octane and ethanol

[Jetblast]

Tetraethyl lead was originally added to combat knock. The fact that it lubricated valve seats and stems was a bonus. It's still used in 100 octane aviation gasoline because you just don't mess with less effective substitutes when you could fall out of the sky.

Yes, that will be a slow change if it ever happens at all.

Lead is great in gasoline but when it was in automotive fuel it literally sprayed out the tailpipe onto roadways, and got washed into adjacent soil and ditches. Anything grown in that soil was toxic. After running a roadside lemonaide stand all summer, little Johnny didn't do so well at math and your goats tried to have big **** time with your cows.

Aviation engines don't care about the environment and there's little reason they should. There are few of them, comparatively speaking, and they blow their lead over wide and often unpopulated areas. Lead is great for them as they must have an optimal power-to-weight ratio at reasonable cost, which is helped by high compression. Since so many piston airplanes are flying around without hardened valve seats, the cost of upgrade would ground a tremendous number of them. General aviation has a hard enough time surviving without that.

 

[KennyV]

Tetraethyl lead was originally added to combat knock.
...

Ah... the good ol days of Ethyl & Regular gas pumps...
and then it got where you could see the air in and around larger cities...

Well now, I guess as Always, around here we burn whatever is coming out of the pumps... 10 - 15 or whatever percent ethanol... if it's there we will use it... In many ways it's good that there are not more choices, because there are always going to be those that will fall for the marketing ploys and buy the more better & most bestes because the fuel distributor claims it will make your happiness factor shine brighter...
Thankfully most of those additive benefits are Quietly taken care of... for us.... and we will continue to use what is placed in the tank...:smile:KennyV

 

[weewilly]

back to the origional question
I n Canada all gasoline pumps have a sticker stating that the gasoline out of a regular (87 octane) pump MAY CONTAIN UP TO 10% ETHANOL if your manuat states that this is acceptable use it,
If you believe you are having performance/milage or other concerns try the premium blend for 2-3 tankfulls and base a cost/expense judgment call; for your unique situation.
Just my $0.02cdn
Weewilly

 

[BlaneR]

It's a fairly simple thing - the smaller the environment (in this case an engine) or the higher the performance tuning (ie. compression), the better fuel you put in it the better it'll run. However, for me, it boils down to the price. I don't see that the higher cost being worth the gain in performance. Maybe on the trimmer it might, as some here have mentioned, since it uses so little fuel. Might have to try that.

As to the 'top tier' stuff that was mentioned - that's about detergent and additives, not octane level. Two totally different discussions.

Peace!

 

[BlaneR]

back to the origional question
I n Canada all gasoline pumps have a sticker stating that the gasoline out of a regular (87 octane) pump MAY CONTAIN UP TO 10% ETHANOL if your manuat states that this is acceptable use it,
If you believe you are having performance/milage or other concerns try the premium blend for 2-3 tankfulls and base a cost/expense judgment call; for your unique situation.
Just my $0.02cdn
Weewilly

We've got the same sticker down here, with that exact wording.

 

[Jetblast]

It's a fairly simple thing - the smaller the environment (in this case an engine) or the higher the performance tuning (ie. compression), the better fuel you put in it the better it'll run. However, for me, it boils down to the price. I don't see that the higher cost being worth the gain in performance. Maybe on the trimmer it might, as some here have mentioned, since it uses so little fuel. Might have to try that.

You're reinforcing the erroneous notion that premium gas is better than regular gas. We're used to the idea that more expensive = better, but higher octane gas is not better than standard octane gas. It's just different. It's made to burn more slowly for a particular set of circumstances, and using it in an engine who's compression ratio doesn't require it cannot possibly improve your performance in any measurable way.

 

[ss69]

I have bought several pieces of equipment (mosty Stihl) the last two years. There is a label attached to the gas cap, that warns "Warranty will be voided if regular gas is used." The dealer tells me that if the engine breaks down (in warranty) an analysis will be done to determine if regular has been used. So, all the 2 to 600cc engines get high test and the 350hp truck gets regular. Go figure.

 

[Jetblast]

I have bought several pieces of equipment (mosty Stihl) the last two years. There is a label attached to the gas cap, that warns "Warranty will be voided if regular gas is used." The dealer tells me that if the engine breaks down (in warranty) an analysis will be done to determine if regular has been used. So, all the 2 to 600cc engines get high test and the 350hp truck gets regular. Go figure.

Using regular gas in an engine with high enough compression to require slower-burning premium can cause detonation, which creates a force similar to beating your piston crowns with a large hammer. Chances are Stihl's analysis starts with a quick check for piston, rod, or bearing damage.

