Vivian Marlowe
Octane is a measure of a fuel's ability to resist knocking or pinging during combustion, caused by the air/fuel mixture detonating prematurely in the engine. The higher the octane number, the more compression the fuel can withstand before detonating. On the other hand, the percentage of alcohol in a fuel blend is measured by the amount of ethanol or methanol present. While ethanol is often referred to as a high-octane fuel, the octane rating of pure ethanol is estimated to be around 100, and it is difficult to measure the octane rating of pure alcohols. However, when ethanol is blended with gasoline, it increases the octane rating of the blend. For example, a blend of 10% 113 octane ethanol with 85 octane gasoline results in an octane rating of 87, which is the standard octane rating for most consumer fuels.
Differences between Octane and Percent Alcohol
| Characteristics | Values |
|---|---|
| Octane definition | Octane is a measure of a fuel's ability to resist "knocking" or "pinging" during combustion. |
| Octane number | Octane numbers are determined by taking the average of two ASTM-specified tests: Research Octane Number (RON) and Motor Octane Number (MON). |
| Octane boosters | Octane boosters include MTBE, ETBE, toluene, and iso-octane. |
| Octane and engine performance | Higher octane fuels improve engine performance and fuel economy in engines with higher compression ratios or supercharging/turbocharging. |
| Octane and health/environmental concerns | Health and environmental concerns have led to a reconsideration of octane sources. Toxic aromatics used to boost octane are being replaced with cleaner alternatives like ethanol. |
| Percent alcohol definition | Refers to the percentage of alcohol (ethanol) in a fuel blend. |
| Alcohol as an octane booster | Alcohols, particularly ethanol, can increase the octane rating of fuels. However, their octane ratings cannot be directly measured using standard ASTM procedures. |
| Alcohol and engine performance | Alcohol fuels, such as methanol, can have high octane ratings. Neat alcohols can be used in diesel engines with ignition improvers, offering similar performance to conventional diesel fuel. |
| Alcohol and emissions | Ethanol reduces greenhouse gas emissions compared to gasoline and has lower toxicity than petroleum-based octane boosters. |
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What You'll Learn
- Octane is a measure of a fuel's ability to resist knocking or pinging during combustion
- Octane ratings of alcohols cannot be measured
- Ethanol is a cleaner-burning alternative to petroleum-based octane boosters
- Octane number at the pump is determined by taking the average of two ASTM-specified octane tests
- Octane boosters include MTBE, ETBE, toluene and iso-octane
Octane is a measure of a fuel's ability to resist knocking or pinging during combustion
Octane is a measure of a fuel's ability to resist "knocking" or "pinging" during combustion. Knocking refers to the premature detonation of the air/fuel mixture in the engine, which can cause significant engine damage. The higher the octane number, the more compression the fuel can withstand before detonating.
The octane number is determined by comparing the knock intensity of the fuel with blends of iso-octane and heptane, with iso-octane being highly resistant to knocking and heptane knocking readily. The percentage of iso-octane in the blend that matches the fuel being tested is the octane number of that fuel. For example, a fuel that has the same knocking characteristics as a mixture of 90% iso-octane and 10% heptane would have an octane rating of 90.
There are different types of octane ratings, including the Research Octane Number (RON), Motor Octane Number (MON), and Observed Road Octane Number (RdON). RON is determined by running the fuel in a test engine at 600 rpm with a variable compression ratio under controlled conditions, while MON testing is done at 900 rpm engine speed and higher engine temperature and speed to further stress the fuel's knock resistance. RdON, on the other hand, is derived from testing the gasoline in ordinary multi-cylinder engines rather than a purpose-built test engine.
The octane rating of a fuel does not directly relate to the power output or energy content of the fuel per unit mass or volume. Instead, it simply indicates the fuel's resistance to detonating under pressure without a spark. Whether a higher octane fuel improves or impairs engine performance depends on the engine design. Engines with a higher compression ratio or those using supercharging or turbocharging require higher octane fuel to keep the mixture from pre-detonating.
While ethanol is often associated with high-octane fuel, the octane ratings of pure alcohols cannot be measured using standard octane test engines. However, blends of alcohols can be tested to determine a "Blending Octane Value" (BOV) by comparing the octane rating of an alcohol/gasoline blend to the octane rating of the gasoline without alcohol. It is important to note that a BOV is not the same as a normal octane rating, and BOVs for methanol and ethanol may overstate octane due to the law of diminishing returns.
