Why can Audi only use 95 octane gasoline?

[Bilbo]

According to the official vehicle manual recommendation, Audi should use 95 octane gasoline. If Audi occasionally uses the wrong gasoline grade, simply switch back to the correct grade after consumption. However, long-term use of incorrect gasoline grades may have the following effects: For vehicles recommended for lower octane, using higher octane gasoline will not cause damage, but the increased octane rating will alter the fuel's ignition point, leading to delayed combustion in the engine. This means both the engine's power output and thermal efficiency will decrease, resulting in poorer performance in practice. For vehicles recommended for higher octane, using lower octane gasoline can cause engine knocking. Because the octane rating is too low, the reduced ignition point may cause premature ignition during the compression stroke. If combustion occurs before the spark plug fires during the compression stroke, resistance will arise during the upward stroke. This resistance makes the engine run very unstably. If the knocking is imperceptible, it only increases noise without significant engine damage. However, noticeable knocking indicates severe engine conditions, affecting not only driving stability but also causing abnormal wear on pistons and cylinders, and in severe cases, cylinder scoring. In addition to checking the appropriate gasoline grade in the vehicle manual, Audi owners can also find this information on the fuel filler cap. Typically, the gasoline grade can also be determined based on the engine's compression ratio. Vehicles with a compression ratio between 8.6-9.9 should use 92 octane gasoline, while those with a compression ratio between 10.0-11.5 should use 95 octane gasoline. However, with the application of new technologies, the compression ratio alone cannot determine the required gasoline grade. High compression ratio engines can still be tuned to use lower octane gasoline because, besides the compression ratio, other factors such as ignition timing, turbocharging technology, and Atkinson cycle technology also play a role. Generally, the higher the gasoline octane rating, the higher the octane number and the better the anti-knock performance. 92 octane gasoline contains 92% isooctane and 8% n-heptane, while 95 octane gasoline contains 95% isooctane and 5% n-heptane.