Quote:
Originally Posted by Yakaru
Interesting, goes against what I was taught; but that was years ago and not the most formal setting. The mileage sub-point is simply one of efficiency -- if the gas is going off too early or too late then the energy is there, yes, but you'll get less mileage since it's not driving the engine's piston as efficiently as it could.
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You're talking about tuning a car for max-power under WOT here. Not about differences in octane or mileage. Optimum spark-timing does make a difference ... for maximum power production under WOT. You want entire mixture fully combusted around 15-ATDC. Any sooner and you're just compressing rods. Any later and mixture is still expanding at bottom of stroke and trying to compress rods on their way back up.
However, under non-WOT operation, such as cruising for mileage, throttle is mostly closed and you've got a rarified mixture in the chambers. It's very thin, even at top of compression stroke. It's extremely easy to ignite this mixture and ignition is not an issue. Efficiency is also maximized with O2-sensor feedback and 14.7:1 AFR. It requires only about 12-15hp to push your car down the road at 65mph. Doesn't matter what octane you use, it's same power required because aero-drag is the same.
I get highest MPG with non-oxygenated (non-ethanol) fuels. Lose 10% MPG with E10. Costco must cut their gas with extra ethanol because I get -15% MPG on their gas. Power-output same in all cases.
REFERENCES
The Chemistry of Hydrocarbon Fuels - Harold H. Schobert_ -_ Butterworth-Heinemann Ltd.
Automotive Fuels Reference Book - Keith Owen, Trevor Coley - SAE#R151
Mixture Formation in Spark-Ignition Engines - H.P. Lenz - Springer-Verlag
Fuel Injection - Jeff Hartman - Motorbooks International
Lean Combustion in Spark-Ignited Internal Combustion Engines - Germane, Wood, Hess - SAE#831694
An Introduction to Thermal Fluid Engineering - Z. Warhaft - Cambridge University Press