Re: More on Top Tier gasoline

Retired VIP wrote:

> On Wed, 30 Jul 2008 18:09:05 -0400, "Mike Marlow" > wrote: > > > > >"doug" wrote in message > >news:_6ydnWYavq19Tw3VnZ2dnUVZ snipped-for-privacy@comcast.com... > >> > >>> wow, how uninformed can one person be. > >>> > >>> > >> I wonder - just how uninformed are you? The 3.8 supercharged engine uses > >> a knock sensor to vary engine operation - higher octane permits it to run > >> at full optimization. Unless you have a foot of pure lead, this results > >> in increased engine efficiency/fuel economy. > >> > > > >Yeah - I know. But - real world experience... I owned a 3.8L Supercharged > >Buick. I saw no significant difference in mileage or performance between 87 > >octane and high octane. The engine adjusts itself to the gas, and optimizes > >for what's in the tank. Granted - that will be somewhat lower for 87 octane > >than for high octane, but I never saw enough of a difference to make a > >difference. Don't know about the other poster Doug - but I'm informed. > >Beyond what the spec sheet says. > > Today's cars don't tune themselves for better performance if you put > in higher octane gas than they need. They have a default point of > operation which is usually their most economical operating tune. Only > if the fuel doesn't meet the engine's requirements does the computer > de-tune the engine. It the engine is designed to run on 87 octane > gas, the engine efficiency will be at it's best if you use 87 octane > gas. You can burn 89 or 91 octane gas in a car designed for 87 with > no drop in performance or mileage but there won't be any increase over > what 87 octane provides.

The problem with that is it is a textbook explanation that is outdated and completely ignores reality. In order to maximize fuel efficiency and performance many modern engines are programmed to run right up to the point where detonation occurs. Ideally you would have a small amount of detonation every firing of a cylinder. Detonation is not something that is black and white, it is not on or off it is dynamic and varies in degree. Modern engines are designed to hold the tune so that detonation is at a point where it maximizes performance. The knock sensor is not the only thing to respond to more than the minimum amount of detonation. The air/fuel trim can also respond due to disruption of complete combustion when detonation exceeds that point of maximum efficiency. That means slight changes in octane do make a difference. Couple that with the fact that octane is a whole lot more complicated than a single number on a pump. Add to that that the number on the pump is not as reliable as you might expect and the reality is that no one can predict the actual mileage effect of going from one brand to the next or one octane level to the next within one brand. You can read a book and have it tell you it won't make a difference but you are only fooling yourself. People who test this in a controlled manner almost always find some increase in mileage as octane increases. Often that increase is too small to pay for the extra cost. It isn't just the fuel that is an unreliable input to your comfortable formulas it is the engine itself that is unknown quantity. Lots of things in the engine also affect detonation and thus how much the engine retunes to accommodate. Even slight changes in engine temperature affect detonation. So does slight changes in the intake air temp as well as atmospheric pressure. When a low pressure system is passing thru everyone's engine is less likely to detonate. And of course Air/fuel mixture and EGR mix both raise or lower the tendency to detonate at any particular moment. And then there are slight defects like vacuum leaks and dirty injectors and dirty or poorly gapped spark plugs that all affect the engines tendency to knock. Carbon accumulation in the combustion chamber can also cause detonation. And what happens then? Well the engine detunes and that may cause more carbon accumulation. A tank of premium at that point could break what might become a slow death spiral. So you tell me you can read a book and read the mfg's specs and tell me exactly how my engine will react to tank of premium. I don't think so.

> Octane is a measurement of how hard it is to catch the fuel on fire, > not how much heat energy the fuel contains.

No it is a rating indicating how much the fuel theoretically has a tendency to detonate. As a counter example to your claim, hydrogen catches fire much easier than any gasoline component yet it has an octane rating much higher than any gasoline component. It is theoretical limit because there is no way to predict with certainty how any particular blend of fuels will work in any particular engine. Testing in an engine is still the method for determining octane even though today they can determine the exact molecular composition of the fuel. If someone really want to know how their particular vehicle responds to any particular gasoline you have to try it and see. It's true the vast majority of vehicles will see only diminishing returns for octane levels above what the manufacture recommends. But how much your engine varies from the norm can only be determined by doing controlled tests.

