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Author Topic: Is premium petrol worth the added cost?  (Read 7513 times)

Offline chris

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Is premium petrol worth the added cost?
« on: 18/05/2012 20:50:54 »
At the petrol pump two "flavours" of unleaded fuel are usually offered, a pricier version and a (relatively!) cheaper option.

But what is the difference, and does the ends justify the added costs?


 

Offline CliffordK

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Re: Is premium petrol worth the added cost?
« Reply #1 on: 18/05/2012 22:27:30 »
As I understand it, the difference is Octane, or essentially anti-knock performance for high compression engines.

If you run a high compression engine, then the Premium gasoline is required to prevent predetonation. 

If you don't run a high performance engine, then it is not supposed to do anything for you, and is not worth the extra expense.
 

Offline Geezer

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Re: Is premium petrol worth the added cost?
« Reply #2 on: 19/05/2012 01:14:34 »
I believe the calorific content of premium fuel is actually a bit lower than regular. If your engine does not complain by knocking/pinging when you accelerate on regular fuel, your engine does not need premium fuel, so don't waste your money on it.
 

Offline dkeizer05

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Re: Is premium petrol worth the added cost?
« Reply #3 on: 19/05/2012 01:32:05 »
My 12th grade Environmental Science teacher once told the class that the premium blend gave your car better performance mileage (kilometerage?).  He said the extra mileage made it well worth the extra cost per gallon (liter?).
 

Offline dkeizer05

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Re: Is premium petrol worth the added cost?
« Reply #4 on: 19/05/2012 01:34:32 »
But then again he was a guy who wore a large gold medallion necklace, wore his shirts with the top buttons open which exposed his mane of chesthair, and situated the desks in the class so that he could look up the girls' skirts from his desk so maybe I shouldn't take his word for it.
 

Offline Geezer

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Re: Is premium petrol worth the added cost?
« Reply #5 on: 19/05/2012 04:51:13 »
Well, he was kinda-sorta right. If your engine has a high compression ratio it might actually be a bit more efficient than an engine with a lower compression ratio, mainly because the expansion ratio is greater, and that allows a bit more work to be extracted from the gas as it expands and cools.

The downside is that a higher compression ratio might require a higher octane fuel to prevent pre-ignition. If it does, it's unlikely that the improvement in efficiency will be sufficient to cover the additional cost of the higher octane fuel.

However, putting higher octane fuel into an engine that does not need it won't make that engine one bit more efficient, and, because the calorific content is lower, it will actually produce slightly less mechanical work per unit volume than it would have done with a lower octane fuel.
 

Offline CliffordK

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Re: Is premium petrol worth the added cost?
« Reply #6 on: 19/05/2012 10:45:06 »
I thought I'd look up on the WWW and see if I could actually find some test results.

Unfortunately, there seem to be very few good research articles on the subject online.  And, perhaps the research that exists needs to be updated to consider modern features like anti-knock dynamic ignition systems.

Here is one study with a 2011 Chevrolet Cruze that found 87 octane (regular):  24.5 mpg & 91 octane (premium):  26.8 mpg

With apparently enough of a difference that the premium gas actually saved a little money.

Here is a Car and Driver Study, and their results table.  Unfortunately they did not list fuel efficiency.  However, two European cars (rated for Premium?) seemed to have significant performance increases on Premium fuel.  Two American cars had more modest performance increases, and one Japanese car had a moderate performance loss on Premium.  Unfortunately, one can not necessarily equate an increase in HP to an increase in fuel economy, as the vehicle may be sucking down more fuel to get the performance boost.

Here is a study comparing E85 to Regular.  The results were quite surprising.  The regular gasoline, 18.3 mpg, and the E85, 13.5 mpg, a 26.5 percent fuel efficiency loss.

Anyway, I think the data would seem to indicate that taxi services and large fleet vehicle services would benefit from conducting a more comprehensive study between the fuel types.
 

Offline chris

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Re: Is premium petrol worth the added cost?
« Reply #7 on: 19/05/2012 15:47:03 »
Gosh; so no clear consensus so far...
« Last Edit: 01/06/2012 09:22:47 by chris »
 

Offline Geezer

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Re: Is premium petrol worth the added cost?
« Reply #8 on: 19/05/2012 17:08:39 »
To further complicate things, modern engines have "knock sensors" that detect the pre-ignition condition. The engine management computer then makes various adjustments to try to prevent pre-ignition, but these adjustments might tend to reduce fuel efficiency, but you don't know it's happening!

Perhaps some vehicles actually warn the driver to use premium fuel when this happens? They could, but I don't know of any that do.
 

