87 vs 91 octane for better mpg?
 

ok I searched a little bit for an answer to my question, but didn't really find anything. A simple question:

will my car (a FEH, so it does not "require" premium gas), get better gas mileage using premium gas (91+ octane) instead of regular gas (87 octane)?

I'm not asking about engine performance in terms of power. I know some cars require premium gas to get peak HP. I'm asking strictly about effect on mpg.

thanks

Re: 87 vs 91 octane for better mpg?
 

Depends on what the engine was made for. If it's a high compression engine than you want high octane. I believe the prius was made for high octane gas but can run on regular and advertise it as such.

Re: 87 vs 91 octane for better mpg?
 

so a high-compression engine will get better gas mileage, and not just max HP, from using premium gas over regular?

Re: 87 vs 91 octane for better mpg?
 

Well the problem is marketing numbers versus engineering numbers. I don't know the forumula off hand to give you the answer you want, but in general, engines with higher compression tend to do better with higher octane fuels in both power and mpg (in the case of atkinson cycle engines, power is sacrificied for efficiency so high mpg but not more power).

There are cars that are more tailered towards lower octane gas and get better power/mpg by using the recommended fuel octane rating such as the HCH.

The atkinson cycle engine (like found in the prius) tends to be on the higher side of compression ratios, which to me means in order to really take advantage of this engine, you need to use higher octane fuel. But the problem is higher octane fuel tends to be more expensive, so marketing kicks in and says you can use regular because you end up saving more even though you might get better mpg using higher octane fuel, but from an engineering point of view, the recommended octane level will be higher than what is listed.

So there is going to be an ideal octane rating for every engine for efficiency and one for power. The problem is those numbers are rarely published.

Re: 87 vs 91 octane for better mpg?
 

There have been many posts/threads on this subject. However, premium fuel has less energy than regular 87 octane so the FE may be slightly lower with premium. Use premium only if the engine requires it otherwise you are just wasting your money.:shade:

Re: 87 vs 91 octane for better mpg?
 

My testing in Huntsville AL revealed 87 octane, Shell, Exxon and Chevron had the highest energy content per gram. All other brands and octanes had about 11% less energy than these. I did see some evidence that at nearly maximum ICE power, the 93 octanes were less bad but not as much energy as the 87 octane Shell, Exxon and Chevron.

Bob Wilson

Re: 87 vs 91 octane for better mpg?
 

Lower octane will give you higher MPG.

The higher octane gas ( these days ) is being cut with ethanol to give the higher octane number... which will give lower MPG... but at the same time it is cleaner burning, and cleaner for your car.

So it's back to a case of pay now, or pay later.

It should be noted that OCTANE is NOT an ingredient. It is only a NUMBER. The number does not tell you what is in the gas. The number tells you how the gas behaves. Ethanol contains zero octane molecules, but has an octane rating of 110. E85 has an octane rating of 105.

Re: 87 vs 91 octane for better mpg?
 

What about all the advice you can find through a Google search? Wherever I look (outside this forum), the only reason to go uptown on octane rating is if your engine is experiencing knock -- premature combustion of the fuel. And if you're getting engine knock at the recommended fuel grade, then you should be taking your vehicle in for servicing. Engines with higher compression ratios sometimes get away with lesser-grade octane if they have working knock-sensors and/or you're driving gently. Higher octane inhibits gasoline from combusting too easily under higher compression; It has no fuel efficiency or power benefits.

I'm not sure if gpsman1 can qualify his statements about ethanol. It's the opposite here, where regular grade gets the highest average percentage of ethanol additive while premium has none.

Re: 87 vs 91 octane for better mpg?
 

I know for sure ethanol is used to raise the octane in the states of Colorado, Nebraska, Kansas, Iowa, South Dakota, Nevada, Minnesota, ( except super premium which can only be used for off-highway use, such as snowmobiles, and boats ).

