Variable Geometry Turbochargers (VGT) also known as Variable Turbine Geometry (VTG) is one of the most extensively used turbine technology in diesel engines today. Turbocharger technology has come a long way in the past decades. Take a small displacement engine; bolt on a turbocharger and voila, horsepower! So, let’s find how how a turbocharger works.
In a turbocharger, the exhaust gas generated by the engine spins the turbine. The turbine wheel rotates the compressor via a shaft which in-turn forces more air (known as boost) into the combustion chamber. With proportionately higher amount of fuel being delivered, the engine can now produce more power without altering its inherent displacement. Turbochargers allow manufacturers to build small yet powerful engines, which produce more power; efficiently and reliably. Sounds great doesn’t it? Indeed!
But, conventional turbochargers also known as Fixed Geometry Turbochargers (FGT) have some drawbacks; too big and they take forever to spool up. This causes a ‘lag’ before the boost kicks in strongly to put you into motion quickly. Although a smaller turbo spools up quicker, it has a tendency to run out of steam just as quick. This results in unfavorable performance at either lower or higher engine speeds.
Feeling a bit confused? Ok, stay with me now!
Technically speaking, this is because the ‘Aspect Ratio (A/R) of a FGT is not variable’. What is an aspect ratio? You may certainly ask! It is the ratio of the exhaust intake area of the turbocharger to the radius of the turbine wheel. Basically, a smaller inlet area allows the turbo to spool up quicker. A larger inlet area increases the spool up time but at maximum flow level it allows the engine to breathe more easily. That’s not all; a conventional turbo requires a valve known as a ‘wastegate’ to regulate excess turbine speed by limiting the flow of exhaust gas through the turbocharger. So how do variable geometry turbochargers get around these issues?
Variable geometry turbochargers solves this issue with movable vanes (fins) to direct exhaust flow on to the turbine blades. The angle of the vanes is controlled via an actuator (a motor that works off pneumatic or hydraulic pressure). The advantage here is that the angle of the vanes is controlled remotely and there is no need for a wastegate. It helps in optimizing the behavior of the turbine across the powerband (engine RPM range). This reduces the ‘lag time’ to spool up the turbine to create boost quickly at lower engine speeds or between gear changes. It also improves the efficiency of the engine by allowing it to burn more air and fuel in every cycle.
Materials generally used to construct a variable geometry turbochargers cannot withstand the heat over prolonged usage. This is why these turbos are commonly found in diesel engines because petrol engines have a higher exhaust gas temperature. In India, variable geometry turbochargers are commonly used in popular diesel cars such as the Hyundai Verna and Maruti Swift.