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Octane ratings of alcohols cannot be measured
Octane is a standard measure of a fuel's ability to withstand compression in an internal combustion engine without causing engine knocking. The higher the octane number, the more compression the fuel can withstand before detonating. Octane ratings do not relate directly to the power output or the energy content of the fuel per unit mass or volume.
Octane ratings are calculated by comparing the results of a test engine with a mixture of 2,2,4-trimethylpentane (iso-octane) and normal heptane that would have the same anti-knocking capability as the fuel under test. The percentage, by volume, of 2,2,4-trimethylpentane in that mixture is the octane number of the fuel. For example, gasoline with the same knocking characteristics as a mixture of 90% iso-octane and 10% heptane would have an octane rating of 90.
The octane rating of a fuel is not directly related to the power output of an engine. Using gasoline of a higher octane than an engine is designed for cannot increase its power output. The added power in such cases comes from the way the engine is designed to compress the air/fuel mixture, and not directly from the rating of the gasoline.
Technically speaking, the octane ratings of alcohols cannot be measured. All octane test engines, as defined in the octane rating procedures set forth by the American Society for Testing and Materials (ASTM), are carbureted. Air/fuel ratio adjustments on octane engine carburetors are limited and cannot accommodate the extremely different air/fuel ratio requirements of pure alcohols. Blends of alcohols can be tested to determine what is called a “Blending Octane Value” or BOV. The octane rating of an alcohol/gasoline blend is compared to the octane rating of the gasoline without alcohol, and some math is done to calculate the effect of the alcohol on the octane of the gasoline. However, a BOV is not the same thing as a normal octane rating.
BOVs for methanol and ethanol typically overstate octane. While a little alcohol can bump octane by a few numbers, twice as much alcohol does not result in twice as much octane increase. So while it may look like an alcohol has a high octane number when a small amount is used (as in determining a BOV), its real octane value is not as high. The best estimates of the actual octane rating of pure ethanol place it at about 100 octane. If you see octane claims for ethanol in the 112 range, be suspicious that the octane number might be incorrectly based on BOV.
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Ethanol is a cleaner-burning alternative to petroleum-based octane boosters
Octane is a measure of a fuel's ability to resist "knocking" or "pinging" during combustion, caused by the air/fuel mixture detonating prematurely in the engine. The higher the octane number, the more compression the fuel can withstand before detonating. Octane rating does not relate directly to the power output or the energy content of the fuel per unit mass or volume.
The term "high-octane" is used to describe a powerful action due to its association with the concept of "octane rating". However, this is misleading as the octane rating of gasoline is not directly related to the power output of an engine. Using gasoline with a higher octane than an engine is designed for cannot increase its power output.
Octane numbers at the pump are determined by taking the average of two ASTM-specified octane tests: the Research Octane Number test (RON) and the Motor Octane Number test (MON). The average is known as the Anti-Knock Index (AKI) and is expressed as (R+M)/2.
Ethanol is often talked about as a high-octane fuel. It is a cleaner-burning alternative to petroleum-based octane boosters. The best estimates of the actual octane rating of pure ethanol place it at about 100 octane. The blending RON of ethanol is about 120-135, and the blending MON is 100-106. The high octane numbers of ethanol could enable the optimization of engines to increase thermal efficiency.
The density of ethanol is 0.79 kg/l, which is slightly higher than that of gasoline. Higher density improves volumetric fuel economy to some extent. The oxygen content of ethanol is 35%, which is significantly higher than that of gasoline, which has an oxygen content of 0%. The oxygen content of fuel determines the stoichiometric air/fuel ratio.
Ethanol is also cheaper than synthetic aromatics, so gasoline blended with ethanol reduces the price at pump. Additionally, ethanol reduces greenhouse gas emissions between 34 to 44% compared to gasoline. A modest increase in ethanol content in fuel from 10 to 15% would result in an anticipated 6.6% reduction in cancer risk from tailpipe emissions.
However, it is important to note that the octane ratings of alcohols, including ethanol, cannot be directly measured. Blends of alcohols can be tested to determine the "Blending Octane Value" (BOV) by comparing the octane rating of an alcohol/gasoline blend to the octane rating of the gasoline without alcohol. However, a BOV is not the same as a normal octane rating, and BOVs for ethanol typically overstate octane due to the law of diminishing returns.
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Octane number at the pump is determined by taking the average of two ASTM-specified octane tests
Octane is a measure of a fuel's ability to withstand compression in an internal combustion engine without detonating prematurely, which is known as "knocking" or "pinging". The higher the octane number, the more compression the fuel can withstand. Octane ratings are based on the pressure at which a fuel will spontaneously combust in a testing engine.
