View Full Version : 2100rpm and 25.9 inches for a non-turbo?
12th Nov 2003, 18:20
As a follow up to my earlier post on economy cruising in an Arrow, I would like to hear some comments on running a constant speed unit and engine at what appear to be unconventional settings.
(The following refers to NON-turbo engines.)
I am referring o POH Arrow cruise data which I have been studying. Now I was always under the impression that manifold pressure should not greatly exceed the rpm divided by 100. However, the POH data tells a different story.
For example, at 55% power, at 2000 ft, it offers two possible options, the first is at 2400 rpm with 20.0 inches of manifold pressure.
The second option is to use 2100rpm and 22.4 inches.
A more extreme example is for a 65% cruise where instead of using 2400rpm and 22.9 inches, it offers 2100rpm and 25.9 inches as an alternative.
Apparently the speeds and fuel flows are the same for each pair of settings.
So what is going on. Can you fly a normally aspirated IO-360 CiC with constant speed unit at 2100rpm and nearly 26 inches without damaging it,? The POH seems to say yes.
Maybe its just not good practise, like cruising your car in 5th at 25mph, or maybe its a case of so many instructors 'choosing' their own 'round' numbers, such as 2400/24, which seems to be so commonly advocated, rather than taking exact figures. Of course there is no need to cruise at exactly 65, or 75 or any particular percent power, but I diverge. - is it safe to fly at 2100/26"?
12th Nov 2003, 18:42
This is one of those recurring themes on this forum.
'Square is good, oversquare is bad'
What are we comparing here. Indeed apples and pears!
The POH is 'the bible' when it comes to operating your mount, so if the POH states you can do it it should be fine.
Although Lycoming state on their material that turbo and helicopter engines use different parts I actually am not so sure about this.
They just last last that bit shorter! (TBO that is)
12th Nov 2003, 23:28
If the setting is permitted in the POH then yes, it's fine. Cruising with a high MP and low RPM has a couple of economy advantages. Pumping losses and frictional losses in the engine are both reduced, leading to better fuel economy. It's also a great deal quieter.
The problem with extreme high MP and low RPM comes when it is used at very high power settings and depends on its interaction with the timing. High MP and low RPM both tend to speed up the combustion within the cycle. That means the peak pressure occurs closer to top dead centre, and that in turn increases the peak pressure as the volume in which the combustion occurs is less. This is exacerbated by high CHTs, which increase the pressure.
In these extreme cases, the peak pressure is so high that it can damage the engine. You can rest assured that the settings in the POH are some way away from this pre-ignition regime.
13th Nov 2003, 00:51
Lycoming must be psychic. This very question is answered on their site - It even uses the 200 HP engine in the Arrow as the example!
They seem to suggest that you should use the setting (from the POH) which gives you the least vibration and noise, but also that you can achieve slightly better economy by going for lower RPM for the reason bookworm stated. Note that this contradicts what you said about the speed and fuel flows being the same. Maybe the difference is so marginal as to make no real difference?
13th Nov 2003, 02:43
I think, Mr Major, it's just that the aircraft manufacturers don't bother and use the most conservative value. The nomograms in the engine manuals often tell a different story.
I don't have the IO-360, but I have the chart for an O-360.
It indicates that at sea level you can get 140 HP (70%) from
25.8" 2200 using 11.1 gph
23.2" 2700 using 11.9 gph
That's worth about £6000 over the life of the engine and we haven't even touched the red knob yet :)
BTW another thing in the O-360 chart is the maximum MP permitted for continuous operation at each RPM.
2000 max MP 25"
2200 max MP 26.8"
2400 max MP 28.9" (which is effectively full throttle)
13th Nov 2003, 18:41
You need to get the Lyco engine operating manual. It probably does not come with the plane (mine didn't) so you have to buy it separately.
It contains graphs showing what MP is allowed at what RPM. These are produced using calculations of crankshaft (or in the case of the IO-540, apparently, prop mounting flange) stresses. It makes for very interesting reading, and I have to warn you it might demolish much of the generalised advice your favourite instructor has been giving you and everybody else on engine management!
The POH should have been written by consulation to the above document.
Not exceeding RPM/100 is nonsense. They are different units (rpm and inches of mercury) so comparing them is nonsense.
14th Nov 2003, 14:46
I know it's a different engine/prop combination, but the 'rule of thumb' for HM's Bulldog with it's IO 360 used to be:
Set Best Power Mixture of 6.5 psi at 1500 ft pressure alt.
To avoid overboosting, do not use a MAP greater than (RPM/100)+ 4.5 in Hg.
So, at 2100 RPM, the MAP limit would be 25.5". An easy way to tell was that if the MAP and RPM needles approached a visually parallel position, you were on the point of overboosting.
However, most folk just flew the little thing with all levers fully forward below 2000 ft.......the taxpayer picked up the bill!
14th Nov 2003, 23:39
One item that was not yet mentioned: LOP or ROP.
