Octane
08-04-2003, 11:36 AM
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Octane
I have an Octane question. If I have 103 octane leaded fuel and mix it with 93 octane will it raise the octane of the 93 and what is the ratio. Thanks Otto
08-04-2003, 12:00 PM
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Yes, maybe. When you mix different fuels the outcome may be different if you try mixing another 2 different fuels. You can/will get many different answers to this question. The only correct way to know for sure is to test the new mix.
08-04-2003, 01:03 PM
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In general the octane response will be linear for most hydrocarbon and oxygenated fuels eg 50:50 of 87 and 91 will give 89.
Attempts to mix leaded high octane to unleaded high octane to obtain higher octane are useless for most commercial gasolines. The lead response of the unleaded fuel does not overcome the dilution effect, thus 50:50 of 96 leaded and 91 unleaded will give 94. Some blends of oxygenated fuels with ordinary gasoline can result in undesirable increases in volatility due to volatile azeotropes, and some oxygenates can have negative lead responses. The octane requirement of some engines is determined by the need to avoid run-on, not to avoid knock.
6.18 How can I increase the fuel octane?
Not simply, you can purchase additives, however these are not cost-effective and a survey in 1989 showed the cost of increasing the octane rating of one US gallon by one unit ranged from 10 cents ( methanol ), 50 cents (MMT), $1.00 ( TEL ), to $3.25 ( xylenes ) [108]. Refer to section 6.20 for a discussion on naphthalene ( mothballs ). It is preferable to purchase a higher octane fuel such as racing fuel, aviation gasolines, or methanol. Sadly, the price of chemical grade methanol has almost doubled during 1994. If you plan to use alcohol blends, ensure your fuel handling system is compatible, and that you only use dry gasoline by filling up early in the morning when the storage tanks are cool. Also ensure that the service station storage tank has not been refilled recently. Retailers are supposed to wait several hours before bringing a refilled tank online, to allow suspended undissolved water to settle out, but they do not always wait the full period.
6.19 Are aviation gasoline octane numbers comparable?
Aviation gasolines were all highly leaded and graded using two numbers, with common grades being 80/87, 100/130, and 115/145 [109,110]. The first number is the Aviation rating ( aka Lean Mixture rating ), and the second number is the Supercharge rating ( aka Rich Mixture rating ). In the 1970s a new grade, 100LL ( low lead = 0.53mlTEL/L instead of 1.06mlTEL/L) was introduced to replace the 80/87 and 100/130. Soon after the introduction, there was a spate of plug fouling, and high cylinder head temperatures resulting in cracked cylinder heads [110]. The old 80/87 grade was reintroduced on a limited scale. The Aviation Rating is determined using the automotive Motor Octane test procedure, and then converted to an Aviation Number using a table in the method. Aviation Numbers below 100 are Octane numbers, while numbers above 100 are Performance numbers. There is usually only 1 - 2 Octane units different to the Motor value up to 100, but Performance numbers varies significantly above that eg 110 MON = 128 Performance number.
The second Avgas number is the Rich Mixture method Performance Number ( PN - they are not commonly called octane numbers when they are above 100 ), and is determined on a supercharged version of the CFR engine which has a fixed compression ratio. The method determines the dependence of the highest permissible power ( in terms of indicated mean effective pressure ) on mixture strength and boost for a specific light knocking setting. The Performance Number indicates the maximum knock-free power obtainable from a fuel compared to iso-octane = 100. Thus, a PN = 150 indicates that an engine designed to utilise the fuel can obtain 150% of the knock-limited power of iso-octane at the same mixture ratio. This is an arbitrary scale based on iso-octane + varying amounts of TEL, derived from a survey of engines performed decades ago. Aviation gasoline PNs are rated using variations of mixture strength to obtain the maximum knock-limited power in a supercharged engine. This can be extended to provide mixture response curves which define the maximum boost ( rich - about 11:1 stoichiometry ) and minimum boost ( weak about 16:1 stoichiometry ) before knock [110].
