Has anyone tried premium fuel w/Prius? Any impact on mpg?

Re: Has anyone tried premium fuel w/Prius? Any impact on mpg

I don't recommend it. The Atikinson cycle engine really doesn't need premium fuel. I don't think you will get enough increase in performance from the increased octane that would offset the additional cost.

 

Fwiw, my second tank of gas was premium and got 4mpg less than the first. I since switched back to regular and am averaging better than the first two tanks. ~60mpg at 150mi. The weather has been warmer, and other variables, but I will not go back to premium.

 

Re: Has anyone tried premium fuel w/Prius? Any impact on mpg

Octane rating how ever it is done is not about the amount of energy (BTU/Gal) in the gas it is about its ability to explode under compression. High Octane gas does not explode as easily as low, so it can be used in high compression engines. Yes such engines often develope higher horse power secondary to larger size (cubic inches) and number of cylinders. This is at the cost of MPG. They go faster but burn more gas. The Prius will not benefit from high Octane gas and in fact it has a very slightly lower BTU/Gal ratio than regular.

 

Re: Has anyone tried premium fuel w/Prius? Any impact on mpg

Gasoline doesn't really explode it ignites and burns.

Shamelessly stolen from howstuffworks

"If you've read How Car Engines Work, you know that almost all cars use four-stroke gasoline engines. One of the strokes is the compression stroke, where the engine compresses a cylinder-full of air and gas into a much smaller volume before igniting it with a spark plug. The amount of compression is called the compression ratio of the engine. A typical engine might have a compression ratio of 8-to-1. (See How Car Engines Work for details.)

The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.

The compression ratio of your engine determines the octane rating of the gas you must use in the car. One way to increase the horsepower of an engine of a given displacement is to increase its compression ratio. So a "high-performance engine" has a higher compression ratio and requires higher-octane fuel. The advantage of a high compression ratio is that it gives your engine a higher horsepower rating for a given engine weight -- that is what makes the engine "high performance." The disadvantage is that the gasoline for your engine costs more.

The name "octane" comes from the following fact: When you take crude oil and "crack" it in a refinery, you end up getting hydrocarbon chains of different lengths. These different chain lengths can then be separated from each other and blended to form different fuels. For example, you may have heard of methane, propane and butane. All three of them are hydrocarbons. Methane has just a single carbon atom. Propane has three carbon atoms chained together. Butane has four carbon atoms chained together. Pentane has five, hexane has six, heptane has seven and octane has eight carbons chained together.

It turns out that heptane handles compression very poorly. Compress it just a little and it ignites spontaneously. Octane handles compression very well -- you can compress it a lot and nothing happens. Eighty-seven-octane gasoline is gasoline that contains 87-percent octane and 13-percent heptane (or some other combination of fuels that has the same performance of the 87/13 combination of octane/heptane). It spontaneously ignites at a given compression level, and can only be used in engines that do not exceed that compression ratio.

During WWI, it was discovered that you can add a chemical called tetraethyl lead to gasoline and significantly improve its octane rating. Cheaper grades of gasoline could be made usable by adding this chemical. This led to the widespread use of "ethyl" or "leaded" gasoline. Unfortunately, the side effects of adding lead to gasoline are:

Lead clogs a catalytic converter and renders it inoperable within minutes.
The Earth became covered in a thin layer of lead, and lead is toxic to many living things (including humans).
When lead was banned, gasoline got more expensive because refineries could not boost the octane ratings of cheaper grades any more. Airplanes are still allowed to use leaded gasoline, and octane ratings of 115 are commonly used in super-high-performance piston airplane engines (jet engines burn kerosene, by the way)."

 

Long story short, you are thinking of things backwards. If your engine cannot compress the fuel properly then you will not see any benefit from the higher octane rating. Certain cars require the higher octane rating (performance cars). Their engines are made for it. You will not see a difference in MPG because the Prius engine cannot take advantage of the higher octane. Jus because the gas is better does not mean you will see a performance improvement.

They also say if your car starts to knock you should increase the octane rating. I am using 86 and getting 56-58 MPG per tank.

The higher octanes usually have more cleaners in them. There was a discussion about this earlier in addition to talking about Top Tier gas.

 

Re: Has anyone tried premium fuel w/Prius? Any impact on mpg

I use Chevron almost exclusively because it has the same amount of cleaners in all grades.

As explained before, octane != E. Do not believe in voodoo that higher octane somehow equals higher HP or mpg just because race cars use it.

 

Re: Has anyone tried premium fuel w/Prius? Any impact on mpg

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That is exactly correct. From the gasoline FAQ at http://www.faqs.org/faqs/autos/gasoline-faq/part3/

"The fuel property the octane ratings measure is the ability of the unburn end gases to spontaneously ignite under the specified test conditions. Within the chemical structure of the fuel is the ability to withstand pre-flame conditions without decomposing into species that will autoignite before the flame-front arrives. Different reaction mechanisms, occurring at various stages of the pre-flame compression stroke, are responsible for the undesirable, easily-autoignitable, end gases."

What octane DOES NOT measure are 1) speed of combustion and 2) energy content of the fuel. Many people think octane measures speed of combustion, or speed of the flame front, but they are wrong.