Historically many designs of blower have been used, from separate pumping cylinders, 'top hat' pistons combining two pistons of different diameter the larger one being used for scavenging, various rotary blowers and centrifugal turbocompressors, including turbochargers. Turbocharging two-stroke engines is difficult, but not impossible, as an exhaust-driven turbocharger does not provide any boost until it has had time to spin up to speed. Purely turbocharged two stroke engines may thus have difficulty when starting, with poor combustion and dirty exhausts, possibly even four-stroking.