It makes a big difference at low RPM!
At the worst possible place (50° ROP) the flame front is travelling fastest, having a richer mixture (100°ROP) or leaner mixture (Peak EGT and lower) slows down the flame front considerably.
What has this to do with the topic? Well, since the spark advance is about 22° BTDC on average engines, this means that part of the combustion takes place BEFORE the piston has hit top center and while it is still travelling upward towards compression.
This situation is obviously not good. Running at low RPM increases the time (in milliseconds) that the F/A mixture can burn before the piston gets over the top. Running at 50°ROP at low RPM at high power is asking for trouble. (fast burning F/A mixture + slow moving piston).
Running at low RPM/high MP at lean mixture is much more beneficial. Low RPM means lower internal friction (gaining maybe 15 HP versus high RPM), the peak power pulse comes later in the piston cycle while the F/A mixture burns slower. This means that more of the power pulse falls after piston TDC giving more mechanical advantage.
So the combination of lean mixture, low RPM in a smooth running engine (are your F/A ratio's balanced between cylinders?) will give savings in the order of 25% or more at very little airspeed loss. Compensate lower RPM and lean mixture with higher MP.
Lower RPM means less noise for the people on the ground too, this can't be bad.
16th Nov 2003, 09:37
I would like your opinion on the GAMI's injectors, because I'm looking for a local dealer to buy some. And I would be interested in being a dealer in Brasil, if it proves it's worth in my plane.
I've read ALL John Deakin's colums in avweb's site and I'm VERY impressed. I've been using some of the knowledge in my plane, but without running LOP (yet). The leaner I could get without the GAMI's injectors is peak EGT, and it proved to be VERY good.
I usually run at 2300RPM, wide open throttle (18 inches at 10,000ft), peak EGT with 10 gal/hr; pretty efficient to me, because it gives me 140KIAS/ 165 KTAS)
For longer flights (above 500 nm and below 700nm), I set the prop at 2100RPM, peak EGT, wide open throttle (18 inches) with 8 gal/hr; it gives me 120KIAS, 140-150KTAS.
The CHT? Well, I get 350F at 2300RPM and 300F at 2100RPM. Unfortunately I don't have a digital monitor, but I'm willing to get one.
About my username in PPRUNE: it's my babe, my Bonanza's serial number.
16th Nov 2003, 13:34
Hello E1453, greetings from E445!
Your settings at peak EGT are fine at these power settings! Try to go even leaner, you may or may not find smooth running. You won't damage anything, but the pax don't like rough running engines. It may help to close the throttle say 1/4". This puts the throttle butterfly valve slightly closed and causes some turbulence in the intake system that helps.
If it runs smooth LOP (unlikely in an unmodified IO520) then you don't need GAMIjectors, just be sure to put the injectors always back in the same cylinder after cleaning.
If it runs rough LOP then there is an inbalance between the fuel/air ratio's.
The front and middle pair of cylinders get additional fuel-laden air from the rear. Induction air on this engine flows from rear to front, and because the intake valve is open one fourth of the time only, while the injectors spray continuously, fuel-tainted air gets to the middle and front cylinders.
So the rear pair of cylinders gets the fuel only from the injectors, the middle pair gets injector fuel plus carryover air/fuel from rear , the front pair gets injector fuel plus carryover from rear and middle. This means the front cylinders are running much richer than the rear, the middle cylinders run leaner than the front and richer than the rear.
It is far easier to compensate this design fault by adjusting the flow of the injectors than to redesign a different intake duct, so this is precisely what GAMI did. Gamijectors compensate for this hidden fuel transfer and permit all cylinders to run equaly rich or lean.
Since the power curve ROP if fairly flat, even engines with mismatched F/A ratios will run fairly smooth. When LOP any mismatch will show up and prevent smooth running LOP.
You have a 30 day return privilege on the Gamijectors. If the engine does't run smooth after installation suspect an induction leak in the rubber couplings of the intake duct.
A great way to test your ignition system is to do it in flight, LOP , bad spark plugs or ignition cables will show up that pass the standard ground runup.
If you have a JPI 700 engine analyser, GAMI will gladly further tweak the injectors (they send a replacement with slightly different fuel flow).
First class products, both the JPI and GAMIjectors.
16th Nov 2003, 19:17
I always try to run LOP, but at 25 LOP my engine starts to run rough, and I'm not confortable flying with this roughness. That's why I leve it a peak EGT.
I know the causes for the mixture unbalance in Continental engines, but I would like to know the causes in Lycoming's engines. Anyone knows?
I always do a mag check in cruise, just before TOD. I read it first time in PerPerch's column, have been doing it since then. The first time I did the check in-flight, mixture leaned, I got surprised. The mags seemed to be so well in the runup check, but in the lean check the right mag started to cause moderate roughness. I checked it out and the plugs were fouled. Thank's Perch!
