What octane fuel do you use?

That's because the advertisements sell it that way. I didn't know any better till I started hanging out with a bunch of car nuts on the web.
It's a similar problem to the anti-lock brake problem. They are advertised so as to appear to give shorter stopping distances (though the ads never state that, which would be a lie). As a result, people with anti-lock brakes don't drive so as to maximize the accident avoidance benefits of the things.

 

Absolutely,
same goes for the vehicle stability control, traction control etc...

 

Higher octane has more 'anti-knock' ability. And the Prius has, what, a really rather high 13 to 1 compression? So 87 octane causes the timing to retard to keep the motor from knocking/hurting whenever its needed (hills?). And retarding the timing loses horsepower which is compensated for by the battery helping... would higher octane on hills/mountains keep the timing from retarding so much and draining the batter so much which keeps the motor from... oh, never mind, fill it with 87 and drive it (says my wife!)

 

Only mathematically, since it is measured from bottom dead center, if you measure it from the intake valve closing, it is 30% lower. And since air was able to rush out until the intake valve closed, that is the compression ratio that matters for Octane Ratings.

 

There is an indian reservation local to me, however they sell fuel with up to 20 percent alcohol in it... I think I will use 89 name brand to be safe... I mean, Im getting great mileage, might as well spend a nickel more per gallon!

 

In other words: Atkinson Cycle.

Tom

 

Toyota calls that an expansion ratio, not a compression ratio, for reasons Jimbo and Tom have already explained.

 

This "no ethanol" claim has me intrigued. But I can't seem to find any factual resource (like @shell.com) that actually says it. Google is filled with people asking for such proof in the light of federal requirements. The only lab analysis I found so far actually has it containing 11%. An Independent Laboratory Analysis of Shell V-Power 93 vs Midgrade Gasolines - bimmerfest - BMW Forums
But then again, I hate bimmers. So i'll throw that datapoint out.
From what I read, actual composition may vary from differing regions and states.

rich

 

Around here (Houston, TX), Shell has ethanol, regardless of grade.

 

In Michigan most gasoline has ethanol. It's our gift to the farmers. Only a few specialized gasolines are exempted. For example, you can purchase marine fuel without ethanol, since ethanol dissolves the GRP fuel tanks found in many plastic boats.

Tom

 

Explain please, for us non-engineers.
Thanks!

 

Octane is a measure of resistance to knock only. The name comes from the test method. A test fuel is determined to have the same knock resistance as a mixture of iso-octane (good) and n-heptane (bad). The vol% isooctane gives you the "octane rating". If your fuel knocks the same as a 90% iC8 / 10% nC7 mix, then it's said to have an octane number of 90. The 100 standard iso-octane is 2,2,4 trimethyl pentane. Ironically, n-octane has an "octane" rating of -18.

A knock sensor listens for incipient knock. It's basically just a microphone bolted to the block. Unfortunately, it's a knock sensor, not a best-power-timing sensor. Best-power timing comes before incipient knock, so they use tables to set timing and back it off if knock is heard. ECU's do not keep advancing until knock is heard, since that would be too far. They only retard from the table values if/when knock is heard. No knock==>table timing values, no matter how good the gas.

So, if the tables are aggressive (too much advance) and the engine is running against the sensor, you won't hear knock. Putting in some 89 or 91 will let it go back to the table values and may get you some more power/economy.

We're talking very small changes though. The table may be a bit aggressive, but it's not likely to be off by enough to see a change in mileage....and certainly not enough to justify 10c or more per gallon.

What will change mileage noticeably is non-ethanol-blended fuel. Non-E10 has about 3.5% more energy per gallon, and that's enough to see. Fortunately, in Oklahoma, we can get non-E10. It's usually more expensive, but at about 2.59 for E10, 2.69 is breakeven for non-E10. You just have to know which stations have non-E10...or go to http://pure-gas.org

In order to see if 87 is not enough for your Prius, you'd have to be able to display "knock retard" from the ECU. I've looked at this on former vehicles...not sure if the Prius will display it on an OBDII scanner or not. If always 0, then 87 is fine. If it occasionally jumps up, then settles back down, then the sensor is pulling back timing.

 

Quote:
Originally Posted by JimboPalmer
Only mathematically, since it is measured from bottom dead center, if you measure it from the intake valve closing, it is 30% lower. And since air was able to rush out until the intake valve closed, that is the compression ratio that matters for Octane Ratings.

For non-technical folks, when the piston compresses the fuel-air mixture, it gets hotter as the pressure rises. There comes a point where the fuel-air mix ignites by itself before the spark-plug fires. This is a bad thing because the piston is still headed up but now, too soon, against the tremendous force of the burning fuel-air. There is a type of burning called 'detonation' that is extremely fast. Regardless, this is how "knock" is created. A higher octane rated fuel delays when this self-ignition occurs.

Often, knock is followed by parts of the engine failing, in particular the pistons:



This last one may not be due to excessive knock but more likely a valve train failure. Still, it isn't everyday you see a valved forged into a piston.

Here is classic piston damage from detonation:


It can damage valves:

and cylinder heads and all sorts of similar stuff.

Bob Wilson

 

When any volume of unburnt fuel/air mix detonates, there is a very rapid rise in local pressure. The pressure it rises to is the same, but just much faster. Combustion noise depends strongly on rate of pressure increase (actually noise varies as the square of dP/dT).

