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  #31  
Old 12-24-2015, 05:35 PM
Bicyclops Bicyclops is offline
 
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Default Detonation margin

Detonation margin can decrease with advance, but only if it causes the peak pressure to occur well before about 17 degrees ATDC. Since the ignition system is only advancing the timing when MAP is low, it shouldn't be possible to detonate under most conditions.

The conditions that reduce detonation margin are high induction heat, high CHTs, and high manifold pressure as well as advanced timing. At altitude, MAP is limited and induction heat is usually reduced. If your CHTs are not above, say, 380 and rising, you are in no danger of detonating and advanced timing is merely decreasing efficiency loss by making up for slower flame spread. If you allow CHTs to rise out of control, it will possibly detonate. In fact, a sudden spike in CHT is a classic symptom of detonation.

Even down low on a hot day, it's hard to make a normally aspirated engine detonate if you keep the CHTs under control. You do that by staying rich, by reducing MAP, and/or by going lean enough to stay out of the infamous red box as defined by higher than 380 to 400 degrees CHT. Remember that, if you reduce MAP and or go well LOP, you are reducing % power which reduces the size of the red box and increases detonation margin.

Auto fuel was mentioned. 100LL burns slower under pressure than auto gas. Octane doesn't increase gas' power output. It just keeps it from exploding all at once when the heat and pressure in the cylinder is approaching its maximum. High octane gas allows us to have fairly high compression ratios, operate at high power settings, and still not detonate. Lower octane would mean that we would have more propensity to detonate under adverse conditions, the most critical of which would be CHT.

Ed Holyoke

Quote:
Originally Posted by StuBob View Post
Ok, so the idea is to advance timing in order to optimize theta-PP? If that's the case, what effect is there on detonation margin?
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  #32  
Old 12-24-2015, 06:30 PM
BillL BillL is offline
 
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Location: Central IL
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Quote:
Originally Posted by Bicyclops View Post
. . . 100LL burns slower under pressure than auto gas. Octane doesn't increase gas' power output. It just keeps it from exploding all at once when the heat and pressure in the cylinder is approaching its maximum. High octane gas allows us to have fairly high compression ratios, operate at high power settings, and still not detonate. Lower octane would mean that we would have more propensity to detonate under adverse conditions, the most critical of which would be CHT.

Ed Holyoke
Heat release (burn rate) of 100LL should be the same than gasoline, but your octane comments are spot on. Temperature/pressure under which a detonation burn front as opposed to deflagration burn front is most assuredly increased (octane).
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  #33  
Old 12-24-2015, 11:21 PM
salto salto is offline
 
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Default Lop

I know this may sound like an old dinosaur talking but before all this fancy gear and in aeroplanes ✈ that didn't have even an egt gauge we just pulled the red knob till it ran rough and then wound it in a couple of turns till it ran smooth. Never had any problems and never had to put all this thought into it. In saying this I will probably fit the latest gizmos and spend half the flight fiddling.
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  #34  
Old 12-25-2015, 10:02 AM
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Toobuilder Toobuilder is offline
 
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Quote:
Originally Posted by salto View Post
I know this may sound like an old dinosaur talking but before all this fancy gear and in aeroplanes ✈ that didn't have even an egt gauge we just pulled the red knob till it ran rough and then wound it in a couple of turns till it ran smooth. Never had any problems and never had to put all this thought into it. In saying this I will probably fit the latest gizmos and spend half the flight fiddling.
Yep. Thats what I was taught too. That initial pull to rough has been renamed the "BMP", and the roughness and back to smooth is now called LOP.

...we've been doing it for years, but now there is mass confusion.
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  #35  
Old 12-26-2015, 12:40 AM
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jjconstant jjconstant is offline
 
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I was taught that way too. The only problem is that if you have fuel injection and went to the trouble of balancing the injectors there is no "rough". You wind up pulling until you feel a very noticable sag in power. You are now very lean of peak. If you didn't use the lean funtion on your engine monitor or pay attention to peak egt you have to start enriching to find peak from the lean side. Nothing wrong with that though, but with balanced injectors you do need the egt to know where you are. The only feel you get is when the mixture drastically affects power and by then you are definitely lean.

On the lean side, the difference between 40 degrees lean and 100 degrees lean is something on the order of 10 kts for me. On the rich side that 60 degree difference is more like 3 or 4 kts. I'm not entirely sure why.

With the carbed training aircraft, I think the onset of rough was still a hair on the rich side or right at peak, so when you went rough and enriched to smooth you were likely around 50-75 degrees rich of peak. At least that was my understanding.
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  #36  
Old 12-26-2015, 01:47 AM
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skylor skylor is offline
 
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Quote:
Originally Posted by jjconstant View Post

On the lean side, the difference between 40 degrees lean and 100 degrees lean is something on the order of 10 kts for me. On the rich side that 60 degree difference is more like 3 or 4 kts. I'm not entirely sure why.
This is APS 101: The power curve on the rich side of peak is relatively flat. i.e. relatively large mixture changes do not result in much change in power. However, on the lean side of peak EGT, power drops off fairely rapidly as fuel flow is decreased. This is why cylinder to cylinder fuel flow imbalance is very noticeable in the form of engine roughness when trying to run lean of peak (LOP), but not noticeable when running rich of peak (ROP) mixtures.

Skylor
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  #37  
Old 12-26-2015, 08:28 AM
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MikeyDale MikeyDale is offline
 
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I'm just getting to know my IO360 with two bendix mags. I can lean past -50 peak EGT but if I go to far I will get a "backfire"! Is this normal?
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  #38  
Old 12-26-2015, 09:33 AM
Aggie78 Aggie78 is offline
 
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Mike,

I don't think you're getting a backfire, but an "afterfire".

The combustion event from what I understand is slower the leaner you go. Sometimes so slow, it's still going on when the exhaust valve opens.

The noise you're hearing is that process occurring out in the exhaust tubing. Not harmful is my understanding.

At least that's what I remember from Mike Busch's leaning webinars on the subject.

Good gouge and worth a listen, well worth the investment of your time.

Rob S.

Webinars found on EAA's video player website....
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  #39  
Old 12-26-2015, 11:25 AM
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MikeyDale MikeyDale is offline
 
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It will sure get your attention! Dont take long for me to get a turn back in!
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  #40  
Old 12-26-2015, 07:48 PM
lr172 lr172 is offline
 
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Quote:
Originally Posted by Bicyclops View Post
At low manifold pressure and high RPM, (where Lightspeed advances the most), the flame spread is slower than it would be at higher MAP/(lower altitude).
Ed Holyoke
I don't believe this is true. Obviously higher RPM's require more advance, as the piston is moving faster relative to the constant flame propagation/flame travel speed. Variable timing started with RPM compensation.

The only major factor that changes flame propogation is the air fuel ratio (aerodynamics associated with head/chamber design also have an impact but are constant for any given design) . Overly rich and lean mixtures burn slower than max power mixtures (12.3-13:1). RPM-based advance is set based upon max power mixtures. This advance setting does not result in optimum ICP, as it has a detonation margin built into it. Vacuum/MAP-based advance is used to advance the timing under lesser "load." This slowly takes out the margin and also accounts for the slower flame propogation (assumes lesser "load" conditions have leaner mixtures). It's all about hitting getting ICP at the proper ATDC point, but also building in detonation margins under high load conditions.

Larry
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Last edited by lr172 : 12-26-2015 at 07:51 PM.
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