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octane
>"Flash Point" is the temperature at which the fuel emits vapors that can be ignited by an open flame. The test procedure is to fill a cup with the liquid. Over the cup is a flame. The distance between the cup and the flame can be varied, and must be specified. A thermometer is immersed in the liquid. The liquid is externally heated until the first flames from the liquid vapors are observed. There are some more details, but that is the essence.
>"Auto-ignition temperature" is the temperature at which the liquid will ignite without any flame or spark.
>In a cylinder, when the spark plug ignites the fuel, the flame front should proceed at a controlled rate through the combustion chamber, neither too fast nor too slowly. If the front moves too quickly, instead of a controlled burn it becomes something like an explosion, or detonation, which is bad for power, driveability and engine parts.
>Detonation becomes more likely as compression ratio increases. In the '60s, cars with only a 9:1 compression ratio required premium fuel, and in the '60s you could buy 100 octane fuel at many stations. Today, 10:1 compression ratios are common in cars that burn 87 octane quite happily, and much higher ratios are used in sports bikes. Engine designers modified the shape of the combustion chamber to get this happy state of affairs.
>Higher compression ratio is good for power, for reasons that require some really hairy thermodynamics to explain. But as a comparison, one of the reasons that diesels get so much more power from a gallon of fuel is that they run roughly twice the compression of spark-ignition engines.
>Ignition advance affects detonation. The spark is advanced as RPM increases, because as the engine speeds up, there is less time in each cycle. If the spark were not advanced, the fuel in the combustion chamber wouldn't be able to burn while during the time of peak compression.
>Computer controlled engines retard ignition when the first sign of detonation is detected. This delays the burn, and reduces pressures in the combustion chamber, and detonation, but it does this at the price of power and efficiency.
>If I ride with a low octane fuel under conditions which produce no detonation, and then repeat the identical ride with a higher octane fuel, the fuel consumption should be nearly identical. If I do the ride under conditions that produce detonation with low octane fuel, then repeat the identical ride with a high enough octane fuel that there is no detonation, I should use less of the high octane fuel.
>When tetraethyl lead was removed as an octane enhancer, various organics were used to replace it. I don't know what the favorites are, but one that helps is ethanol = grain alcohol = Everclear. Iowa subsidizes gasohol made with ethanol, so 87 octane, regular gasoline is always about 4 cents per gallon more than 89 octane gasohol.
>"Flash Point" is the temperature at which the fuel emits vapors that can be ignited by an open flame. The test procedure is to fill a cup with the liquid. Over the cup is a flame. The distance between the cup and the flame can be varied, and must be specified. A thermometer is immersed in the liquid. The liquid is externally heated until the first flames from the liquid vapors are observed. There are some more details, but that is the essence.
>"Auto-ignition temperature" is the temperature at which the liquid will ignite without any flame or spark.
>In a cylinder, when the spark plug ignites the fuel, the flame front should proceed at a controlled rate through the combustion chamber, neither too fast nor too slowly. If the front moves too quickly, instead of a controlled burn it becomes something like an explosion, or detonation, which is bad for power, driveability and engine parts.
>Detonation becomes more likely as compression ratio increases. In the '60s, cars with only a 9:1 compression ratio required premium fuel, and in the '60s you could buy 100 octane fuel at many stations. Today, 10:1 compression ratios are common in cars that burn 87 octane quite happily, and much higher ratios are used in sports bikes. Engine designers modified the shape of the combustion chamber to get this happy state of affairs.
>Higher compression ratio is good for power, for reasons that require some really hairy thermodynamics to explain. But as a comparison, one of the reasons that diesels get so much more power from a gallon of fuel is that they run roughly twice the compression of spark-ignition engines.
>Ignition advance affects detonation. The spark is advanced as RPM increases, because as the engine speeds up, there is less time in each cycle. If the spark were not advanced, the fuel in the combustion chamber wouldn't be able to burn while during the time of peak compression.
>Computer controlled engines retard ignition when the first sign of detonation is detected. This delays the burn, and reduces pressures in the combustion chamber, and detonation, but it does this at the price of power and efficiency.
>If I ride with a low octane fuel under conditions which produce no detonation, and then repeat the identical ride with a higher octane fuel, the fuel consumption should be nearly identical. If I do the ride under conditions that produce detonation with low octane fuel, then repeat the identical ride with a high enough octane fuel that there is no detonation, I should use less of the high octane fuel.
>When tetraethyl lead was removed as an octane enhancer, various organics were used to replace it. I don't know what the favorites are, but one that helps is ethanol = grain alcohol = Everclear. Iowa subsidizes gasohol made with ethanol, so 87 octane, regular gasoline is always about 4 cents per gallon more than 89 octane gasohol.
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