Have you noticed a reduction in MPG from using regular ?

Non hybrid engines use what's called the Otto Cycle which I think produces better power from the fuel. Hybrid engines use the Atkinson Cycle (like the diet I guess) which produces better ecomomy/emissions. It all has something to do with crankshaft position, ignition timing, valve timing and two timing - scratch that last one - a feeble attempt at humor.

http://en.wikipedia.org/wiki/Atkinson_cycle

I would assume Consumer Reports findings were for cars using the otto cycle. Perhaps hybrid engines can take advantage of higher octane but if they could you would think manufacturers would let you know about it. I'm sure it wouldn't be something they'd try to hide.

I'm curious about this as well. Up until now I have always used mid-grade on my previous cars. My other car is a 95 Honda Civic that pings badly with cheap regular. So far I have not noticed any difference between mid-grade and regular gas in the Altima Hybrid.

The Atkinson Cycle is really just a 4 cycle engine with the timing of ignition and valves that enable a longer power (combustion stroke) to extract more energy from the power event. The longer time for the combustion to complete (related to eariler ignition and later opening of the exhaust valve) lends it's self to the use of hight octane rather well. I think that the reason it is not stressed is that the manufactures want to say uses Regular gas very badly. I give a comparasion test run but need to wait until I am finished with the break in period. If any of you have completed that a well controlled test. Same rout in both directions at same temperature winds etc would be very valuable to the entire community. Google Atkinson Cycle for additional information. I had a previous engine with this setup it had a scroll belt driven supercharger and produced smooth power and goor economy which is why they use it.

More famous last words

Your vehicle is designed to run on unleaded 87 octane fuel (85 in high altitudes) (R+M)/2 method [1], and is not calibrated to derive any advantage from slower and cooler burning higher octane. The slower flame propagation of high-octane fuel could be detrimental to your engine. Running fuel labeled as “Premium “ will also result in excessive engine deposits and higher emissions and may result in engine misfires and other server check engine indicators. Check engine indicator as a result of running high-octane fuel premium may void your factory engine and emissions warranty.

As an example:

A by-product of this is that Atkinson cycle engines actually run best on lower-octane fuels. The slower flame propagation of high-octane fuel is detrimental to such an engine, and Prius owners who have made the mistake of running premium fuel in their cars have been rewarded with misfires, check engine and hybrid system warning indicators.

Page 204 of the 2005 Owner's manual. "The Prius is designed to run on unleaded 87 octane fuel (85 in high altitudes) (R+M)/2 method [1], and is not calibrated to derive any advantage from slower and cooler burning higher octane, such as higher MPG. The Atkinson cycle effectively prevents it, so engine deposits and higher emissions can be expected due to incomplete burn and the catalytic converter not running hot enough to efficiently clean it. "

Octane Myths

• High octane gasoline improves mileage.


In general, if your car is designed to run on 87 octane gasoline, high octane gasoline will not improve mileage. If switching to high octane gasoline does improve mileage, you might find that your engine, or its control systems, need repair.


• High octane gasoline gives quicker starting.


No, it doesn’t.


• High octane gasoline increases power.


If your car is designed to run on 87 octane gasoline, you shouldn’t notice any more power on high octane gasoline. Again, if it does make a noticeable difference, your engine, or the engine’s electronic control systems, may need repair.


• High octane gasoline has been refined more – it is just a better product.


Additional refining steps are used to increase the octane; however, these additional steps do not necessarily make the gasoline a “better” product for all engines. They just yield a different blend of hydrocarbons that burn more slowly. The additional steps also increase the price

We got our very best MPG in some ways (like the top half of the tank) when it was filled with high octane. I actually suspect this was a dealership trick and they "topped off" the tank to the moon.

I don't like being tricked, but I do want to prove to myself one way or another whether or not this was a trick or better MPG from high octane.

I don't think the dealership was trying to convince me high octane worked better. I think they were topping off to make it look like their hybrid was better than it is, but either way - we don't know unless we run a tank of high octane to see where things come out... making sure the tank isn't topped off.

Dear Xavier

I owned a vehicle with Atkinson cycle. For those on informed as to it characteristics it is simply a modification as to the valve timing and ignition timing that provides a somewhat longer number of degrees for the power stroke. It is USUALLY used with supercharged engine as the Atkinson is not efficient in clearing out the exhaust gasses and the supercharger helps.

Moder engines Ours included utilize adaptive Spark timing. this means that the point of ignition is governed by a computer that has the time to spark supply partially controlled my a :knock detector" this is an acoustic or vibration accelerator" that works with the main computer so as the spark is advanced providing a longer power delivery time it finally gets to the point that the gasoline is either self igniting from compression or the spark is ingoting it too soon. Too soon means that the piston is still compressing and the expanding gases from the combustion event are being forced to compress and this is detected as a "Knock" the detector "hears" this and retard the Spark timing to the point this no longer occurs.

The actual velocity of the flame front or duration of the combustion event is not changed by the amount of antiknock compound in the gasoline and dos not effect the efficiency of the Atkinson cycle engine. I it only allows the spark to be a small bit advanced .

I agree with Marianne's idea to test the premium under controlled circumstances and run a fare test to see the difference one way or the other. The difference in fuel cost of about 10% most likely will not provide an additional 3 mpg and so the test will not be positive and may even prove negative but fact are forth a million opinions mine or others. IT matters not that I designed fuel injection systems for a living for some of my career. What matter is what the mileage does.

All said respectively and in good spirit.