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slicey
06-17-2005, 02:33 AM
OK, i'm the proud owner of my very 1st blower and trimmer-stihl's bg85c and fs80r. what i'm curiuos to learn is; is there an emperical performance increase when using a higher octane fuel, like 92 octane for example? or should i just save the extra money towards my 2-cycle oil? thanks!

Lawn Masters
06-17-2005, 10:49 AM
I dont notice any real difference in performance, but these arent really high compression engines either, unlike a Stihl 066, which is something you'd run nothing but premium in.

topsites
06-17-2005, 10:52 PM
Well perhaps it doesn't get you extra rpm's but I used to run cheap AND even name-brand 87 octane and what I noticed more than anything, I went through a pull-cord about once/month between all my machines. I must've replaced a dozen pieces of string that year, thou yes I could brag they all started in 1 pull...

Then I switched to Amoco Premium and now:
- Not only do they start in 1 pull but they start a LOT easier. I replace maybe 1 starter cord (maybe 2) the entire year AND I never get that sharp pain in the neck from over-exerting myself while pulling on the WB's 15hp Kohler. Matter of fact, I can pull-start the Kohler with my left hand like a princely phaggot and it starts.
- On the mix, I notice I almost never have to use the choke on the backpacks AND my sparkplugs last longer (as do fuel filters on the WB's).
- It smells much, much better all the way around.

There is one other thing:
Octane is to fuel like what carbonation is to soda. If your fuel is in a can and the can gets HOT (like from the sun), when you open it, air pressure escapes with a hiss - That's your octane blowing away. Yes, you can watch fuel develop bubbles when it's in the heat, and I swear it acts like soda, and as those bubbles develop, your fuel loses octane.
So the longer fuel sits around, the more octane it loses, just like soda loses carbonation, fuel will eventually go flat (and it smells bad) thou it does take a long time, still, the loss of even a few percents of octane has me convinced: Amoco 93-octane for everything.

There is actually a TON more, I could tell you about the issues of dirt in fuel affected by the tanker truck re-filling your service station's underground tanks in ADDITION to how much the fuel MOVES (i.e.: If a type of fuel is purchased a LOT, then it MOVES a LOT). A fuel has to move some but not too much in order to retain cleanliness - If it moves too little, it goes bad over time... If it moves too much, sediment and debris gets stirred up in it but more so because the truck has to re-fill the tank fairly every day.
Yes, underground service station fuel tanks hold about 10 thousand gallons of gas and a busy station fuels out about 3-5 thousand gallons (or more) per day while a tanker truck holds around 9 thousand gallons (I can't remember if it's 8700 or 9200, but it's in that range).

The small difference in price between cheap 87 and nice 93 octane is recovered many times over by easier starts, dependable runs, and critical fuel-dependent parts longevity.

grass_cuttin_fool
06-18-2005, 08:34 AM
It has something to do with the amount of alcohol blended in the gas. If it has a certain % of alcohol then the oil doesnt mix very well with it. The higher octanes have less alcohol and less chance of the oil seperating from the gas. I think this is correct, I have never used anything but 87 octane but the next can I mix will be atleast 89 octane from now on, Its not worth taking a chance on messing up my trimmers and blowers

mcdansley
06-18-2005, 04:01 PM
What about your thoughts on using a higher octane fuel in 4-cycle engines (mowers)? I have a 6 hp Kawasaki, and currently use 87 octane with Sta-Bil added to it. Is there any advantage to using a higher octane fuel?

KLLawn
06-18-2005, 08:26 PM
McD,
I try to use at least 92 octane on my Echo bp blowers and all my lawnmowers. A few years ago on lawnsite , viewers discussed this very issue and all said that Echo OPE benefits from higher octane. I started using 92+ and it was a big difference. I think Echo manuals state the use of a higher octane fuel.

HK45Mark23
06-18-2005, 11:43 PM
Gas is an explosive. This means that it burns faster than the speed of sound.

Octane ******s the explosive properties of gas. In doing so it transforms gas into a propellant witch burns slower than the speed of sound.

In this state the burn time of gas is greatly reduced proportionately to the percentage of octane fortification.

