Variable nozzle turbos were first launched in the industry by Garrett in the mid 90’s and are now found on most diesel engines. It was the introduction of variable nozzle (also known as variable geometry) technology in turbos at the same time as common rail fuel injection systems, which led to the huge increase in popularity of small displacement diesel engines in passenger cars.
Over the last 20 years regulations to reduce emissions have continued to tighten. As a result turbo technology has increased in complexity, and the settings and control advancements, such as variable nozzles have become more critical to the correct operation of the turbo.
A VNT turbo needs to be set correctly so that it produces the correct boost pressure as the engine speed and power demand changes. With incorrect boost pressure, both engine performance and emissions will suffer, and may result in an ECU shutdown putting a vehicle into limp-home mode.
This change has presented new challenges to turbo repairers as the correct setting of the turbo on later models now required specialist equipment in the form of an air flow rig
When a turbocharger is matched to an engine, the engineers have to balance the low-speed response with the high speed efficiency. The variable nozzle is designed to change the exhaust gas inlet area with engine speed to closely match the desired boost requirements of the engine. For low-speed response, the nozzle vanes move to the ‘closed vane’ position to reduce the nozzle area – this increases gas speed through the turbo giving improved response at low engine speeds – similar to squeezing the end of a hose pipe to make the jet of water more powerful. As the engine speed increases, the actuator moves the nozzle vanes to the fully open position to maximize the exhaust gas flow.
During the manufacture of a new complete variable nozzle turbo, vane positions are set using accurate air flow equipment, which ensures that the ‘minimum vane opening’ position is set to allow a specific mass of air flow through the vanes. If the vanes are too closed, this can cause choking of the engine and over speeding of the turbine. If it is set to open, the turbo will have too much ‘lag’ and not respond as well as it should
When we repair or rebuild a variable vane turbo – it’s the same process. The need to set the accurate air flow of a turbo has been well understood by reputable repairers, who recognised it is not acceptable to guess at the vane setting position.
As a specialist turbo repair shop, we believe it is vital to run the most up to date G3 Air flow equipment and provide our customers with the flow report- ensuring the best results from your variable vane turbocharger.
