Premium or Regular?

We usually characterize the modern Atkinson cycles as:

• ~8.5 to 1 - compression stroke (variable valve controlled)
• 13 to 1 - expansion stroke

I have been able to detect octane effects in both the 1.5L and 1.8L engines using hill climbing data. This data also shows the relative energy content. The easiest to replicate is to find a steep hill and run climb tests using the built-in MPG gauge:


You can see where the 89 octane continued to provide engine power at higher speeds while the 87 octane 'flat lined' ... probably due to knock avoidance. You'll also notice where they over lap, the 87 octane delivered more power per unit of fuel.

Bob Wilson

 

Bob, how about the error in your measurements? Are these differences significant?

 

Only for planning purposes. These are derived from 'indicated' MPG and now we have the 5% correction formula. To get true numbers, .95*indicated, works.

I'll update the chart later tonight. However, the relative effect of 87 and 89 octane will remain. From what I can tell, there does not appear to be any benefit from a higher octane fuel like 91 or 93 octane.

What will be fun is validating the model ... so I'll have to take the wife over to East Tennessee / Western North Carolina ...

Bob Wilson

 

Does the E10 affect the measurement ?

 

I don't know enough to quantify the effect.

We are stuck with E10 in North Alabama. By the time we switched, my usual source of straight gas was converted so I missed the straight gas vs. E10 testing window.

Given E10 has become all but universal, I'm resigned to using E10 as the base fuel. I'd tried mixing E30 but it was such a supply problem that it wasn't worth the effort. We also have a three sources of straight gas in Huntsville but I only know about them because I'm local. If I were to drive out of my area, I would have to start a new search.

Bob Wilson

 

I think you just proved to everyone that you are old.

 

I agree with your conclusion with respect to octane requirements. Your results are consistent with predictions for engines with effective compression ratios of less than 10.5:1 or thereabouts.

As far as error, I was inquiring about the precision of your measurements.

It use to be that some of the premium fuels had more substantial additive packages than regular fuels, but I think those differences are not as great as they used to be. Nonetheless, high mileage cars can probably benefit from running a tankful of premium every so often from a valve deposit point of view.

 

According to my local Chevron jobber, there is still a higher percentage of techron in premium, there is more to it than merely just octane. I believe if you stick to tier 1 fuel and or like you say an occasional dose of the good stuff, or toss a bottle of techron in the last tank before every oil change your good.
All that being said, I am not abdicating use of premium fuel in a Prius, just that there is more to it than just octane and there is a difference between fuels just like there is in clothes, foods and everything else you buy.

 

i used 93 last week just to see how it drives (i've been using 89 for the past 2 gas fill ups) and 93 sucks, i only got 300+ for full tank compared to 89 that gives me 400+ so this time i filled it up with 87 and ill see what happens.

 

FWIW, using more octane than required for an engine may in fact be less efficient. The reason is octane is a measurement of how hard the fuel is to ignite, not how many calories per unit the fuel contains or how "powerful" the fuel is etc. High octane fuel will burn slower, Higher compression will speed up this burn rate up and more spark advance will also take advantage of it.
Bottom line, all engines are designed to require a certain amount of octane, less than the required amount may cause damage, and more octane than required is not necessarily beneficial, it in fact may reduce efficiency.

 

Very true...isn't it a shame that many oil co adds push the higher octane by implying that it offers more vrooom ? I guess profit must be tied in somewhere.

 

I guess it may have started way back in WWII, the BIG advantage we had over the Axis powers was that we had high octane fuel and they didn't Chemists explore WWII ‘miracle’ aviation fuel | Innovation
The high octane fuel allowed the boost pressures to be turned up in these engines. High octane will allow for higher power if the engine is designed and or modified for it, so I guess implying that "premium" fuel is better isn't really lying? You know it used to be called Ethyl, from the Tetryl Ethyl Lead it had in it. I'm sure changing it's name to "premium" was done as a marketing thing long before the lead was removed.

 

If we can formulate Premium Electricity we won't need improved batteries.

 

Best mileage in my 2010 so far has been with Shell Premium.

http://priuschat.com/forums/gen-iii-2010-prius-fuel-economy/68738-tank-mpg-12.html#post959087

 

I saw that and am kinda scratching my head about it. How many miles are on the car? Did anything that you can think of beside's the octane of the fuel change for that tank? Like tire pressure, cooler wx and not running the AC, different types of trip etc? What brand of fuel did you use before the Shell Premium?
See I don't know enough about this engine to make intelligent / informed statements, but on many other engines the ignition timing is variable and many engines can take advantage of higher octane by increasing timing advance to just shy of detonation, usually the more spark advance the more efficient the engine within limits of course just like everything else. Don't know enough about this engine and or Atkinson cycle engines.

 

My drive has remained the same -- a 53 km commute during the week in each direction with a little bit of misc driving on the weekend. Shell says their premium gas contains no ethenol, so that might be a factor. It's been fairly cool up here this year, so I've used the air conditioner only rarely. I set the tire pressure to 40/38 quite a while back so I don't think that has anything to do with it. Car has about 4000 miles [6000km] on it. I've used Shell 87 consecutively for a few tanks, then PetroCan 87, and finally Shell Premium 91.

I'm currently running with a full tank of Shell 87. The car seems to run equally well with both Shell 87 and Shell 91, so I'll probably just stay with the cheaper Shell 87. The mileage may not be quite as good, but for the cost (about 13% less), Shell 87 is cheaper with only about 5% worse mileage. I'm one of the people who believes that Shell did something right with their newest "Nitrogen" additive, so I'll probably just stay with it. I've run it in a couple of cars now and they both ran better with it.

For sure, I'm convinced that the 2010 Prius mileage is certainly not worse when running premium fuel, at least with Shell premium fuel (the opposite of what has been suggested by others in several previous threads). Whether the mileage is better with premium fuel is debatable, since I might have just had a good two week run (or maybe the car is finally breaking in).

I tried running PetroCan 87 in one tank and was disappointed when I immediately noticed that it tended to lug at low engine RPMs. Also, I had to drive very carefully to keep the mileage up.

Unfortunately, it looks like the season for top mileage is about to end. It's been a bit cool here in the mornings for the last couple of days (13C or 55F), and I notice that the mileage on my trip from home to work is already beginning to drop .

 

With fond childhood memories! arty:

 

I think you have found the answer, ETH or ethanol will slightly reduce your mileage, possibly more than the higher octane does.

 

Not possibly; definitely reducing your mileage more than the higher octane.

Tom

 

The novelty of ‘beat the mpg’ wore off awhile ago for me, so consequently I “just drive†my prius, and whatever mpg I get, I get. There are lots of hills where I live, and usually I just fill up with Costco 87 resulting in an overall mpg of ~47. This past week I noticed a gas station (Conoco) advertising ‘ethanol free’ gas, and since I was feeling a little saucy, I pulled in for a fill up. Remembering that premium might perhaps improve mpg going up the hills, I also elected to fill up using premium. Perhaps the wind was at my back, but my trip from Yakima to Seattle and back yielded an overall trip mpg of nearly 60mph. Crazy.