While this is occurring, the piston is also moving upwards, compressing the air-fuel mixture and simultaneously increasing its temperature since temperature increases as pressure increases.
With lower octane fuels, the air-fuel mixture can also ignite due to this compression. When this flame front collides with the flame front ignited by the spark plug, an audible "knock" is heard. When the air-fuel mixture within an engine ignites due to compression rather than from the spark plug it is known as "pre-ignition" [4] If the fuel combusts and burns before it is ignited by the spark plug, it burns incompletely.
The leftover fuel from this incomplete ignition causes residue to stick to the inside of the fuel chamber, eventually leading to the sounds from ones engine known as engine knock. For a helpful video explaining engine knock, click here.
Generally, the owners manual that comes with new vehicles has manufacturers recommendations as to which octane rating a vehicle should use to prevent this harmful pre-ignition and knocking due to build-up.
Most auto manufacturers recommend 87 octane gasoline, but advise using a higher octane gasoline if knocking is an issue with the lower grade. The most important and significant difference between methane and iso-octane for this problem is the postquench oxidation of unburned hydrocarbons.
Methane shows fast oxidation of unburned fuel and intermediate hydrocarbons whereas postquench oxidation for iso-octane is slow especially for the intermediate hydrocarbons. Furthermore, the Soret effect which is usually considered to be of minor importance appears to be important in modeling the rate limiting diffusion process.
This is caused by different directions of the thermal diffusive transport for certain species. A short mechanism for iso-octane applied previously for flame propagation studies is found to be inadequate to describe the hydrocarbon evolution after quenching. The octane number of a fuel is measured in a test engine, and is defined by comparison with the mixture of iso-octane and normal heptane which would have the same anti-knocking capacity as the fuel under test: the percentage, by volume, of iso-octane in that mixture is the octane number of the fuel.
Because some fuels are more knock-resistant than iso-octane, the definition has been extended to allow for octane numbers higher than The octane rating of a spark ignition engine fuel is the detonation resistance anti-knock rating compared to a mixture of iso-octane 2,2,4-trimethylpentane, an isomer of octane and n-heptane.
By definition, iso-octane is assigned an octane rating of and heptane is assigned an octane rating of zero. This does not mean, however, that the gasoline actually contains these hydrocarbons in these proportions. It simply means that it has the same detonation resistance as the described mixture. Octane rating does not relate to the energy content of the fuel. For example, the "octane" used as a standard gasoline engine fuel against which gasolines are rated by definition, octane is actually not n -octane, but rather 2,2,4-trimethylpentane - a branched isomer of octane - in contrast to the straight chain version.
Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more.
What is the meaning of n in n-octane and similar compounds? Ask Question. Asked 7 years, 3 months ago.
0コメント