why 91 octane in the prius?

Octane ratings are greatly misunderstood. Many people think higher octane fuel produces more power - it does not. Octane is a hydrocarbon which can be burned in an internal combustion engine (ICE). Compared to gasoline, which is a mixture of several hydrocarbons, the most prevelent being naptha, octane burns slowly, and it takes more temp. and pressure to get it to burn as quickly as required in an ICE. Octane was chosen to "rate" fuels resistance to exploding in ICEs (you want the fuel to burn, not explode!).

The Prius ICE is set up to use 87 octane fuel. If you try to use, say, 92 octane fuel, it will burn more slowly. This -may- result in unburned fuel being expelled through the exhaust valve, or at the very least, some of the fuel being burned -after- it can expend its' energy by pushing the piston. The energy will be lost out the exhaust. I use 92 octane fuel in this example because there are so many variables, saying 89 octane fuel would do this wouldn't be provable.

So, for maximum efficiency, use the fuel recommended for the "state of tune" of the engine, in this case 87 octane rated fuel.

There are other reasons to use different fuels however. In my case, I use 89 octane Huskey/Mohawk "mid-grade" fuel because it contains "up to" 5% ethanol, and it burns cleaner - not in a Prius, in an SUV. I'll let you know how well it works in my Prius when I get it. I also use 92 and 94 octane rated Huskey/Mohawk "premium" fuel in another car (93 RX-7) because it calls for 92 octane rated fuel minimum, and because it contains "up to" 10% ethanol and burns cleaner than "normal" premium fuels.

Fuel containing ethanol will produce a little less energy when burned in an ICE, so you can expect 5% or so lower mileage. If you want to use it, just keep telling yourself you are using "up to" 10% less non-renewable resources (ethanol is renewable).

BTW, "up to" is in quotation marks because that's what it says on the pumps.

 

<div class='quotetop'>QUOTE(NuShrike @ Jul 29 2006, 04:47 PM) [snapback]294410[/snapback]</div>

Funny that you mentioned that.

I filled up yesterday with 87 and I am getting about 45 mpg

 

<div class='quotetop'>QUOTE(hschuck @ Jul 29 2006, 06:27 PM) [snapback]294390[/snapback]</div>

The Prius ICE is actually an Atkinson cycle NOT a Miller cycle


<div class='quotetop'>QUOTE(Stairman @ Jul 29 2006, 04:50 PM) [snapback]294372[/snapback]</div>

I work for Cadillac and the standard V8s ALL call for 89 NOT 87 octane

<div class='quotetop'>QUOTE(NuShrike @ Jul 29 2006, 07:47 PM) [snapback]294410[/snapback]</div>

How do you figure? Lowering the octane last two tanks and the problem is gone. That's as complete as it gets

 

Actually, the Prius specs do call for 91 Octane fuel, but that's 91 RON, though.. In North America, pumps are labelled with MON+RON/2 ratings, so the 91 RON is really equivalent to about an 87 pump rating.. This leads to a topic that never gets talked about and that's the sensitivity of the fuel (ie, the spread between RON and MON numbers).. I'd like to know how octane numbers are adjusted for ethanol-blended fuel.. It would seem that adding ethanol would allow refiners to produce a lower octane base and boost it up with ethanol, resulting in a final mix that was higher sensitivity and lower energy content, while charging you a premium to buy it.. Yet another reason I'm still not convinced about the supposed benefits of ethanol..

That said however, not all fuels with higher octane numbers are necessarily lower energy content; it is very much possible (actually incredibly simple) to formulate a fuel with both a higher octane number as well as higher energy content.. This is likely why many people do report MPG gains after trying certain brands of premium fuel.. I know that I've seen up to 4-6 MPG improvement on the original Shell "V-Power", but this seems to have disappeared with their "V-power 2", which I suspect has extra ethanol in it, which is one of the octane boosting additives that in fact decreases energy in the fuel.. The Shell mid-grade seems to give me about 2 MPG better mileage, and timing doesn't get retarded quite so much on hard acceleration, so there is some benefit..

 

<div class='quotetop'>QUOTE(BobZ @ Jul 30 2006, 03:40 PM) [snapback]294753[/snapback]</div>

Thanks for the correction (I knew that, brain not properly engaged).

 

Since filling up with 87, I can't get it above 46

When this tank is done, for fun, I am going to fill up with 100 and see what I get.

 

I'm experimenting with "Shell Optimax" fuel. It's higher octane I think, but contains better additives apparently which might be from where the benefits come. Hard to say if it's working though, since I don't do regular commutes and my driving varies quite a bit from tank to tank. Might there be additives that speed up the combustion hence making higher octane burn like lower octane?

If UK Fuel is 95 or 97 RON, how would that compare with US 87 octane?

 

<div class='quotetop'>QUOTE(Stairman @ Jul 29 2006, 04:50 PM) [snapback]294372[/snapback]</div>

We are focused on cars here, so the 89 octane applications may be missed. Many marine engines require 89 octane gas to counter the higher load (has to do with ignition timing, thermal loading, and knock affects). There are also many commercial engine applications that require 89 octane. This is not an oil company thing, it is simply that gas stations sell gas for all applications, not just the vehicle you drive up in.

 

Again, whether you might get better mileage from a higher octane fuel depends very much on what additives were used to boost the octane rating..

