Gasoline Brand Preference

Just a quick thought...

Didn't Karl Marx write the book on Kapitalism? [Das Kapital]


iPhone ? - now Free

 

What is you guy`s opinion on engine knocking using 87 or lower grade? ive experienced that with my previous rides that when i put 87 which was recommended the engine starts knocking and it eventually goes away after switching to and continuously sticking with 91 or 93 premium grade.

 

The Prius engine has a "knock sensor" and tunes the engine for the octane of the gas. My earlier gasoline studies confirmed it will tune for a higher octane fuel on a maximum acceleration, steep hill climb about a 2-3% effect . . . making it still less than the performance quality, 87 octane fuel. Quality 87 octane fuel has more energy gallon than premium and crappy 87 octane works just like premium.

For those who drive most of the time with the accelerator on the floor up an 8% grade, premium will perform less than a quality 87 octane gas with the advantage of higher prices. Drivers who seek to maximize premium gasoline performance should always drive with the accelerator to the floor board and to comply with speed limits, stomp the brakes, an extreme form called "Pulse and Brake" designed to demonstrate the advantages of premium in a Prius.

I've run the tests and burning premium fuel has no measured effect in ordinary driving. You have to be at the extreme power range such as maximum acceleration up a hill or around a race track at speeds of 100 mph or higher . . . only to perform worse than a quality 87 octane. But there is a higher octane fuel available.

Run straight E85 and ignore the "Check Engine" light. E85 has an octane rating between 105-110. The "Check Engine" light is just saying it can't trim for the higher octane. There are 'spoofing' systems that will run E85 without the "Check Engine" light.

Bob Wilson

 

So what does that mean?
Seriously I'm confused if u r saying 87 is best or premium is? Or maybe its too late for me to understand a since its way past my bedtime...

 

• 87 octane straight gas always works, the best choice
• some 87 octane straight gas have more energy than others
  • higher energy, straight gas does best - but is not labeled!
• 89, 91, and 93 octane straight gas tested the same as low energy straight gas but costs more
• the Prius engine detects it has higher octane gas and self-tunes BUT it can not tune enough to overcome the low energy of premium
• E85 is the highest octane fuel at the pump
  • generate a harmless "check engine" light that can mask a real problem
  • energy content is ~70% of straight gas, worst possible mileage
  • price is only 5% down from E10 gas, not cost effective
• E50 (you have to mix it 50/50 with E10 and E85)
  • Prius runs without "check engine" light
  • energy content is ~85% of straight gas, next worst mileage
  • price is only down 2.5%, not cost effective

The Prius engine keeps the intake valves open for about 1/3d of the compression stroke pushing some of the fuel-air mix back into the intake manifold. Then the intake valves close giving about an 8-to-1 compression stroke which is well within the compression range for 87 octane. But there is more, a knock sensor.

The knock sensor is a specialized microphone that listens for the high-frequency sound of a knock, the 'ping.' The Prius engine controller adjusts the spark advance to bring the gas to the edge of knock. The more advanced the spark, the more power BUT the early combustion can force even a low octane fuel to knock. So the engine constantly listens for and adjusts the spark advance to keep the engine 'tuned' for the best performance for that tank of gas. So what happens with extra octane gas?

The engine knock sensor and controller will advance the spark to maximize the amount of energy that can be gotten from that tank of gas. But the energy gain from advancing the spark is not enough to make up for the low energy content of premium gas or even E85.

In summary:

• higher energy content, 87 octane gas is best
• all higher octane gas have lower energy content
  • Prius will tune the spark advance to get as much energy as possible but not as much as a higher energy, 87 octane gas

Bob Wilson

 

Thanks for the primer! But... your linkie no workie. It'd be cool if you could dig it up.

 

This should work:
Different MPG with different brand of gas? | Page 2 | PriusChat

Measured at 4,100 rpm in a 1.5L, 2003 Prius:

• 14000 Watt sec/g (100%) - higher energy fuel power
• 13000 Watt sec/g (92%) - lower energy, premium fuel tuned
• 12500 Watt sec/g (89%) - lower energy fuels

Measured at 3,200 rpm:

• 14700 Watt sec/g (100%) - higher energy fuel power output
• 13300 Watt sec/g (90%) - lower energy and premium fuel

The energy output per gram is a complex but reproducible effect. It was not limited to just one brand:

So on my fuel lists:

1. Shell, Chevron, and Exxon 87 octane were best fuels
2. BP, Texaco, Conoco and Citgo 87, lower energy

NOTE: these were measured in Huntsville Alabama at least 4-5 years ago before our area went E10. I have no interest in repeating these tests because: E10 is not a hard ratio; summer-winter blends, and; refinery feedstock and processes. To do a proper study, I would want each sample analyzed, at least a molecular-weight survey, along with a better analysis than just burning in our Prius.

