Why premium is wasted in a Prius

Our Atkinson engined Prius works best with regular gas, 85-87 octane, compared to premium, 91 octane or higher. The reason is the lower octane gasoline tends to have more energy, produce more power per unit, than higher octane gasolines. But let's start by discussing octane.

When the piston goes up in the cylinder, it compresses the fuel-air mixture which heats it. If this increase in temperature is high enough, it can cause the whole fuel-air mixture to ignite all at once. This is the same process that works in a diesel engine on a smaller scale around the high-pressure, fuel injector. So many diesels have a sharp knocking sound but they don't ignite the whole charge at once. But in a spark ignition engine, like our Prius, spontaneous ignition would be a detonation, an explosion, that can tear the engine apart. But our Prius engine plays a trick.

As the Prius engine piston goes up, the intake valve is kept open letting some of the fuel-air escape back into the intake manifold. So instead of a 13-to-1 compression stroke, the intake valve remains open and only closes so there is an 8-to-1 compression stroke ratio. The fuel-air mixture does not get hot enough to detonate, to ping like a diesel. This is why regular 85-87 octane fuel works in our Prius. If the engine had a 13-to-1 compression stroke, even 91-93 octane fuel would likely knock.

As for the fuel-air mixture that was pushed back into the intake manifold, there will always be another piston drawing in fuel-air. So no fuel-air is pushed out the throttle body but this small fraction passes from one cylinder to the next. Nothing is wasted.

Now close to top-dead center when the fuel-air is fully compressed, the spark ignites the mixture that burns very rapidly, heating the combustion gas to a very high temperature and pressure. This smooth increase in gas pressure pushes the piston down, expanding it 1-to-13 until the exhaust valves open.

As the piston moves down under pressure, it takes energy out of the hot gas and the expanding gas cools. Otherwise, the exhaust gas would be so hot it would melt the exhaust valves, manifold, and catalytic converter. But the amount of power is a function of how hot the fuel-air mixture burns, the energy content of the gasoline.

Not all gasolines have the same energy content, even less if mixed with ethanol. However, burning straight gas is better than E10. Then during winter, butane is often dissolved in the gas to make cold-starts easier. There are high-energy gasolines, expensive but available at race tracks that have even higher energy content. Sad to say, measuring the energy content of gasoline at the pump is what we call a 'hard problem'. Worse, it is made in batches and often the final output is driven by the quality of feedstock, the crude oil, and processing. But as a general rule, my testing revealed higher octane gasolines tend to have about 3-5% lower energy than lower octane gasolines from a few oil companies.

Notice I used the word "testing." This is the true way to find out if one or another brand is better than another. Here is how anyone can test the different grades and companies:

1. Have a 1 gallon spare can with the next gasoline to test.
2. Run the car out of gas so there won't be any of the last sample in the tank.
3. Add the 1 gallon spare can and drive to the station to get 5 more gallons.
4. Wait until the temperature is as close to 70F, dry, and little or no wind.
5. Drive at least 20 minutes to the largest, tallest hill.
6. At the bottom, stop the car and do a forced charge until the engine stops, 80% SOC.
7. Reset a trip meter!
8. Wait for traffic to clear on the hill.
9. Floor the accelerator and keep it there until you reach a known landmark at the crest of the hill.
10. Note the trip meter values, audio recording is a good idea or a camera recording the run.
11. Repeat for three runs.
12. Calculate the fuel burned.
13. Buy a 1 gallon of the next gas to test.

I used a variation of this test to find the optimum speed to climb a hill:

The same methodology works for gasoline by making sure as many variables as possible are controlled and the only one that changes is grade and source of gasoline.

Now I used a modified version of this test using a recording, Prius-aware scanner, that allowed me to calculate the power generated as a function of fuel burn. This meant I did not have to make so many hill-climbs. I wanted to share a method that would work for anyone. Using my modified method, this is what I got before our area was E10 captured:


So the key point is, regular gasoline tends to have more energy than premium. Our Prius can exploit a higher energy gasoline but premium is a poor choice for energy content gasoline.

Bob Wilson

 

I can find no evidence that premium has more energy per unit than regular gas from some vendors. At best, premium seems to have the same energy content as regular . . . no energy or fuel economy advantage.

Bob Wilson

 

I'm curious, how do you measure the 'clean'?

The only thing that is normally removed is the spark plug, roughly every 60-70,000 miles. The first set we removed were working fine and I could only see a little electrode wear. Otherwise, everything looked pretty good and I suspect we could have just re-gapped the plugs and run them for another 60-70,000 miles.