The heat energy will be > about the same in all grades of gas. If ethanol is used as the octane > booster then 91 octane will have slightly less heat energy since > ethanol contains about 30% less heat energy than gasoline.

Even when no ethanol is present on average higher octane fuel contains less energy than low octane. It is just in the nature of the gasoline fractions that the ones that contribute the most to detonation (for example heptane) also contain the most energy. But the energy content is for all practical purposes irrelevant. If energy content was all that was important for fuel economy then you would burn diesel in your gas engine.

-jim

> The octane requirement of an engine is determined by the engine's > compression ratio, valve timing, ignition timing, fuel/air mixture, > etc. The engine computer can control some of these in all engines but > it can only de-tune to compensate for a fuel that doesn't meet the > engine's requirements.

> A supercharged or turbocharged engine runs at an effectively higher > compression ratio under boost than it's spec sheet ratio. Boost > forces more fuel/air mixture into the engine which will raise the > effective compression ratio. So a boosted engine will almost always > require premium fuel. > > Jack

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Reply to
jim
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"C. E. White" wrote:

That is right. Drivers tend to respond differently to how the car behaves. Some drivers respond to less power by stepping down on the gas pedal. Other drivers do just the opposite - when they sense a flat spot in the acceleration curve as they step on the gas pedal they back off and stay just below that point. Other drivers are pretty much oblivious to acceleration and respond to the cars speed.

Knock is not something that is turned on or off like a light switch. In a perfect world you would want an engine designed to knock to some extent at all times (except when decelerating) That would maximize the amount of energy from each load of fuel and air. That means delivering the maximum amount of pressure as early as possible from a given amount of fuel. But there is a limit and going beyond that point does increase the pressure pulse and that seems like a good thing for fuel economy but too great a pressure spike (knock is just a pressure spike)) also destabilizes combustion so going beyond this ideal point reduces fuel economy. No matter what amount of throttle the driver is trying to deliver to the wheels you still have this balance point where any additional leaning of the mixture or advancing of the timing will result in too much knock (i.e. too big a pressure spike) and not enough will waste fuel. When the engine is under light load for maximum fuel efficiency the timing advances and fuel mixtures become leaner this brings the charge right up to that ideal knock point. But where that point is can't be predicted - it is arrived at by content feedback. And there are many things that determine the knock point besides fuel octane rating. Engine temperature, inlet air pressure and temperature, EGR volume, compression ratio, spark plug gap and secondary voltage are some of the things that effect the knock point in any engine.

In automotive engineering terms what you ask about is called "octane sensitivity". And one of the problems with the widespread use of ethanol today is that ethanol is an extremely octane sensitive fuel. Ethanol used under light loads has an octane capability that is really no better than mid grade gasoline. But under heavy loads its octane rating is much much higher than premium fuel. That means adding ethanol as an octane booster isn't going to help the fuel economy of the guy "who is very gentle" compared to someone who drives more aggressively and fast.

There are limits to how much engines can respond to more octane and when you reach the limit and go beyond usually means less mpg. But given that most cars today seem to be deigned beyond the limit of base

87 octane fuel and most labeled 87 octane fuel is actually slightly lower than that labeled 87 octane, it is a fairly safe bet that most newer cars will get some more miles from higher octane. But if it isn't more than %5 better then it isn't really going to be worth it. Most of the time the differences are going to be small and hard to detect unless you are very meticulous about keeping records and driving in exactly the same way.

-jim

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Reply to
jim

....

Actually hydrogen is not a good example to use. It has a high RON rating (research method) and a low MON rating (motor method). In an Otto Cycle engine hydrogen tends to pre-ignite and therefore acts like a low octane gasoline.

And as I said before - how you drive will have an effect as well. Someone who almost never drives at WOT is likely to see less of a difference than someone who routinely drives hard.

I was taught that higher octane gasoline had a lower energy content than regular when I was in college - 30+ years ago. However since then I have seen many credible sources saying this is not true.