The significance to this information is; if gas was at time of detonation to explode then the release of energy in the form of expanding gasses would be extremely rapid and the yield would be low. But if you ****** the burn time and hence the expansion and release of the gasses then you’re able to extract more energy from the fuel and over a greater period of time.

Look at it like this. If you were to use gas with no octane, the explosion would be over before the piston could have moved even a quarter of an inch. Faster than the speed of sound the gas would have burned and the expansion and released of the gasses would be over. The yield of gases would also be less.

On the other hand the higher the octane the slower the burn hence a longer burn time and also pressure from expanding gasses for a longer duration during the power stroke.

In all the manuals I have it says to use the lower grade fuel. But it would stand to reason that any internal combustion engine would benefit form higher octane. Higher octane is more efficient and easier on the components of the engine.

Oldtimer
06-19-2005, 09:18 AM
I have used Amoco and only Amoco as a shop fuel for 20 years and recommend it above all others. I use 89 octane with 50:1 Shindaiwa mix oil in everything. There is no straight gasoline allowed in my shop and there are no fueling mistakes possible. The oil goes in the drums before they are hauled to the gas station.

It seems that the smaller the engine the more sensitive it is to fuel. This is usually the result of discount station, big box store fuel or old fuel that the customer has stored for a few months. When fuel goes thru several heating and cooling cycles, especially in small vented containers, it loses it's light ends and small engines become very difficult to start.

My estimate is that 90% of the 2 stroke engine problems we see are fuel related. The customer says "my car runs just fine on the 87 octane from the big box store so my weed eater should too". Yesterday a customer brought in a small Stihl line trimmer and felt he had a warranty repair because it was hard to start and wouldn't idle. We changed out the fuel and it was perfect.

Oldtimer

bill w
06-19-2005, 10:09 AM
OK, i'm the proud owner of my very 1st blower and trimmer-stihl's bg85c and fs80r. what i'm curiuos to learn is; is there an emperical performance increase when using a higher octane fuel, like 92 octane for example? or should i just save the extra money towards my 2-cycle oil? thanks!

My understanding of octane is that it is a measurement of resistance to pre-ignition and detonation. Most 2-strokes that I am famiiar with call for 89 octane fuel...mid-grade. Any higher octane rating is a waste of money. Any lower octane rating is a risk of a burned piston.

topsites
06-20-2005, 02:20 AM
What about your thoughts on using a higher octane fuel in 4-cycle engines (mowers)? I have a 6 hp Kawasaki, and currently use 87 octane with Sta-Bil added to it. Is there any advantage to using a higher octane fuel?

*ALL* my base runs 93-octane: Truck, wb's, everything... but it's all carbed.
Course on EFIs you need to read your manual and use whatever it calls for - carbureted: run 93, EFI: run what the manual says.

topsites
06-20-2005, 02:24 AM
Gas is an explosive. This means that it burns faster than the speed of sound.

Octane ******s the explosive properties of gas. In doing so it transforms gas into a propellant witch burns slower than the speed of sound.

In this state the burn time of gas is greatly reduced proportionately to the percentage of octane fortification.

The significance to this information is; if gas was at time of detonation to explode then the release of energy in the form of expanding gasses would be extremely rapid and the yield would be low. But if you ****** the burn time and hence the expansion and release of the gasses then you’re able to extract more energy from the fuel and over a greater period of time.

Look at it like this. If you were to use gas with no octane, the explosion would be over before the piston could have moved even a quarter of an inch. Faster than the speed of sound the gas would have burned and the expansion and released of the gasses would be over. The yield of gases would also be less.

On the other hand the higher the octane the slower the burn hence a longer burn time and also pressure from expanding gasses for a longer duration during the power stroke.

In all the manuals I have it says to use the lower grade fuel. But it would stand to reason that any internal combustion engine would benefit form higher octane. Higher octane is more efficient and easier on the components of the engine.

Maybe in your world it works like that, but in my world higher octane burns faster - hence the pinging of advanced ignition on some carbed models.
IF fuel burned slower at higher octane then it would produce LESS push for the piston, and if so, then why do all hi-performance autos call for premium? Surely they want fast ignition, especially when revving engines around 6000-7000 rpm and higher, a slow burn wouldn't be able to sustain those higher rpm's thus racing cars actually run 98-103 octane and such.