Octane numbers at the pump are determined by taking the average of two ASTM-specified octane tests: the Research Octane Number test (RON) and the Motor Octane Number test (MON). The average is known as the Anti-Knock Index (AKI) and is expressed as $(R+M)/2$. In the United States, pump gas is sold based on AKI, and the octane number displayed on pumps is typically lower than in other countries. The AKI in Canada, the US, and Mexico is 4 to 6 octane numbers lower than elsewhere due to the difference between RON and MON.
The RON test involves running the fuel in a test engine at 600 rpm with a variable compression ratio under controlled conditions, and comparing the results with mixtures of iso-octane and n-heptane. The MON test is similar but uses a higher engine speed of 900 rpm, a preheated fuel mixture, and variable ignition timing to further stress the fuel's knock resistance. The MON is generally 8 to 12 points lower than the RON.
The octane ratings of pure alcohols cannot be measured using standard ASTM test engines due to their different air/fuel ratio requirements. However, blends of alcohols can be tested to determine a "Blending Octane Value" (BOV) by comparing the octane rating of an alcohol/gasoline blend to the rating of the gasoline without alcohol. BOVs for methanol and ethanol tend to overstate octane due to the law of diminishing returns. While a small amount of alcohol can increase octane by a few numbers, doubling the amount of alcohol does not result in a proportional increase in octane.
In summary, the octane number at the pump is determined by averaging the RON and MON, which are measured using ASTM-specified tests. The displayed octane number varies by country, with the US and a few other countries using the AKI, while most other countries use the RON. The octane ratings of pure alcohols cannot be directly measured, but blends can be tested to determine a BOV.
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Octane boosters include MTBE, ETBE, toluene and iso-octane
Octane is a standard measure of a fuel's ability to withstand compression in an internal combustion engine without causing engine knocking. The higher the octane number, the more compression the fuel can withstand before detonating. Octane ratings do not relate directly to the power output or the energy content of the fuel per unit mass or volume.
Octane boosters are additives that improve the octane rating of fuel. Typical "octane booster" gasoline additives include MTBE, ETBE, toluene, and iso-octane.
Methyl tert-butyl ether (MTBE) is a gasoline oxygenate that was once widely used as an octane booster. It is still used in Europe, the Middle East, Africa, Asia, and Latin America. MTBE has a detrimental environmental impact by contaminating water supplies, and it increases NOx emissions. However, it has a high octane value, is easily incorporated into gasoline stock, and reduces exhaust emissions.
Ethyl tert-butyl ether (ETBE) is another octane booster with a high research octane number (RON) of 105-123 and a motor octane number (MON) of 95-105. It has excellent fuel properties and is compatible with current cars and the fuel distribution chain. It also has a low vapor pressure, which makes it predictable when blending with gasoline.
Toluene is another octane booster that has been used in the past. It is a petroleum-based synthetic octane enhancer that can be harmful to the environment.
Iso-octane is also used as an octane booster, although it is not the most knock-resistant substance available today.
It is important to note that the octane rating of alcohol fuels cannot be directly measured. Blends of alcohols can be tested to determine a "Blending Octane Value" (BOV), but this is not the same as a normal octane rating.
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Frequently asked questions
Octane is a measure of a fuel's ability to resist "knocking" or "pinging" during combustion, caused by the air/fuel mixture detonating prematurely in the engine. The higher the octane number, the more compression the fuel can withstand before detonating.
The percent alcohol refers to the amount of alcohol in a fuel mixture. For example, E15 contains 15% ethanol, while E85 contains 85% ethanol. The octane number is a measure of the fuel's ability to withstand compression without detonating prematurely.
The octane rating of an alcohol/fuel blend is compared to the octane rating of the fuel without alcohol, and some math is done to calculate the effect of the alcohol on the octane rating of the fuel. This is called the "Blending Octane Value" or BOV. However, it is important to note that BOVs for methanol and ethanol typically overstate octane due to the law of diminishing returns. While a small amount of alcohol can increase the octane rating, doubling the amount of alcohol does not result in twice as much increase in octane.
Whether a higher octane fuel improves or impairs an engine's performance depends on the design of the engine. In general, higher-compression engines may yield higher power when using fuels with higher octane ratings. Higher octane fuels can also improve fuel economy in engines with a higher compression ratio and/or those that use supercharging or turbocharging.
