Can You Directly Add 95 Gasoline Before Using Up 98 Gasoline?
2 Answers
It is possible to add 95 gasoline before completely using up 98 gasoline. Occasionally mixing them is fine, but it is not recommended to do so frequently. If the car owner intends to switch gasoline grades, it is best to wait until the fuel gauge light comes on before refilling, or to extract the remaining gasoline and then switch to another grade, rather than mixing them directly. Different grades of gasoline have different octane levels. The higher the grade, the higher the octane level, and the better the anti-knock performance. Knocking is an abnormal combustion phenomenon in the engine. When the piston is in the compression stroke, it moves upward to compress the combustible mixture, causing the temperature and pressure of the mixture to rise. If the gasoline's anti-knock performance is poor, the combustible mixture may ignite before the spark plug fires, leading to knocking. The burning mixture pushes the piston downward while the piston is still moving upward, resulting in reduced engine power, increased fuel consumption, and potential engine damage. The gasoline grade represents its anti-knock performance. The higher the grade, the higher the octane level and the better the anti-knock performance. 95 gasoline contains 95% isooctane and 5% n-heptane, while 98 gasoline contains 98% isooctane and 2% n-heptane. Anti-knock performance refers to the gasoline's ability to resist knocking during combustion in the engine. It is the primary indicator of gasoline's combustion efficiency. Knocking occurs due to abnormal combustion in the engine. The octane level is the measure of gasoline's anti-knock performance; the higher the octane, the stronger the anti-knock capability. Higher compression ratios and combustion chamber pressures require gasoline with better anti-knock performance to prevent knocking. Occasionally using the wrong gasoline grade is not a problem as long as you switch back to the correct grade afterward. However, long-term use of the wrong gasoline grade can have the following effects: For vehicles designed for lower grades, using a higher grade gasoline will not cause damage, but the increased octane level may alter the fuel's ignition point, leading to delayed combustion. This reduces the engine's power output and thermal efficiency, resulting in poorer performance. For vehicles designed for higher grades, using a lower grade gasoline can cause engine knocking. The significantly lower octane level lowers the fuel's ignition point, causing premature ignition during the compression stroke. If the mixture ignites before the spark plug fires, it creates resistance during the upward stroke, making the engine run very unstably. Mild knocking may only increase noise without noticeable 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.
Well, I've personally experienced this issue. That time, I had about a quarter tank of 98-octane fuel left and I directly topped it up with 95-octane. Not long after driving, the engine started knocking and a fault light came on. After inspection, it turned out to be engine knocking—mixing different octane ratings caused the engine control unit to fail to adjust properly, resulting in slight damage to the spark plugs and cylinders. The repair cost me a few thousand. The lesson learned is that the manual always recommends using the specified octane rating; mixing fuels carries significant risks. If the engine requires high-octane fuel, like in high-performance cars, randomly mixing fuels can easily lead to problems. Now, I always wait until the fuel is nearly empty before refueling with new fuel—it's worry-free and safe. Don't gamble with safety to save a little money; the top priority when driving is safety.
