"C. E. White" wrote:
> "Retired VIP" wrote in message
> news: snipped-for-privacy@4ax.com...
> > 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.
> >
> > Octane is a measurement of how hard it is to catch the fuel on fire,
> > not how much heat energy the fuel contains. 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.
> >
> > 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
>
> In the past, Ford claimed that the 5.4L V-8, which was intended to run on
> regular fuel would have slightly higher fuel economy and slightly more power
> if premium fuel was used. Their claim was a maximum improvement of around 3%
> increase in fuel economy and maybe an extra 5 horsepower. I don't think must
> people could reliably detect either change. I owned 2 Expeditions with the
> 5.4L V-8 and tried both regular and premium for extended periods and I never
> could see any difference in mileage between the two. And there is no way I'd
> ever be able to tell the difference between 265 hp and 270 hp just by
> driving. I have also compared regular and premium in my Nissan Frontier
> (4.0L V-6). Nissan says the truck is designed to run on regular fuel, but
> that for maximum economy and performance premium should be used. The first
> time I ran premium, the mileage records indicated a significant increase in
> fuel economy, However, when I switched back to regular, the mileage stayed
> the same. Since then I have tried premium for other extended periods, and
> the mileage has not shown any improvement compared to regular.
>
> One factor you guys don't seem to be considering is the effect of the > driver.
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 sensors adjust the engine parameters based on detecting knock.
> The PCM doesn't know the type of fuel, only how the engine is reacting as
> indicated by the presence of knock. What do you suppose happens in the case
> of a driver who is very gentle and never provokes the engine to knock even
> when running on regular?
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
I am guessing it will adjust the engine parameters
> in the same manner as if the engine was running premium. I suspect this
> might be why some people claim significant difference when running premium,
> while other (like me) see no difference.
>
> Chevron used to have a really good set of web pages discussing gasoline.
> Unfortunately they have replaced those with some advertising inspired
> drivel. As I recall, the old Chevron pages addressed the question of energy
> content of premium and regular. They claimed premium contained around 3%
> more energy than regular on average, but that the energy content of gasoline
> in general varied by around this same amount, so in some cases a particular
> batch of premium might have less energy than a particular batch of regular. >
> At least in North Carolina, gasoline samples are routinely checked to verify
> the octane.
>
> The gasoline FAQ
formatting link
is an> interesting, if not always accurate source of information on gasoline.>
> Regards,
>
> Ed White
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