From

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: "Gasoline with a higher heating value (energy content) provides better fuel economy. Traditionally, premium gasoline has had a slightly higher heating value than regular, and, thus, provides slightly better fuel economy, but it is difficult to detect in normal driving. There can be even larger differences in heating value between batches of gasoline from the same refinery, between summer and winter volatility classes, or between brands of gasoline from different refineries because of compositional differences. The differences are small and there is no practical way for the consumer to identify gasoline with a higher-than-average heating value."

Other interesting references:

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It is my opinion that if your car was designed to run on regular, running premium is a waste of money. Although the engine may be able to adjust some parameters to increase performance and fuel economy, the changes will be minor. On the other hand, if your car was designed to run on premium (because of turbocharging/supercharging, a higher compression ratio, or other inherent design factors) then running regular may drastically effect your performance and fuel economy even if the PCM can adjust parameters to protect the engine from excessive pre-ignition.

Regards,

Ed White

Reply to
C. E. White

Right that is a very definite issue with hydrogen but not an fatal one. Pre-ignition is not the same thing as detonation altho it may cause detonation. Engines have been designed to deal with that. That means high compression engines that take advantage of hydrogen's octane and are very efficient compared to gasoline powered engines.

Yes depending on what exactly the fuel is composed of. The current octane rating system is not intended to help motorists with fuel economy. It is intended to help the oil companies and auto mfgs. sell there products.

That looks more like a biased opinion not a credible source. The API is doing a lot of bad mouthing of fuels that contain less heat content but produce more useful output energy. If you are interested, the API and SAE do publish figures on heating values for the various classes of components in gasoline. So you can look at the raw data and judge for yourself rather than reading the spin they put on that data.

When they say traditional they mean 50 years ago when the world was awash in high quality sweet crude oil and large amounts of lead were used for boosting octane. The reality is octane is achieved in much different way today and if you look at the class of components that go into premium compared to what goes into regular from many refineries the components with higher octane tend to also have less energy. That doesn't mean it isn't possible to create blends where regular has the same heat content as premium - it just isn't the current reality. But the heat content really has nothing to do with fuel economy. Put some diesel in your gasoline engine and see how much that extra heat content helps your mileage.

The point I'm making is not that premium is cost effective, but that you really don't know how any particular engine will respond without trying it - there are just too many variables for sweeping predictions to be valid. And this is more true today than in the past because of the changes engine management systems and changes in fuel. Not because premium is better but because regular in many places is worse than it was in the past.

-jim

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Reply to
jim

Define "grade".

Gasolines with good additive packages that include good detergents to keep the fuel and combustion systems free of crap--now, those are good grades of gasoline.

Funny thing--the manufacturer of your car doesn't specify what GRADE of gasoline you should use.

I think you mean "octane" instead of "grade". Octane is not a measure of "goodness". It's a measure of the fuel's ability to resist knock and preignition. It's a property of the fuel--like color, it doesn't make the fuel inherently "better" or "worse". It's just a property.

You've bought into the fuel company "premium" hype.

Reply to
Elmo P. Shagnasty

yeah yeah, I meant Octane.

Reply to
RT

Grade is a term used by the petroleum refining industry to classify types of gasoline. Basically gasoline is pumped into pipelines for distribution, The grading system is a way to determine the value of the product so that exchanges and sales can be made. Octane is the main criteria for determining each grade.

Most people agree that if you go below the octane requirements for a given engine that your mileage will go down and if you purchase gasoline that goes above the octane requirements you won't get any benefit. That seems like a pretty simple proposition but in reality it is a whole lot more complicated. It's complicated by the fact that you really can't be sure what the exact octane requirements are with out careful real world testing.

I suppose you think there isn't any difference in quality of beef cuts or that there is no such thing as "better" or "worse" when it comes to types of wine?

