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InläggPostat: sön 08-02-03 19:22 
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Blev medlem: tis 06-12-19 15:54
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Using the Bernoulli equation deltaP = (density/2)*velocity^2 and assuming that deltaP should be the same for gasoline and E85 and using the inverse of the air to fuel mass ratios as a measure of the mass flow rate and then converting them to velocity in m/s by dividing with the jet area and the fuel density the resulting expression for the jet area ratio becomes: A_E/A_G =(AFR_G/AFR_E)*sqroot(rho_G/rho_E) The diameter ratio is then the square root of the area ratio.

Using AFR_G = 15, AFR_E = 10, rho_G =740 and rho_E = 780 the area ratio is 1,46 and the diameter ratio is 1,21.

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InläggPostat: mån 08-02-04 13:18 
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Hi

Thanks GL for this Bernouilli based calculation


In the meantime I got in touch with the author of the Wikipedia Synthesis "E85 in standard engine"
It turns out that the Wikipedia recommendation of extending the jet area by some 27-30%, then the jet diameter by some 13-14% is not the result of any mathematical calculation but the conclusion drawn from a practical and experimental standpoint .

In short :
- Many folks are very happy with that 30% increase in fuel flow
- E85 is a very forgiving fuel and will run well lean of ideal mixtures. Some people do this intentionally at light throttle and idle to improve fuel mileage.
- in the case of a person that is drilling jets it is much better to be safe and start small.

Mail details hereafter
Best
Bye


1/ Initial Question from Ceyal
Hello Hotrod
I read carefully your excellent Wikipedia topic "E85 in standard engines".
But … that is why I am writing you today
When running a RON95 gasoline designed engine with E85, 2 different strategies can apply

Strategy 1 : maintain stoech AFR
-------------------------------------
When running stoech RON95 : 1 kg of gasoline = 14.7 kg of air. When running stoech E85 : 1kg of E85 = 9.765kg of air i.e. 14,7 kg of air = 14.7/9.765 kg of E85 fuel .
In volume , the density of both E85 and RON95 should be taken into account and 14.7 kg of air = (737/780) * (14,7/9,765) = 1.422 kg of E85
So the volume of fuel to be injected should be increased by some 40% which is the basis for Carb engine, Jet size calculation in the most famous Etanol Swedish web site http://etanol.nu/ombygg-forgasare.php (sorry in Swedish)

Strategy 2 : maintain amount of heat produced
-----------------------------------------------------
To run the car similar, the same amount of heat is needed, whichever the fuel … which wikipedia "E85 in standard engines" reads : E85 contains only 72% of the energy compared to gasoline"
So the volume of fuel to be injected should be increased by some 28%

Consequence of Strategy 2 on a fuel injected engine with O2 sensor : no impact
-------------------------------------------------------
This additional 28% volume of fuel is in fact some 14% leaner than stoech … a margin the O2 lambda sensor is able to cope with.

Consequence of Calculation 2 on a Carb engine
----------------------------------------------------
To inject some 28% more fuel, the area of the jet should be increased by some 28% and the diameter by some 13% fully in line with Wikipedia, E85 in standard engine text
The mixture will remain lean because a stoech mixture should require a jet area increased by some 42% and the diameter by some 18% … and running lean is not the optimum, isn't it ?…

Questions
------------
On which basis did you wrote that the fuel rate should be increased by some 25%-30% in a Carb engine ?
Is it in the Heat calculation as done in calculation 2 above ?
Or is it another different rationale ?
From your standpoint, which rationale is the correct one
- the one described in Wikipedia with a jet Size extended by some 28%
- the one des cribbed in www.etanol.nu with a jet size extended by some 40%
- another one ?
Thanks



2/ Answer 1 from HotRod, the Wikipedia article main contributor
Sorry I did not get back to you sooner, but have not had the time until now. First off thanks for the kind words regarding the wiki article, but it has been modified many times since I made my major contribution (about 50%) of the original content.

Experience has shown that you want to increase fuel supply on a volume basis by about 30%. A fuel injector or a carburator does not meter fuel by mass but by volume. The 30% increase seems to be a very good starting point for most cars. Each engine will vary slightly in the exact fuel mixture it prefers but increasing by 30% will get you very close to a usable mixture.

E85 is denser than gasoline so on a volume basis you will flow more mass of fuel. Ethanol also has a lower viscosity than gasoline so the carburetor jet or fuel injector will flow slightly more volume at a given pressure with E85 than with gasoline. The net effect is that a 30% increase in volume flow will give you approximately 40% increase in mass flow.

