What are the consequences of filling a car designed for 98 octane with 92 octane gasoline?

[HarleyAnn]

Using 92 octane gasoline in a car designed for 98 octane can cause engine knocking due to their different octane ratings: 92 octane gasoline contains 92% isooctane and 8% n-heptane, while 98 octane gasoline contains 98% isooctane and 2% n-heptane. Additionally, 98 octane gasoline has better anti-knock properties. Using 92 octane gasoline with lower anti-knock properties can easily lead to engine knocking. Gasoline is a mixture, and the octane number we commonly refer to indicates its anti-knock index (i.e., octane rating). Strictly speaking, 92 octane and 98 octane gasoline should not be mixed, although their chemical compositions are relatively similar. To ensure vehicle safety, it is best to use them separately. In some regions, there is even lower-grade 90 octane gasoline. Mixing it with higher-grade gasoline can cause knocking, insufficient power, and reduce the lifespan of spark plugs, as well as increase exhaust emissions. 98 octane gasoline typically has a density above 0.74, while 92 octane gasoline has an octane rating equivalent to a mixture of 92% isooctane and 8% n-heptane. The advantages of 98 octane gasoline are "two highs and two lows": higher anti-knock properties and power output, and lower harmful substances and exhaust emissions. 98 octane gasoline has strong anti-knock properties and will not ignite under high pressure, ensuring the engine operates at the optimal ignition timing without affecting power output. The octane number represents the fuel's anti-knock properties—the higher the number, the better the anti-knock performance. Anti-knock performance refers to gasoline's ability to resist knocking during combustion in the engine, which is the primary indicator of gasoline's combustion efficiency. Knocking occurs when gasoline burns abnormally in the engine. The octane rating is the measure of gasoline's anti-knock performance—the higher the octane rating, the better the anti-knock performance. Higher compression ratios and combustion chamber pressures require gasoline with higher anti-knock properties to prevent knocking. Gasoline with different octane ratings has varying octane values. Higher-octane gasoline has higher octane values and better anti-knock properties. Knocking is an abnormal combustion phenomenon in the engine. When the piston is in the compression stroke, moving upward to compress the air-fuel mixture, the temperature and pressure of the mixture increase. If the gasoline's anti-knock properties are poor, the air-fuel mixture may ignite before the spark plug fires, causing knocking. The burning mixture pushes the piston downward while the piston is still moving upward, leading to reduced engine power, increased fuel consumption, and potential engine damage. Occasionally using the wrong octane gasoline is not a problem as long as you switch back to the correct grade afterward. However, long-term use of the wrong octane gasoline can have the following effects: For vehicles designed for lower-octane gasoline, accidentally using higher-octane gasoline will not cause damage, but the increased octane rating may alter the fuel's ignition point, leading to delayed combustion. This can reduce the engine's power output and thermal efficiency, resulting in poorer performance. For vehicles designed for higher-octane gasoline, using lower-octane gasoline can cause engine knocking. Since the octane rating is significantly lower, the gasoline's ignition point decreases, causing premature ignition during the compression stroke. If the mixture ignites before the spark plug fires, resistance will occur during the upward stroke. This resistance makes the engine run very unstable. If the knocking is mild, it may only increase noise without obvious engine damage. However, severe knocking indicates serious engine issues, affecting not only driving stability but also causing abnormal wear on pistons and cylinders, and in extreme cases, cylinder scoring.

[McMax]

After filling my car, which requires 98-octane gasoline, with 92-octane fuel, I immediately noticed something was wrong. The engine started making a knocking sound upon ignition, which became even louder when accelerating or climbing hills, as if someone was hitting a metal can inside. This is because lower-octane fuel isn't suitable for high-compression engines, causing premature ignition and engine knocking. Prolonged use can damage pistons or valves, leading to repair costs of several thousand dollars. I also observed increased fuel consumption—previously, a full tank could last 500 kilometers, but now it's nearly empty before reaching 400. The power output has noticeably weakened. If you accidentally add some 92-octane fuel, it's advisable to top up with 98-octane fuel as soon as possible to mix them, or use a fuel additive to balance the octane level. In short, don’t compromise on fuel quality to save a few bucks—small savings now could lead to major headaches later, both in repair costs and safety risks.