Mixture Control
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I agree about the articles being mostly concerned with big engines, well monitored, CHT and EGT gauges etc. However, today is try things out day with more confidence.
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Gave it a go today. First time I've used the red knob. On the ground I killed it once or twice, not quick enough pushing back in as it faltered.
Airborne pretty much a non event as the speed keeps it turning. I noticed the lack of throttle response when lean but a quick push back to rich was no problem, ref the "mixture rich first when increasing throttle". Getting that back to front did no harm except to run rough till I richend it.
All in all not a problem.
Airborne pretty much a non event as the speed keeps it turning. I noticed the lack of throttle response when lean but a quick push back to rich was no problem, ref the "mixture rich first when increasing throttle". Getting that back to front did no harm except to run rough till I richend it.
All in all not a problem.
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If a carburettor engine runs rough when leaned, it means all cylinders are not developing same power due to uneven fuel/air distribution. You may help this by applying a little carb heat. Also changing throttle position may help. Maybe your spark plugs are not performing well enough.
To test the ignition system do a LOP may test in flight. At safe altitude, check one mag, take hand off switch and wait a few seconds. If the engine would run very rough or quit, pull the mixture to cut-off, put may switch back to both and slowly reopen mixture. I improved my lop running dramatically by switching brands of spark plugs to Tempest.
To test the ignition system do a LOP may test in flight. At safe altitude, check one mag, take hand off switch and wait a few seconds. If the engine would run very rough or quit, pull the mixture to cut-off, put may switch back to both and slowly reopen mixture. I improved my lop running dramatically by switching brands of spark plugs to Tempest.
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If these engines are running richer than necessary because of the engine/carb design, I'm thinking that if the throttle is set to max static revs on the ground with mixture fully rich, then leaned to best revs, this would not be " fully rich" but would depend on elevation, pressure, density, temp etc? and should give best take off power on that particular day/place?
I'm not trying to rock the boat suggesting oddball ideas and I doubt if this would save anything worthwhile in fuel but, if the theory is correct then it would help me get my head completely round understanding this.
I'm not trying to rock the boat suggesting oddball ideas and I doubt if this would save anything worthwhile in fuel but, if the theory is correct then it would help me get my head completely round understanding this.
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If these engines are running richer than necessary because of the engine/carb design, I'm thinking that if the throttle is set to max static revs on the ground with mixture fully rich, then leaned to best revs, this would not be " fully rich" but would depend on elevation, pressure, density, temp etc? and should give best take off power on that particular day/place?
I'm not trying to rock the boat suggesting oddball ideas and I doubt if this would save anything worthwhile in fuel but, if the theory is correct then it would help me get my head completely round understanding this.
I'm not trying to rock the boat suggesting oddball ideas and I doubt if this would save anything worthwhile in fuel but, if the theory is correct then it would help me get my head completely round understanding this.
What you're suggesting is entirely correct (for a fixed pitch prop - for a CS prop you'd use a slightly different method as you can't use RPMs as your indicator). You can run up the engine to full throttle, then lean for best power (max RPM) to get the best performance out of your engine. However, in that situation the engine is developing 100% power so it'll heat up very quickly to beyond the maximum allowable values (CHT mostly) which may lead to detonation and other sorts of engine damage. So don't do this when you care about your engine.
Having said that, that procedure is exactly what you would do in a high density altitude (approximately > 5000' DA) situation, such as when you're taking off from an airfield in the mountains. In that case, the density altitude is such that the aircraft cannot produce more than about 70% of rated power anyway, and you need all the power you can get from the engine in order to fly before you run out of runway.
(And to help your understanding even further: I was flying as a passenger in a kitplane with very sophisticated engine controls. We took off from a sealevel airport and even during the early stages of the departure the pilot started leaning. However, he did not lean for peak EGT, max RPM or anything like that. He only leaned just a bit and used the CHT temperature as his most important indicator. For every twist of the vernier mixture control his CHTs would rise a few degrees, and he leaned so that his CHTs were just below the yellow bar. Through this, he achieved a few percent more power, and a bit of fuel saving, without the risk of engine damage. So yes, you can even lean when the engine is operating at 100% power, but you really have to be careful and know what you're looking for. And it would be foolish to attempt this in a carbureted engine with just one CHT probe.)
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Backpacker.
Thank you for that. It confirms my beliefs.
Years ago in the days of SU carburettors where you could adjust the mixture by screwing the needle valve up or down, not getting full power or running too hot if it was too lean etc.
Though too rich seemed to equate to too cold, lean ran too hot. This was of course in water cooled engines so the heat/cooling issues were solved otherwise.
Thank you for that. It confirms my beliefs.
Years ago in the days of SU carburettors where you could adjust the mixture by screwing the needle valve up or down, not getting full power or running too hot if it was too lean etc.
Though too rich seemed to equate to too cold, lean ran too hot. This was of course in water cooled engines so the heat/cooling issues were solved otherwise.