I've followed the Gen I forum and Yahoo Groups pretty closely looking for evidence of engine problems and only read of two: sticky throttle plate and injectors. The sticky throttle plate problem has become much rarer since E10 became universal. I suspect E10 has a powerful cleaning effect on gasoline gum.

As for injector cleaning, the same thing. In fact for both throttle plate and injector cleaning, I've always recommended an annual cleaning with a kit from Toyota: Cleaning Prius Throttle

How does E10 rate as a cleaning compound compared to traditional additives? The reason I ask is this warning about after market engine cleaners: Gas Additives, Fuel System Treatments and Ethanol E10

Understand I have no problem if you personally prefer a high-detergent gasoline. If you think high octane gasoline has even more cleaning additives than regular, it would help if you'd cite the reference(s). Personally I would just mix up equal parts of E85 and E10 which gives effectively a tank of engine cleaner. I've tested tanks of E50 and it works without raising a Check Engine light in both our NHW11 and ZVW30. Due to the high ethanol content, mileage will be pretty poor but you'll have a clean engine when done.

Notice I like to use the phrase 'I tested' in my postings. I'm OK if you want to reference someone else's testing especially if they included the protocol. I'm a little leery of unilateral claims without reference to some testing.

Bob Wilson

 

No problem, sometimes what I write is not clear. Hopefully it is clear now. But in descending quality of gasoline energy:

• High energy racing and EPA test fuel - best MPG, highest energy per unit, ~$25/gal
• Straight 87 octane from Shell, Exxon, or Chevron (measured in Huntsville AL)
• Straight 91-93 octane from anyone and the other vendor 87 octane fuels
• E10 regular, E10 premium - not tested(*)
• E85 - worst mileage, cleanest engine(*)

Bob Wilson

* - E10 and E85 are approximate ratios as there is no hard threshold of the blend.

 

The USA has a mandate for ethanol, up to 10%, in our fuel. In addition to oxygenating it for reduced SMOG, ethanol also has a cleaning effect often over looked. But I am sympathetic to the difference found in Aussi fuel.

Before our area adopted E10 as the standard, I had found the sticky throttle plate in our 2003 Prius. Worse, it was one of the more common faults reported in the NHW11, Gen 1, forum. I can easily believe that this problem would return if we reverted to straight gas at the pumps.

I would like to suggest running a fuel economy test with both the low and high octane fuel. I don't know if you have different oil company sources but I would suggest testing each vendor. I was able to find a measurable difference using OBD data.

Bob Wilson

 

I tried testing gasoline density before E10 arrived and could not find a correlation between the measured energy content and specific gravity. BTW that led to seeing the dissolved butane coming out of solution.

We can do an easy field test to measure the ethanol content as it has a fixed energy content. Doing partial fractions, we can then calculate the energy content of the gasoline . . . for that sample. But there is no guarantee we'll have the same ethanol content in the next tank.

One of the reasons for my study were the reports that diesel has significantly more energy content than gasoline yet we know diesel has a significantly lower octane rating. So I was wondering if regular octane gasoline might have higher energy content than premium octane gasoline and that turned out to be the case in some brands. Other brands had no difference in energy content and were always in the lower energy content group.

Gasoline is not just one hydrocarbon but a mix driven by different refinery designs, operators and feedstocks. It may be possible to do a molecular weight assay and get a first-order approximation of the energy content but this is not something one carries in their pocket or the trunk. Then comes the practical problem of how to use it in the field. With established gas stations, it is likely reproducible but on the road, all bets are off.

In a perfect world, gasoline would be sold not only by octane but energy density. Sad to say price and vendor name is not sufficient to tell. It takes testing. However, it can be derived from OBD analysis of our Prius data.

I am sure there are regional differences. The East Coast refineries get their feedstock from different sources than the Gulf and West Coast refineries. The East Coast refineries were sold to new operators a couple of years ago so their procedures may have changed. Then as part of refinery maintenance, their internal processes change too along with the ratio of ethanol. But the raw data is available from the OBD data of the ZVW30.

The ZVW30 engine controller metrics include these are the primary metrics for measuring energy content:

• injector volume
• rpm
• torque

To get reproducible results, these also would be needed:

• coolant temperature (for warm-up status)
• outside air temperature (for gasoline density)

Mapping these into a table, each trip after warm-up should provide enough, raw data samples to provide metrics of the gasoline energy content without having to worry about E10 content. If a Scangauge II had the ability to do math on several variables, it could read out the energy content. Some of the Android scanners might have that ability. Regardless, I'll probably do a study with my miniVCI someday.

Bob Wilson