Bob Wilson

 

I use Shell gas about 75% of the time since it's top tier gas, and happens to be one of the lowest prices on my commute $3.169 yesterday). Other times, I buy gas at Kroeger when I have a discount (.40 to 1.00 off per gallon), or when I'm travelling and don't have a choice where I fill up.

 

All gas by octane is the same depending on what part of the earth it is "siphoned" out of, stored refined, and transported, the additive by gas station brand is what makes it different, so all gas is really the same; just depends on how old it is because fuel breaks down when stored too long. Store bought additives (Techron) help a lot especially between oil changes.

We all know winter fuel is different and terrible, and makes the effects of 10% ethanol worse. Whoever came up with the winter fuel idea and got it passed instead of making fuel efficient cars should be forced to drive on 10% ethanol winter fuel all year round for the rest of their life, fixing the same car same engine, and while I am teaching my 5 year old numbers have my kid make up the price that person would pay for a gallon of gas ;-)
JK but winter fuel sucks; there is no need for it unless you live somewhere that gets below 32f 0c.

 

I use an app on my phone called GassBuddy to locate the cheapest fuel in the area I happen to be in. It has very consistently lead me to the cheapest fuel. Of course, I have to factor in other variable such as Arco charging extra to use a card, getting 1% cash back for using my credit card, and other discounts such as 3 cents off per gallon at Fred Meyer when using a rewards card.

The Shell's/Chevron's in my area are normally 35 cents per gallon more expensive than the station directly across the street (Arco/7-Eleven). There is no way Shell gasoline can deliver 10% more mpg to account for the price difference.

Looking at my app today, the lowest price for regular is $3.46 at Arco or Costco. The most expensive is $3.80 at Chevron.

Bob,

Would you kindly explain this a little further? Are you saying that you measured BP at 11% less energy content than Chevron? That would translate to about 5mpg difference in a Prius, which is very significant. How did you measure the energy content? I see data points from the same gas vary by more than 2,000 joules/gram. This suggests inconsistency in the testing method, or inconsistency in energy density from the fuel source.

I appreciate your scientific approach to answering questions. Thanks!

 

The general protocol:

• Run the tank dry with 1 gallon can of the next test gas. After adding the gallon, drive directly to the gas station selling the test gas and put in at least 5 gallons. This ensures the gas in the tank is from the same station and as close to the same batch as possible.
• Use a Graham miniscanner recording data into a laptop
  • use ICE rpm, MG1 torque to measure the shaft power, converted to Joules/sec
  • use injector timing and ICE rpm to measure the fuel burned per gram
• Do a maximum accelerator, hill climb up an 8% grade
  • stay as close to 70F as practical while avoiding busy traffic times
• Fill the 1 gallon can with the next test gas

Recording the data as fast as possible, there are six data points every second from both the hybrid vehicle ECU and engine ECU over the OBD. Sad to say, the six separate data items have to be 'averaged' in time to calculate shaft power and fuel consumption leading to unavoidable sampling errors.

Do not forget these tests were done in 2007 when none of the Huntsville stations had E10 stickers. Now all the pumps have the E10 stickers so I've lost interest in follow-up studies.

Bob Wilson

 

Before E10, I used to hear people say Arco gas is "watered down". Can I assume it was E10 fuel before it was mandated? Perhaps energy densities are more similar across the brands now that E10 is required.

I would think the best method of comparing energy densities would be to take a known quantity of liquid at a known temperature, and heat it by burning a known quantity of fuel and measure the temperature rise.

 

Gasoline is a complex mix of different hydrocarbons and near as I can tell, handled in batches. The feedstock, crude oil is an even greater mix of hydrocarbons. It is entirely reasonable that the processing of any given batch is driven by organic chemistry aimed at yield and octane ratings more than anything else.

I agree. Just be careful as open flame, burning gasoline has a risk. It is very volatile and if the liquid gasoline is heated, the gasoline can boil and this would greatly expand the flame.

Assuming you plan to heat a water bath, it is important to carefully control the starting and ending temperatures. One approach would be to pre-heat the water bath to boiling and then switch the boiling bath to heat from the known, gasoline sample. Measure the start and stop weight and then calculate the heat from the amount of water that boiled away.

But consider the logistics of dealing with open flame and boiling water including getting start and stop weights. This is not a trivial problem.

Bob Wilson