Sunday, September 7, 2008

9-3 doesn't increase timing advance with increasing ethanol content

This plot is of the timing advance, which was in all three data file Alcohol gave me. Essentially, it's not showing any more advance with more ethanol.

Now, I don't know jack about timing advance, if ~32 degrees of advance is a lot, if you should expect a lot of advance on the highway, etc. But I was hoping to see the Saab Trionic ECU be able to increase advance, since higher ethanol means higher octane, and less detonation.


Anonymous said...

Guys that are racing E85 are all over the board with timing on E85.
Generally though they find alcohol at slightly rich of stoich to be similar to gas and too much advance creates negative work in compressing an already burning/expanding fuel. From what I understand ethanol's laminar flame spread is slightly faster than gas as long as we are slightly rich of stoich, not lean- then it it slower than gas.
What I am not sure of is how this will apply in low % ethanol/gas blends.


Anonymous said...

Buzz- I do not remember if I gave you the MPG's on the three fuels I ran during the test period- all were done in 90-92 degree fair weather, no wind, 72 mph ave, on the level interstates of IL and IN with the exception that the E27.5 and E20 included some running in gentle hills of WI also.

E27.5 = 31.7 mpg
Premium93 (could have been E0 or E10) = 31.8 mpg
E20 = 31.5 mpg

These were taken from the on board dash calculator. Car is a 6 speed manual 2004 Saab Aero 2T. Short trip mileage city around home is generally down around 21-23 mpg when trips average only 2-5 miles.


buzzcut said...

That's outstanding mileage for the cruise set at 70. And no drop in mileage with increasing ethanol.

I wonder how that happens. We know that it isn't because the increased octane allows more ignition advanve. The advance is no different from one batch to the next.

Anonymous said...

My theory is this Buzz- while my LTFT's are higher on ethanol that is over the base fuel map at that given point and I suspect if I took the time to study it the position on the map may be primarily based on the grams of air intake. If that is the case the ECU is comparing the airflow against the amount of time is is injecting. Yet since the air intake is less than required for gasoline and the throttle should be a little more closed on E85 (since it contains part of it's own air) the ECU is really in a different (lower) place on the fuel map than it would have been on pure gas. Given that ethanol expands more than gasoline in combustion the piston is actually being pushed down with less BTU content but equal volumetric volume of E85. This combustion expansion gain on ethanol was a fact passed on to me by a retired Chrysler engineer who spent the entire '80s and '90s seeing what he could do with ethanol in their existing engine families.

Well- that is my theory anyway- someday I should take time to break down the log to see if it holds water.