Hmmm, wonder if I should run a racing fuel in the WB? LOL, too bad it's almost $10/gallon.

If you don't believe me, then maybe you should run Diesel which has an Octane rating of around 40, wow that should burn REAL fast.

HK45Mark23
06-20-2005, 03:13 AM
Your world or my world physics are the same.

I understand this concept due to my experience with propellants.

I study ballistics and the properties affecting projectile ballistics. Gun powder is a propellant. Gee it is interesting how a propellant that does not burn at a rate higher than the speed of sound can propel a bullet faster than the speed of sound. 1,100 feet per second is the speed of sound.

What you fail to realize is that this still happens at an extremely high rate of speed. If the burn time is too fast then you get less fuel efficiency. This meaning that you will not burn all of the fuel efficiently. Also you will not get the release of gasses to last the entire duration of the stroke. This also means that half way through the power stroke the fuel is spent and the cylinder is now only carried by its own inertia.

Even with ballistics, you are looking to get just the right chemical propellant compound to burn every grain completely at the time that the projectile breaches the crown.

It then becomes necessary to know the amount of drag that the barrel has on the projectile by knowing the contact surface aria and the length of the barrel, this is extremity important. When you take in all the info, drag, weight, length, burn rate, maximum pressure rating then you are able to produce the most efficiency.

You want to have a chemical that will reach that maximum pressure rating for the duration of the time that the projectile is still traveling through the barrel. Only then are able to produce extremity high rates of speed.

If that the projectile is half way down the tube and your propellant is spent than the projectile drags the rest of the way to crown.

In contrast if there to still propellant burning even after the projectile has breeched the crown then that means that your burn rate was to slow hence you wasted powder and chamber space and lost total efficiency.

If either scenario occurs your speed will suffer and your hitting ability will also suffer.

For instance a 180 grain 10 mm projectile from a 6” barrel can be projected to a velocity of 1,300 fps.

Please understand that if you take the speed squared times the weight in grains divided by the constant of 450,400. you will then know the pound feed of energy.

For instance in this case (1300 * 1300) * 180 ÷ 450,400. = 675.33

Or 1300 *1300 = 169,000. * 180 = 30,420,000. ÷ 450,400. = 675.33 ft. lbs. of energy.

My apologies but octane actually ******s detonation and in return prolongs the burn time. This actually extends the burn causing a longer duration of expanding gasses and therefore longer duration of pressure.

And finally this all happens in a fraction of a second. Please if you don’t believe me call a professor of physics or maybe even a high school physics teacher.

Did you know that in a diesel truck cylinder the force in which the diesel ignites is that of close to 1-3 sticks of dynamite?

A diesel engine and a gas engine are very different in many ways. #1 Diesel ignites under pressure. No spark necessary. #2 Diesel releases much higher amounts of gasses. #3 The BTUs are much higher when diesel ignites.

The comparison of diesel to gas is not apples to apples. Well it is like diesel to gas. ;-)

HK45Mark23
06-20-2005, 04:09 AM
My source

http://www.prime-mover.org/Engines/GArticles/octane.html

Read for your self!

The octane number assigned to a motor fuel has very little to do with the actual chemical "octanes" in the fuel and everything to do with how well the fuel resists detonation (which is directly related to the amount of energy (heat) required to get the fuel burning in the first place).

Therefore, it is possible to assign octane VALUES to fuel which contain no octanes whatsoever.


WHY DO WE CARE ABOUT OCTANE? WHAT IS PREIGNITION? WHAT IS DETONATION?
The octane value of a fuel is an empirical measure of its ability to resist detonation and, to a limited extent, preignition. Technically, octane ratings measure a fuel's ability to resist the spontaneous ignition of unburnt end-gases under controlled test conditions.

What is preignition?

Preignition occurs when the fuel/air mixture in a cylinder ignites before the spark plug fires.
It can be caused by burning contaminates (such as carbon, or a spark plug of the wrong heat range) in the cylinder or by extreme overheating.

What is detonation?