I also use the climb leaning technique.
As for the descent, I set the manifold at 16 inches and rpm to 1800, then enrich the mixture slightly to 50 ROP (just to keep CHT's above 280F). In a 500FPM descent, I get 130KIAS, aproximately.
Do you know how many hours of operation it takes to the GAMI injectors pay for thenselves?
How much gas could I save after installing the injectors?
17th Nov 2003, 22:37
I have GAMIs too - very good. Significantly smoother running at say 10% LOP (in terms of GPH fuel flow).
But I should point out that I don't think you can purchase them unless you have a multi-cylinder engine analyser like an EDM700, because to order the injectors you need to do the GAMI test during cruise, noting the EGT profile versus the common fuel flow.
Deakin's stuff is very good and should be compulsory reading for everyone operating an injected aircraft. It's amazing how many instructors try to tell you to do something which they learnt many years ago on some twin, and which is simply wrong in the particular aircraft. Leaning during full power climb for example.
17th Nov 2003, 23:58
IO540 and E1453,
You can purchase the Gamijectors only, but in my experience (have set number 23 since early days) I would first install an engine analyser and then order the 'jectors.
Having the analyser will also permit to fine-tune the 'jectors.
I had a long discussion with my IRE last week, explained the whole concept of LOP to him, he actually bought me a drink and is looking further into the matter. he is an active airline pilot who came up through the GA ranks and still instructs and examines GA pilots. He was open minded and listened carefully to my explanations.
Reason for this was that my friend just did his IR checkride with him in his A36, during the initial climb he reduced RPM (for noise)while leaving MP and Mixture wide open. This prompted a comment from the IRE, but now he is ready for more knowledge and he enjoys it.
18th Nov 2003, 00:21
How could you usefully order GAMIs without the EGT data?
There are huge variations between the EGT profiles between engines of the exact same type.
It is sometimes necessary to have a second go at it, with GAMIs. I was lucky. A friend had to get a couple of injectors done again.
All this is the result of appalling bad QA at Lycoming - selling engines priced well into five figures, with injectors picked out of a box and chucked in...
18th Nov 2003, 02:49
You're obviously a Lycoming man! My involvement in Lycs ended in about 1993, analysers or Gamijectors were not yet invented then.
On the Continental, things are pretty predictable. The intake duct design fault is standard equipment and well understood.
I ordered the Gamis very early on, no JPI installed yet. My engine ran smooth up to 40°LOP (on the stock EGT). This by itself is a proof that the injectors are well matched and that there are no induction leaks.
Having a JPI however opens up a whole new world, helps fine tune the engine and helps troubleshoot in real time.
For example if you would have a detonation event on cylinder number 2 (rear left on TCM) how would you know before parts start flying out of the cowling? Hint: factory EGT probe is on right hand exhaust collector, and factory CHT probe is on number 3 cylinder. Detonation is identified by quickly rising CHT and falling EGT. The alarms in the JPI would inform you as soon as anyone parameter goes out of spec (set by yourself to conservative limits).
I have my CHT alarm set to 380°F, if that temperature would be reached I would take action immediately.
Given a choice, I would install the analyser first.
15th Feb 2004, 02:50
I have a question about dirkdj's last reply on this topic:
Why does the EGT drops during a detonation event?
The rising CHT is obvious, but I can't figure out the reason for the EGT falling.
15th Feb 2004, 16:32
This is a good question indeed!
During normal combustion you have two spark plugs igniting the charge creating two flame fronts advancing towards each other. This advance can be slower of faster depending on the mixture strength (very rich or very lean mixtures burn slower).
In case of detonation, the unburnt charge between the advancing flame fronts is compressed, heated and explodes by itself in a violent way, not a controlled burning.
So, even if the absolute temperature inside the cylinder will be higher during the detonation; because it is happening sooner in the cycle and by the time the exhaust valve opens at the end of the cycle, the EGT probe will take a 'snapshot in time' showing somewhat lower EGT temperature, regardless of the destructive event taking place inside the cylinder just a few milliseconds earlier.
The danger is that someone mistakes the classic signs of detonation on a graphic engine monitor (rapidly rising CHT AND falling EGT) for a simple CHT probe failure and fails to take immediate action (go full rich and reduce power).
For the same reason, a very very lean mixture will show an increase in EGT because the charge is still burning when the exhaust valve opens, this does not mean that the cylinder gets very hot but that the charge burns very slowly. At his very very lean setting I experienced in a M231 the power was so low as to be of no practical benefit, but the engine still ran smooth about 100°F LOP.
Some reasons for detonation would be:
-improper octane rating of fuel
-magneto timing too far advanced
-CHT too high (faulty baffling, high power-low airspeed)
-full power-full rich mixture not rich enough
(if the CHTs don't stabilize after take-off have your mechanic set the full rich even richer to at least the redline or above on the fuel flow instrument.) This is a major cause of short engine life.