I suspect it is also used for this in Illinois, and Indiana, but I don't visit those areas. I visit all the others above frequently.

With the 110 octane rating, and costs of well under $2 per U.S. gallon, it is the cheapest booster for the fuel blenders/distributers.

Some states mandate ethanol must be in all blends.
Some mid-west states ( NE, IA, KS ) do not put it in the lowest grade.
And in Minnesota, the ethanol capitol of the world, I cannot even find 87 octane anymore... 89 is the lowest sold. My car gets a jolt every time I'm in CO and UT when I fill up with 85 octane. 85 always boosts my MPG. But you have to be over 3500 feet elevation to find/buy 85 octane.
:)

Re: 87 vs 91 octane for better mpg?
 

The bottom line is pretty simple....

Use what the manufacturer recommends. They are telling you what the engine was designed to run on. Using anything else will likely have a negative effect on both your performance and fuel economy.

(I'm sure someone somewhere will post that in THEIR car THEY got better performance using a different fuel rating than recommended. In reality, your car will run best on what is recommended unless there is a problem with your vehicle...in which case the right action is to get it fixed not change fuel rating.)

Re: 87 vs 91 octane for better mpg?
 

The engine is tuned and designed to run on 87 octane! Octane is a NUMBER or rating that is used to express the resistance to uncontrolled pre-ignition, generally a slower combustion. If the engine is designed for a faster combustion (lower octane) then you will actually lose mpg by running a higher octane rating. (before you flame me this is for a layman's understanding of octane)

Re: 87 vs 91 octane for better mpg?
 

Using 93 octane in my insight
gives me more power during lean-burn.
That results in higher MPG.

Re: 87 vs 91 octane for better mpg?
 

You might consider updating your profile to show the vehicle and geographical area. Also, I didn't see a mileage record in the Insight database. Of course it isn't required but it often gives folks a clue about how Insights work.

What year? How many miles? How long have you had it?

Bob Wilson

Re: 87 vs 91 octane for better mpg?
 

This is all assuming the manufacturers are giving you a number from the engineering group. Marketing can always have a say in the number if they feel it is in the best interest of the company to market the car at a lower octane level. The engineering group will sign off saying it can be tuned to the lower octane level but actually can perform better at the higher octane level. I believe the Prius engine (atkinson cycle engine) is such a case.

Re: 87 vs 91 octane for better mpg?
 

The Atkinson type engine behaves the opposite of a high compresson race engine. The lowest octane available is the best choice. I use 85 octane in the Rocky Mountain states, and it works great when I drive a tank down to near sea level.

Octane is "anti-knock"... and the lower compression due to the atkinson cycle makes knocking almost impossible with even the lowest octane.

( I heard Poland sells 80 Octane, and Russia... 78 Octane... I'd like to try it some time... but it's pretty far for me to get some! ) -John

Numbers below from 85 Octane... 4,000 pound SUV too!

Re: 87 vs 91 octane for better mpg?
 

Are you sure about lower compression in an atkinson cycle? I could have sworn it had a higher compression than average.

I believe the Prius has:

Toyota Prius hybrid electric (front-wheel drive) with a compression ratio of 13.0:1

That's pretty high.

Re: 87 vs 91 octane for better mpg?
 

That's the "theoretical" compression ratio that you'd get if the closing of the intake valve wasn't delayed. But it is significantly delayed (by 32 degrees in the Camry Hybrid's 2.4-L ICE versus the non-hybrid's 2.4-L ICE). It's the expansion ratio that's represented by this number; the effective compression ratio is around 10:1, as in all low-octane ICEs. Calling it a "compression ratio" is what's throwing you off. The big expansion ratio is what gets more of the thermal energy out of the gases before they're discharged (hence the higher thermodynamic efficiency) — but it also breathes in less fuel per stroke, and so results in the lower power of the "Atkinson"-type ICE for its rated size. It's really more like a 2-L than a 2.4-L ICE power-wise.