Detonation occurs when the flame-front in a cylinder does not proceed smoothly from the point of ignition (the spark plug) to the opposite side of the cylinder.
It refers to the spontaneous ignition of the entire charge in the cylinder. This ignition is often caused by the extreme pressure rise in the cylinder that occurs when the charge is first ignited (by the spark plug).



WHAT ABOUT FUELS?
There are six things to consider when comparing hydrocarbon fuels:

1. Volatility. In short, what's the fuel's propensity to vaporize. This effects the ability to easily mix the fuel with air and the fuel's tendency to vapor-lock. It also determines the pollution characteristics of the fuel where evaporative pollution is a concern.
2. Pre-ignition & knock resistance. Referred to as "Octane value." How much energy does it take to get the fuel burning - how much does it resist auto-ignition from compressive heat? Also, what is the rate of burn of the fuel (which affects the rate of pressure rise)?
3. Energy content. How much energy can be extracted from the fuel as a percentage of its volume or mass.
4. Heat of evaporation.
5. Chemical stability, neutrality, and cleanliness. What additives does the fuel contain to ****** gum formation? Prevent icing? Prevent corrosion? Reduce deposits?
6. Safety
The first three factors are often confused and interrelated when, in fact, they measure three completely separate things. There is no natural collelation between them.

General rules:

Heavy fuels (diesel, jet): Low volatility, low knock resistance, high energy per volume
Light fuels (gasoline): High volatility, high knock resistance, low energy per volume

Note that gasoline, partially, makes up for its (relatively) low energy-per gallon by the fact that a gallon of gasoline weighs less (by about 15%) than a gallon of jet fuel.

Octane rating is in no way correlated with engine power or efficiency. There is more potential energy in a gallon of diesel fuel than a gallon of gasoline, yet the diesel fuel has a much lower octane value (more on that below).


HOW DO YOU DETERMINE OCTANE?
Ok, then, how is octane rating determined? First, you go out and get a suitable supply of the fuel which you wish to test. Then, you get yourself some heptane (made from pine sap) and some iso-octane (a petroleum derivative). Finally, you and your buddies arbitrarily, agree that iso-octane has an octane rating of 100 while heptane has an octane rating of 0.

Next, you call up Waukesha Motors and order yourself an ASTM-CFR test engine. Make sure you have about $250,000 available on your VISA before you order it. This single-cylinder wonder has a four bowl carburetor and a movable cylinder head that can vary the compression ratio between 4:1 to 18:1 while the engine is running.

You fill the ASTM-CFR full of your mystery fuel and, for automotive fuels, you run two test protocols using the ASTM. One protocol is called the motor protocol and the other the research protocol. You vary the compression ratio until the onset of knock and write down all kinds of various scientific parameters.

Next, you run your reference fuel, made up of various proportions of heptane and iso-octane through the ASTM-CFR. You keep varying the proportion of heptane to iso-octane until you get a fuel that behaves just like (knock-wise) your mystery fuel. Once you get that, you say to yourself "How much heptane did I have to add to the iso-octane to get the mixture to knock in the ASTM-CFR just like my mystery fuel?" If the answer is, say, 10% heptane to 90% iso-octane, your mystery fuel has an octane number of 90.

How do the motor and research protocol differ? Mostly in input parameters. In the motor protocol (ASTM D2700-92), the input air temp is maintained at 38C, the ignition timing varies with compression ratio between 14 and 26 degrees BTDC, and the motor is run at 900 RPM. In the research protocol (ASTM D2699-92) the input air temperature varies between 20C and 52C (depending on barometric pressure), timing is fixed at 13 degrees BTDC, and the motor is run at 600RPM.

The motor method, developed in the 1920s, was the first octane rating method devised. After its introduction, many more methods were introduced. During the 1940s through the 1960s one of those methods, the research method, was found to more closely correlate with the fuels and vehicles then available. However, in the early 1970s automobiles running on high-speed roads, such as the German Autobahn, started destroying themselves from high-speed knock. It was found that the difference in ratings between the research and motor method, known as the fuel's sensitivity was important as well. The greater the fuel's sensitivity, the worse it performed from a knock point of view in demanding, real-world, applications.

