I think its not the caloratic heating value you compensate for with the fuel-map for E85 but the different stociastic AFR that for E85 is lower than for petrol.
ahto skrev:
As I understand at idle advanced ignition, at lean burn more or less the same as gasoline and slightly advanced at high rpm and engine load. Is it right?
From what I’ve read E85 can still burn with leaner and richer mixtures as petrol but for both the flame speed is slower of E85 burn slower. Only stoich or slightly rich mixtures can bur as fast or faster than petrol would, but unless the engine is equiped with a knocksensor, the gasoline map is usually somewhat retarded from max effeciency att full load to make sure the engine will not suffer from preignition/detonation. So even at max load it is not unlikely that the engine needs ignition retarding and might stil gain from more advance.
If you look at
http://www.faqs.org/faqs/autos/gasoline ... amble.html you see the
Stoich. Flame Speed. for ethanol is not given but it is higher for methanol it is 0.43 m/s and lower for Unleaded Gasoline 0.34 (m/s) so ethanol is likely to be faster at stoch as well. Interestingly propane and methane (the intended use of the box) are burning even a little faster 0.45 (m/s) as you can see further down. So even though the flame speed is supposed to be 30% faster still the ignition advance is benefitial!
Besides the fact that the gasoline ignition timing probably is less than optimal in order to be on the safe side, another reason for the need of extra ignition advance could be that LPG, CNG and ethanol are more difficult and slower to ignite as gasoline, so that lag between the start of the spark and the start of a flame is greater as with gasoline and this might ofset the difference in flame speed?
When the octane of a fuel is lower and you are closer to detonation and self ignition you should have a mixture that ignites easier and faster than with a fuel that has a higher octane number. This is already noticable with gasoline of different octane ratings; in contrast to what many people think the lowest octane (that does not cause detonation) is usually more efficient as a higher octane fuel unless the ignition advance (or turbo boost pressure) is increased/adaptive.
ahto skrev:
This ignition map is for engine at operation temperature but from your discussion I understand that more optimized ignition could improve cold engine behavier (my be even cold start?). Any ideas what could be done there?
Yes, I have that impression. At cold start / cold running the burning is likely to be extra slow as the mixture is lean as ethanol is only partly evaporated. once ignition occures the heat of mostly the gasoline helps further the evaporation and ignition of the ethanol. As you want peak pressure at a certain point after TDC the slower burning cold engine needs more ignition advance as with petrol I suspect that otherwise more heat goes into the exhaust instead of moving away pistons..
I also think that a cold engine running on E85 would need even more advance when the engine load increases at low rpm. I have have often experienced that a cold engine running on E85 ”hesitates” or even dies when opening the throttle suddenly when it runs just fine on idle or full throttle at high rpm. When I do the same with the extra advance (15° now) the cold engine responds much more like it was warm or running on gasoline. When running on LPG or CNG -which this box was intended for- the evaporation is never a problem, in contrast to ethanol LPG and CNG are much better at becoming a gas (again) in the cold engine as gasoline is so there is no need for cold engine ignition compensating (or cold start fuel enrichement) when running on those fuels.
With more ignition advance before TDC the gas compression when the spark ignites is smaller, this might be a good thing because the energy required the start a spark with less compression is smaller than with a higher gas pressure and maybe the duration of the spark will become greater as a result. Longer spark duration could increase the chances the mixtures ignites under difficult circomstances?
Of course I do not yet know how much it helps when it is really cold but I have a feeling it can make a difference in drivability just like it does now.
This weekend I’ll make a trip of 2x 160 km and will hopefully have gained more experiance.