Variable Geometry Turbo (VGT) Systems

VGT systems, otherwise known as Variable Geometry Turbo Systems and Variable Nozzle Turbo boost control systems have been installed to a majority of diesel engine vehicles since the beginning of the 1990's. One feature of the control system is the variable turbo boost control, this has been designed in order to minimise "turbo lag" and to also provide a nearly constant turbo boost pressure at all engine speeds. The turbocharger in the boost control system has been fitted with a finned diffuser piston which in turn is operated by a vacuum controlled capsule, this allows you to achieve and obtain the turbo boost control effect.

Developed to increase the speed of the exhaust gases which strike the exhaust turbine at reduced engine speeds, the finned diffuser piston can also be changed to minimise the speed of the exhaust gases which strike the exhaust turbine at excessive engine speeds, this is done by changing the angle of he finned diffuser on the exhaust turbine.

At low engine speeds, the diesel engine management system control module (PCM) provides a control signal to a vacuum solenoid valve, this valve then opens which allows vacuum to be applied to the controlled capsules; this then leads to the finned diffuser piston being pulled towards the boost pressure control piston. When this takes place, the cross-section area between the exhaust gases and the exhaust turbine is closed, allowing the exhaust gases to pass through narrow cross-sectional areas before passing through to the exhaust turbine. Leading to an increase in the speed of the exhaust gases passing across the exhaust turbine, which in turn also increases the speed of the intake air turbine and increases the rate of boost pressure at  low engine speeds.

However on high speeds, the vacuum is released  on the vacuum controlled capsule. The finned diffuser is pushing open due to the spring tension inside the vacuum controlled capsule. When this happens, the cross-sectional area between the exhaust gases and the exhaust turbine is fully open. This then allows the exhaust gases to pass through a much larger cross-sectional areas before passing through to the exhaust turbine. This reduces the speed of the exhaust gases passing across the exhaust turbine, which in turn reduces the speed of the intake air turbine and reduces the rate of boost pressure at high engine speeds.