Remember, at the pumps, the results of the motor and research numbers are averaged together to get the value you see. The fuel's sensitivity is not published. Highly cracked fuels have high sensitivity while paraffinic fuels often show near zero difference between the two. While the fuel's sensitivity is not published at the pump it can be a valuable indicator as to the fuel's real world octane performance. Remember, the octane tests are conducted in a lab using a special test engine; the lower the fuel's sensitivity, the more likely it is that the fuel will, indeed, behave as expected. Generally, the closer the fuel's research rating to the published rating the more reliable the published rating. Because the motor and research methods primarily differ in terms of input parameters (the test engine is the same for both), the greater difference that a fuel exhibits between its motor and research test will be due to differences in input parameters (intake temp, timing, etc.). A fuel that has an octane rating that varies with intake parameters is said to be more "sensitive."


HOW DO YOU DETERMINE AVIATION GASOLINE OCTANE?
The octane of aviation fuel is not measured in exactly the same was as is automobile fuel.

Once again, you start with your trusty ASTM-CFR engine. First you set up the ASTM-CFR for the motor method and use that method to determine the motor rating of your fuel. You then correct that rating to the "Aviation Lean" rating using a conversion table. Below about 110 motor octane (a performance number of 110), the aviation lean and motor octane numbers will differ by only about 1 or 2 points. Above 110 motor octane the differences can be significant. Next you pull out another version of the ASTM-CFR engine. This one has a fixed compression ratio but allows you to supercharge the intake manifold. You pressurize the intake to higher and higher values until the onset of knock. Other than that, the parameters are the same as for the motor method used for automobiles. The supercharge method is then used to assign the Aviation Rich value of the fuel. Supposedly the pressurization method (as opposed to changing compression ratios) is a throwback to the 1950s and 60s when supercharging was common in aircraft engines. The engineers were particularly concerned with the fuel's behavior under boost.

Because of the different ways in which automotive and aviation gasoline octane is measured one must be very careful when comparing absolute numbers. 100 octane avgas is not equal to 100 octane autogas (but it's close!). Note that the lean number rating of an aviation engine will correspond very closely to its autogas (mogas) motor rating requirement (see the above paragraph). Thus when shopping for autogas for your 91/96 O-360, you should look for a filling station at which the motor octane rating of the fuel is at least 91.

You should also be careful leaning the engine as this may cause its octane requirements to go above what the autofuel can provide. Look for an autofuel with an octane number as far above the lower aviation octane number as you can. If you can get one which is at or above the rich octane requirement (the higher number) then you should be a-OK.

Thus an engine rated for 80/87 aviation should have no trouble whatsoever running on 89 octane (or higher) unleaded. Engines rated for 91/96 should run on at least 91 (motor) octane unleaded but note that this is lower than the rich limit requirements (96) of the engine. Therefore it is especially critical to limit leaning with such an engine/fuel combo when running at high power settings.

HK45Mark23
06-20-2005, 04:11 AM
HOW DO THEY GET OCTANE NUMBERS ABOVE 100?
Often it's done by pure extrapolation. A more reliable method, however, is through the use of so-called performance numbers. Briefly, these are arrived at by determining the instantaneous mean effective cylinder pressure (IMEP), using the fuel under test, at the highest boost that does not cause knocking. This number is then multiplied by 100 and the resultant is divided by the IMEP at the highest boost that does not cause knocking on the 100 octane equivalent fuel.

Note that, technically, there is no such thing as an octane number above 100. If you're at a party, avoid saying things like "110 octane gasoline" because people will get up and walk away from you. You should say, instead, "a gasoline with a performance number of 110." That will bring the help scurrying over with more champagne.


HOW DOES LEAD AFFECT OCTANE?
Tetraethyl lead raises the octane rating of a fuel not because it adds more "octanes" to the fuel but because it makes the fuel knock at a higher compression ratio in the ASTM-CFR. According to the latest research, octane ratings go down with fuels comprised of long, straight, hydrocarbon chains (paraffinic fuels). Fuels with branching hydrocarbon fuels, and aromatic fuels, have a higher octane ratings.

