A higher compression ratio implies higher overall temperatures operating inside the cylinder. Because of this, the risk of detonation is much bigger, so the octane rating of the fuel used in the mixture needs to have a much higher octane rating. A higher compression ratio will necessarily require a higher octane rating as well. Although the diesel engines operate at much higher compression ratios than the normal petrol engines, they don’t have such high numbers of octane ratings because they don’t compress fuel but only the air. Fuel is then injected directly into the cylinder and mixed with air under high pressure.