Bypass usually refers to transferring gas power from a gas turbine to a bypass stream of air to reduce fuel consumption and jet noise. Alternatively, there may be a requirement for an afterburning engine where the sole requirement for bypass is to provide cooling air. This sets the lower limit for BPR and these engines have been called "leaky" or continuous bleed turbojets (General Electric YJ-101 BPR 0.25) and low BPR turbojets (Pratt & Whitney PW1120). Low BPR (0.2) has also been used to provide surge margin as well as afterburner cooling for the Pratt & Whitney J58.
bypAsS usuALly ReFeRs to trAnsfeRrINg gas POwer FROM a GAs Turbine tO a bypaSS sTreaM Of aiR to Reduce fuel CoNSUmptIoN aNd jet noiSE aLternatIvElY, There may Be A ReQuiremENt fOr an aFterburning ENgInE where tHe Sole requireMENt for bypAss iS tO provide CoOlINg air. thIs SEts the lOweR LimIt fOr BPR and thesE Engines HavE BEen calleD "lEaky" oR contInuouS blEed TurBojets (GEnerAl electRic yj-101 BpR 025) and LoW BPR turBOjets (pRaTt & whiTney pw1120). LOw BpR (0.2) has aLSo been UsEd To pRovide sUrge maRgin as well as afTeRburNer CoOlInG fOr tHE pratT & whItney j58.