Does high octane "suck" in the prius?

<div class='quotetop'>QUOTE(windstrings @ Jun 4 2006, 08:04 PM) [snapback]265798[/snapback]</div>

Ethanol is an octane booster and is being phased in to perform that function in place of MTBE. Yes, having ethanol mixed w/the fuel should yield worse gas mileage (it does w/E85 on flex fuel vehicles vs. straight gasoline or E10).

 

zzz

 

I've tested this theory over the past week. It seems to give the car a tiny boost... but only at higher speeds in a way. It's strange. maybe it was all plecibo effect.

Then again, i was running 100 octane.

 

<div class='quotetop'>QUOTE(windstrings @ Jun 4 2006, 08:49 PM) [snapback]265815[/snapback]</div>

I guess I should clarify since my statement was a little confusing.

I meant to say if you run a flex fuel vehicle on E85, it should yield worse mileage than if it ran on E10 or E0 [straight gasoline]. By extension, I'd imagine that a non-flex fuel vehicle should get worse mileage running on E10 compared to running on E0.

 

Lower octane is a good thing by itself because it's heptane that gives us the majority of the reaction. Octane is like the carbon rods in the reactor that slows down the chain reaction to control it.

The rest is as been said, petro diluted with something other than heptane or octane, you have to look at the energy content of it, and ethanol has less per volume.

However, there's this statement from Wikipedia which states:
If gasoline is run at its prefered max power air fuel mixture of 12.5:1, it will release approximately 19,000 BTU of energy, where ethanol run at its prefered max power mixture of 6.5:1 will liberate approximately 24,400 BTU, and Methanol at a 4.5:1 AFR liberates about 27,650 BTU.

My interpretation is gasohol is not being run at the proper compression to extract the maximum possible energy out of ethanol due to the mixing.

 

<div class='quotetop'>QUOTE(NuShrike @ Jun 5 2006, 07:13 AM) [snapback]265904[/snapback]</div>

Those aren't compression ratios, they're Air-to-Fuel ratios. So, what it is saying is that it takes almost twice as much ethanol to extract only 26% more BTU's than gasoline.

 

<div class='quotetop'>QUOTE(windstrings @ Jun 5 2006, 11:47 AM) [snapback]265979[/snapback]</div>

No, you need to reread those ratios: its basically saying that if you inject 92% MORE ethanol, you will get 26% more BTU's. That's still a loss, by my math. As Jack Straw said above, it works out that ethanol has 65-67% the BTU content per gallon. (However, as I think someone posted in another thread, the math is slightly different if you calculate by weight, instead of volume.)

 

I think I understand it now, ethanol and methanol can burn hotter than gasoline, but only at a super-rich AF mix which counter-balances the "burn efficiency".

So it would still take an engine designed for this which the Prius engine is probably not. And, since engines operate on a volume basis, the energy by mass would probably be only a metric for trivia.

 

<div class='quotetop'>QUOTE(windstrings @ Jun 5 2006, 08:24 AM) [snapback]265967[/snapback]</div>

I, too have been looking for sound info on the ethanol mix when I fill up. I thought California law required a label on the pump showing quantity or % of MTBE or alcohol, years ago. The proprietor of a Valero station could not tell me, and the pump had no specs posted. But she gave me a telephone #, which I still have not called: 800 242 4020. My mpg averages have declined, even with slightly higher average daytime temps when I do most of my driving, so I strongly suspect some less efficient mix.

 

<div class='quotetop'>QUOTE(DieselConvert @ Jun 6 2006, 12:59 AM) [snapback]266527[/snapback]</div>

Gulp, let me stick in a quarter's worth here. The reason that effective mileage is worse with ethanol than with pure gasoline is that the heating value of ethanol is lower than that of gasoline. To go the same distance, with the same weight car, at the same speed requires more ethanol than gasoline because a gallon of ethanol doesn't have as much "umph" as does a gallon of gasoline. We mechanical engineers measure this as "heating value," a concept sort of like "how high can you raise the temperature of something by putting it into a fire burning ethanol vs one burning gasoline?" Looked at another way, a gallon of gasoline can release more energy than can a gallon of ethanol.

The effective octane rating is just a measure of how likely the engine is to make a "pinging" noise under heavy load, such as climbing a hill. Gasoline with an octane rating of 87 will not prevent the engine from "pinging" while climbing a hill as well as will gasoline with an octane rating of 92.

Given the kind of driving you do, if your engine doesn't "ping" while burning 87 octane gasoline, it is a complete waste of money to buy 89 or 92 octane fuel. Your engine doesn't need it in the kind of driving conditions you expose it to, in this example, so you shouldn't spend the extra $0.20 per gallon to buy the higher octane rating.

If it DOES ping, however, in your driving conditions, by all means put in fuel with a higher octane rating. The word "ping" is a very gentle-sounding word used to describe all kinds of very bad stuff going on inside the combustion chamber!

When my wife's V-6 mini-van was brand new, we drove from Northern California to LA on Interstate 5. There is a very steep grade to climb, "The Grapevine," and if any section of highway in California would cause her new engine to "ping," this was it. I cringed when she started up the grade, while burning the factory-recommended fuel with an octane rating of 87. I was amazed when the engine effortlessly flung the car up the mountain without making a single sound out of the ordinary. It is perfectly happy with 87 octane fuel. Unfortunately, this didn't stop my wife from ignoring my mechanical engineering degree and putting in 89 octane mid-range fuel "just to be nice to the engine." What finally persuaded her was when the price of fuel went upwards of $2.75 a gallon!

