The higher the compression ratio the higher/lower the grade of fuel required?

• A. lower
• B. unchanged
• C. higher
• D. The higher the compression ratio, the higher the grade of fuel required.

Answer: (D)

Raising the compression ratio increases the pressure and temperature in the cylinder when ignition occurs. That hotter, denser mixture is more prone to knocking or pre-ignition unless the fuel can resist detonation. Higher octane fuel has greater resistance to knocking, so as compression goes up, you need a higher grade of fuel to prevent pinging and ensure smooth operation. If you stick with a lower-octane fuel in a high-compression engine, you’re more likely to experience detonation and potential engine damage. The idea that the required fuel grade would be lower or stay the same doesn’t fit because the tendency to knock grows with compression, requiring a fuel with greater detonation resistance.