Power calculations

[GlennB]

Hi all,

So, why not cheekily make a feature request as my first post?  It goes like this:

The % power display on our Skyview is astoundingly accurate, when compared with the formula that applies when LOP.  Specifically, that's (FF x 14.9 / Rated HP).

There's only one problem with that in our installation.  Our engine is the brilliant Aerosport IO-375 with 8:1 compression ratio, for which the multiplier should instead be 13.7.  This makes the LOP % power calculation incorrect, by about 8% of the displayed value.

My proposal is a simple one: an option in the engine setup menu allowing the choice of multiplier or, even easier, “Standard / Low compression”.  This would benefit the many owners of non-standard CR engines, including turbos.

I know this could also be achieved by simply lying to the system about the installed HP.  That wouldn't affect the calculation at T/O power, as the ROP calculation is done differently, but equally it wouldn't fit with the Dynon quest for perfection.  I suspect it would also affect the ROP / PK / LOP trigger points.

Responses, corrections to my arithmetic etc, all welcome...

 

[mmarien]

That's an interesting thought. I always though the % power was independent of the horse power, and based on RPM, MAP and FF. So does adding electronic ignition, fuel injection and compression ratios, etc change the formula?

 

[GlennB]

That's an interesting thought. I always though the % power was independent of the horse power, and based on RPM, MAP and FF. So does adding electronic ignition, fuel injection and compression ratios, etc change the formula?

Correct, some of the time!

When ROP, the power developed is dependent on mass airflow, because by definition you have an excess of fuel.  So MAP and RPM determine % power.

LOP, there's an excess of air, so the power depends just on how much fuel is supplied.  You can simply multipy the FF by a constant to get the HP produced, and divide that by the rated HP to get %.  Ignition and injection won't change that, but compression ratio will.

The issue with a low-compression engine is that its horsepower per unit of fuel (sfc) is less.  The Skyview appears to be set up to assume a standard CR, which would make it a bit inaccurate in calculating the % power when LOP.

(Apologies for the diversion into an APS ground school  )

 

[mmarien]

The discussion is much appreciated. Had me searching all over the Internet trying to determine the HP of my modified o320. :-/ In the end it didn't much matter. I do display % power on my engine page but adjust power/cruise settings according to AFR rather than % power. So % power is more a curiosity than a useful tool.

If there is a method to display a more accurate % power I'd support that. But unless the HP is directly related to the compression ratio it would be difficult to come up with a HP rating for a modified engine that is needed in the calculations. I suspect there are more than a few non standard engines in the experimental world.

 

[dynonsupport]

You don't need to know the HP of your engine to get % power set up.

The first thing to realize is that it's % power, not absolute power. So it doesn't matter if your modified o320 creates 150HP or 200HP. 87% power is 87% of max power for that engine.

The neat thing about the ROP calculations is that they don't really care about compression ratios and efficiencies. The numbers that make 75% power on an 0-320 make 75% power on an IO-360 angle valve and make 75% power on a IO-720. Now that 75% might mean 100HP, 137HP, and 300HP, but it's all 75% for that engine, so it's a pretty valid load indication.

The HP number can be tested and adjusted without knowing the exact HP. Start with an HP estimate- I assume you can get within 25%. Then lean the engine manually at about 70% power, finding peak on the EGT temps. If the automated LOP/ROP reads peak too early or too late, you can adjust the HP number until everything matches.

The reason we use "HP" in the menu is it gives you a place to start. If we gave a unitless number, then doing it for an O-200 vs. an IO-720 would be a pain. However, it can be non-intuitive- if you have a more efficient engine, you will end up with a number in there that is LOWER than the stock number even if the engine produces more power. So we definitely see how it can be confusing.

Now, on the lean side, I do see how we could ask about both HP and "efficiency" of the engine. We'll consider that. For now, this means you can get the ROP accurate, and you can get the auto LOP/ROP switch right, but it may be optimistic or pessimistic about your % power once LOP with non-standard compressions. The nice thing is that it will never be wrong about you being LOP or ROP, so as long as you don't go LOP before you're down to a safe % power, you're not being misled into a dangerous setup.

