Variable-geometry turbocharger

Variable-geometry turbochargers (VGTs), (also known as variable nozzle turbines/VNTs), are a family of turbochargers, usually designed to allow the effective aspect ratio (A:R) of the turbo to be altered as conditions change. This is done because optimum aspect ratio at low engine speeds is very different from that at high engine speeds. If the aspect ratio is too large, the turbo will fail to create boost at low speeds; if the aspect ratio is too small, the turbo will choke the engine at high speeds, leading to high exhaust manifold pressures, high pumping losses, and ultimately lower power output. By altering the geometry of the turbine housing as the engine accelerates, the turbo’s aspect ratio can be maintained at its optimum. Because of this, VGTs have a minimal amount of lag, have a low boost threshold, and are very efficient at higher engine speeds. VGTs do not require a wastegate.

VGTs tend to be much more common on diesel engines as the lower exhaust temperatures mean they are less prone to failure. The few early gasoline-engine VGTs required significant pre-charge cooling to extend the turbocharger life to reasonable levels, but advances in material technology have improved their resistance to the high temperatures of gasoline engine exhaust and they have started to appear increasingly in, e.g., gasoline-engined sports cars.[1]


Volvo FM VGT diesel engine with EGR emission technology

Most common designs

The two most common implementations include a ring of aerodynamically-shaped vanes in the turbine housing at the turbine inlet. In general, for light-duty engines (passenger cars, race cars, and light commercial vehicles), the vanes rotate in unison to vary the gas swirl angle and the cross sectional area. In general, for heavy-duty engines, the vanes do not rotate, but instead the axial width of the inlet is selectively blocked by an axially sliding wall (either the vanes are selectively covered by a moving slotted shroud or the vanes selectively move vs a stationary slotted shroud). Either way, the area between the tips of the vanes changes, leading to a variable aspect ratio.


The vanes are controlled by a membrane vacuum actuator, electric servo actuation, 3-phase electric actuation, hydraulic actuator or air actuator using air-brake system pressure.

Main suppliers

The invention introducing the VNT was developed under Garrett (Honeywell) (called Allied-Signal at the time). Several companies supply the rotating vane type of variable-geometry turbocharger, including Garrett (Honeywell), Borg Warner, and Mitsubishi Heavy Industries (MHI). The rotating vane design is mostly limited to small engines and/or to light-duty applications (passenger cars, race cars and light commercial vehicles). Main supplier of the sliding-vane-type of variable-geometry turbocharger is Cummins Turbo Technologies (Holset).

Other common uses

In trucks, VG turbochargers are also used to control the ratio of exhaust recirculated back to the engine inlet (they can be controlled to selectively increase the exhaust manifold pressure until it exceeds the inlet manifold pressure, which promotes exhaust gas recirculation aka EGR). Although excessive engine backpressure is detrimental to overall fuel efficiency, ensuring a sufficient EGR rate even during transient events (e.g., gear changes) can be sufficient to reduce nitrogen oxide emissions down to that required by emissions legislation (e.g., Euro 5 for Europe and EPA 10 for the USA).

Another use for the sliding vane type of turbocharger is as downstream engine exhaust brake (non-decompression-type), so that an extra exhaust throttle valve is not needed (turbo brake). Also, the mechanism can be deliberately modified to reduce the turbine efficiency in a predefined position. This mode can be selected to sustain a raised exhaust temperature to promote “light-off” and “regeneration” of a diesel particulate filter (this involves heating the carbon particles stuck in the filter until they oxidize away in a semi-self-sustaining reaction – rather like the self-cleaning process some ovens offer). Actuation of a VG turbocharger for EGR flow control or to implement braking or regeneration modes in general requires hydraulic or electric servo actuation.

History and examples of use

One of the first production cars to use these turbos was the Japanese 1988 Honda Legend and used a variable geometry turbo with an integrated water cooled intercooler installed on its 2.0 L V6 engine. There was also the limited-production (only 500 produced) 1989 Shelby CSX-VNT, equipped with a 2.2-liter Chrysler K engine. The Shelby CSX-VNT utilised a turbo from Garrett, called the VNT-25 because it used the same compressor and shaft as the more common Garrett T-25. This type of turbine is called a variable-nozzle turbine (VNT). Turbocharger manufacturer Aerocharger uses the term variable-area turbine nozzle (VATN) to describe this type of turbine nozzle. Other common terms include variable-turbine geometry (VTG), variable-geometry turbo (VGT) and variable-vane turbine (VVT).

In 1991, Fiat puts a VNT in the Croma’s direct-injected turbodiesel.[2]

The Peugeot 405 T16, launched in 1992, used a Garrett VAT25 variable-geometry turbocharger on its 2.0-liter 16-valve turbocharged engine.

The 2007 Porsche 911 Turbo has twin VNT turbochargers on its 3.6-liter horizontally-opposed six cylinder gasoline engine.