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"The fuel that's not burned helps cool the engine through evaporation." Not very convincing explanation:
The reason a lean or very rich mixture cools the CHT/EGT is that they slow down the flame front. The fuel/air mixture takes longer to burn, placing the power pulse later in the cycle, where the piston is moving more downwards already, expanding more, cooling down more.
Peak EGT and peak CHT are not coincident, they do not occur at the same mixture. The important one is the CHT since it reflects the long term heat dissipation of the head. Peak EGT is already past peak CHT and is a good place to run at 65% or below.
The reason a lean or very rich mixture cools the CHT/EGT is that they slow down the flame front. The fuel/air mixture takes longer to burn, placing the power pulse later in the cycle, where the piston is moving more downwards already, expanding more, cooling down more.
Peak EGT and peak CHT are not coincident, they do not occur at the same mixture. The important one is the CHT since it reflects the long term heat dissipation of the head. Peak EGT is already past peak CHT and is a good place to run at 65% or below.
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Trying hard to remember the sixties and referring to the BSA 650 twin I had as a "fuel cooled oil burner". Leaning the mixture too much made it run hot. If the ignition was too far retarded or too far advanced it ran hot, though retarded was worse, I think?
The Hilman Imp boiled dry every ten miles on the way from Edinburgh to Cornwall so we stopped in the middle of the Borders and re set the timing, using fence wire and the interior light bulb. The radiator cap could be removed after 100 miles engine still running!
Happy days.
The Hilman Imp boiled dry every ten miles on the way from Edinburgh to Cornwall so we stopped in the middle of the Borders and re set the timing, using fence wire and the interior light bulb. The radiator cap could be removed after 100 miles engine still running!
Happy days.
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Here is another excellent starting page:https://www.avweb.com/news/pelican/182179-1.html
Pay attention to the graph that is near the bottom of this page. There is a lot of information there how EGT/CHT/FF and HP relate to each other. Hint: you can only see EGT (maybe) and CHT in the cockpit, you have to navigate by those.
Pay attention to the graph that is near the bottom of this page. There is a lot of information there how EGT/CHT/FF and HP relate to each other. Hint: you can only see EGT (maybe) and CHT in the cockpit, you have to navigate by those.
Hi Crash One, I don't think any of this carburettor tuning procedure is relevant to car or Motor-cycle engines.. This is because the aero engines operate for long periods of time at a fixed high throttle opening.
Motor Bike riders are constantly varying the throttle to maintain cornering stability etc, and require instant response to a snap throttle opening. This requires a richer mixture to be already present, to ensure correct power delivery.
Also I have an almost Concours vintage motor cycle that has always been run rich. It suffered a blocked slow running jet one day, and caused the chrome exhaust pipe to become coloured because of the increased EGT... A replacement exhaust pipe will cost me over £50.
There are other IC Engines that use much different tuning techniques to ensure safe rpm operation.
.
Motor Bike riders are constantly varying the throttle to maintain cornering stability etc, and require instant response to a snap throttle opening. This requires a richer mixture to be already present, to ensure correct power delivery.
Also I have an almost Concours vintage motor cycle that has always been run rich. It suffered a blocked slow running jet one day, and caused the chrome exhaust pipe to become coloured because of the increased EGT... A replacement exhaust pipe will cost me over £50.
There are other IC Engines that use much different tuning techniques to ensure safe rpm operation.
.
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Quite right carb tuning is irrelevant. I was just remembering that it could be done in days of yore and by ear, no computers.
By the way, ref to blue exhausts, I thought we used to polish it out with Sovol Autosol? Toothpaste was just as good. Depends how bad it is I think.
By the way, ref to blue exhausts, I thought we used to polish it out with Sovol Autosol? Toothpaste was just as good. Depends how bad it is I think.
Thanks for the Toothpaste tip, it did remove most of the colouration.
Just thinking on about the other engines which will not tolerate lean mixtures... Model aero engines are always tuned for Maximum revs, then Richened half a turn to loose about 1000 rpm... say from 15000 to 14000. If this is not done, the first time the throttle is quickly advanced, the engine will suffer a 'lean cut' and will stop.
I think you need to pay very close attention to running a GA engine lean, especially if you have set it in the cruise at FL100, and then descended a few thousand feet.
Most cars these days have the economy mode that runs the mixture very lean, with the aid of the exhaust lambda probe, but this adversely affects the car's acceleration compared with the normal power mode.
Just thinking on about the other engines which will not tolerate lean mixtures... Model aero engines are always tuned for Maximum revs, then Richened half a turn to loose about 1000 rpm... say from 15000 to 14000. If this is not done, the first time the throttle is quickly advanced, the engine will suffer a 'lean cut' and will stop.
I think you need to pay very close attention to running a GA engine lean, especially if you have set it in the cruise at FL100, and then descended a few thousand feet.
Most cars these days have the economy mode that runs the mixture very lean, with the aid of the exhaust lambda probe, but this adversely affects the car's acceleration compared with the normal power mode.
