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87 vs 91 octane for better mpg?

  #21  
Old 01-29-2008, 06:21 PM
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Default 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!
 
  #22  
Old 01-30-2008, 04:39 AM
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Default 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.
 
  #23  
Old 01-30-2008, 08:45 AM
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Default 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
 

Last edited by SPL; 04-04-2008 at 11:53 AM. Reason: Added the word "effective."
  #24  
Old 04-02-2008, 02:39 AM
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Default Re: 87 vs 91 octane for better mpg?

Originally Posted by SPL View Post
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 auto body part 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 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
Thats the compression ratio of the camry??? Do you know the compression ratio of the Honda Civic??
 
  #25  
Old 04-02-2008, 09:00 AM
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Default Re: 87 vs 91 octane for better mpg?

the actual compression ratio achieved is much lower than the above "theoretical" ratio
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.
 
  #26  
Old 04-02-2008, 09:31 AM
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Default 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
 

Last edited by SPL; 04-02-2008 at 09:34 AM.
  #27  
Old 04-02-2008, 10:24 AM
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Default 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.
 
  #28  
Old 04-02-2008, 10:48 AM
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Default Re: 87 vs 91 octane for better mpg?

Originally Posted by Mark E Smith View Post
. . . I am nit picking this because this was going to be my thesis for my ME degree in 1979.
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
 
  #29  
Old 04-03-2008, 10:40 AM
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Default 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
 
  #30  
Old 01-15-2013, 02:31 PM
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Default Re: 87 vs 91 octane for better mpg?

Originally Posted by SPL View Post
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.
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?
 

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