Premium Gas and MPG

I think this topic has become so complex as to be unanswerable. It used to be much simpler, or perhaps our understanding of it was simpler.

Follow the recommendations in the owners manual. The engineers who designed the car and engine know much more about it than any of us.

 

One chemist to another, I'd say the answer is not quite that simple, but it's a good first approximation.

Recent article in GasBuddy seems to suggest Premium prices are really high now, but I think they hyped it up by not showing cents/gallon like they should have.

Premium-to-Regular Spread in a Tug of War: How shale oil production and turbocharged engines are aff - Gasbuddy Gas Prices

 

If gas station layouts corresponded to the manufacturer's recommendations, there'd be a row of pumps offering nothing but regular, and one (lonely and dusty) premium pump, off in the corner beside the air pump.

 

Yes, people tend to think of gasoline as a defined substance but in reality it's a complex mixture whose composition may vary from source to source and over time even from a single source. While "Chevron is the best" may not be literally true, it's very possible that cheaper gasoline may not be as reliable as gas from brand name sources. With consistency in mind, I try to buy Shell, failing that from one of the other name brands.

Case in point: we were driving a Chevy Tracker from Antigua east to the Atlantic coast of Guatemala, with a diversion to Honduras en route. We filled up at our usual no-brand local station in Antigua, where we had not had problems with the gas. As we drove up over the ridge into Honduras, the Tracker started pinging really badly, something it did not usually do even at the higher elevations around Antigua. As soon as we returned to Guatemala, we filled it at a Shell station, and the pinging stopped almost immediately.

The point is that, while cheap gas may be fine most of the time, it may be less reliable than name-brand gas that one pays a bit more for. And I'm happy to take part of the money I save by getting 60-70mpg and spend it on gas which is (hopefully) more reliable.

 

I try to do Top Tier Gasoline, but for me that is easy that's almost all we got here except a few off brand. If Iived elsewhere in Virginia, I'd have fun trying different stations but no fun here (EPA RFG zone).

 

True about the engineers, but they are designing engines for multiple countries that likely have a different octane for regular gas. Then there is the negative effect that any fuel besides regular on a cars sticker has on sales for your non-performance, typical car. Which can lead to the owners manual recommending an octane lower than what the engineers designed for.

I've noticed the price difference gets closer around this time of year in the past. Hopefully it will be close enough for my next fill up.

 

Ditto, use the gas you are directed to use in the manual and save all that cash for a new Prius.

 

The easy answer is to measure the vehicle efficiency, the power per unit of fuel, over the OBD:

• K * ICE_rpm * MG1_Nm / MAF = power per unit of gas
  • K - a constant that handles SAE or metric units, sign of MG1, and MAF units
  • ICE_rpm - engine revolutions per minute
  • MG1_Nm - the counter to engine torque
  • MAF - air mass flow

The harder problem is making sure the fuel in the tank is a good sample:

1. Drill a hole in the fuel tank and install a drain valve while the tank drains on the drill, the person making the hole, and into a gasoline safe catch container. Close the valve and add the new sample, at least a couple of gallons. Run the test recording the data. WARNING: this is the most hazardous technique.
2. Disconnect the fuel line from the injector rail and attach piping to a gasoline safe container. Power up the fuel pump until the tank is empty. Turn off the pump and add the new sample, at least a couple of gallons. Run the test recording the data. WARNING: this is the second most hazardous technique due to the spillage when opening fuel rail.
3. Run the car down to the normal refill level. Add enough of the sample gasoline to dilute the old fuel. Repeat until the partial fraction ratios show the test sample is 90-95% of the remaining fuel. For example, if the typical refuel is when 1 gallon is remaining:
   1. 50% after first addition of one sample gallon, 1/2
   2. 33% after second addition of one sample gallon, 1/3
   3. 25% after third addition of one sample gallon, 1/4
   4. 20% after fourth addition of one sample gallon, 1/5
   5. 1/n ## where "n" is the number of these cycles
   6. WARNING: this will take a little time.
4. Put a spare gallon of sample in spare can in car. Drive the car until the engine stops because there is no fuel in the tank. Add the sample, enough to run the test, with a spare gallon of the next sample in a spare can in car. WARNING: this requires detecting when there is no gas remaining and using the traction battery to park in a safe place, a mile or so.