 

Hi Windstrings,

I think there is some confusion here. Probably due to the piecemeal nature of a newsgroup. While the ethanol requires less air to burn, that does not mean anything about the miles per gallon. What it means is the energy content of the fuel is less, because there are less molecular oportunities to make water and carbon dioxide.

All this stoiciometric information means is how fast you need to push the particular fuel into the intake air stream. Since ethanol has less energy per gallon than gasoline you need to feed more of into the same cylinder to react with air in the cylinder. Thus the lower stoiciometric ratio.

There is nothing earth shattering here.

As Ethanol is higher octane, if your engine could dynamically sense this and adjust the compression ratio up, the engine might get better efficiency. The Prius might be able to do this, but I do not know for sure. It can adjust the mixture ratio automatically - allot of cars can these days. But to get more efficiency one needs to optimise the compression ratio. I know the Prius has a lower compression ratio than expansion ratio, but I do not know if the compression ratio is variable, or variable enough to accomodate pure ethanol - I doubt it.

Even if the Prius could optimise the compression ratio for the fuel, if the fuel has less energy per gallon and you ask the Prius to do the same work, the mileage will be worse.

The reason for Ethanol in the fuel is to help complete combustion, and reduce unburned hydrocarbons. This happens, as during the burning the OH groups are split off the Ethanol molecule. The OH groups are very chemically active, and help to catalise other reactions in the burn, before being consumed as well.

I am guessing an advantage of the 13:1 expansion ratio is that slower burning fuels, as higher octane fuels usually are, might have a chance to burn for longer. The Prius engine might be a "stroker" configuration (smaller cylinder with more travel for the same displacement) to achieve the 13:1 ratio. That is, the length of the power stroke represents more degrees of rotation on the crank and thus more time at a given RPM. So, possibly the Prius engine can burn ethanol more efficiently than Otto engined cars. But, even still, in comparison to gasoline, the mileage will be worse when burning ethanol.

 

Hi All,

About octane. Let me paraphrase the content of an excellent article from the AOPA magazine of some 20 years ago.

The purpose of octane is to raise the flash point of gasoline. That is, it raises the temperature at which it will spontaneously combust. In a gasoline engine, the fuel/air mixture does not explode, it is supposed to burn smoothly as a "flame wall", starting at the sparkplug and progressing to the extremities of the cylinder in a smooth manner. As the flame wall advances, the burned gasses behind it expand, increasing the pressure in the unburned portion of the cylinder. If the pressure in the unburned volume of the fuel/air mix in the cylinder becomes sufficiently high, the temperature of the mixture will rise above its flashpoint and the entire remaining volume of the cylinder will "explode" (called detonation or pinging) rather than do a smooth burn. This is why a small amount of pinging may occur if you use too low an octane under a high load condition (i.e. going up a hill) because high loads cause higher pressures in the cylinder.

Thus, the purpose of high octane gasoline is to ALLOW the existance of higher compression high perfomance engines. It does NOT increase the energy content of the gas itself, or increase the perfomance of any given engine. (The only caveat here is that some high octane gasolines also have additional useful additives to keep the engine clean. If these additives are not present in the cheaper gas, there may be some very minor advantage to using it.)

Now there are some interesting ramifications of these facts. First, if you use LOWER octane than your engine is designed for, you will eventually burn out rings and valves. This occurs because the temperature of burning gasoline is high enough to melt the steel walls of the cylinder. The only thing that keeps this from happening is that the flame wall is only in contact with any one point on the cylinder wall for a fraction of a millisecond and the adjacent, cooler parts of the cylinder draw off the extreme point of heat before it can do any damage. However, if detonation occurs, the hot gasses remain in contact with the metal of the cylinder, rings and valves long enough to cause damage. I saw this happen in a flying club I belonged to where they insisted in filling up the club truck with 80 octane aviation fuel. Within a year, the truck had no rings and was burning more oil than gas.

The second interesting ramification is that if you mix half a tank of high octane with half a tank of low octane you get a full tank of LOW octane. This occurs because as the burn compresses the unburned mixture ahead of the flame wall, the low octain molecules will detonate. This will ignite the surrounding high octane molecules. Voila! Ping!

So, if you cannot find the octane that your engine was designed for, always put in a HIGHER octane gasoline, never a lower one. If you use a lower octane gasoline, it will take forever to get it out of the mix in your gas tank and in the meantime, you are damaging your engine.

Dave H.

 

<div class='quotetop'>QUOTE(windstrings @ Jun 6 2006, 12:12 PM) [snapback]266686[/snapback]</div>

No, you're still thinking about it backwards. The total volume of the cylinders in the engine are the same, and they WILL be filled with something. So, with gasoline, if the total volume of the cylinders is 1350cc's, 1250cc of that will be air, and the other 100cc will be gasoline. However, with ethanol that same 1350cc will still get filled, since we haven't changed the volumetric efficiency/pumping efficiency of the engine, and the 1170cc of air will need 180cc of ethanol to provide the correct 6.5:1 A/F ratio, that's 80% more per every 2 revolutions of the engine! Yes, you *might* be releasing 26% more energy/BTU's in each cycle, which will slightly offset some of that increase in fuel, but it's still less efficient per gallon.