The Koenigsegg One:1 (a reference to the car’s HP to curb weight ratio of 1:1,) launched in 2015, uses twin variable geometry turbochargers on its 5.0-liter V8 engine, allowing it to produce an impressive 1361 horsepower and become the world’s first “Megacar.”

VGTs have been used on advanced turbo diesel engines for many years, primarily to compensate for performance loss when the engine is equipped with an EGR.

Typically VGTs are only found in OEM applications due to the level of coordination required to keep the vanes in the most optimal position for whatever state the engine is in, however there are aftermarket VGT control units available (such as the Suprock Technologies Turbo Commander) and some high-end aftermarket engine management systems can control these turbochargers as well.


  1. “Increasing the Efficiency of an Engine by the use of Variable Geometry Turbochargers” (PDF). IJIRSET. Retrieved 11 April 2016.
  2. “Turbo Pioneer”. Retrieved 2014-01-22.
Pressure wave supercharger A pressure wave supercharger (also known as a wave rotor) is a type of supercharger technology that harnesses the pressure waves produced by an internal combustion engine exhaust gas pulses to compress the intake air. Its automotive use is not widespread; the most widely used example is the Comprex, developed by Brown Boveri. Valmet Tractors of Finland were one of the first to use the device when they fitted it to the 411CX engine which powered their 1203 model of 1980. Although it provided a useful increase in performance it was considered too expensive to be incorporated into later models.  Ferrari tested such a device during the development of the 126C Formula One car. The system did not lend itself to as tidy an installation as the alternative twin-turbocharger layout, and the car was never raced in this form. A more successful application was in the RF series diesel enginefound in the 1988 Mazda 626 Capella; ultimately 150,000 Mazda diesel cars were fitted with a Comprex superchar...
Intercooler An intercooler is any mechanical device used to cool a fluid, including liquids or gases, between stages of a multi-stage compression process, typically a heat exchanger that removes waste heat in a gas compressor. They are used in many applications, including air compressors, air conditioners, refrigerators, and gas turbines, and are widely known in automotive use as an air-to-air or air-to-liquid cooler for forced induction (turbocharged or supercharged) internal combustion engines to improve their volumetric efficiency by increasing intake air charge density through nearly isobaric (constant pressure) cooling.   The intercooler (top) of this 1910 Ingersoll Rand air compressor extracts waste heat between the two compressor stages. Air Compressors Intercoolers are utilized to remove the waste heat from the first stage of two-stage air compressors. Two-stage air compressors are manufactured because of their inherent efficiency. The cooling action of the intercooler i...
Twincharger Twincharger refers to a compound forced induction system used on some piston-type internal combustion engines. It is a combination of an exhaust-driven turbocharger and an engine-driven supercharger, each mitigating the weaknesses of the other. A belt-driven or shaft-driven supercharger offers exceptional response and low-rpm performance as it has no lag time between the application of throttle and pressurization of the manifold (assuming that it is a positive-displacement supercharger such as a Roots type or twin-screw and not a Centrifugal compressor supercharger, which does not provide boost until the engine has reached higher RPMs). When combined with a large turbocharger — if the "turbo" was used by itself, it would offer unacceptable lag and poor response in the low-rpm range — the proper combination of the two can offer a zero-lag powerband with high torque at lower engine speeds and increased power at the higher end. Twincharging is therefore desirable for small-displacement mo...
Turbocharger A turbocharger, or colloquially turbo, is a turbine-driven forced induction device that increases an internal combustion engine's efficiency and power output by forcing extra air into the combustion chamber. This improvement over a naturally aspirated engine's power output is due to the fact that the compressor can force more air—and proportionately more fuel—into the combustion chamber than atmospheric pressure (and for that matter, ram air intakes) alone. Turbochargers were originally known as turbosuperchargers when all forced induction devices were classified as superchargers. Today the term "supercharger" is typically applied only to mechanically driven forced induction devices. The key difference between a turbocharger and a conventional supercharger is that a supercharger is mechanically driven by the engine, often through a belt connected to the crankshaft, whereas a turbocharger is powered by a turbine driven by the engine's exhaust gas. Compared with a mechanically driven ...
Supercharger A supercharger is an air compressor that increases the pressure or density of air supplied to an internal combustion engine. This gives each intake cycle of the engine more oxygen, letting it burn more fuel and do more work, thus increasing power. Power for the supercharger can be provided mechanically by means of a belt, gear, shaft, or chain connected to the engine's crankshaft. Common usage restricts the term supercharger to mechanically driven units; when power is instead provided by a turbine powered by exhaust gas, a supercharger is known as a turbocharger or just a turbo - or in the past a turbosupercharger. Roots type supercharger on AMC V8 engine for dragstrip racing History In 1848 or 1849, G. Jones of Birmingham, England brought out a Roots-style compressor. In 1860, brothers Philander and Francis Marion Roots, founders of Roots Blower Company of Connersville, Indiana, patented the design for an air mover for use in blast furnaces and other industrial app...