Regular Octane Fuel?

[Nets46]

Any problem using regualr octan fuel? any engine knock?

[SnoutDog]

played with using 87 and 89 octane... did notice a performance difference. Use the search tool and you will see a long discussion about this topic.

Premium Gas Vs. Regular Gas

[DirtyLittleSecret]

Pro's and Con's...just remember that 87 has more energy than 91. Use the lowest number that doesnt knock. A little knock under heavy load is normal.
Words to live by.

[Kansas Law Dog]

DirtyLittleSecret previously said:

...just remember that 87 has more energy than 91.

DLS - I don't understand this comment. Fuel does not burn instanteously when the spark plug initiates an electric arc. It takes a few microseconds for the air/fuel mixture to combust fully within the cylinder. Low octane fuel achieves this combustion quicker than high octane fuel. This is why modern engines adjust the timing (retard the spark) when engine knock is indicated by the knock sensors. So the cycle of the the piston is beyond TDC (top dead center - and in its power stroke) when combustion occurs fully within the cylinder. Timing of fuel combustion doesn't have a direct correlation with energy.

Premium grade (91) fuel is a blend of hydrocarbon molecules that have more branch chains and aromatics (toluene / xylene) in the fuel than Regular grade (87) fuel. Aromatics are denser than branch chains, which are denser than straight chain hydrocarbons. For this reason Premium grade fuel generally weighs more per gallon than regular grade fuel (not much), but it is noticable when measuring the NHV (net heating value) of gasoline.

My experience with gasoline blending at the four refineries I have worked extensively with during my career, shows that Premium grade contains a slightly higher BTU/gallon than Regular grade. Please note that blending gasoline is complex because there are many specifications that must be meet, but the primary distinction between premium and regular is blending the greatest proportions of refinery process streams containing the highest octanes in the premium fuel.

Does this higher energy in Premium exceed the economic cost of premium? I cannot answer that. Attached is a write-up I posted earlier this year - I did not write it.

Ever wondered what causes an engine to knock? Or what spark advance is for? All of the information has been lifted directly from reliable references (although the author changed a bit of the description). See references at the end.

The "Normal" Spark-Ignition Cycle

The basic "ideal" spark-ignition cycle for one cylinder consists of four parts. First, the Intake stroke: the fuel and air mixture is drawn into the cylinder through the intake valve as the piston nears the bottom of its downward travel. The pressure and temperature in the cylinder remain near outside conditions. Second, the Compression stroke: the fuel and air mixture is compressed as the piston goes up. The pressure in the cylinder goes up, and so does the temperature. At the end of the Compression stroke, the spark plug ignites the fuel mixture and it (in the "ideal" version) combusts instantly. Third, the Power stroke: the cylinder is now full of hot gasses that are at much higher pressure and temperature than the unburned compressed mixture was. This hot burned gas mixture pushes the cylinder down, which pushes the crankshaft around, which pushes your car forward. Fourth, the Exhaust stroke: these burned gas mixture is pushed out the exhaust valve into your exhaust manifold. The cycle then repeats.

This is a very simplified version of what happens in a real engine. The only difference that is really important as far as knock is concerned is that the fuel does not burn anywhere near instantaneously at TDC (top dead center). It takes about 0.5 ms (around 7.5 degrees of crank angle at 2500 RPM) after the spark occurs for the fuel to start burning beyond a small ball around the spark plug. This region of burning then spreads throughout the cylinder, and the burning is mostly complete somewhere around 30 to 50 degrees of crank angle after the spark. Now, clearly, if we ignite the fuel mixture directly at top dead center, the piston will have traveled down quite a distance before a considerable part of the fuel burns. This fuel, then, is unable to help push the piston down as much, and thus the engine is less powerful. Because of this, the spark is usually set to occur before the piston hits TDC, so more fuel will be burned when the piston begins moving downward. If the spark is advanced too far, though, the burning fuel starts pushing against the piston during the compression stroke and hindering things instead of helping. The optimum spark timing is obviously a trade-off between these two effects. Small changes don't make a particularly big difference in power, so usually the minimum advance needed to get the maximum torque is used.

Knock in Spark-Ignition Cycles

As you recall, the fuel-air mixture gets to a fairly high pressure during the Compression stroke. This, along with the heat of the engine, causes the mixture to become very hot. In a diesel engine, this temperature is high enough to cause the fuel to spontaneously ignite. In a spark-ignition engine, however, the mixture is kept somewhat below the ignition threshold.

Now, consider what happens when the fuel starts to burn. There is a fairly large amount of mixture that does not normally begin burning immediately. As the mixture burns, it compresses and heats up the mixture even more than it is already heated and compressed. If the pressure and temperature of this unburned gas gets high enough, it can spontaneously ignite without waiting for the flame to spread to it. This is known as "knock" or specifically "combustion knock."

In the case, a considerable amount of the mixture has spontaneously ignited in only few degrees of movement of crank angle. This same fuel normally takes almost 30 degrees of crank angle to burn. The result of this "almost instantaneous" burning is that the pressure in the cylinder is raised dramatically and suddenly. This causes a pressure wave which reverberates throughout the cylinder, causing adverse effects to the combustion cycle, high stresses on the engine, and annoying pinging noises. If it happens before the piston reaches TDC, it also pushes down hard against the piston on its compression stroke and considerably less on the power stroke. This causes a sudden loss of power and bucking of the vehicle, not to mention very high stresses on the engine.

