The myth about premum is that, because it costs more, it is better. Unless you need the "octane" rating for the particular vehicle to prevent "pinging," it is NOT better. Gasoline is composed of hydrocarbon chains of 6, 7, or 8 carbons linked together. Octane means the mix ratio contains more 8 chain carbion molicules. Higher octain means that it will not "pre-detonate" under higher pressures under higher compression ratios. It does nothing for lower compression engines. Under usual conditions, I do not purchase premium because; It is a slower seller, which means that it is less "fresh" than regular and has been in the underground tank longer to collect more water through condensation. To achieve higher octane, premium has a larger percentage of ethanol. If you look on the pump at the gas station, there is a sticker saying that it can contain up to 10% ethanol. Ethanol provides less mileage and collects condensation water more than gas without or with a lower precentage of ethanol in the mix. Ethanol must be used, since MTBE as an octane booster (methylbutyltertiaryether) was banned in 2005 in many states. So, regular is better functionally and financially in any model Prius. If you look at the gas pump, you may not see an "ethanol" sticker on the regular digital indicator of the pump, but you do for midgrade or premium.
You do realize that the Prius has a 13:1 compression ratio so it is not a low compression engine. But, yeah, if its not a required thing, don't use it.
Somebody went to a deal of great effort to print the owners manual. It actually tells you, well ya gotta read it, the damn book does not speak to you, what octane gas your new Prius needs. My Gen II manual, which I actually read, says 87, so that's what I use......
Like I said in my original post. 87 octane regular is the gas most likely to have the least ethanol in it. More gas vs. ethanol means more energy per gallon and more miles per gallon.
I read that the Compression Ratio is regulated at around 8:1 while the Expansion Ratio is 13:1 This appears to be something to do with the Atkinson cycle. I am no expert!
The compression ratio of the Prius isn't anywhere close to 13:1. The expansion ratio is 13:1, but the compression ratio is about 8:1. This might be a good time to read up on the Atkinson Cycle. Tom
Great post.. However, Be aware and stay way from HIGH Ethanol gas.. Hard to do since "regular" gas may not indicate the ethanol content. NO E15 - E85! Up to E10 Gasoline only - per the manual, and gas cap.
All of my cars always got better mpg AND 1/4 mile performance using regular gas instead of premium....except my turbo cars. My bone stock SRT4 Neon was getting the epa rated 30 mpg but dropped to 26 when using regular. When bone stock and not driven hard you can use regular in these cars, but the reduced mpg actually cost about the same as using premium when figured out. I started using premium in that car only from then on. As far as the E10-E85, everyone I know loose at least 25% mpg running that. My 95 Neon and 84 Horizon (that I drag race, stock compression) both loose power and economy using any ethonal.
Good point. Toyota's spec sheet is designed for Otto cycle engines. They need to add another line. Tom
Good post. Oddly enough, it's the opposite here. Regular has "up to 10% ethanol" but midgrade and premium don't (some stations... others have up to 10% in all grades).
It's politics that results in variations. Ethanol was highly subsidized to appease the "corn farmer lobby" In the Midwest, here in the US. Prior to the ethanol era, Gasohiol contained methanol, which costs about 46 cents a gallon to produce from land fills, natural gas and coal. Methanol is likely to become the 'liquid fuel" to be sold at erstwhile gas stations, after petroleum is depleted.
The 2010_Prius_Product_Info_FINAL.pdf files I saved, two versions from March 2009, correctly list this as Expansion Ratio. So they did understand it at some point in history.
I thought this case, (not using premium when only regular is required,) was true for cars without variable valve timing. Since there is VVT-i in the prius then why wouldn't it improve performance? I understand there is a case, although hazy in my mind, that some higher octane fuels contain less energy, but even my 2004 Sienna manual says it will perform better on higher octane fuel...
Toyota is correct. The theoretical compression ratio is 13:1. That is the total available volume divided by the final volume with the piston at TDC. However, with the Atkinson Cycle engine and VVTi the effective compression ratio can be regulated around 8:1 This is done by keeping the inlet valve open for a period during the compression stroke, allowing some of the mixture to flow back into the inlet manifold. This effectively reduces the compression ratio to a value that can be regulated around 8:1 hence 87 Octane is OK. The expansion ratio is still 1:13 as the valves are closed during the expansion stroke.
The valve control mechanism on this engine cannot close the intakes early enough to achieve anywhere near a 13:1 compression ratio.
The intent is to take advantage of leaving the intake valve open so that you get good mileage and control the open period with VVTi so that you can adjust the open period to get the best of mileage or power as required by the driving conditions.
Using premium fuel in a Prius when ethanol isn't present could result in better fuel economy, there's no question. There will be no benefit to performance as the Atkinson cycle engines have less chance for detenation when compared to a typical Otto cycle engine. The Atkinson cycle engine is very efficient and makes reasonable horsepower , but lacks torque. Add Ann electric motor to the party and you've really improved. Add premium to your sienna whe you're going to tow for better low end performance....like acceleration up a steep grade. In that climb, the engine's under a heavy torque load and will benefit from the high octane's resistance to detonation. If the engine doesn't hear pinging, it'll advance spark and valve timing to generate more torque. Back to the Prius, the electric motor is doing the heavy torque work. The engine's spinng the generator to make power for that motor in addition to helping move the car. Premium's pointless since the torque load on the gas engine is light. Buy premium gas to escape ethanol otherwise save the cash.