You can see the comparison of these two engines on page EG-3 of the attached file from the 2007 Camry Hybrid "New Car Features Guide."

Stan

Re: 87 vs 91 octane for better mpg?
 

Hmmm... cool. Can you explain the formula on how to take theoretical compression ratio and convert it to the "actual". I understand the delay, is it because the valve is open that the volume of the chamber is actually bigger therefore the compression ratio is smaller? So when cylinder is at TDC there is a valve (exhaust) that is still open?

Re: 87 vs 91 octane for better mpg?
 

It is possible to work out the geometry but in the Prius community, we have access to the Toyota papers and maintenance manuals. The commonly believed values are:

8-to-1 - compression stroke
13-to-1 - expansion stroke

If you want to work out the geometry, the ratio should be roughly proportional to:

((intake_delay_close_in_degrees) / 180 degrees) * (max_cylinder_volume/min_cylinder_volume)

Due to the geometry of the connecting rod, there will be some variance but for a rough approximation, this should be close enough.

Bob Wilson

Re: 87 vs 91 octane for better mpg?
 

Using 93 octane in my insight
gives me more power during lean-burn.
That results in higher MPG.

.............because I can avoid using rich-burn when climbing gentle slopes (I left out that key piece). That's the compression-to-expansion ratio. Prius has an abnormally large expansion, but the same compression as any other car, so there won't be any pre-ignition of the fuel even with 85 octane.

Re: 87 vs 91 octane for better mpg?
 

I have never used anything but 87 octane in any of the vehicles i have owned and have never had any knocking problems. I don't think it's worthe the extra money for 91-93 octane gas. Like what was posted above..........use what the owners manual recommends. You can't go wrong!:D

Re: 87 vs 91 octane for better mpg?
 

True however...

The owner's manual recommends using conventional oil.

But everyone knows synthetic is better. Sometimes you can improve upon the advice given in the manual.

Re: 87 vs 91 octane for better mpg?
 

livvie — What I was getting at is the following. If we take a conceptually ideal Otto-cycle engine, with intake and exhaust valves each open for precisely 180 degrees of crankshaft rotation, the compression and expansion ratios are equal, and set by the stroke length and chamber dimensions. If we now retard the closing of the intake valve so that it only closes partway up the compression stroke, then some of the fuel/air mixture that has already been sucked into the cylinder is blown back out into the intake manifold before the valve closes, and so after the intake valve closes the actual compression ratio achieved is much lower than the above "theoretical" ratio. This is what the so-called "Atkinson"-cycle hybrid engines do. The Camry's hybrid ICE delays its intake valve closing by more than 30 degrees compared to that of the non-hybrid ICE. This makes its effective compression ratio still around the 9.8:1 value specified for the non-hybrid engine, and thus suitable for regular gasoline. However, because of this strategy, its effective expansion ratio is now larger than its effective compression ratio, and this is what leads to its greater thermodynamic efficiency. In reality, both intake and exhaust valve openings and closings do not occur at the precise top or bottom of the piston's stroke, and this complicates computing the actual achieved compression and expansion ratios, as bwilson4web has indicated. I believe that page EG-3 is saying that the Camry hybrid's effective compression ratio is around 9.8:1, and its expansion ratio is around 12.5:1. But I haven't tried to verify these numbers, and without very detailed information about the geometry it would be impossible to do so.

Stan

Re: 87 vs 91 octane for better mpg?
 

Thats the compression ratio of the camry??? Do you know the compression ratio of the Honda Civic??

Re: 87 vs 91 octane for better mpg?
 

The compression RATIO stays the same, the compression psi is reduced. This is an important point. A lot of energy is wasted in a conventional as heat because the expansion rate of the engine is to small ( lower compression ratio). But to high a compression ratio causes to high a compression psi and you get uncontrolled ignition. The Atkinson/Miller cycle reduces compression psi while maintaining a high compression ratio.