Oxygenates and alkyl lead affect the pre-flame reaction pathways by ******ing branching sequences. Lead was previously believed (by many, including myself) to work by slowing the flame front, thus leading to a slower pressure rise in the cylinder. While general flame-front propagation speed does affect octane ratings, lead does not significantly affect it.


WHAT OTHER FACTORS AFFECT OCTANE REQUIREMENTS?
Combustion chamber design, localized hot spots, piston speed, and a host of other factors can all contribute to a engine's propensity to ping.

Additionally, in the aviation world, altitude extremes and super/turbo charging affect octane requirements. Increased induction pressures (such as would be encountered in a turbo/supercharged engine) cause more rapid flame-front propagation. Likewise, decreased exhaust pressure (as would occur at altitude) also tends to increase flame-front propagation speed. Both of these effects can combine to raise octane requirements - especially at altitude.

Note that the latter effect also affects the proper fuel/air ratio for BEST ECONOMY operation. The reason is left as an exercise for the reader.


HOW DOES THE OCTANE OF DIESEL AND/OR JET FUEL COMPARE TO GASOLINE?
Diesel and Jet fuel (along with kerosene) have, indeed, terrible octane numbers; typically about 15-25 "octane". They tend to ignite easily from high compression. Their use in a gasoline engine will quickly destroy the engine.

Diesel fuel is rated by its cetane number which is determined, like octane, by running the fuel in a test engine. Instead of heptane and iso-octane they use napthalene (cetane rating = 0) and n-cetane (cetane rating = 100). In total opposite to octane ratings, the higher the cetane rating the higher the fuel's propensity to knock!

Just as using a fuel with an octane number higher than necessary in a gas engine will gain you noting, using a fuel with a cetane number higher than necessary in a diesel engine gets you nothing. On the other hand, where using a fuel with too low an octane number in a gas engine will result in a damaged engine, using a fuel with too low a cetane number of a diesel engine will just result in a rough-running (or not running at all) engine with no damage.


WHAT'S THE DEAL WITH DIESELS?
Why can diesel engines tolerate a low octane fuel? In all gasoline engines, (including injected gasoline engines!) the fuel/air mixture is present in the cylinder the entire time the piston is travelling upward on its compression stroke. This means it could be ignited at any time whereas we only want it to be ignited when the spark plug fires, some time just before the very top of the stroke. Furthermore, we want a nice, even, steady, pressure rise in the cylinder as a result of ignition. This means that we want the flame-front to travel linearly from the source of ignition (the sparkplug) to the other side. We do not want combustion to occur randomly within the mixure as that may cause a too-rapid pressure rise which will throw off all our calculations about where the piston should be and when.

First, In a diesel engine there is NO fuel in the combustion chamber as the piston starts up on its compression stroke. Instead, fuel is injected at high pressure (up to 3000PSI!) into the combustion chamber at the exact moment when ignition is desired. In a diesel engine with a compression ratio of around 20:1 (compared to 7:1 for many modern gas engines), the heat of compression will have raised the combustion chamber temperature to arond 1000-1500F. The injection time takes about .002-.004 seconds during which the fuel spontanously ignites from the heat of compression at just the right time. Even so, a diesel fuel with too low a cetane rating may not ignite, or may ignite poorly - especially on cold days starting a cold engine.

The second critical difference is that Diesels are set up to burn the fuel in a slightly different way.

In a gas engine, you typically set it up so that the mixture is ignited before the piston hits the top of the stroke. What you're aiming for is for the mixture to be fully burned around the top of the stroke - thus combustion pressures are maximized at the top of the stroke and gradually fall off as the piston moves downward on the power stroke (and increases the volume in the cylinder). Diesels, on the other hand, are set up to inject fuel very close to the top of the compression stroke. The fuel spontenously ignites (auto-ignition) and, actually, knocks just like it does in does in a gasoline engine (hence the classic diesel "knocking"). The combustion pressures in the diesel increase evenly as the piston goes down. The net result is that the diesel piston "feels" a constant pressure on it as the piston travels from top dead center to bottom dead center whereas a normally operating gasoline engine piston "feels" a constantly decreasing pressure as it travels to the bottom of the stroke. The net result is that the diesel feels a lot lower PEAK pressure while the pressure is maintained over a longer period. The gasoline engine feels a much higher peak pressure which starts to fall off immediately as the piston travels downward. The implication, for the latter, is that it periodically operates very close to the capabilities of the base metals. Anything, such as knocking, which increases those peak pressures even more is apt to push beyond the capabilities of the base metals and result in engine damage.