The awesome thing about Dynon's implementation of all of this is that % power is actually valid LOP. In my IO-360, I run 26" MAP (when I can) and lean to 9GPH or less. People look at me askew when I am at 26" and ROP, showing 80%+ power and I tell them I'm going to lean. However, yank the mixture, and suddenly you're at 70% power even though things like MAP, RPM, OAT, etc didn't change. No other EFIS I know of can do that, which is why everyone seems to think they need to fly around at 24" before they can lean.

Mmarien, I'm not sure what AFR has to do with deciding your % power. Most people use % power as a way to tell them when it's safe to lean. If you don't have any idea what % power you're at, how do you decide when you can go to a lean AFR? I see they are both useful tools, but I don't see how AFR replaces % power in any way.

 

[edwalker]

Good summary, thanks. So, can your remind an old Lycoming driver why my new LSA Rotax 912 ULS does not have power calculations as part of the setup in SV?

 

[dynonsupport]

A few simple reasons:

1) Rotax does not publish power charts. They consider them proprietary.
2) You have no mixture control on a Rotax. Whatcha gonna do with the information?

But you know, #1 is a biggie.

 

[mmarien]

Mmarien, I'm not sure what AFR has to do with deciding your % power. Most people use % power as a way to tell them when it's safe to lean. If you don't have any idea what % power you're at, how do you decide when you can go to a lean AFR? I see they are both useful tools, but I don't see how AFR replaces % power in any way.

Doesn't. I just don't use it much. I do the same as you (and Mike Busch). At WOT and ROP and leveling off I just turn down the RPM and pull the mixture lean. As you say the % power drops below the safe 75% when I lean quickly so I don't care much what the % power is before I do that. Rather than % power I use the AFR for setting power peak (12.6) and efficiency peaks (15.4). Robert from EFII said that 13.1 AFR is the power peak for his system on a Lycoming but 12.6 has better cooling on climb out. The following chart is a good indication of AFR at those points. Without the AFR gauge I would have to lean until the air speed drops off to determine peak efficiency.

Lycoming has the same chart (below) but without the ratios. They go by delta EGT which is harder to determine and you have to find peak first which may put you in the danger zone if you lean slowly at high power settings.

 

[GlennB]

Now, on the lean side, I do see how we could ask about both HP and "efficiency" of the engine. We'll consider that. For now, this means you can get the ROP accurate, and you can get the auto LOP/ROP switch right, but it may be optimistic or pessimistic about your % power once LOP with non-standard compressions. The nice thing is that it will never be wrong about you being LOP or ROP, so as long as you don't go LOP before you're down to a safe % power, you're not being misled into a dangerous setup.

This is the point I raised in the original post.  Yes, the % calc is accurate when ROP, which is probably where that's most important.

The LOP % figure is also spot on, if a couple of assumptions are correct.  The main one is that the engine's SFC is at the usual number of 0.4 lb/hr/HP.  For our low-compression engine it's closer to 0.44 or so, which causes the % figure to overread.  The Skyview thinks the engine is making more HP than it really is, for the fuel flow it's seeing.

This is certainly on the safe side if we are to use its display to define and avoid the APS "red box".  And it's not a huge issue if it's a bit out anyway.  It would just be a nice tweak, to raise the accuracy of this parameter to that of all the others 

 

[Bob_Redman]

Thank you Dynon & MMarien.

I assume that your explanation is true for the D120 ?

Regards & thanks,

Bob Redman

 

[mmarien]

The HP number can be tested and adjusted without knowing the exact HP. Start with an HP estimate- I assume you can get within 25%. Then lean the engine manually at about 70% power, finding peak on the EGT temps. If the automated LOP/ROP reads peak too early or too late, you can adjust the HP number until everything matches.

