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n45bm(at)yahoo.com
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 Posted: Sat Jul 16, 2011 6:26 am Post subject: Mixture |
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Group,
This thread is from the Corby Starlet Yahoo group. Thought it might be interesting to some of you.
Bernardo
Hi Russ,
I've been thinking of using some form of mixture control too. A mate with a 2.2 Jab in a Sonerai introduces a bit of vacuum into the balance tube which lowers the fuel level in the bowl resulting in leaner mixture. The greatest EGT increase he has been able to achieve is 30 degrees C. What he does is have a tube connected to a point like a static vent about 2" above the highest point in the wing camber in the fuselage. This runs through a valve/tap (lawn mower type fuel tap) then into a 't' in the balance tube. Opening the tap introduces a small amount of negative pressure from the low pressure above the wing. When you close the tap off things return to factory standard. Hope this makes sense.... What have you been doing?
Laurie
Yes, my system works on that principle although I have variable control. It’s important to realize that changing the air pressure above the float bowl fuel surface does not change the level – the float still has total control of this. The metered fuel through the jets is controlled by the pressure difference between float bowl surface and venture. Thus changing the float bowl pressure varies this difference for any given carby air flow and air density.
I would not recommend heading down this path because you finish up with too many variables working. First of all the normal carby adjustments are all still happening with changes in air density, power settings etc. My system still has a vent to the cowl area so any change in cowl flap also changes mixture. Change in airspeed will also change mixture. A range of other variables also come into play.
My aim was to investigate the engine behavior and temperatures under varying mixture conditions and I have achieved a reasonable understanding of what will happen.
I have a Grand Rapids EIS fitted which quickly warns me if anything untoward is happening. Believe me the EGT and CHT can move rapidly with slight changes in flight conditions and mixture. I wouldn’t fiddle too much without a monitoring system of at least this caliber. My findings back up the tuning figures recently mentioned from the US Jabiru distributors which are more stringent than the EGT range in our manuals for full power.
In general, the engine works well with quite lean mixtures but is a dogs breakfast to manage on a trip because of all the variable changing all the time. Better to be flying the plane. I think there are good benefits to be achieved, but with a carby that offers proper and accurate mixture control. I think the Rotec carby will offer this.
On a final note, it’s worth reading a lot of engine management articles that can be found applying to the bigger traditional aircraft engines. Check out “Pelicans Perch” and articles from the GAMI injector research. Some of these articles quickly made me realize how quickly we can destroy our engines when drifting into unacceptable sets of conditions.
Russ
[quote][b]
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vettin74(at)yahoo.com
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| Posted: Sat Jul 16, 2011 1:21 pm Post subject: Mixture |
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We use the HACman mixture control from green sky adventures with great results. It is a kit for about $170. We can usually save a gallon an hour above 8,000ft when leaned to the jabiru specs, we do not run lean of peak
Sent from my iPhone
On Jul 16, 2011, at 9:24 AM, "Bernard Melendez, Jr." <n45bm(at)yahoo.com (n45bm(at)yahoo.com)> wrote:
[quote]Group,
This thread is from the Corby Starlet Yahoo group. Thought it might be interesting to some of you.
Bernardo
Hi Russ,
I've been thinking of using some form of mixture control too. A mate with a 2.2 Jab in a Sonerai introduces a bit of vacuum into the balance tube which lowers the fuel level in the bowl resulting in leaner mixture. The greatest EGT increase he has been able to achieve is 30 degrees C. What he does is have a tube connected to a point like a static vent about 2" above the highest point in the wing camber in the fuselage. This runs through a valve/tap (lawn mower type fuel tap) then into a 't' in the balance tube. Opening the tap introduces a small amount of negative pressure from the low pressure above the wing. When you close the tap off things return to factory standard. Hope this makes sense.... What have you been doing?
Laurie
Yes, my system works on that principle although I have variable control. It’s important to realize that changing the air pressure above the float bowl fuel surface does not change the level – the float still has total control of this. The metered fuel through the jets is controlled by the pressure difference between float bowl surface and venture. Thus changing the float bowl pressure varies this difference for any given carby air flow and air density.
I would not recommend heading down this path because you finish up with too many variables working. First of all the normal carby adjustments are all still happening with changes in air density, power settings etc. My system still has a vent to the cowl area so any change in cowl flap also changes mixture. Change in airspeed will also change mixture. A range of other variables also come into play.
My aim was to investigate the engine behavior and temperatures under varying mixture conditions and I have achieved a reasonable understanding of what will happen.
I have a Grand Rapids EIS fitted which quickly warns me if anything untoward is happening. Believe me the EGT and CHT can move rapidly with slight changes in flight conditions and mixture. I wouldn’t fiddle too much without a monitoring system of at least this caliber. My findings back up the tuning figures recently mentioned from the US Jabiru distributors which are more stringent than the EGT range in our manuals for full power.
In general, the engine works well with quite lean mixtures but is a dogs breakfast to manage on a trip because of all the variable changing all the time. Better to be flying the plane. I think there are good benefits to be achieved, but with a carby that offers proper and accurate mixture control. I think the Rotec carby will offer this.
On a final note, it’s worth reading a lot of engine management articles that can be found applying to the bigger traditional aircraft engines. Check out “Pelicans Perch” and articles from the GAMI injector research. Some of these articles quickly made me realize how quickly we can destroy our engines when drifting into unacceptable sets of conditions.
Russ
[b]
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