Occasionally using 98 octane gasoline in a car designed for 95 octane won't cause issues, but long-term mixing of different octane ratings can lead to delayed combustion in the engine. This means both the engine's power output and thermal efficiency will decrease, resulting in poorer performance and potentially advancing the vehicle's major maintenance timeline. The appropriate gasoline grade should be selected based on the engine's compression ratio. Cars with compression ratios between 10.0-11.5 should use 95 octane gasoline, while those with higher compression ratios should use 98 octane. However, with modern technologies, compression ratio alone doesn't determine gasoline selection. High compression ratio engines can be tuned to use lower octane fuel due to other influencing factors like ignition timing, turbocharging technology, and Atkinson cycle technology. Generally, higher octane gasoline has higher octane numbers and better anti-knock properties. 98 octane gasoline contains 98% isooctane and 2% n-heptane, while 95 octane contains 95% isooctane and 5% n-heptane. Occasional use of wrong octane fuel only requires switching back to the correct grade after consumption. However, long-term use of incorrect octane fuel can cause: Using lower octane fuel in high-performance vehicles may cause engine knocking. The significantly lower octane rating reduces gasoline's ignition point, causing premature ignition during compression stroke before spark plug firing. This creates resistance during piston's upward movement, making engine operation unstable. Mild knocking only increases noise without obvious engine damage, but severe knocking indicates serious engine issues affecting driving stability and causing abnormal wear on pistons and cylinders, potentially leading to cylinder scoring. Gasoline selection considerations: Choose gasoline with appropriate volatility based on ambient temperature. Highly volatile gasoline facilitates cold starts and smooth engine warm-up, but excessive volatility may cause vapor lock in fuel lines, preventing normal fuel pump operation and causing engine stalling.