I did this test and adjusted the HP until the ROP/LOP was reporting LOP at 14.7 AFR. All the EGT's peaked at about 14.9 AFR but I'm thinking the AFR gauge is more accurate, or perhaps the average peak as the O2 sensor in the collector pipe and the LOP from the LEAN function doesn't switch until the last EGT peaks. At that point three of the cylinders are already LOP.

I had to adjust the HP back to 150 HP to get the ROP/LOP to switch at the correct place. The o320 has 9.1:1 pistons so it's probably making about 163 HP. So while the ROP/LOP switch is correct, I assume the %power is not as it depends on the HP according to GlennB's formula.

So, if I want an accurate %power, would I get that if I changed the HP back to 163 or approximately the real number? The ROP/LOP probably isn't a concern as I use the AFR gauge to determine ROP/LOP.

 

[Steve_N]

A few simple reasons:

1) Rotax does not publish power charts. They consider them proprietary.
2) You have no mixture control on a Rotax. Whatcha gonna do with the information?

But you know, #1 is a biggie.

Rotax power curves are indeed published in their operators manuals. Example (see section 5):

http://www.flyrotax.com/portaldata/5/dokus/d05649.pdf

 

[dynonsupport]

Those don't look like the power charts I am used to. Help me out:

At 4700 RPM, 21" of MAP, 4,000' MSL, and 20C, what is the % power I am producing? I can't see how I can get to that answer from those tables, but I am probably being dense.

 

[Steve_N]

Well i'm just a grunt pilot pointing out there are published power curves and I'm not sure Rotax are trying to hide anything.

Using my 912ULS as an example on page 5-7 they give a formula for compensating from ISA to various altitudes and temps but I guess that is the stuff you calculate anyway for Lycoming and laws of physics will be the same in US/Austria 

So your question is really at a given RPM and MAP what is the power produced.  Most Rotax are turning fixed pitch props so MAP is less important thus data a bit more elusive I grant you with only one curve offered in Fig 5. 

I have a CS prop and page 5-6 shows the RPM/MAP/% settings Rotax recommend we operate at, with the resulting outputs shown.  I would say most of us operate within those 55%-WOT datapoints shown.  If there is anyway you can interpolate between those data I think you produce a % number at least as useful as you get from the Lycoming graphs which I think we all agree will only be delivered with "ideal" engine conditions.

Not sure if that helps :-/

 

[Battson]

The HP number can be tested and adjusted without knowing the exact HP. Start with an HP estimate- I assume you can get within 25%. Then lean the engine manually at about 70% power, finding peak on the EGT temps. If the automated LOP/ROP reads peak too early or too late, you can adjust the HP number until everything matches.

I am interested in this - can you explain in more detail why the horsepower, %power, and peak EGT are interrelated?  I read your earlier comment about horsepower not relating to the other two variables, apart from at peak, but I just can't put all those variables together in my head to see how HP should changes if the calculation says peak EGT happens early / late compared to the %power prediction?

Am I missing something?  Can you explain in more detail please?

To an earlier question, I believe SV and D100 series all use the same %power calculation.


Post Script:
There also seems to be an implicit assumption in some of the Dynon Employee posts above. An assumption that a pilot can always run the engine LOP below 70% power, as if 70% is some special safety margin. I do not think that assumption is correct for 100% of test cases. As stated, 70% power LOP occurs at throttle / RPM settings which would give closer to 80% power if the engine was ROP. Such high settings can generate a lot of pressure in the cylinder and thus a lot of heat. To achieve 70% LOP you also need to be reasonably close to the peak, 60*F LOP might not do it...
Whether you can safely run LOP at such high power settings depends on the engine installation and the day you are flying. With a normal type installation, a hot summers day could cause the CHTs to run too hot and cause alarm, but with a modern cowling you might not see this kind of warning that you're damaging the cylinders. Mike Busch or Savvy Aviator discusses this at length.

 

[dynonsupport]

I'm not sure what your question is. We listed how one can verify that the LOP/ROP auto detection is working, and that is to lean while watching the EGTs and verifying that the automatic LOP/ROP switches from ROP to PK to LOP as your EGT's peak as well. They're related because differential EGT is one measure of being lean or rich, and our calculation is another, and both should tell you the same thing.

