Octane Question

[A_Harman]

Originally Posted By: SilverSurfer
Originally Posted By: A_Harman
Originally Posted By: SilverSurfer

I think that graph might need to be adjusted just a little bit. How is detonation going to occur if the cylinder pressure is decreasing as indicated in the graph? I suppose heat could be increasing due to burning gasoline, but pressure is going down due to piston movement? That is the only thing I can think of.



The detonation occurs ATC because the end gases reached autoignition temperature, then ignited all at once, sending shock waves bouncing around the cylinder. That is what causes the spikes on the negative slope portion of the cylinder pressure curve.


If the remaining charge ignited all at once, shouldn't we see an instantaneous pressure spike?


I don't think the slope would get to be vertical, as there would be a finite time for the burning to complete. And as you said, the cylinder volume is increasing, so there is some fast burning occurring to cause cylinder pressure to rise during expansion.

 

[ragtoplvr]

The chamber, heat and pressure is not uniform. So one place auto ignites, then pressure rises faster and another auto ignites and so on. There is some heat decomposition that is needed for auto ignition. In the 60's Honda discovered on the racing motorcycles that detonation did not seem to occur above 10000 RPM. They had high compression for the time at 10.5 to one or higher.


Rod

 

[y_p_w]

Originally Posted By: MNgopher
YLocal refiners had been making 84 octane E0 for years - ethanol use was mandated here in the 1990's.

There's one commodity blendstock called RBOB, for "Reformulated Blendstock for Oxygenate Blending". I think it meets requirements for 87 octane EPA RFG if blended with 10% denatured fuel ethanol.

 

[Clevy]

Originally Posted By: bigt61
Remember, that octane is an explosion inhibitor - lower octane is more volatile and has higher energy content (BTU). High octane is needed in high performance engines so they can squeeze the air-fuel tighter(high compression) for a bigger bang which equals more horsepower. Low octane fuel will explode too easily causing pre-ignition (it ignites all by itself from being squeezed - like a diesel) This pre-ignition causes power loss and engine damage in a gasoline engine. Octane additives are expensive - therefor the price difference.



Not quite.

The btu difference is moot if it even exists.
Octane is resistance to PRE-IGNITION,not spark induced burn.
High compression will cause lower octane fuels to explode early which is like hitting the piston with a sledgehammer while the piston is still on the upstroke. Serious damage can occur in a very short time period.
More octane just means less volatility in that compression or hot spots are less likely to initiate burn.

Originally Posted By: zach1900
Originally Posted By: bigt61
Remember, that octane is an explosion inhibitor - lower octane is more volatile and has higher energy content (BTU). High octane is needed in high performance engines so they can squeeze the air-fuel tighter(high compression) for a bigger bang which equals more horsepower. Low octane fuel will explode too easily causing pre-ignition (it ignites all by itself from being squeezed - like a diesel) This pre-ignition causes power loss and engine damage in a gasoline engine. Octane additives are expensive - therefor the price difference.

Exactly, anybody I've spoken to off BITOG was under the impression that high octane = faster combustion. when the opposite is true.



Nope.
They both burn at the same speed. Octane is merely a measure of resistance to PRE-IGNITION.
Regular 87 octane and premium 91 octane will burn at the same rate once ignited. Well close enough anyway. If there is any difference it will be minutiae

 

[y_p_w]

Originally Posted By: SilverSurfer
Originally Posted By: Benzadmiral
Originally Posted By: SilverSurfer
Hello all. First post on BITOG!

Anyway, I am curious how gasoline is blended with ethanol. If I buy E5 or E10 pump gas that is rated at 91...is that 91 octane that was mixed with ethanol (114 octane equivalent), to produce something higher than 91? Or is it a lower octane gas blended with ethanol to produce a final product of 91 octane?

Please, no need to argue about the merits of ethanol in gasoline. It has been beaten to death and I am not interested in that thread of debate.

Hello, Silver,

From reading here I think it's the latter. No way would the fuel companies give you more than you're paying for!


Thank you for the reply. This what I figured, but then that begs the question, what is the initial octane of 87 E10? Did they have to intentionally retool their refineries to produce a lower octane gasoline to start with?

The refinery doesn't really have to be "retooled" for anything. They're set up to produce different "streams" that are then stored. Some of these streams are used for gasoline, some for kerosene, some for diesel, and some for lubricants. Each of the gasoline streams are of a different octane rating. After the streams are produced, it's a matter of blending to achieve the desired octane rating.

