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MG MGA - E10 and air/fuel
|For those interested in this subject, I have calculated the air /fuel ratio required to completely burn fuel to CO2 and H2O ("theoretical")|
results: gasoline 15.06
Assumptions: gasoline is C8H18 octane, E10 is 10% by weight (not sure if it is actually volume % but makes little difference)and ratios are by weight.
Based on this, might there be a case for running a richer needle than original?
|Art, I thought petrol was a mix of hexane and heptane with only a touch of octane and, at the other end, pentane? Mike|
|You could be right mike. I don't think using heptane as the model compound would make all that difference as it has almost the same C/H ratio. The end result is that E10 needs to run about 4% more fuel for the induced air.|
|Well, if I remember right, stoichiometry for complete combustion of air and gasoline mix seems to be about 14.5:1 by weight (3/4 ton of air for 100 wt of fuel).|
Also alcohol contains about 40% less energy than gasoline (by volume). That should translate to a 4% reduction of maximum power output using E10. It follows that you have to burn (and pay for) about 4% more E10 than straight gasoline for similar driving results. It has always bugged me that E10 is usually not 4% cheaper than straight gasoline.
In the late 90's in particular E10 was the normal fuel in the Chicago area (local emissions regulations) while 50 miles or farther out it was almost universally straight gasoline with no alcohol. In some places non-alcohol fuel is still available, but is increasingly harder to find. I hate E10, so while I was commonly traveling outbound nearly every weekend I would fill up in the outlying areas whenever possible.
I can also report that aside from some vapor lock issues in hot weather, there didn't seem to be any need to readjust the carburetors when regularly switching back and forth between these fuels. Furthermore, trying to readjust the carbs would screw up running of the engine. I was particularly sensitive tho these issues, because I would occasionally intentionally tweak the carbs slightly richer for autocross competition, and put them back to normal afterward for touring. Correct adjustment for the carbs seemed to be the same for either type fuel (at same altitude).
I don't know exactly the correct A/F ratio for stoichiometry involving alcohol. If I have to burn 4% greater volume of liquid fuel to drive a given distance with E10, then it follows that the carburetors will be mixing in the 4% additional air volume to go with it. If this is not changing the stoichiometry mixture for the gasoline, then 90 large parts of air would burn with 90 small parts of gasoline while 10 large parts of air will burn with 10 small parts of alcohol. That's the same A/F ratio by volume (not by mass).
Does that give any hint to answering the question?
|When we were forced to convert to E10 I had to richen the mixture by about two flats on the jet adjustments. I also had more problems with vaporization of the fuel on hot days. I have insulated the fuel lines and the float chambers on the carburators which has helped with that problem.
|Barney, the stoichiometric numbers are as I calculated. Mike, for heptane it is 15.11, so ~ same as octane. 14.5 is the usually quoted number, I suppose for normal engine operation, which is incomplete combustion (some CO in the exhaust). But my point is that energy content aside, the carbs are designed to suck a certain amount of fuel per pound of air induced. With E10, that air will draw in the same pounds of fuel, but the fuel now needs less air to burn it, so it must run leaner, by about 4% of fuel. From your experience it would seem that this is not enough of an effect to matter in practice, at least with E10.|
|Ed, nice scenery! But no carbies.|
|For stoichiometric combustion of gasoline, I usually figure 14.8:1 (air:fuel), and pure octane is 15.2:1. With gasoline, better milage can be obtained at 16:1. but uneven mixing in the cylinders can cause a cylinder to run weaker, not a good thing. With SUs the best power seems to be about 12:1, the excess fuel cooling the incoming fuel/air, and burning all the air. At idle, with the incoming charge diluted by the relatively greater exhaust mix, with our intake runners we need about 13.2:1 (less air, more fuel is richer). This wide band of mixtures is what our carburetors are designed to handle both with the tapered needle in the jet, and the damper which momentarily enriches the mixture for acceleration.|
While it seems the loss of 4% BTU with 10% alcohol should be noticable, it isn't. More fuel is used because using the alchohol is closer to its best power mixture (we're at 13.2:1 instead of octane's optimal 15.2:1 for idle) and it varies as our carb needles move up and down with air flow and is modified by our dampeners. 10% alcohol does require more fuel per mile, and provides more power. Really. We are running on the rich side of peak, and lean mixture will give more power, but use more fuel as the alcohol is looking for a rich 9:1. The alchohol leans out the mixture on a car set up for regular gasoline. This is more noticable at 5000 feet, where we would expect a gasoline engine to suffer.
The higher latent heats of alcohol can create a cold weather starting problem. Modern cars deal with this by heating the intake manifold for smoother running in cold weather. Alcohols burn more completely, which promotes the flame front, much as lead used to in fuel. The result is approximately the same mixture required, although you will see more power, and less fuel milage. This does not hold for mixture greater than 10% alcohol with gasoline. At 20% or greater you have to start modifying your jets and needles.
Blended fuels are more stable in cold weather, which is why storing our cars and lawn mowersover the winter is less problematic than storing our snow blowers over the summer!
|Dave, thanks for that superb explanation.|
|Art, Here is a photo of the carbs. I used heat tape that is used for wraping headers. I figured if it keeps heat in, it should also keep heat out. The zip ties are in case the glue on the tape doesn't like fuel. As they say, "belts and braces". It has made a difference on those hot days when stopped in traffic. It will still start running rough if stopped too long, but it takes a lot longer to happen.
This thread was discussed between 28/12/2009 and 29/12/2009
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