In order for us to detect LOP/ROP accurately, one of the math inputs we need is an "HP" number. It's really a fudge factor that adjusts for a lot of small variances, but does start with the primary knowledge of HP. % power is of course just current HP over rated HP.

We're vary familiar with GAMI, Mike Busch, John Deakin, and more. This is a complex topic, and everyone has their own opinions. Dynon is not telling you how to run your engine, we're just giving you the tools to operate it the way you want.

In my personal opinion, I do believe that 70% is a generically safe place for a normally aspirated piston engine to be run LOP. It doesn't matter if that would be 110% if you were ROP. All that matters is your current power. You can lean an engine that is running 80% power down to 65% power without touching the blue or black knobs and be safe. That's the whole point of our advanced % power algorithm in fact- it doesn't just tell you your % power IF you were ROP, it tells you your % power now, which is what you really want.

Personally, for me, on an IO-360, if you are at or below 26" MAP and at or below 9.0 GPH, you are safe because the fuel is limiting your power. Doesn't matter that if I gave it more fuel I might be at much more power.

--Ian Jordan, Dynon Avionics

 

[Battson]

So, if I have understood your posts - correct me if I am wrong - how the computer calculates the LOP/ROP change over, means that if the measured EGT's reach peak but the ROP / PK / LOP doesn't change at the exact same time, then you're saying the horsepower setting is wrong?   In other words you could determine a HP rating using that information? 

I guess what I really want to know, is can I determine exactly how much HP my engine is making, using those variables discussed above.

WRT my post script - I just wanted to air the fact that there's nothing special about the 70% (or other) power level for leaning, in terms of a safety margin. In my larger IO-540 engine installation that %power might cause overheating on a warm summer day, so I just wanted to point that out. For instance, suppose I choose 60% for my engine - why is 70% right and 60% wrong? [rhetorically]. There's no evidence to suggest which %power is related to the safe maximum, other than operational experience, with a given installation.  Really the physical limitations on running LOP seem to be related to cylinder peak pressure (as detailed by all those people listed above) or the CHT. For instance, Mike B started out talking about %power quite a lot, but lately seems more focussed on CHT only, maybe there's some new research or evidence behind that, I haven't asked him yet.  But a different and interesting use for %power is Lycoming recommends always running full rich above 75%, again for cylinder head cooling reasons I assume.

Personally I have been working off percent power in combination with EGT to choose a "safe" leaning spot, using all the information I could find, such as the Red Fin Diagram. But when I questioned where the upper safe limit should be, or questioned why I chose the "safe" limit I have chosen - I could find no scientific evidence to suggest a sensible maximum point for leaning past peak, other than one which would have to be measured in cylinder peak pressure [impossible given today's engine instrumentation].   At least that is my understanding, I am always seeking new knowledge on this subject. 

 

[dynonsupport]

You cannot determine the actual HP of your engine. We use HP because it lets people get close to start, but ironically if you have a more efficient engine, you lower the HP number, not raise it. It's not related to what you will find on a dyno. It's just an accurate way of knowing if you are LOP or ROP.

 

[DBRV10]

Perhaps I can help here;

First of all I must declare I am part of the Advanced Pilot Seminars team, and Andrew Denyer and myself run the APS Engine Management Made Easy classes in Australia, and along with John Deakin, Walter Atkinson and George Braly in the USA do our best to help people understand these topics on various internet forums. The downside is there is no easy way to educate fully unless a lot of ground level things are understood, and even our online class is only about 70% as effective as the live class room. So if you really want a very good education, register for the next class in Ada OK, in March.

So to answer the questions above as best I can here,

There is no reason whatsoever to apply a limit on % power and running LOP. In fact we can and do run on the dyno in Ada a turbocharged IO550 in excess of 100% power and LOP……and VERY safely. The secret is doing it right. Lycoming have published this 75% limit because often they could not actually get their engines to run properly LOP (same as CMI) and then the pilot would richen up a bit for smooth running and then be at a high power and right in the middle of the "Red Box". By the way the Red Box was an APS initiative back 14 years ago. It has been refined since then.