For example my car comes with 420 cc/min fuel injectors, and by swapping in 550 cc/min injectors it have a very good fuel mixture for E85 with no other changes. Giving me a gasoline AFR if 11.5:1 on 100% E85 at wide open throttle. The ECU adapted to bring the mixture to stoichiometric under closed loop fueling.

I hope that helps!
Larry

3/ Second Mail exchange from Ceyal
Thanks for the answer
Originally Posted by hotrod :" ... regarding the wiki article, but it has been modified many times since I made my major contribution (about 50%) of the original content."
Are you the author of the section dedicated to Carburettor ? (the one which states that fuel volume and jet area should be increased by some 27-30%)

Originally Posted by hotrod : "E85 is denser than gasoline ... The net effect is that a 30% increase in volume flow will give you approximately 40% increase in mass flow."
OK but to run stoech on E85, a car needs some 50% fuel mass increase when compared to Gas(ratio of AFR 14.7/9.8) .. so with 40% the car will run lean

Originally Posted by hotrod "... my car comes with 420 cc/min fuel injectors, and by swapping in 550 cc/min ...The ECU adapted to bring the mixture to stoichiometric under closed loop fueling."
Agreed : no problem for fuel injection systems; as you mentionned the O2 sensor and the ECU will adapt to run stoech. The problem is for Carb engine.

A carb engine does not have any O2 sensor ... so to run Stoech it needs instantenously +50% more mass fuel (14.7/9.8) i.e some +43% more volume fuel because as you mentionned of E85 density when compared with Gas ... so instantaneously at least some 13-15% more volume fuel than indicated in Wiki article.

With Winter E85 (E70) ... a carb engine needs some + 38%mass fuel i.e. 33% more volume fuel to run Stoech

Best
Bye


4/ Answer 2 from Hotrod
Carburetor tuning is, and always has been an experimental process. You take your best first guess on jet sizes install them and see what the engine says it wants. Lots of people have used the 27% - 30% increase in jet area rule of thumb to make that first guess. After that, only your engine can tell you what to do. Carburetors have more than one circuit, you have power valves, accelerator pumps, main circuits, adjustable idle air bleeds etc. That rule of thumb number is only a first guess --- no two cars will want exactly the same jetting for best performance. In most carburetors your idle and low speed mixture is controlled both by fixed jet sizes and adjustable mixture controls so precise jet sizing is not even necessary on the idle circuit.

The proper way to tune the engine is to use a wide band O2 sensor and actually measure if you are getting lambda 1 mixtures at idle and light throttle cruise.

The difference between 1.50 and 1.40 increase in fuel flow is only 7% which is plenty close enough for the first guess on your jetting.

Everyone I know that has converted their carburetors has had good luck using the 30% jet area increase rule as the starting point. Especially since some of them are drilling out fixed passages, it is much better to be too small on the first guess than it is to be too large. If you are lean with that first guess, you increase the size. You are not going to get an "exact" increase in jet size for all carburetors because no two engines, and no two carburetors will want exactly the same jetting for best performance.

E85 is a very forgiving fuel and will run well lean of ideal mixtures. Some people do this intentionally at light throttle and idle to improve fuel mileage. Don't get obsessed with finding a precise answer to a question with a hundred unknowable factors.

Larry


5/ Last mail exchange from Ceyal
Thanks for your answer
Before sending these two emails, I was wondering if there were any mathematical justification for the 27-30% value .. while some other well known swedish E85 web site was recommending to drill the jet size by 40%, based on calculation based on the AFR and fuel density

Now I understand that the 27-30% is not the result of any precise math calculation but the result of a practical analysis made on various carb engines, considering that in fact there is no harm to run a little bit lean with E85.

Hope this summary is correct

So Thanks again for the wonderful global synthesis you made available in Wikipedia. I use it a lot and give it very often as a reference to some colleagues asking basic questions on E85

Best

6/ Last answer from Hotrod
Yes you are correct! 27% -30% is simply a recommended starting point that typically gives good overall results including good fuel mileage. After you get the engine running with that baseline tune, then you can evaluate what additional changes you need to make.

Many folks are very happy with that 30% increase in fuel flow, others who are interested in max power and high boost may use richer mixtures than the theoretical +50% increase, because E85 can run much richer than gasoline can.

Especially in the case of a person that is drilling jets it is much better to be safe and start small. Then redrilling to a larger size as necessary once they know how much additional fuel the engine wants.

It is simply a difference in how conservative you make your initial recommendation. They are simply suggesting a starting point about 8% richer than I am.


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