Whether high octane is "better" or "worse" depends on what you are interested in. Octane certainly is the single most important property with respect to fuel economy. The reason the price is higher and people pay more for premium is because A) It costs more to produce B) In some vehicles it is cheaper to run using premium. C) regular may cause damage to some vehicles. (note: I'm not saying B) applies to every person who purchases premium and C) doesn't apply to very many modern engines)

Here is a list what J.H.Gary's "Petroleum Refining Handbook" says about the impact of octane on gasoline engines. The assumption here is if everything else would be kept constant this is how these factors affect a typical engine's octane requirements:

Combustion chamber deposits 0-12 RON

Altitude (air pressure) 3 RON for every 1000 ft. Spark timing 3 RON for every 2 degrees Engine speed 1 RON per 300 RPM Engine Temperature 1 RON per 10°F

Intake air temp 1 RON per 20°F Humidity 1 RON per 20%RH

He doesn't say anything about spark plug voltage and gap, EGR concentration, and Air/Fuel ratio, or engine load which also are all known factors that alter the engines octane requirements.

Engine management and it's sensors are designed to deal with a good many of these factors so for instance timing is used to offset many of the other factors. But sensors that measure these factors aren't perfectly accurate or reliable. Just add up the possible normal deviations and it is easy to see even in engines that are working 100% as designed it is possible to deviate from the norm by more that 2 octane points. And what about the millions of cars that aren't operating 100% as designed?

In isolation anyone of these factors is pretty simple to understand, but when you combine everything together it becomes very complex interactive system. Anyone (including the car's manufacturer) who tells you they can predict with certainty what the effect of purchasing higher octane fuel will have on any particular car's fuel economy at any particular time or place is talking thru their hat. There are just too many different factors beyond the scope of their knowledge for such a statement to be anything but a guess.

-jim

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Reply to
jim

You would be mistaken, just as you're mistaken when you compare cuts of beef to gasoline.

Reply to
Elmo P. Shagnasty

yeah. Gee, you're right. Honda's engineers are pretty much stupid, and don't know anything.

Sure. Uh-huh.

Reply to
Elmo P. Shagnasty

No. If you had ask I would respond that Honda's engineers are among the best. But as far as I know all that they say about mileage estimates on their vehicles is something to this affect: "Your actual mileage may vary". It would be pretty pointless for the engineers to enter into a technical dialog with customers about the numerous factors that contribute to the wide range of mileage that their customers get from the same make and model vehicle. I mean look at your response - it appears the only thing technical that you were able to grasp and respond to was the analogy I made between grades of beef and grades of gasoline.

-jim

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Reply to
jim

Ed, that Car Talk page is a great link. Thanks for providing it. I'm copying part of it into the newsgroup for permanent archiving.

CarTalk.com passage:

It's true that premium gas does contain special or extra detergents, but in our opinion they're of no additional benefit.

Our advice is to ignore any sales pitch about the super-special detergents that come in the premium-grade fuel.

These days, all of the gasoline from the major gas companies contain more than enough detergents to keep your engine clean. Period.

The only reason you might have a use for the extra detergents in premium, is if you have a noticeably dirty engine, and need to scrub carbon and other crud off engine components. (And, if you're doing that, we'd instead suggest that you toss in one of the many fine engine additives.)

You said "major gas companies." Is there a difference in the gas between the big names, and the El-Cheapo gasoline at my local Costco or Stupey-Mart?

There is. A number of years ago, some of the major auto manufacturers were ticked off with the deposits from gas that met EPA detergent standards. In other words, they were afraid they'd take the heat for problems, when their customer's engines clogged up from the twigs and dirt that was getting deposited on their fine handiwork.

So, they set their own guidelines, which they call "Tier 2." All of the gas from the major gas companies meets the Tier 2 standards. The gas at your local Quickie Mart probably does not. The difference? In some tests, after about 10,000 miles, there was a minor amount of crud built up on engine components that ran exclusively on Quickie-Mart gas. Do we think this is an issue? Not really. But, would we advise running your car for 100,000 miles only on Quickie-Mart gas? Not if you're concerned about the longevity of your vehicle. We recommend switching back and forth between Tier 1 and Tier 2 ? maybe every other tank full ? to keep your engine running clean.

Reply to
Built_Well

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