 

[KennyV]

You're reinforcing the erroneous notion that premium gas is better than regular gas. We're used to the idea that more expensive = better, but higher octane gas is not better than standard octane gas. It's just different. It's made to burn more slowly for a particular set of circumstances, and using it in an engine who's compression ratio doesn't require it cannot possibly improve your performance in any measurable way.

You are absolutely correct... something about the word "Premium" will make some folks think that it equates to better...
There is no mistake looking at spark plugs and pistons from a high compression engine that has been ran with low octane... Wont take a lot of chemical analysis...

But if you are not running high compression or a LOT of boost... you are not going to need or gain from octane increase... :smile:KennyV

 

[Black Bart]

This guy has it right and rather than type all this I just did a copy paste but I agree with what this guy posted he has it right and he explains some of the myths about fuel.

Within practical reason premium fuel and regular fuel burn at the same rate. Premium fuel resists detonation better than regular so the explanation that it burns slower or faster was invented to explain this in-explicable phenomenon. That is called an old wives tale. The burn rate is the same. Period.

This gets murky, however.

As long as we are talking pure gasoline then premium and regular burn at the same rate.

There are very special blends of gasoline that do have different burn rates....like the magic stuff of formula 1 legend. We are talking pump gas here, right?

There are other ways of improving the octane rating of the fuel than just making better gasoline. Adding lead, as mentioned, will increase the octane rating and will not affect the burn rate. Adding alcohol (either methanol or ethanol) or some other oxygenates will also improve the octane rating of fuel. Alcohol burns slower than gasoline so gasoline spiked with alcohol will burn slower than straight gasoline. I believe this is the reason for some of the confusion and explains some of the "testimonials" from experts that claim "their motor made more HP on regular fuel than premium" and that "regular has more energy than premium." Likely they tested with some premium gasoline spiked with alcohol and they didn't change the jetting and spark timing accordingly.

As a matter of fact I have seen many engines on very accurate dynos run on both premium and regular fuel (these were engines designed for regular) and the power was exactly the same and the optimum spark timing was exactly the same. If the fuel burned slower or the burn rate changed then the optimum spark would change accordingly. Proof that the burn rate stays the same.

Normally aspirated engines at altitude are running somewhat throttled due to the lower atmospheric pressure. The cylinder pressure of an engine at WOT at altitude will be like the same engine at 2/3 throttle at sea level. That is why octane requirements at altitude are reduced and premium fuel at altitude will always be about 3 or 4 points lower than fuel at sea level.

Don't confuse octane rating (the fuels resistance to detonation) with volatility (the vapor pressure of the fuel). Volatile fuel will boil or generate vapor at lower temperatures and/or volatile fuel will create more vapor at a temperature than a lower volatile fuel would. Volatility affects the fuel's ability to vaporize or atomize in the intake tract and combustion chamber. Winter fuel is much more volatile than summer fuel. High altitude fuel is much less volatile (within the same season) than low altitude fuel. Volatility can also affect HP significantly depending on the engine and fuel system as the better the fuel atomizes the more completely it can burn and the more oxygen in the air is used. Many times power differences blamed on premium/regular are really due to differences in volatility without the tester realizing it. Aviation fuel is very low volatility even though it has high octane rating. I have heard lots of people claim av gas "won't make power" without realizing why....it's the volatility, dummy.

Some racing fuel can have very low volatility, also. Low volatility helps resist vapor lock ..... vapor forming in the suction side of the fuel line as the fuel pump tries to pull the fuel from the tank. I know of one specific case where a racer ran some race gas and pump gas and found more power with the pump gas and presented the results as proof that the lower octane fuel had "less energy in it". Wrong. The race gas was too dead to atomize completely and hurt the air/fuel distribution to the point that the power dropped. The pump gas was more volatile, atomized well and distributed well and made more power.

Old wives tales die hard....but there is usually an explanation out there to explain how they got started.


BTW....it is detonation or pre-ignition. "Pre-detonation" is not a word and not a condition.

Detonation is the spontaneous combustion of the end gas in the chamber. Detonation happens after the spark ignites the charge in a normal fashion. Causes a very very rapid pressure spike in the chamber that "rings" the structure of the chamber much like hitting it with a hammer. That is what you hear. It is NOT the sound of the flame fronts colliding. Another old wives tale.

Pre-ignition is a situation where the charge is ignited BEFORE the spark plug ignites the mixture. You will never hear pre-ignition. You'll hear the piston breaking up and parts chewing each other but not the pre-ignition that caused it. Pre-ignition is always deadly because the piston is coming up while the charge is burning and creating huge pressure trying to stop it. Piston looses.

Detonation often occurs near the exhaust valve because the exhaust valve is hot and heats up the mixture making it prone to spontaneously combust and cause detonation. Not because it ignited the tail end of the mixture. If that happened it would have lit it off much sooner and been pre-ignition.