makes a difference. The effect on the GAMI spread could make it better or worse, depending on where you start. Attempt to tune the exhaust and not have to balance the F:A ratios has been largely unsuccessful. The reason is the occult transfer of fuel from one cylinder to another in the INDUCTION system.

Originally Posted by rhurt

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

The certification effort is continuing but I do not have any estimate on the timeline. GAMI might, But I don't. Based on testing, I think the BSFC(min) may come in about .37, which is a substantial improvement and not far from the operational BSFC(min) of the diesels.

I hope that answered you adequately.

[rhurt]

Thank you Walter. That is a great wealth of information! It sounds like taking a seminar would be time and money well spent.

For clarification though, about prism: Is the BSFC of .37 for the NA engine? And does prism measure and tune the peak ICP?

Finally, do you know anyone who has a rig for measuring crankshaft torque in flight?

[Walter Atkinson]

Originally Posted by rhurt

Thank you Walter. That is a great wealth of information! It sounds like taking a seminar would be time and money well spent.

For clarification though, about prism: Is the BSFC of .37 for the NA engine? And does prism measure and tune the peak ICP?

Finally, do you know anyone who has a rig for measuring crankshaft torque in flight?

Several thousands have taken the seminar and the only comments we get are that it was well worth the time and expense. We'd love to have you join us. The next class is in Colorado Springs, August 9 & 10.

As far as PRISM is concerned, the answers to your questions above are yes, and yes.

Crankshaft torque is usually measured in flight at the geared accessory case on the nose of the engine where there is a reduction gear for the prop. I am unaware of any such device for the engine you are flying to be used in flight. GAMI can do it on the engine test stand.