I have no control over the ethanol ratio in local gasoline samples. WARNING: measuring the ethanol ratio requires handling gasoline and non-tank disposal of the left-overs. I am OK with using E85 to dilute local gas to get a reasonable estimate of the ratio.

Gasoline is a complex mixture of hydrocarbons and what comes out of the pump is difficult to quantify. However, a mass spectrometer or gas chromatograph can quantify the molecular weights. More sophisticated metrics can separate the compounds and provide an exact definition of the sample gasoline but these are a little expensive. WARNING: this requires handling small samples of gasoline.

A simpler approach is to switch to racing fuels sold in 55 gallon drums that have exceptional purity. However, the last time I checked they were very expen$ive and have no relationship to the curious mix that comes out of a retail gasoline pump. WARNING: keeping gasoline on your property is not trivial and may require compliance with local ordinances and insurance.

Of course you could just switch to different gasolines. If you want to prove premium is best, use regular during the coldest part of the year and then switch to premium as the temperatures rise. If there is a season where air conditioner is used, switch back to regular. Do the reverse if you want to prove regular is best. WARNING: fooling yourself does not fool others.

Bob Wilson

 

Think this post will quiet the comments.

Remember a gas tank blowing up and catching the whole shop on fire. Gas tank repairs were banned. Take it somewhere else!

 

If we knew 3 things about the fuel in the tank: vol (gallons), wt (pounds), and %ethanol
we could probably figure it all out. Main thing we are missing is weight, we know volume and %ethanol is close to 10% give or take. I keep wondering if there is enough data on the OBD2 reader to back out pounds O2 burned then we could calc pounds Carbon burned/gal, and that probably is best correlation to MPG.

How does this E85 method work? Normally (and I don't do this often) I'll take a small sample of gaso and mix with water to get the ethanol out.

 

The three-way-catalytic converter requires oscillation about a 14.7 stoichiometric ratio measured in gm/sec. So it is easy to calculate the mass of fuel burned.

As for E85, I assumed it was truely E85 (or someone is getting screwed.) So it is easy enough to 'mix a tank' and doing partial fractions figure out the E?? value. My earlier tests show that E50 is the threshold between working or getting a check engine light within a few minutes.

Bob Wilson

 

Octane is to control knock. My understanding is that it actually slows flame spread. There is NO mileage benefit from higher octane and may even be reduced if it is done with ethanol that has lower energy density than straight gasoline!

 

Your argument is half-correct but there is a flip side. Octane itself does not impact MPG, but you have alluded to another parameter, energy content, which can be reduced by adding ethanol or increased by removing ethanol. Also there are a few other ways to manipulate energy content. Normally the consumer has no control over energy content variations. Especially in RFG areas energy content is pretty much same for all grades (StL is probably RFG).

PS- ethanol is not lower density by weight, but it is lower "energy density"

 

You buy fuel by the gallon, not by the pound. When buying fuel with higher ethanol content, you're getting fewer BTUs per gallon. Ethanol content in most fuel mixtures is there to replace MTBE and other undesirable chemicals that have been used for controlling knock. The main down-side is the lower energy, but also the problems with effect on fuel system components and its affinity for water!

 

No.
1-(½)^n would be the fraction of sample in the mixture in the tank, so you'll have about 94% undiluted sample in the tank after your fourth "cycle."

 

I know I just had to drain the fuel out of my lawn mower yesterday. You were saying Premium could not be more energy/MPG than regular, only less. I would say it could go either way, but its not a big deal.

 

Is there any kosher way to dispose of contaminated gas? Just curious.

 

You're in Canada. Call the dump and ask to speak with the hazardous waste people. Or in the middle of the night pour it into the gas tank of that old F-150 down the street. They'll never notice.

Incidentally there's this stuff called "Stabil" that you can add to the gas in the fall to keep it from going bad over the winter. And fill the tank to the top.

 

The effect is likely to be small for narrow octane spreads. 87-91 is not very much and the effect would probably be "buried" in the "noise level." E-85, on the other hand... I drove a Volvo SUV in Athens a couple of years ago. Apparently vehicles there are high compression. The only fuel available was 94 octane! It was the equivalent of $8 per gallon but the SUV gave me about 29 miles to the gallon as close as I could tell!

 

Need a haz waste collection center, my County has one open every day for used oil etc.
Since the free water phase sinks to the bottom of the gaso, I am going to try to reuse mine in the lawn mower with some additive to clean it up.