When a normal burn happens, the pressure in the cylinder is roughly the same everywhere (there are gradients, but they are small relative to total pressure).

When a section detonates, the rise is so fast that there is a very powerful pressure wave that heads out across the combustion chamber, and it bounces around, reflecting off all the walls. This is what generates the ping noise you can hear. Since the combustion gasses are so hot, the local speed of sound is very fast, so these pressure (sound) waves bounce around quickly and generate high frequency sound.

The pressure waves also do something else. They destroy the boundary layer of gasses near all the metal walls. The combustion gasses are normally much hotter than the metals can handle. So the metals are cooled on the outside (either by coolant or oil). They are also insulated on the inside by a very thin stagnant layer of gas that naturally forms between the metal and the combustion gasses. Disrupt that layer and the heat transfer coefficient goes way up.

That's why severely pinging engines can overheat...much more heat is transferred to the coolant, by the destruction of this insulating layer. That's what happens if you're lucky. You can pull over and let it cool down. If you're not lucky, the piston will overheat until it melts, and there's no warning light for an overheating piston.

That's how pinging leads to engine damage.

 

You shouldn't have a problem unless you hit one of those 115deg days but that hasn't happened for a couple years. I lived in Lincoln for 5 yrs.

Next time you hear the pinging, determine if you are at partial throttle or at 3/4 throttle or more. If only at partial throttle try to give it a bit more input and see if the pinging goes away. The car's computer system will keep the engine safe so you don't have to worry. My 2000 Corvette would sometimes ping at partial throttle while merging on the freeway on-ramp near Woodland and that was due to being in a high gear at low rpm and lugging the engine. If I gave it more throttle input the pinging went away immediately. I wonder if the Prius can experience the same situation despite the different transmission and engine coupling.

 

I can't see using anything other than 87 octane.
To make a higher octane fuel requires more crude oil. Didn't we buy our Prius to use less fuel?

 

Greatly simplified*, in an Otto cycle engine, the intake valve opens as the piston is at top dead center and air (and fuel) rushes in as the piston descends. Near bottom dead center, the intake valve closes and the air (and fuel) is compressed as the piston ascends. The compression ratio is the room for air at the bottom divided by the room for air at the top. In an Otto cycle engine this is often roughly 10 to 1.
http://www.tpub.com/content/doe/h1018v1/img/h1018v1_38_2.jpg

Just before top dead center the spark plug fires and the fuel burns and expands. It forces the piston down to bottom dead center where the exhaust valve opens, wasting any further expansion, by venting it to the exhaust pipe. The piston rises again pushing the remaining exhaust out, and we are ready to go around again.

The Prius Atkinson cycle engine differs in that the intake valve does not close at bottom dead center, but 1/3 of the way up. So less air (and fuel) is trapped than math would suggest. When the plug fires, the remaining air expands and comes much closer to being fully expanded when the exhaust pipe opens, with less work is lost to making loud exhaust.

http://www.mazda.com/mazdaspirit/env/engine/img/mirror2_img02.gif (look at the top half)

Highly compressed air and fuel can ignite on its own, and this will tend not to be well synchronized with the piston, so work will be lost as heat trying to expand at inappropriate times. Detonation, Knock, Ping, Self ignition, etc. drives up engine temperatures and pressures dramatically. If the Prius had a 15 to 1 compression ratio, it would need a fuel with a high resistance to Ignition, a high Octane Rating.

It does not, as 1/3 rushes back out of the cylinder as it rises, so it does fine on Regular. (87 Octane)

* In actual engines, you can take advantage of inertia, and open an close the valves slightly off dead center, as the column of air will keep moving. Many modern engines vary the valve timing at different RPMs to capture this air. The Atkinson cycle is not even close, and the air reverses direction.
[ame]http://en.wikipedia.org/wiki/Variable_valve_timing[/ame]

Any clearer?

 

How many of you have added naphthalene "mothballs" to your tank back in the racing days?

 

I didn't mess around with that kind of stuff. I just added a methanol injection kit.

 

Okay, your point #2 makes perfect sense. And I admit I'm wearing the tin foil hat on this one.

But point #1? Do you remember how long it took Toyota to officially state it was an error? Getting official confirmation from Toyota as to what grade- took forever.

So yeah, I think I'm wrong and paranoid. But that doesn't mean I'm not right...even if I'm wrong. Plus let's say you are releasing a new Generation Hybrid...at the same time a competitor is releasing their insightful new hybrid competition. Let's say you invest huge amounts of money into this new generation vehicle. Then let's say, someone realizes that the new fangled beltless engine you have been promoting actually seems to run better on premium. But your competition is promoting it's vehicle as the Hybrid For Everyone!

You don't want your vehicle to be labeled as the Hybrid for the rich and elite. You don't want to inch your price upwards while your competition is getting cheaper...you don't want to say you have to negate your gas savings by purchasing premium. So you become vague about what "grade" gas IS actually recommended. You pick a grade that actually doesn't exist...then you see how things play out.

Typo? How does fuel grade become a Typo? Nobody proof reads those things? It's such an obvious standard how does that get missed?

I wouldn't be so suspicious but it took Toyota so long to come back with the answer. It seems to me to be a question that should of never had to be asked, and then when the "typo" was discovered, getting confirmation about recommended fuel grade seemed so slow in coming...

Okay...I'm likely wrong. I know that. But I'm going to stick with my paranoia on this one. Visit me in the padded cell...