Mixture Control

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pilotrs

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PilotRS
Do Jet Engines have a mixture control, the red knob, or is that only found on piston engines?
 
pilotrs said:
Do Jet Engines have a mixture control, the red knob, or is that only found on piston engines?
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They do but it's automatic. GA piston engines with carburetors or mechanical fuel injection such as RSA or Continental's system measure air volume rather than weight and have no barometric sensor so they have to be manually adjusted. There are many piston engined airplanes going back to at least the mid 30's that had automatic mixture control.
 
If I understand correctly the limit is maximum ET, which is affected by ambient air temperature and air density. The only adjustment is the power control. Back off of maximum power enough to limit exhaust temperature to the desired level.
 
Jet engines, like diesel engines, have no throttle or mixture control. The power control varies the amount of fuel being injected into the combustors (or diesel cylinders) and there is always more than enough oxygen for it. The gasoline engine uses a throttle plate to limit the amount of air entering, since too much air and too little fuel entering with it (it's not injected into the cylinder) would result in an overlean mixture that might not burn, since the fuel is spread out all through the charge of air.

Diesel and jet fuels burn as they are injected. It can't get too lean.
 
pilotrs said:
Do Jet Engines have a mixture control, the red knob, or is that only found on piston engines?
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Just to add to the above, you may also see a 3rd set of levers for certain turboprop engines called condition levers. These levers provide different idle speeds and provide a fuel cut-off function similar to recip engine mixture controls.
Here is a link that gives a bit more info and pics.
https://www.faa.gov/regulations_pol...ation/airplane_handbook/media/16_afh_ch14.pdf
 
I recall the Beech B-50 Twin Bonanza has automatic mixture controls. Leave the throttles at the firewall and the altitude compensating carbs are at auto rich. Back of a tad and the cyl head temps go up.
 
Maxnr said:
I recall the Beech B-50 Twin Bonanza has automatic mixture controls. Leave the throttles at the firewall and the altitude compensating carbs are at auto rich. Back of a tad and the cyl head temps go up.
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All of the normally aspirated Twin Bonanzas (save for a few that have been modified) have PS5-BD pressure carburetors, which have automatic mixture control. Some folks still lean them a little, but they do a pretty good job. A small number of supercharged Twin Bonanzas with Simmonds fuel injection don't even have mixture levers at all - simply a guarded switch for mixture on or off for each engine - no way to lean.
 
Dan Thomas said:
Jet engines, like diesel engines, have no throttle or mixture control. The power control varies the amount of fuel being injected into the combustors and there is always more than enough oxygen for it.
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There is always enough oxygen for combustion, but the mass of air flowing through in excess of that used for combustion is what determines the temperature. (More air = cooler temps)
 
Dan Thomas said:
...The power control varies the amount of fuel being injected into the combustors...
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Via the Fuel Metering Unit normally controlled by the FADEC which in turn is ultimately controlled by the EEC. Lots of parameters being monitored so the proper "mixture" is constantly optimized automatically. When you see guys in their "chipped" diesel pickups "rolling coal" they are overriding the ECU commands to dump more fuel into the cylinders than they can burn so they can make complete asshats of themselves by belching out clouds of black smoke into the faces of complete strangers.
 
Silvaire said:
When you see guys in their "chipped" diesel pickups "rolling coal" they are overriding the ECU commands to dump more fuel into the cylinders than they can burn so they can make complete asshats of themselves by belching out clouds of black smoke into the faces of complete strangers.
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Hence the old saying that propane is nitrous for diesels (and without the smoke of over injecting diesel).
 
MauleSkinner said:
There is always enough oxygen for combustion, but the mass of air flowing through in excess of that used for combustion is what determines the temperature. (More air = cooler temps)
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In gasoline engines, more air = hotter; thus over leaning an engine will over heat it. Running rich pushes unburned fuel out the exhaust, and that unburned fuel absorbs and takes heat with it. A stoichiometric air fuel mixture of 14.7 to 1 is the theoretical perfect mixture of air and gasoline, but it also runs hotter than engines can tolerate. Max Power is something like 12 to 1.

Funny thing though, if you lean further "lean of peak", you start to cool things off.

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bradg33 said:
All of the normally aspirated Twin Bonanzas (save for a few that have been modified) have PS5-BD pressure carburetors, which have automatic mixture control. Some folks still lean them a little, but they do a pretty good job. A small number of supercharged Twin Bonanzas with Simmonds fuel injection don't even have mixture levers at all - simply a guarded switch for mixture on or off for each engine - no way to lean.
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This was not a civilian "50". It was a military U8-D. I was unable to complete the check-out due to the airplane getting orders to report to Davis-Monthan
 
In a sense the only thing you are controlling on a diesel or turbine engine is the mixture itself.. the throttle on these is technically always wide open.
 
Tantalum said:
In a sense the only thing you are controlling on a diesel or turbine engine is the mixture itself.. the throttle on these is technically always wide open.
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Yes, there is no throttling of the air, but the mixture is not controlled in the same way that it is in a gasoline engine. The combustible range of gasoline is 8:1 to 18:1, in weight of air to weight of fuel, and if you get too rich or too lean it stops firing. In the turbines and diesels the fuel is injected and it burns as it leaves the injector; it's not dispersed all though the volume of air before combustion as it is with the gasoline engine, so we can't say that we're controlling its mixture. It burns as it encounters available oxygen molecules; it doesn't wait for a spark. In the turbine it's a continuous burn as air and fuel meet continuously, and in the diesel it's near TDC as the fuel is injected and the heat of compression sets it off as it enters the cylinder.

In a turbine only about 25% of the air is used for combustion. The rest is used for cooling.
 
Maxnr said:
I recall the Beech B-50 Twin Bonanza has automatic mixture controls. Leave the throttles at the firewall and the altitude compensating carbs are at auto rich. Back of a tad and the cyl head temps go up.
Click to expand...
I had the same carb on the GO-435 that was on my Navion. Apparently, while the carb was a stock item on the Navion, the altitude compensator was not so we found nothing about its operation in the manuals. After trying to figure out why the mixture control didn't seem to do much, we ended up calling the overhaul shop who told us that we weren't to mess with the mixture below 12,000'.

The only thing worse than one geared Lycoming engine is two geared Lycoming engines.
 
flyingron said:
I had the same carb on the GO-435 that was on my Navion. Apparently, while the carb was a stock item on the Navion, the altitude compensator was not so we found nothing about its operation in the manuals.
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The military U-8D (B-50) had a pair of GSO-480's. Not my favorite airplane.
 
Maxnr said:
The military U-8D (B-50) had a pair of GSO-480's. Not my favorite airplane.
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In fact, the U-8 was the plane should have clued us in to the fact there might be something up with the carb. It had a placard that said not to touch the mixture below 12,000. The mechanic had one of those sitting right outside his hangar and had seen the notation.
 
flyingron said:
I had the same carb on the GO-435 that was on my Navion. Apparently, while the carb was a stock item on the Navion, the altitude compensator was not so we found nothing about its operation in the manuals. After trying to figure out why the mixture control didn't seem to do much, we ended up calling the overhaul shop who told us that we weren't to mess with the mixture below 12,000'.

The only thing worse than one geared Lycoming engine is two geared Lycoming engines.
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Yeah, the PS5BD manual indicates there are versions with and without the automatic mixture control unit, which is just bolted to the front of the carb.
 
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