Pre-Ignition in Spark-Ignition Cycles

Pre-ignition is term used for ignition that occurs before the spark plug is fired. It is mentioned it here because it can lead to knock. Usually pre-ignition occurs because of some hot spot in the engine that causes the fuel-air mixture to be heated past its auto-ignition point. There are many possible hot spots in the engine. If deposits are built up on the spark plug, they will be heated to a very high temperature when the spark fires. However, they do not lose their heat quickly, and can remain hot enough to cause pre-ignition in the next compression stroke. Exhaust valves are generally quite hot under normal operation, and if something happens to over heat them or keep them from being properly cooled, they can cause pre-ignition. Finally, carbon deposits on the cylinder walls can stay hot like burning coals and cause pre-ignition. Even if pre-ignition does not cause knocking, it causes higher temperatures and pressures in the cylinder that can cause rings to fail and even cause aluminum-alloy pistons to melt.

What to do About Knock

Knock occurs because the pressure and temperature in the cylinder are high enough when combustion is occurring that the unburned fuel/air mixture spontaneously ignites. Thus, these temperatures and pressures must be kept below a certain level to prevent knock from occurring. Several factors that affect this temperature and pressure, and thus affect knock are:

Engine compression ratio. If the mixture is compressed more, then the pressure gets higher, and knock is more likely to occur. Thus, a higher engine compression will cause knock to be more likely.

Spark Advance. The maximum pressure and temperature in the cylinder occur some time after the spark occurs. If the spark is delayed so that this maximum pressure occurs after TDC, the downward motion of the piston will somewhat counteract the pressure rise from combustion, causing the maximum pressure to be lower. Thus, retarding the spark causes knock to be less likely.

Pre-Ignition. Pre-ignition has the same effect as increasing the spark advance. It causes knock to be more likely.

Combustion Chamber Design. This has a fairly complicated relationship with knock. First, if the distance from the spark plug to the far corners of the combustion chamber is reduced, combustion occurs more quickly. This increases the power, and also reduces the amount of time that knock has to occur. Second, if the exhaust valve is placed close to the spark plug it will not heat up the later unburned gas as much, and thus will reduce the likelihood of knock. Third, cooler areas such as the intake valve and areas close to both the piston and cylinder head can be placed in the parts of the cylinder that burn last, thus cooling this unburned gas and reducing knock. Finally, increasing the turbulence in the engine (up to a point) increases the burn rate and thus reduces knock. Turbulence can be increased by valve design or by creating areas of mixture that are squeezed between the piston and head, and are forced at high velocity into the middle of the combustion chamber.

There is one significant factor other than the cylinder pressure and temperature that affects knock. That is the temperature at which the fuel/air mixture will spontaneously ignite. Since the ratios of fuel and air are fairly well determined by other considerations, the main variable here is the fuel. If the fuel is able to withstand higher temperatures before self-igniting, it will clearly be less likely to knock. The resistance of fuel to knocking is quantified as the Octane rating. The Octane rating is defined by the percentage of iso-octane in an iso-octane/n-heptane mixture that is required to match the knock behavior of the given fuel in two different tests. Both of these tests are done on a standardized variable-compression single-cylinder engine, at two different speeds and intake temperatures, and the results are averaged.

Why use High-Octane Fuel

A direct quote from Stone (p. 80): "The attraction of high octane fuels is that they enable high compression ratios to be used. Higher compression ratios give increased power output and improved economy [assuming the same power of engine]. The octane number requirements for a given compression ratio vary widely. There are even wide variations in octane number requirements between supposedly identical engines."

Raising the octane level of your fuel will primarily help prevent knock. If your engine is not currently knocking, you won't see any significant advantages of using higher octane fuel. On the other hand, if the engine is designed from the beginning for higher octane fuel, it will have considerable advantages in power and economy.

References
Lichty, L. C., Internal-Combustion Engines, Sixth Edition, McGraw-Hill, 1951
Stone, R., Introduction to Internal Combustion Engines, Second Edition, Society of Automotive Engineers, 1993.

The author of this page is Brooks Moses, 1997.

[for realsss]

Wow. That's some informative stuff.

[Boxhead]

As a former refiner myself, KLD nailed it, and quite well. KLD, by any chance did you work at the former Texaco refinery in El Dorado? I spent a couple of years there when it was the Equilon refinery.

[Basstrack17]

i'd say avoid any potential issues, and run the 91-93. It's maybe 3.00 per tank more, and especially now that gas has dropped 50+ cents per gallon ( at least by us ). it seems SOOOOO affordable...
We're used to the premium need anyways in the wife's TC, so this way if both rides need it, no chance of forgetting and running something we shouldn't be.

[lllateralus]

For the record, my FJC pinged with 85 degree outside temps in 5th and 6th gears. (using 87 octane)

89 octane has been run through my FJC for 10,000 miles except that once. Never again.

[tnegiet]

Basstrack17 previously said:

now that gas has dropped 50+ cents per gallon

Our regular is still over $3.00 a gallon!!!....something stinky in Denmark (eastern Washington)!!!

[Basstrack17]

our regular is 2.51-2.56; premium ( 91-93 octane ) 2.71-2.76

And after what it's been the past year.....that's a bargain!!