Originally Posted by Floydian
Originally Posted by RDY4WAR
I found the email exchange I had with the tuner of that car. I asked him the same question of how the ethanol fuel could handle as much spark advance and pressure as the leaded gasoline with a much lower octane rating. This was his response.
"As the piston moves up the cylinder and compresses the air/fuel mixture, the pressure rises which means temperature also rises, causing the fuel to vaporize. During this phase of vaporization, ethanol absorbs significantly more heat out of the surrounding air in the cylinder than gasoline. So while C85 has a lower knock resistance than C16, the temperature in the cylinder at the time of ignition/combustion is also lower, allowing it to withstand the severe conditions. It's also why ethanol fuel is harder to ignite."
That would explain it. Alcohol does absorb temperature when it evaporates.
Although I wonder why isn't ethanol rated higher in terms of octane numbers?
I believe it's because the octane rating is based on a ratio of iso-octane at a controlled temperature. Giving ethanol an octane rating of 106 would mean that it would have pre-igntion / detonation issues at around the same temperature as gasoline with a 106 octane rating. Let's say this temperature is 540*F. (just throwing out a number) However, the ethanol will take longer to reach 540*F in the chamber because of its higher capacity for heat absorption. So take 2 identical engines, down to every spec, one with 106 octane gasoline and the other with 106 octane ethanol. Tune the gasoline engine to the very edge for peak power, right before the point of knock with cylinder temperatures of 540*F. Then tune the ethanol engine to mimic the gasoline engine's tune with the same spark timing, cylinder pressure, coolant temp, oil temp, etc... the ethanol engine may only be at say 460*F in the chamber. It would take the timing and pressure of a gasoline engine pushed to the edge of an octane rating 10 (or more) points higher to get a comparable ethanol engine up to that 540*F chamber temperature.
Another thing is that once ignited, the fuels burn rather differently. On C16, the T/A made the best power at .82 lambda. On C85, it made the best power much richer at .72 lambda. With C16, any richer than .82 lambda showed an increase in exhaust gas temperature (EGT) and a progressive drop in power. With C85, the EGTs remained constant as the air/fuel ratio was richened up all the way down to .72 lambda with a progressive increase in power. At .70 lambda on C85, the EGTs started going up and a slight drop in power accompanied it.
It must be noted though the above only worked when we changed out the spark plugs. The engine was run on NGK -9 heat range plugs on C16. After several pulls on C85, these plugs looked fouled. Going 2 heat ranges hotter to -7 plugs made the C85 more responsive and tolerant of richer air/fuel ratios. What we learned was that the more heat you could keep in the cylinder, the more ethanol you could dump in there to absorb it, and the more the ethanol liked it.
Methanol works the same way but to a greater extreme. Have a look at NHRA Top Alcohol dragsters that run methanol fuel with an octane rating of only about 110. However, they withstand 60+ psi boost pressure at 9,000+ rpm with 40+ degrees of spark advance to make in the neighborhood of 4,000 hp.