If you add ethanol for instance, yes, you boost the octane rating, but you decrease the net energy content of the fuel. This is generally true of most oxygenated additives: better octane rating, lower emissions, lower energy content because some of the potential carbons are swapped with oxygen..

If you on the other hand, add some complex hydrocarbon molecule with lots of carbons, you similarly increase octane, but due to the high number of carbons, you're also increasing the net energy.. Now the hitch comes in that although many high MW carbon compounds will work to increase octane and net energy, many of them may have temperature range issues, they may be damaging to engine seals and other components, may cause increased emissions or may be poisonous to the catalytic converter, etc, so although it is dead easy to make a fuel additive that is high octane and high energy, you do have to use some discrimination when developing additive packages that are compatible with the engines and also emissions regulations..

 

Ignorance is the only reason. In addition, higher octane does not prodce a bigger bang, actually it produces less of a "bang". Higher octane fuel simply has additional ingredients added to the gas to SLOW down the burning or exploding of the air/gas mix. Slowing the burning helps prevent the gas from burning so fast that it will be burnt before the piston crosses over top dead center. If the mixture burns too fast, the explosion will effectively "kick" the piston as its coming up and that is the knock you hear. High compression, in addition to things like hot spots in the cylinder, tend to create conditions that cause the gas mixture to burn too fast. Tetraethyl lead and other compounds slow the burn, allowing the piston to pass top dead center rather than exploding too far before top dead center. Retarding the spark has the effect of having the spark wait a bit longer, which tends to aleviate detonation also, but at the expense of efficiency and power. Advancing the spark has the effect of lighting off the gas mix earlier and earlier, depending on the adjustment, which has the possible negative effect of detonating the gas too early, causing detontation. In addition "premium" gas (so named to encourage ignorant consumers to overbuy and overspend),with more additives to retard burning, tend to leave more deposits. So the trick is to buy the lowest octaine gas recommended for that engine. Gas mixtures vary per station and by season. So, the real trick is to adjust (advance or retard) the spark get get a slight "ping" when lugged down at about 25 mph in high gear, but NO more. That is perect tune for that tank of that gas. Lugging produces the worst conditions for knocking, so all other driving will not produce it.
So, preignition is as it says, premature ignition for that type gas, and so the gas is burning too fast or the ignition is too advanced or a combination. Check that out with someone who really knows, not someone who spends their life gathering opinions to box up.

 

I use 94 or 95RON petrol (gas isn't a liquid it's a gas) which is the standard in Australia for E10 petrol (10% ethanol)
E10 is 3 to 4 cents per litre CHEAPER in Australia.
I have not noticed a fuel consumption increase and I wouldn't expect to see any more than 1 or 2% better consumption if I switched to 91RON petrol so I save about 3% per litre and lose 1% to 2%. IN the process I contribute a little less CO2 because the ethanol is from sugar not corn.

 

in europe you have oct 95 and oct 98
a higher oct means a higher poweroutput from the engine.

looking @ ethanol its oct 85 and thats wy eth. cars have less power then a same engine sizecar with oct 98.

 

No, a higher octane means the engine can be made to produce more power but if an engine is designed for lower octane fuel higher octane will have no benefit. The lowest you can use is 95 so that is the one to use, Prius can go as low as 91RON or 87 what ever the American way to measure it is.

Higher octane fuel has slightly less energy per litre I believe.

 

ok

you say 95? buth 91 ot 87 is ok to?

 

US 87 is equal to AU91
Lowest octane in the UK is 95 because they don't have many cars with tractor engines like the pushrod engines used until recently in AU and USA so they use 95RON as a minimum allowing more efficient engines to be used. If that is all you have then you can't use 91RON or 87PON.

Have a read, it's about the difference between RON and PON.
http://en.wikipedia.org/wiki/Octane_rating

 

and the us 87 = germany 91? or 95?

 

From the link in post #35:

"In most countries (including all of Europe and Australia) the "headline" octane that would be shown on the pump is the RON, but in the United States, Canada and some other countries the headline number is the average of the RON and the MON, sometimes called the Anti-Knock Index (AKI), Road Octane Number (RdON), Pump Octane Number (PON), or (R+M)/2. Because of the 8 to 10 point difference noted above, this means that the octane in the United States will be about 4 to 5 points lower than the same fuel elsewhere: 87 octane fuel, the "regular" gasoline in the US and Canada, would be 91-92 in Europe. However most European pumps deliver 95 (RON) as "regular", equivalent to 90-91 US (R+M)/2, and even deliver 98 (RON) or 100 (RON)."

 

so you can say that 87 us is the same like 91/92ron
i now that in germany you have ron91
and because the prius can work with this without any problem ( if i am currect reading it here ) its cheaper to go for ron91 then ron95

 

all the octane rating in gas determines how much the fuel can be compressed before it spontaneously ignites.

if your gas ignites due to the compression, that is where you get knock. you want the gas to ignite because of the spark from your spark plug.

the tune in the prius is set up to work with 87 octane gas and that is what should be run in it.

the 87 is just a percenage in the gas mix. it is saying that the gas is 87% octane, and 13% heptane.

heptane ignites with very little compression, and octane can handle a great amout of compression.

when toyota (or whoever) builds the map for the ecu, they determine what rated gas is needed for the spacific compression of the motor built and tuned. going over this will not result in anything good.

 

so the european models willl be adjusted for the higher octane gas.