Flex-Fuel Tuning — VE Delta for Pump vs E85

I see where the secondary VE table is trending more efficient as the ethanol content increases and I’m missing why this is or how it’s even possible?

Are we using the increases VE as a workaround for some other actual property variation, since the VE is mechanically fixed, regardless of fuel type?

Please help me fill in this gap in my understanding and I thank you.

~Rich Laws

Akron, OH

I expect the combustion temperature is going down, leading to denser air in the cylinder. The amount of temperature change depends on the fuel, some air-mass models don't account for this, so it shows up as an increase in VE with more ethanol present.

MoTeC uses Engine Charge Cooling Gain, and this table can have an axis for Fuel Composition. I think that is the proper way to account for the fuel properties.

Also, in this webinar: https://www.hpacademy.com/previous-webinars/116-modelled-multi-fuel-flex-fuel-tuning-link-g4/

Andre mentioned that the viscosity / density of the Ethanol fuel is also different. If the ECU doesn't adjust for this (ie, different injector flow characteristics and/or, fuel density), you may find that you end up with slightly different VE values.

It may be partially due to the evaporation of the alcohol content cooling the air more, increasing it's density before entering the cylinder?

Yet another reason to use something like "mass ratio", rather than VE.

Thanks for the replies, gentlemen.

~Rich L

Akron, OH — US