In a cylinder, the piston displacement plus clearance volume, divided by the clearance volume. This is the nominal compression ratio determined by cylinder geometry alone. In practice, the actual compression ratio is appreciably less than the nominal value because the volumetric efficiency of an unsupercharged engine is less than 100%, partly because of late intake valve closing. In spark ignition engines the allowable compression ratio is limited by incipient knock at wide-open throttle. Knock in turn depends on the molecular structure of the fuel and on such engine features as the temperature of the combustible mixture prior to ignition, the geometry and size of the combustion space, and the ignition timing. For example, isooctane, benzene, and alcohol can be burned at much higher compression ratios than n-heptane. In compression ignition engines critical compression ratio is that necessary to ignite the fuel and depends on fuel and cylinder geometry.