Re: 87 vs 91 octane for better mpg?
 

kerpal — I don't have data on the Honda Civic, but any internal combustion engine running on regular gasoline must have an effective compression ratio no higher than ~10:1, otherwise you'd get bad pre-ignition.

Mark E Smith — This is admittedly partly a semantic question, but since the intake valves are held open during a significant part of the compression stroke of an Atkinson/Miller-cycle ICE, its effective compression ratio is greatly reduced, and is much less than the length of the stroke would suggest. To call the simple number given by the length of the stroke the "compression ratio" is misleading in this context, as it doesn't represent the true compression ratio. To get greater heat energy from the burnt gases, it's important to make the expansion stroke as long as possible. The Atkinson/Miller idea achieves this by shortening the effective compression stroke relative to the expansion stroke (or, if you prefer, by lengthening the expansion stroke relative to the effective compression stroke). I'd prefer to call the geometrically-computed number of 12.5 the "expansion ratio," for that is indeed what it more nearly represents. The actual compression ratio is more like 9.8:1.

Stan

Re: 87 vs 91 octane for better mpg?
 

You are confusing compression ratio and compression!

See wiki article http://en.wikipedia.org/wiki/Compression_ratio

My point is important, the compression (psi) determines the uncontrolled combustion not the compression RATIO. This is the reason A/M engines get better fuel economy. They extract more energy from the burning of the fuel and less goes out the tail pipe and into the cooling system. Its a thermodynamics thing. I a nit picking this because this was going to be my thesis for my ME degree in 1979.

Re: 87 vs 91 octane for better mpg?
 

This poses and interesting question, could a diesel be built that "Atkinsonized" the compression stroke with a greatly increased expansion cycle?

Today's diesels are rated from 14 to 20 to 1 compression ratios and typically that is also their expansion ratio. I am wondering if one could be built that had say at 28 to 50 expansion ratio but the same compression ratio?

The idea is to maximize the expansion ratio of a compression ignition engine to further reduce the exhaust gas temperature when the exhaust valve is opened. Actually this begs the question: What is the maximum ratio of an Atkinson cycle before other effects limit the effectiveness?

If we took our Prius engine and put a modified intake cam that added degrees to the intake open angle, we would expect to see a further reduction in ICE power output. But at a minimum, it might avoid the inefficient specific fuel combustion that occurs above 4,150 rpm (in the NHW11 engine.)

Bob Wilson

Re: 87 vs 91 octane for better mpg?
 

Mark E Smith — But that was precisely my point! It's a semantic issue. I'm not confusing "compression" with "compression ratio." What I'm saying is that, for greater thermodynamic efficiency, the power stroke should be longer than the compression stroke. It extracts more of the heat energy from the hot gases. The Atkinson/Miller-cycle engines achieve this difference between the strokes by delaying the closing of the intake valves. My point was really that the "compression ratio" number is not meaningful for such engines. It really represents the "expansion ratio," and not the factor (i.e., the ratio) by which the unburned gases are actually being compressed.

Stan

Re: 87 vs 91 octane for better mpg?
 

My insight which uses regular 87 gasoline is 11.5 to 1. Wonder why it does not knock?

The Chevy Volt (which uses premium) is 11 to 1 supposedly so it can get better MPG.

What do you suppose the Prius would be like if its Atkinson engine was used in a standard car without the electric motor assist? Anybody ever made an Atkinson-powered mass production car?

Re: 87 vs 91 octane for better mpg?
 

Which version of Octane rating are you using RON, MON, AKI, R/M2? the reason I read through this thread was to find some answer to this question but instead just found bickering over what fuel was better to use on merrit of marketing.

all modern engines include a knock sensor that will retard the timing if you use a fuel that is not got a high enough octane rating, that doesn't mean that you get what the manufacter expected out of the engine. to do that you have to know which octane rating system they were using when they rated the engine and suprise saying Amarican isn't an option, we don't have the world standard and allot of different engines are made in different places.


Just my two cents worth.
HBP