Fuels that contain ethanol are hygroscopic and tends to absorb moisture over time out of the air. This can be a problem, typically with lawn equipment, and it does not store as well. Air fuel ratio runs slightly leaner in things like chainsaws and mowers need a little bit of tuning to compensate. Another issue is retail fuels labeled as E10 have been found at times to contain far higher levels than advertised. Most small engine repair folks suggest avoiding it altogether if possible. For whatever reason it seems to increase small engine carburetor problems. Straight gasoline in the amounts used by homeowners for cutting the grass, snoblowers and weedwackers can avoid ethanol fuels without breaking the bank.
At any given operating condition (load, RPM, temperature, etc.), there is an ignition timing that results in the maximum possible amount of the burned fuel’s energy being converted to mechanical work. This is called the maximum brake torque (MBT) spark timing. If the timing is advanced further, an excessive amount of the mixture’s energy gets lost to the cylinder walls as heat; if the timing is retarded further, an excessive amount of the mixture’s energy lost as heat in the exhaust gas.
It’s often the case that the engine cannot be operated at MBT timing because some portion of the mixture will detonate, so the spark timing is retarded to keep that from happening.
Premium (higher octane) fuel will enable greater spark advance without detonation (possibly all the way up to MBT timing), resulting in more of the fuel’s energy being converted to mechanical work during the power stroke. So for a given engine power requirement - and assuming that the premium fuel in question has the same energy density as regular - you wouldn’t need as much fuel.
If the engine is such a detuned piece of crap that it’s already running at MBT timing, or if the engine doesn’t utilize knock sensing to adjust spark timing, then switching to premium won’t make a difference in fuel economy.
I know when I had my Mustang GT, the manual said it was perfectly OK to use regular (87 octane) but they only guaranteed rated horsepower on premium (91 or higher.)
my motorcycle (Harley) requires premium, but that’s largely because it’s air cooled and runs all kinds of hot.
As mentioned, the mpg difference if it occurred would by the engine using thermodynamically optimum ignition timing with higher octane gas, if the knock restriction prevented that with lower octane gas. Assuming this is the case, and assuming a modern car which adjusts for it. The old conventional wisdom that higher octane than manufacturer’s reqt was a pure waste of money was correct when cars didn’t adjust themselves. Now it’s not strictly true, either wrt power or mpg. But it’s more likely I think to be practically as good as true for naturally aspirated cars with 87 octane recommended. Where it might tend to be less true is with combinations of turbocharging, changing turbocharger control and/or need for >87 to begin with.
Again semi-off topic, I’m satisfied it’s a fact my BMW 328i puts out non-negligibly (not necessarily lots) more HP with the turbocharger tuner on 93 than 91 (basic recommended), and that it gets higher* highway mpg with the tuner than t did before (similar reason to the gas octane discussion, closer to ideal thermodynamic efficiency if more exhaust gas goes through rather than around the turbocharger). I’m pretty sure it gets slightly higher mpg now on 93 than on 91, for same ethanol content, not quite as clear.
*~20% more mpg than EPA highway rating now, but it was already above without the tuner, I’ve gotten at least slightly above EPA highway in every car I’ve had on interstates, where I take it nice and easy. Where there are curves and occasional full throttle for quick passing on two lane roads, not as far above.
Indicative figures only. If everything was perfect, a higher octane fuel made from octane would give you slightly better mileage than a lower octane fuel made with heptane
Just becase I wondered. My car (regular) got about 3% better mileage on premium (which costs about 20% more.) It’s not a high performance engine: I don’t think it can change the compression ratio. It might be running leaner with the high octane fuel. I’ve never seen my suppliers quote the actual energy density of their fuels.
Not gonna happen on a modern car in good working order. The catalytic converter gives poor conversion efficiency if the mixture deviates much from stoichiometric; the ECM pays attention to the exhaust oxygen sensor and dithers the fuel injection rate to keep the air/fuel ratio dithering tightly back and forth between a smidge rich and a smidge lean, regardless of fuel composition (within the range of fuel compositions the car is designed to tolerate). This why modern cars can run straight gasoline, E5, or even E10 and meet EPA emission requirements using any of them. Flex-fuel vehicles are designed to run on anything from straight gasoline to E85.