As long as you run appropriately Rich enough or Lean enough, then it is OK with the laws of physics. Remember the laws of physics determine what happens to your engine. Not my words, or those of John Deakin or the manufacturers either. Sadly the OEMs have printed some scientifically incorrect and poor advice over the years.

Here is the guide to what is appropriate, regardless of what engine you have.

________________________________________
Red Box = No Fly Zone
• At and below about 60% power, there is no red box. Put the mixture wherever you want it.
• At about 65% power or so, 100ºF ROP to Peak.
• At about 70%, 125ºF ROP to 25ºF LOP.
• At about 75%, 180ºF ROP to 40ºF LOP.
• At about 80%, 200ºF ROP to 60ºF LOP.
________________________________________


________________________________________
Outside the Box
• At 65% power, use richer than 100 ROP, or leaner than peak EGT.
• At 70%, use richer than 125ºF ROP, or leaner than 25ºF LOP.
• At 75%, use richer than 180ºF ROP, or leaner than 40ºF LOP.
• At 80%, use richer than 200ºF ROP, or leaner than 60ºF LOP.
________________________________________

For a graphical education on the red box we publish this on the APS site for free! http://www.advancedpilot.com/redbox.html
There is a short video and an interactive demo at the bottom of the page.

As you can see there is an infinite number of settings and there is no really good "Cookbook" approach.

I routinely operate at about 83% power, WOT/2450-2500 and 80dF LOP at low level. CHT's in the 300-320 range!

LEAN FIND in the Dynon works very well, and it is an awesome training and diagnostic tool, however in real life there is a better way. Here are some words I wrote on the Beechtalk site some time back, I hope they are useful here. Once you get good at this, you will not need to use the lean find function except for do a GAMI Lean test occasionally.

Assuming you know what power you are going to be making, i.e. something from 10dF through to 80dF LOP, the simple procedure is this, set your EMS to display the RAW DATA for the cylinders EGT and find the one that peaks last from the rich side, or first from the lean side, this is your richest cylinder. Lets call it 'Cx'.

Level out, accelerate, things start cooling off, close cowl flaps, hopefully accelerate a bit more, do a BMP (Big Mixture Pull) until you feel the speed loss. You may have gone too far but do not worry. I then set about doing a few cockpit chores, which might be checking estimates, setting heading bugs, whatever for a minute or three. During this time the CHT's have cooled off a bit. Then with the EMS set to display EGT raw data, watch Cylinder X, slowly wind in the mixture watching the EGT for that known to be first to peak cylinder. As soon as it peaks you remember what the egt was then roll back leaner to the desired LOP offset.

Once you get good at BMP's you will often just land right there anyway.

The other option is to use the lean find and let it alert you to which one peaks first, then when it says 020 being 20ROP, you need to add that to the RAW DATA number of that cylinder, and then work back to the lean side.

Hope that helps. But I bet a weekend in Ada in March would help a whole heap more in understanding WHY to do things, not just monkey see/do.

Hope that helps.

Regards

David Brown
Advanced Pilot Seminars
And Dynon advocate!

 

[Battson]

Thanks very much David - that's exactly where my understanding of best practice leaning is at too. Although I am used to expressing that information using a "fin" rather than a box, like this example (although I use the same numbers you have presented):

http://sparky.supercub.org/photopost/data/500/medium/SR20LOPRedFin.jpg
Of course the trick to using that information, is knowing what peak EGT is - which changes with altitude / temp / air pressure / all engine settings.

One question I have always had... why are the limits of the box set where they are?  In other words, what are the consequences of infringing upon the box?

It would also be interesting to know why this should apply to most any typical air cooled ICE used in aircraft regardless of cylinder compression ratio, cooling, ignition timing, etc?   Surely an engine high higher compression making more real power has different limits?