Has anyone ever demonstrated a substantial power gain by varying ignition timing in cruise on Lyco or Conti engines?
This is in connections with MPG improvements claimed here and especially here.
Some of these claims represent a lot of HP and I an amazed the engines would be so compromised in cruise to achieve something else (like CHT control during max power climb).
I don’t see why it would not be true. The relationship between ignition timing and BMEP (average pressure on the piston which translates to useful torque) is well documented like here:
It sais “efficiency” on the ‘y’ axis but you can translate that directly to BMEP or power for ideal fixed conditions (they never are but it illustrates the point)
The problem is the ignition advance for that peak will be heavily dependant on manifold pressure: lower pressures will drive lower flamefront propagation (a slower combustion event) hence a higher timing advance is required for maximum efficiency and maximum power. Ie cylinder pressure has to be distributed during the crank turn in such a way as to maximize its usefulness. Think of it this way: a lot of combustion pressure at or before TDC or at or after BDC is useless and even detracts from BMEP.
Typical Lycosaurus fixed-timing is tailored to max power at max MP, and you would see a significant advantage at medium powers by increasing advance (in the graph example above easily 10% improvement by a 10 deg advance). Furthermore, even typical fixed-timing is even less advanced than required for max MP, in order to increase detonation protection, which only makes matters less efficient at lower MP’s
Hence you should see an improvement by having the right advance for the circumstance, more so on NA (vs turbo) engines at high altitude. The lower the MP the more improvement you should see by having optimal variable timing.
But this is digressing from the OP: I have no practical experience: in the air I drive traditional contisaurus and on the ground I drive electric…
This is the spark advance (vs baseline timing) curve for the Electroair EIS-61000 system where you see at MP lower than 24", ignition is progressively advanced up to 15 degrees more vs standard mag.
So it is really at the lower MP’s that you will see any differences.
Apologies if this is already in the ref’d threads
Vacuum advance was discussed previously with a very thorough study linked. The point I thought was interesting is that on a NA RV-8 at cruise, climbing another 2000 ft or less from something like 7000 ft provided the same aircraft efficiency gain and mpg as an appropriate increase in ignition advance at unchanged altitude. I figured advancing timing at low MP (high altitude) would be worthwhile but the guy doing the study basically said it’s not worth the trouble.
One can wonder what the OP is getting at, but clickbait… the subject of EI and its benefits has already been beaten to death in many different threads here.
In a nutshell:
but clickbait
Clickbait? Why would I do that, when we don’t carry adverts? That’s a pretty bad accusation. I asked a question. It’s related to GA, for a change…
your bird, and its certification (or lack of), allows you to install EI, and you will
Far from assured; many factors to consider. There are also contra reports e.g. this one, from 2008, on the US RV forum
While I agree EI may be of some limited benifit and aids in starting, it’s not like adding a CS prop to an engine, not even close. The HP gains are usually greatly exagerated by the guys making the EI’s. I’ve taken an IO-540 from EI back to Slicks with very little, to NO difference in performance & fuel burn. Certainly, IMHO not worth price or the risk involved. Granted, I’m pretty much turned off to EI’s right now due to recent in flight experiences, the materials used in making the EI’s, and other problems I have heard about in talking to other pilots. After market EI on Lycomings is just not worth the risk at this point in time.
The relationship between ignition timing and BMEP (average pressure on the piston which translates to useful torque) is well documented like here:
OK; I see that, but I didn’t realise that the typical ~20 deg advance is done for the max power condition, not a typical cruise condition. But even for that, I can’t find a reference, for the IO-540. I am finding various speculations for the current advance. There is some discussion from 15-20 years ago, when a lot more people used to discuss stuff properly in forums.
This is a good article which shows a possible 10kt increase, albeit with quite some CHT increase. There is a curious altitude dependency
There are other factors; should the engine be producing more power than approved by the manufacturer? An example is the Continental O-200. It’s timing advance was reduced by Continental to prevent damage to the valve train, which had been attributed to too much power being produced with the more advanced timing. Another factor for large Continentals is the increased risk of kickback during a start, which can cause expensive damage to the starter, and indeed, the whole engine if it actually starts following a kickback start. Detonation margins may also be affected, if you detect detonation in flight, it’s too late, your engine has been damaged.
If the engine manufacture (who will be considering a broad range of factors) approves a change in timing then it’s fine. In the absence of manufacturer approval, many factors must be considered, as a failure resulting from too advanced timing can be very expensive!
At low manifold pressure and high rom, which is the condition under which more ignition advance is advantageous, the engine will be making well under rated power in any case. That is the condition for high altitude cruising flight.
The original fixed ignition timing spec continues to be used for wide open throttle, low altitude operation because that’s what it is designed for. Everywhere else it is non-optimum for the engine, although not greatly so.
Another post on the RV forum from 2008
I wondered that too, as I have an IO-360-A3B6D (angle-valve 200HP) in my -6 that’s spec’d at 25 deg, when most 8.7:1 angle-valves are spec’d at 20 deg. I heard from a friend who allegedly asked a Lycoming engineer why that was, and he said it was so that Mooney (the original application of my engine) could make the performance numbers it promised. Apparently, the engine made just enough more power at 25 deg to make the numbers. That begs the question of if the engine could tolerate more timing, why weren’t ALL of the 8.7:1 engines re-rated at the higher advance? As far as I can tell, Mooney 201 J’s didn’t have any better cooling than the usual spam can of the day and therefore weren’t any more or less tolerant of higher advance.
It does make one wonder… but it does tend to confirm that the ignition advance is indeed done for the max power case.
Yes, the fixed ignition timing is set for max power. In a gasoline engine development program the timing is varied to determine the angle for maximum power at whatever RPM makes most power based on other factors, always with full throttle. If the optimum is a relatively small advance angle it is indicative of a good combustion chamber design and small bore. That relatively small advance angle then allows use of relatively low octane fuel. Conversely if the angle for most power is relatively large, indicative of large bore and other combustion chamber design factors then higher octane fuel is required. Sometimes a compromise is made with ignition advance set slightly less than optimum for power, to allow for lower octane fuel with the same safety margin.
Once the optimum advance angle for maximum power is set, real time spark timing adjustments can provide more advance to account for lower MP and thereby slower flame front propagation. That increases partial power efficiency. For a car engine this is called vacuum advance and it often means huge ignition advance at small throttle openings (e.g. 50 degrees) For a plane it can mean more advance at high altitude cruise, with the throttle wide open but with low ambient air pressure.
Also, on an engine with a wide loaded speed range, in combination with vacuum advance a smaller timing advance angle is used at low rpm than high rpm – for fairly obvious reasons. A 3D ignition timing map results. However aircraft engines are nearly constant speed engines so RPM variation is not as important. Impulse couplings are enough to retard the timing for startup at low rpm, when turning on the starter motor.