The 115/145 grade is being phased out, but even the 100LL has more octane than any automotive gasoline.
( I didn't write this, just a quote )
Attempts to mix leaded high octane to unleaded high octane to obtain higher octane are useless for most commercial gasolines. The lead response of the unleaded fuel does not overcome the dilution effect, thus 50:50 of 96 leaded and 91 unleaded will give 94. Some blends of oxygenated fuels with ordinary gasoline can result in undesirable increases in volatility due to volatile azeotropes, and some oxygenates can have negative lead responses. The octane requirement of some engines is determined by the need to avoid run-on, not to avoid knock.
6.18 How can I increase the fuel octane?
Not simply, you can purchase additives, however these are not cost-effective and a survey in 1989 showed the cost of increasing the octane rating of one US gallon by one unit ranged from 10 cents ( methanol ), 50 cents (MMT), $1.00 ( TEL ), to $3.25 ( xylenes ) [108]. Refer to section 6.20 for a discussion on naphthalene ( mothballs ). It is preferable to purchase a higher octane fuel such as racing fuel, aviation gasolines, or methanol. Sadly, the price of chemical grade methanol has almost doubled during 1994. If you plan to use alcohol blends, ensure your fuel handling system is compatible, and that you only use dry gasoline by filling up early in the morning when the storage tanks are cool. Also ensure that the service station storage tank has not been refilled recently. Retailers are supposed to wait several hours before bringing a refilled tank online, to allow suspended undissolved water to settle out, but they do not always wait the full period.
6.19 Are aviation gasoline octane numbers comparable?
Aviation gasolines were all highly leaded and graded using two numbers, with common grades being 80/87, 100/130, and 115/145 [109,110]. The first number is the Aviation rating ( aka Lean Mixture rating ), and the second number is the Supercharge rating ( aka Rich Mixture rating ). In the 1970s a new grade, 100LL ( low lead = 0.53mlTEL/L instead of 1.06mlTEL/L) was introduced to replace the 80/87 and 100/130. Soon after the introduction, there was a spate of plug fouling, and high cylinder head temperatures resulting in cracked cylinder heads [110]. The old 80/87 grade was reintroduced on a limited scale. The Aviation Rating is determined using the automotive Motor Octane test procedure, and then converted to an Aviation Number using a table in the method. Aviation Numbers below 100 are Octane numbers, while numbers above 100 are Performance numbers. There is usually only 1 - 2 Octane units different to the Motor value up to 100, but Performance numbers varies significantly above that eg 110 MON = 128 Performance number.
The second Avgas number is the Rich Mixture method Performance Number ( PN - they are not commonly called octane numbers when they are above 100 ), and is determined on a supercharged version of the CFR engine which has a fixed compression ratio. The method determines the dependence of the highest permissible power ( in terms of indicated mean effective pressure ) on mixture strength and boost for a specific light knocking setting. The Performance Number indicates the maximum knock-free power obtainable from a fuel compared to iso-octane = 100. Thus, a PN = 150 indicates that an engine designed to utilise the fuel can obtain 150% of the knock-limited power of iso-octane at the same mixture ratio. This is an arbitrary scale based on iso-octane + varying amounts of TEL, derived from a survey of engines performed decades ago. Aviation gasoline PNs are rated using variations of mixture strength to obtain the maximum knock-limited power in a supercharged engine. This can be extended to provide mixture response curves which define the maximum boost ( rich - about 11:1 stoichiometry ) and minimum boost ( weak about 16:1 stoichiometry ) before knock [110].
The 115/145 grade is being phased out, but even the 100LL has more octane than any automotive gasoline.
( I didn't write this, just a quote )
08-04-2003, 06:35 PM
#8
AT Cult Member #3
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Originally posted by 36spectre
Sounds good but what did he say.Otto
Sounds good but what did he say.Otto
Only kiddin' VonBongo. Thanks for the thorough(sp) explanation....now where are my 800mg Ibuprofens......