Knock in a gasoline engine tends to occur at the end of combustion, when pressures inside the cylinder have reached, as a result of spark ignition, very high values - values high enough to auto-ignite the fuel.

Knock in a diesel engine happens at the beginning of combustion as a direct result of piston compression only. It is what allows further combustion as the piston moves downward. This continued combustion keeps the cylinder pressure constant as the piston moves towards BDC.


WHAT DOES A LOW OCTANE VALUE MEAN, TO ME?
In the absolute worst case, if the fuel is too low octane, it may spontaneously ignite before the spark plug fires due to thermal rises from the heat of compression or from hot spots in the cylinder itself. This kind of ignition is called pre-ignition (as opposed to knocking) and is a pathological case which will just turn an engine to scrap. Diesel fuel is low enough octane that mixing it with gasoline can cause pre-ignition!

What usually happens, and what we usually call knocking or pinging is that the fuel/air mixture does not ignite before the spark plug fires but does ignite spontaneously after that. The sparkplug fires and this causes an immediate, rapid, rise in combustion chamber pressure. This causes fuel on the other side of the flame-front to ignite before the flame-front reaches it. In turn, this causes combustion chamber pressure to rise even more rapidly. The result is an explosion inside the combustion chamber as opposed to the desired rapid burning.


WHAT DOES A HIGH OCTANE VALUE MEAN, TO ME?
A high octane rating ensures that it takes a REALLY hot ignition source to ignite the fuel (such as a spark plug or the flame-front itself) and not just the rise in pressure & temperature that's a result of normal combustion. Note that the thermal rises in the cylinder are in direct proportion to the compression ratio of the engine (more below). The higher the compression ratio, the higher the octane of the fuel that's needed.

Again, if the mixture in a gasoline engine ignites before the spark plug fires, we call that "pre-ignition." Pre-ignition can damage an engine before you finish reading this sentence. To reiterate, what we're really concerned with is called "knock" and that's the spontaneous ignition of the fuel-air mixure ahead of the flame-front as a result of the rise in cylinder pressure caused by the onset of ignition (caused by the firing of the spark plug).


WHAT ABOUT AIRCRAFT?
Now, back to aircraft. We want to make aircraft engines with the following characteristics:


1. Very high power/weight ratio
2. Low specific fuel consumption (so we don't need to carry around heavy fuel)

The easiest way to do this, without involving lots of complex machinery that might fail and add weight, is to raise the compression ratio of the engine. An engine's efficiency is in direct proportion to its compression ratio. Unfortunately, raising the compression ratio means we need to protect against knock/detonation. How do we do this? We use high (100 octane) fuel!

wow that took two postings

CincyWolf
06-20-2005, 08:54 PM
I've been getting a kick out of this. Using premium octane on engines designed for regular octane will not result in better performance - only spending more money. "

SHHHHHHH. not so loud say the oil companies! You'll kill the goose that laid the golden egg!" They have tried to convince us for years that premium will give you better results, will burn cleaner and should be used if you really care about your car. The cleaner part could be true because they add in more detergents but that is an expensive way to get more detergents.

This debate has gone on for years. The fact is that high octane fuel will burn more slowly as the explosion wave propagates across the combustion chamber allowing the full power stroke and higher output in engines that are designed for the higher octane. Compression ratio and ignition timing advance curve being two of the most important design factors. Fuel with octane ratings too low will ping and knock because the charge explodes faster than it should. In extreme cases when ignition timing is also off you might get premature ignition. HK45Mark23 has the right definitions.

In practical terms for most modern car engines they will run fine on good quality regular octane fuel even if specified for premium. This is because the engine has a knock sensor and it compensates by adjusting timing. Note I'm not saying you will get optimum performance but with the 10-20% upcharge in premium fuel over regular of the same brand you likely would save money on regular for most driving purposes. You should try this out before assuming you are stuck paying for premium gas.