Offline damocles

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Re: Is premium petrol worth the added cost?
« Reply #9 on: 20/05/2012 15:01:04 »
Here is some simple thermochemistry: ethanol added to petrol forms pretty close to an ideal solution -- that is, all of the properties change by amounts that are pretty much proportional to the amount of ethanol additive.

Ethanol only produces about two thirds as much energy per unit mass as petrol in its combustion reaction. As an additive it is a very effective and low-cost way to increase octane rating, but it actually decreases energy output (by about 33% times the additive proportion on a mass basis, or about 28% on a volume basis).

Thus, on an engine designed and tuned to run efficiently on 91 octane fuel, if you run it on a 15% ethanol/85% petrol mix, you might expect to get 15% times 28%, which is 4.2% fewer km/L or about 4.5% more L/100 km.

So unless you are saving at least 5% at the pump price of the 15% ethanol blend, it is not worth using in a standard low-compression motor. Ethanol also puts out significantly more carbon dioxide per energy unit produced when it is burnt.

Of course premium blend petrol -- the 94 or 96 or 98 octane stuff that sells at a rather higher price than the standard 91 octane -- has extra added aromatics and branched chain compounds in the mix from the refinery and does not contain ethanol. Unless you have a motor designed and tuned to perform with a higher octane fuel, you are simply wasting your money (and some valuable materials)
 

Offline Geezer

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Re: Is premium petrol worth the added cost?
« Reply #10 on: 20/05/2012 18:49:06 »
Between government agencies, the automobile manufacturers and the petrochemical industries, octane rating has become more mucked up than Hogan's goat. They have managed to sufficiently confuse consumers into thinking higher octane ratings are somehow worth more money when it is far from clear that is the case.

IMHO opinion, all fuels should be sold on the basis of cost of energy purchased so that consumers have a fair basis for comparison. The secondary consideration is the suitability of a particular fuel for a particular engine, and that is something the engine manufacturer should advise the consumer of so that the consumer is not left to guess at the pump.


 

Offline CZARCAR

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Re: Is premium petrol worth the added cost?
« Reply #11 on: 20/05/2012 21:46:40 »
q= DOES THE HIGH OCTANE DETERIORATE THRU TIME & HOW LONG DOES IT TAKE?
 

Offline damocles

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Re: Is premium petrol worth the added cost?
« Reply #12 on: 20/05/2012 22:22:00 »
q= DOES THE HIGH OCTANE DETERIORATE THRU TIME & HOW LONG DOES IT TAKE?

a= YES & A MILLENNIUM OR MORE.    ;D

In practical terms, the answer is no. All blends of petrol are made of reasonably stable organic compounds. But every carbon/hydrogen/oxygen compound is subject to

(1) Oxidation: attack by free radicals, mostly by hydroxyl radicals which are present in tiny quantities in the air, starts a complicated reaction that eventually leads to carbon dioxide and water as products.

(2) Microbial degradation: attack by microbes leads to a variety of products in the early stages; ultimately to methane, soot (elemental carbon), carbon dioxide, and/or water.

In normal environments it would take perhaps a century or two before either of these forms of degradation became noticeable in a normal hydrocarbon-based petrol, and longer before they became significant. Ethanol is more subject to these reactions than hydrocarbon fuels. If the high octane has been achieved by using an ethanol blend, the degradation could be much faster. Wine left exposed to the air is significantly oxidized after just a few days. Petrol is a less comfortable environment for microbiota, but a similar rapid degradation may be possible.
« Last Edit: 20/05/2012 22:28:06 by damocles »
 

Offline CliffordK

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Re: Is premium petrol worth the added cost?
« Reply #13 on: 20/05/2012 22:23:21 »
q= DOES THE HIGH OCTANE DETERIORATE THRU TIME & HOW LONG DOES IT TAKE?
Octane would be essentially CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH3, (C8H18) although Ethanol is used as an anti-knock agent, and thus contributes to the octane rating.

I assume the octane would be reasonably stable in the fuel.  However, the volatility of different compounds could vary. 

Ahh, here's from Wikipedia:
The alkanes from pentane (C5H12) to octane (C8H18) are refined into petrol, the ones from nonane (C9H20) to hexadecane (C16H34) into diesel fuel, kerosene and jet fuel. Alkanes with more than 16 carbon atoms can be refined into fuel oil and lubricating oil.

That would also include rings such as cyclohexane, benzene, and toluene. 

This would indicate that the octane is actually one of the heavier components in gasoline, and thus it may have lower volatility than other compounds.  And, thus, if you sat a gallon of gasoline outside, uncovered for some time, the octane concentration may actually increase somewhat.
 

Offline damocles

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Re: Is premium petrol worth the added cost?
« Reply #14 on: 20/05/2012 22:50:05 »
The "octane" that "octane rating" is based on is NOT the straight chain compound
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH3 listed in the previous post, but an isomer
(CH3)3-C-CH2-CH-(CH3)2

While ethanol may be used to increase octane rating, the traditional way to do it has been to add more branched-chain compounds (like isooctane) and aromatic compounds (mostly toluene) into the blend.

