How much does ethanol affect gasoline

[MRC01]

Ethanol has a natural octane that is lower than that of other additives such as Toluene, Xylene or MTBE. Ethanol also has a wide Ron/Mon spread, meaning its MON octane (under heavy load) is significantly lower than its RON octane (under normal cruise conditions).

So if you replace MTBE, Xylene or Toluene with ethanol and keep the blend ratios the same, you will end up with a lower octane fuel than you had before. On top of that, the actual octane under heavy load will be yet even lower due to ethaonol's wide Ron/Mon spread.

For comparison, here are some (R+M)/2 octane ratings for common additives:

ethanol: 101
MTBE: 118
Xylene: 116
Toluene: 114

Note that the other additives not only have higher octane, but not revealed in these numbers is that their Ron/Mon spread is narrower.

The bottom line is, most engines will make less power with ethanol gas and there is no point to running any octane higher than what the engine was designed for.

 

[chefwong]

The closest station (mobil) is going under an construction improvement, so they have been closed for awile.

For the last 5 fillups, I've been using Sunoco Ultra 94 just to see if the car would behave differently. I would expect the latter but it seems to be a slight decrease of power as opposed to just *Super + Mobil fillup*.

I know gas is just gas and it's just the additive package that makes the difference but usually at a standstill for example, on the highway and making a quick lane change to the next lane or the latter of just accelerating from a red light......the X5 (2003 4.4) just seems to have more pep with the Mobil gasoline.


Could it be the 10% ethanol that they add now that it's wintertime in the fuel that is dropping the octane of the fuel ?

 

[wwillson]

chefwong,

Pure ethanol is 113 octane, so it wouldn't decrease the octane of the gasoline. I have a '02 Suburban that I regularly run E85 (85% ethanol), it runs better on the E85 than it does on straight gasoline. It has much better low-end torque, but gets about 15% less miles to the gallon.

Wayne

 

[JTK]

The sunoco stations in my area run 10% ethanol in all grades year-round. I like it because it takes the ping & rotten egg exhaust smell away from my 2002 rodeo 3.2L. I dont notice a change in MPG with it.
Joel

 

[nick778]

Forgetting about emissions considerations, is a fuel better with or without MTBE? Does it enhance or degrade the fuel's inherent burn rate and/or performance/power potential? Does it offer anythng positive in the way of cleaning the fuel system. Another words, are we better off with or without it nonwithstanding the alledged claims of emmissions reduction?

 

[batterycar]

MTBE pollutes the water supply. We're better of without it.

 

[SHOZ]

Aren't Ethanol and MTBE used as oxygenators for fuel? Not really added to increase the octane. And with O2 sensors why do they need oxygenated fuels anyway? Won't the O2 sensor see the O2 and just increase the fuel mixture?

 

[Pete]

Well, don't you all forget that in New York, as here in Connecticut, MTBE has been banned as of Jan 1. Since sometime this fall (going by when my gas stopped stinking), MTBE has been gone in my area. Currently 10% ethanol is required in all gasolines, but I believe that ethanol has been used for many years, year-round.

Now my point: Sunoco has changed thier octane offerings in our states, now selling 87, 89, and 91, instead of 93. The 94 octane is gone, replaced by 93. Mobil and other brands's Super remains at 93 octane, if pump labelling is to be believed.

There is plenty of info on the net about ethanol and MTBE.....

 

[MRC01]

quote:

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Originally posted by SHOZ:
Aren't Ethanol and MTBE used as oxygenators for fuel? Not really added to increase the octane. And with O2 sensors why do they need oxygenated fuels anyway? Won't the O2 sensor see the O2 and just increase the fuel mixture?

---

They serve double duty, both to increase octane and to oxygenate.

You're right that they don't have any effect on a FI car in full loop (cruise) mode, other than to lower fuel economy. That's because the exhaust O2s detect the lean condition and the computer rings up the fuel department for more fuel.

But in open loop mode (WOT), they do lean out the mixture even in FI, because the computer isn't listening to the exhaust O2 sensors.

On carberated engines, oxygenates lean out the mixture under all conditions, though you can compensate for it with jetting.

In all cases, oxygenates do provide more complete combustion. This lowers emissions.

Ethanol is cleaner than other oxygenates like MTBE. The problem is that ethanol's octane is so much lower than other oxygenates, that you have to blend more of it to get the octane you had before. But there's a limit to how much you can add to the fuel. That's why high test drops from 92 to 91 octane in places like CA where ethanol is used. The other problem is that ethanol's Ron/Mon spread is wider than other oxygenates. So even though the gaoline's (R+M)/2 octane only drops by 1 point, the effective octane under heavy load (e.g. for racing) drops even more. That's why engines designed to burn 91 octane fuel without pinging, often ping on 91 octane ethanol fuel.

 

[MRC01]

quote:

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Originally posted by nick778:
Forgetting about emissions considerations, is a fuel better with or without MTBE? Does it enhance or degrade the fuel's inherent burn rate and/or performance/power potential? Does it offer anythng positive in the way of cleaning the fuel system. Another words, are we better off with or without it nonwithstanding the alledged claims of emmissions reduction?

---

MTBE compared to what? Ethanol? All gasoline (even low test) needs SOME kind of octane boosting additive, so the only question is which one and how much of it to use.

Compared to Ethanol, MTBE is easier on the fuel system components and provides higher octane, better torque, power and efficiency.

But MTBE is more toxic than ethanol, which is one of the reasons MTBE is being phased out and replaced with ethanol.

Other reasons might have something to do with the $$$$ corn farmers are going to make when fuel is legally required to have ethanol in it.