I (and others) noted on another thread that oil marketers used to just get non-oxygenated fuel that was 87/91/92/etc octane rated and then added ethanol or MTBE. The effective octane rating was higher than what was listed on the pump. The industry calls a higher effective rating (often to provide a cushion) "octane giveaway".

These days there's a higher demand for premium - especially on the West Coast with lots of luxury and performance cars that recommend premium. All the higher octane rating streams are needed.

Blending is an exercise in using up all the streams to make a salable product and to meet all the demand. There's a lot of streams that are under salable octane rating, even when blended with ethanol. There's one blend called "reformulated blendstock for oxygenate blending". It's specifically meant for blending with 10% ethanol to create an 87 octane regular unleaded.

 

[SilverSurfer]

I emailed QuickTrip about the ethanol based 91 octane (QT is serving me the E0 91. They stated:

"The 91 octane that we offer in your city is a blend of 89 octane and ethanol. The ethanol brings the grade to an octane level of 91."

So this equates to 92% gasoline and 8% ethanol.

 

[Brigadier]

Originally Posted By: SilverSurfer



But yes, you have identified why I am asking the question! I have E0 91 octane and E10 91 octane available locally to me (no reliable 93 octane sources). So what do I pick? Where is the power?


IMO, the E0. I have run Cenex E0 92 vs. Shell/Costco/Chevron E10 92's, and the performance edge goes to the E0 92. It is definitely noticeable with the butt dyno, and the MPG.

 

[TiredTrucker]

Depends on the ECM programming and a myriad of other factors. The Vortec 5.3L in the GM pickups would get documented, by the OEM, 6 more HP and 8 lb more torque by when using E85 compared to regular gas. They could advance the timing and other variables and tweak a little more out of the motor. Whether is was worth it to the owner was another story altogether.

While mpg is important and everyone likes a higher mpg number, it is the cost per mile that actually is truly important. Depending on the price spread and the mpg difference per engine, sometimes the ethanol blends actually give a lower cost per mile. It is one thing to get, say, 21 mpg on E0 regular at 2.39 a gallon which is a hair over 11 cents a mile, while you might only get 20 mpg on E10, at 2.19 a gallon which is a hair under 11 cents a mile. Basically break even. When the spreads get larger on price, it can make a difference. In my case, I notice no appreciable mpg change from E0 to E15 with my pickup. So, I go for the E15 right now that is 20 cents a gallon cheaper.

 

[Brigadier]

Cheaper because last I heard, ethanol is federally subsidized to the tune of $.51/gal. So the tax payers are paying for part of your gas bill.

I guess it is down to $.45/gal now.

http://www.taxpayer.net/library/article/corn-ethanol-subsidies-are-alive-and-well

Originally Posted By: TiredTrucker
Depends on the ECM programming and a myriad of other factors. The Vortec 5.3L in the GM pickups would get documented, by the OEM, 6 more HP and 8 lb more torque by when using E85 compared to regular gas. They could advance the timing and other variables and tweak a little more out of the motor. Whether is was worth it to the owner was another story altogether.

While mpg is important and everyone likes a higher mpg number, it is the cost per mile that actually is truly important. Depending on the price spread and the mpg difference per engine, sometimes the ethanol blends actually give a lower cost per mile. It is one thing to get, say, 21 mpg on E0 regular at 2.39 a gallon which is a hair over 11 cents a mile, while you might only get 20 mpg on E10, at 2.19 a gallon which is a hair under 11 cents a mile. Basically break even. When the spreads get larger on price, it can make a difference. In my case, I notice no appreciable mpg change from E0 to E15 with my pickup. So, I go for the E15 right now that is 20 cents a gallon cheaper.

 

[TiredTrucker]

Mandate of use is not a subsidy. Re-read the article. If mandates are a subsidy, the seat belt manufacturers, TPMS manufacturers, and the list goes on are subsidized. There are some true subsidies in the pipeline for retailers to modify and install ethanol blender pumps, but that is not an ethanol subsidy. Ethanol producers don't sell at the retail level and no one is forced to use higher blends of ethanol that would be available at those pumps like E15, E20, E30, and E85.

And if you read closely, the subsidies were initially .40 a gallon, then .51 a gallon, and then .45 a gallon, then from the same article you linked to....

While VEETC was finally eliminated at the end of 2011 (in addition to the tariff on imported ethanol)

So, one actually needs to read what is written without interjecting their own preconceived ideas into the article.