Of course, nothing I say will convince those that view Premium Gas as the only way to get the most out of their engines. Most premium gas is sold to people that have had this notion passed down from father to son, some just believe they are special so deserve to burn premium and yet others have cars that really need it. Older cars in particular, even those that could run 87 octane for years might need a step up to a higher octane because of carbon buildup on the cylinder heads causing the compression ratio to increase.

Two final observations/tips:
1) If you are convinced or really do need a higher octane fuel for any of your internal combusion engines you can mix 50/50 Regular and Premium and be money ahead because the octane is a pure average at the pump. A 50/50 mix of 87 and 93 yields 90 octane and it is highly likely that this mix would be significantly less per gallon than 89 octane gas. You can change your ratio until you get the optimum mix that stops ping/knock and the lowest cost. This might not be quite as convenient but you can save real money over the course of a year and the life of your car.

2) As far as your lawn equipment from mowers to 2 stroke engines it all depends on your equipment, it's age and state of tune. It was all pretty much designed for 87 octane but conditions can change so moving up might actually result in a performance increase so it is worth a try. If you really want to separate fact from hype - do a blind test across across all your equipment and see if you or others in your crew can really tell a difference. I'm willing to bet that in most cases you can not but if you do see a positive you'll have science and real data instead of hype on your side.

As high as fuel costs are you really owe it to yourself to really make sure you need Premium fuel before spending that extra money. It is probably more important to buy fuel from a dependable source so it is clean and water free, use it quickly to keep it fresh and buy a premium 2 stroke oil for premixes. Stale fuel is the #1 cause of starting and performance problems with small engines - of course I didn't need to tell anyone on this forum that well known fact.

bill w
06-20-2005, 11:02 PM
Coupla quick questions, CincyWolf.

What are the fuel prices where you are for regular, mid-grade, and high test? Around here, there is a 5% and 10% upcharge for the higher grades ($2.00...$2.10...$2.20).

Do your 2-stroke manuals call for 87 octane? Mine call for 89 octane...Echo and Stihl.

Oh, one more...are you saying that premature ignition is a factor of timing?

CincyWolf
06-20-2005, 11:49 PM
bill w,

The crazy gas stations around here always have a non-linear price escalation so it is even better to mix gas here than your example in Virginia even though that example would still give you 90 octane at 50/50 for $2.10 vs. 89 octane. Around here you get the bonus octane AND for less cost - sometimes 5-10 cents/gallon.

Some call for 87 and some for 89. I've found gas rated at 87 that pings less than gas rated at 89. Most fuel is the same in the gas pipelines and then gets gasoline additives blended in at the depot before being loaded on the delivery tanker. Sometimes they add too much or too little octane additive at the depot so many times its hard to tell a consistent difference between the regular and plus versions. Each batch is not tested.

True pre-ignition is an extreme condition caused by an overheated combustion chamber that causes the fuel to ignite well before it is supposed to ignite. This can be from a plug with the wrong heat range, a very lean air/fuel mixture, carbon hot spots on the cylinder head or on the exhaust valves etc. Timing advanced too much can sound almost as bad as pre-ignition but it is a different cause and is really severe detonation vs. pre-ignition. In my haste I didn't state it right in the first post. Thanks for catching that.

Also - there are many legitimate reasons to use higher octane fuel, I just think its a hoot when I hear some of the reasons why people choose to do so.

bill w
06-21-2005, 07:23 AM
Thanks for clearing that up, CincyWolf. One more question...do you find greater consistency of octane at the name brand stations vs. the non-name brand outlets? That seems to be my experience around here.

CincyWolf
06-21-2005, 07:27 PM
That is my experience too. Name brands usually have better quality control, if for no other reason they have much more to lose with the equity they've built up on the Branded product.

Of course, there always are exceptions.

Holistic Landscapes
06-23-2005, 08:01 PM
I CAN ANSWER ANYTHING ABOUT GASOLINE NOW! THIS SITE IS AMAZING I always wondered why I would get sprayed with gasoline once in a while... when it was hot out.... KEEP GOING!

KathysLGC
06-24-2005, 10:45 AM
Finally! A thread worth reading for every one.... Keep it up guys. Informative threads like this should read by all.