Engine Performance: System: Turbocharger: Operation
At idle and light throttle, turbocharged engine operates like any other diesel engine. When more power is required, the usually wasted exhaust gases from exhaust manifold enter turbocharger turbine housing and flow through turbine blades.
Exhaust flow and turbine speed increase as engine RPM increases. The impeller turns with turbine and forces air into compressor housing and intake manifold. The faster impeller and turbine spin, the more pressurized air is provided to engine.
If boost pressure were to go too high, engine damage would result. The wastegate opens when exhaust pressure exceeds a predetermined limit and allows exhaust gases to by-pass compressor. A drain pipe is connected from wastegate diaphragm housing to impeller housing. This prevents build-up of oil or blow-by from crankcase ventilation system.
A boost pressure safety valve (blow-off valve) is used for protection in case wastegate fails. The blow-off valve is connected to intake manifold, and opens whenever boost pressure exceeds 11.5 psi (.8 kg/cm2 ). The excess boost pressure is vented to intake pipe after the air cleaner. Some vehicles are equipped with a warning lamp that is connected to blow-off valve. The warning lamp will light whenever blow-off valve operates.