Why can diesel engines achieve higher compression ratios than gasoline engines?

Diesel engines can achieve higher compression ratios than gasoline engines because they compress only air, rather than a pre-mixed air-fuel charge.1

In gasoline engines, the air and fuel are mixed before or during the intake process. If the compression ratio is too high, this mixture can spontaneously ignite ahead of the flame front, causing a phenomenon known as knocking, which can damage the engine. Consequently, the compression ratios of gasoline engines are limited by the fuel's octane number, which measures its resistance to this premature ignition.23

Because diesel engines induce only air into the combustion chamber during the intake stroke, they are not restricted by these pre-ignition problems. Instead, the air is compressed to a very high temperature—typically exceeding 526 °C (979 °F)—and the fuel is injected only when the piston is near the top of its stroke. This allows diesel engines to operate with much higher compression ratios, generally ranging from 14:1 to 22:1, which contributes to their superior thermal efficiency.1