Ethanol blend petrol is (or ought to be) cheaper than traditional petrol, but significantly less efficient. Aromatic-enriched premium petrol is significantly more expensive than standard 91 octane petrol, and (as the previous postings have suggested/established) marginally less efficient in a motor designed and tuned for 91 octane petrol.
 

Offline Geezer

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Re: Is premium petrol worth the added cost?
« Reply #15 on: 20/05/2012 22:51:47 »
q= DOES THE HIGH OCTANE DETERIORATE THRU TIME & HOW LONG DOES IT TAKE?

a= YES & A MILLENNIUM OR MORE.    ;D

In practical terms, the answer is no. All blends of petrol are made of reasonably stable organic compounds. But every carbon/hydrogen/oxygen compound is subject to

(1) Oxidation: attack by free radicals, mostly by hydroxyl radicals which are present in tiny quantities in the air, starts a complicated reaction that eventually leads to carbon dioxide and water as products.

(2) Microbial degradation: attack by microbes leads to a variety of products in the early stages; ultimately to methane, soot (elemental carbon), carbon dioxide, and/or water.

In normal environments it would take perhaps a century or two before either of these forms of degradation became noticeable in a normal hydrocarbon-based petrol, and longer before they became significant. Ethanol is more subject to these reactions than hydrocarbon fuels. If the high octane has been achieved by using an ethanol blend, the degradation could be much faster. Wine left exposed to the air is significantly oxidized after just a few days. Petrol is a less comfortable environment for microbiota, but a similar rapid degradation may be possible.

So, basically you are saying I'm wasting my money when I put "fuel stabilizer" in my boat's gas(petrol) tank when I lay it up for the Winter?
 

Offline damocles

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Re: Is premium petrol worth the added cost?
« Reply #16 on: 20/05/2012 23:28:09 »
q= DOES THE HIGH OCTANE DETERIORATE THRU TIME & HOW LONG DOES IT TAKE?

a= YES & A MILLENNIUM OR MORE.    ;D

In practical terms, the answer is no.

*** [snip] ***

Ethanol is more subject to these reactions than hydrocarbon fuels. If the high octane has been achieved by using an ethanol blend, the degradation could be much faster. Wine left exposed to the air is significantly oxidized after just a few days. Petrol is a less comfortable environment for microbiota, but a similar rapid degradation may be possible.
So, basically you are saying I'm wasting my money when I put "fuel stabilizer" in my boat's gas(petrol) tank when I lay it up for the Winter?

I am not necessarily saying that! There would be a few provisos.

(1) You are operating in the US, and reading between the lines of wikipedia articles and similar sources, ethanol is routinely added to increase octane rating there.

(2) Fuel in a situation where it makes a thin layer exposed to the air is much more subject to both radical and microbial action than it would be in a normal container. The additive may or may not make some difference in your boat's tank, but it would certainly be important for any fuel left in or near the carburettor. You should run your motor for a few minutes after adding the "fuel stabilizer" to make sure that the stabilizer finds its way to this area.

(3) As well as the two forms of chemical degradation I have outlined, evaporation of fuel, condensation of water, and uptake of other pollutants from the atmosphere may be important factors for storage of fuel in a semi-open situation like a petrol tank/fuel line (as opposed to a closed jar or jerry-can). I have no idea whether or how your stabilizer might address some of these problems.
 

Offline Geezer

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Re: Is premium petrol worth the added cost?
« Reply #17 on: 21/05/2012 08:04:06 »


I am not necessarily saying that! There would be a few provisos.

(1) You are operating in the US, and reading between the lines of wikipedia articles and similar sources, ethanol is routinely added to increase octane rating there.

(2) Fuel in a situation where it makes a thin layer exposed to the air is much more subject to both radical and microbial action than it would be in a normal container. The additive may or may not make some difference in your boat's tank, but it would certainly be important for any fuel left in or near the carburettor. You should run your motor for a few minutes after adding the "fuel stabilizer" to make sure that the stabilizer finds its way to this area.

(3) As well as the two forms of chemical degradation I have outlined, evaporation of fuel, condensation of water, and uptake of other pollutants from the atmosphere may be important factors for storage of fuel in a semi-open situation like a petrol tank/fuel line (as opposed to a closed jar or jerry-can). I have no idea whether or how your stabilizer might address some of these problems.

I was winding you up : )

Stabilizer does seem to work, but how does it do that?
 

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Re: Is premium petrol worth the added cost?
« Reply #17 on: 21/05/2012 08:04:06 »

 

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