Originally Posted by jgetti

I think you mean 70% of the heating value of gasoline. The thermal efficiency is a function of the engine characteristics,,, not the fuel. Bottom line is, it takes more ethanol to make the same amount of heat as gasoline. This is not all a bad thing though.

The reason gasoline isn't as efficient as diesel is because the compression ratio you can run is limited by the auto ignition temperature of gasoline. Thermal efficiency is directly related to the compression ratio your engine runs at. So, yes, ethanol takes more fuel to make the same amount of heat, but since ethanol also has a much higher octane number (higher auto ignition temperature), you can raise the compression ratio much higher than if you were running gasoline. This can raise the thermal efficiency of burning ethanol to higher than that of burning gasoline.

Fuel efficiency of OF AN ENGINE is governed by two things,,, how much energy is in the fuel (heating value), and how effectively you turn that energy into mechanical power (thermal efficiency).

If you build a dedicated engine (not flex-fuel) to run on ethanol with a higher compression ratio that takes advantage of the 105 octane of E85, then you can get similar fuel efficiency with ethanol even though the heating value is lower, because the thermal efficiency of the engine is higher.