Variable Geometry Turbocharger (VGT)

Turbocharging  is the most common used technology in internal combustion engines for forced intake air induction. The main components of a turbocharger are the turbine and the compressor. The role of the turbine is to use the thermal and kinetic (twin-scroll turbochargers) energy of the exhaust gases and convert it into mechanical energy. The role of the compressor is to use the mechanical energy and compress the intake air in order to increase its density.

For a better understanding on what is a (fixed geometry) turbocharger and how turbocharging works, read the articles:

Due to the geometry and different speed range operation, there is a mismatch between the exhaust gas flow of the internal combustion engine and the radial flow of the turbocharger. If the geometry (flow area) of the turbine is designed to match the full speed and load of the engine (large area), at low and medium speeds, the response of the turbocharger will be poor. If the geometry of the turbine is matched for a fast response (small area), when the engine will operate at high speed, the choke limits might be reached and the turbocharger might overspeed or the intake air pressure may exceed the maximum limit.

An ideal turbocharger should be able to provide the required intake air pressure (boost) regardless of the operating point of the engine (speed and torque). This is not possible due to the fact that the speed of the turbocharger shaft depends on the mass flow of the exhaust gases, which depends on the engine operating point.

For a fixed geometry turbocharger, at low engine speed, the exhaust gas mass flow is low, therefore the speed of the turbocharger shaft is low, which means low air boost. On the other hand, at high engine speed, the exhaust gas mass flow rate is high, the speed of the turbochrger shaft high as well, which translates in high intake air boost (pressure).

Fluid flow through a pipe

In order to understand the working principle of a variable geometry turbocharger (VGT), we need to recall some hydrodynamics laws.

Imagine you have a pipe with variable diameter along its length.

Image: Continuity of fluid flow

A [m2] – area
v [m/s] – speed
p [Pa] – pressure

In the larger area A1 the fluid is going to have a certain mass flow rate [kg/s]. Since the mass of the fluid is conserved, in order to be able to pass the same mass through the smaller area A2, the speed of the fluid must be increased.

The following laws applies to a fluid flowing through a pipe with variable diameter:

$A_1 \cdot v_1 = A_2 \cdot v_2 = \text{const.} \tag{1}$

This means that, in order to have the result of the product constant, if the flow area decreases, the speed of the fluid must increase. This is called the continuity law.

There is another relationship involving also the pressure of the fluid. Assuming that the pipe is horizontal, or the flow sections are at the same height, the following relationship is applicable:

$\rho \cdot \frac{v_2^2}{2} + p_2 = \rho \cdot \frac{v_1^2}{2} + p_1 = \text{const.} \tag{2}$

ρ [kg/m3] – fluid density

This means that, in the section with lower fluid speed, in order to maintain a constant sum between terms, the pressure must be increased. This is called Bernoulli’s law.

To summarize, for a fluid flowing through two cross-sections with different areas, the following relationships are true:

$\begin{split} A_1 > A_2 \\ p_1 > p_2 \\ v_1 < v_2 \end{split}$

Turbocharger A/R ratio

An important geometric characteristic (parameter) of a turbocharger is the A/R ratio, where A stands for the turbine/compressor inlet cross-sectional area and R for the radius of the turbo centerline to the centroid of the area A.

The A/R ratio (area divided by radius) applies both for the compressor and turbine, but the major impact on the turbocharger performance in linked to the turbine A/R ratio.

 Image: Turbocharger A/R ratio (1) Image: Turbocharger A/R ratio (2) Credit: Honeywell Garrett

The flow capacity of the turbine depends on the A/R ratio of the housing and has significant impact on the overall performance of the turbocharger.

A small A/R ratio will increase the speed of the exhaust gas as it enters the turbine wheel, the compressor will spin faster and provide an increase of intake air boost. A negative effect of a small A/R ratio is the tangential flow of the exhaust gas into the turbine wheel, which reduces the flow capacity of the turbocharger. The effect is an increased backpressure in the exhaust manifold at high engine speeds, which translates into difficult gas exchange (exhaust gas vs. intake air) of the engine and reduced peak power.

A large A/R ratio will improve the flow capacity of the turbocharger at high engine speeds, reducing the backpressure in the exhaust manifold. This will improve the engine’s capacity to “breathe” (exchange gas) at high speed and push the peak power towards higher values. The drawback is that, at low and medium engine speeds, the exhaust gas velocity will be lower (because of bigger flow area) and the increase of intake air boost will be slower (turbo-lag).

Image: BV50 – variable geometry turbocharger (VGT) for gasoline engines
Credit: BorgWarner

For a better understanding, let’s take as example two turbochargers with different A/R ratios and the same base engine (6 cylinders with 3 liter capacity).

 A/R ratio Turbocharger characteristics Engine/vehicle performance 0.83 low flow capacityhigh flow velocityfast air boost responsehigh exhaust backpressure Engine:high low-end torquefast torque responselimited peak powerVehicle:fast accelerations from standstillfun to drivesuitable for urban driving 1.22 high flow capacitylow flow velocityslower air boost response (turbo-lag)low exhaust backpressure Engine:small low-end torqueslow torque responsehigher peak powerVehicle:poor accelerations from standstillhigher maximum speedsuitable for extra-urban driving

In a nutshell, variable geometry turbochargers (VGT) are combining the benefits of a small A/R ratio and a large A/R ratio into one unit, bringing together the advantages of both types.

Types of variable geometry turbochargers

Variable geometry turbochargers means variable A/R ratios. The only plausible way of getting a variable A/R ratio is by varying the cross-sectional area A of the exhaust gas flow. The radius R will always be constant.

Compared with fixed geometry turbochargers, variable geometry turbochargers are designed to:

• increase intake air boost pressure at low engine speed
• improve the response time of the turbocharger during transient engine operation phases
• increase the availability of the maximum engine torque
• prevent over-boosting at high engine speed
• reduce exhaust gas emissions and improve fuel economy

Depending on the turbocharger manufacturer, there are several technical solutions available in the automotive industry. Regardless of the mechanical system used, the outcome is the same: use movable components to provide a variable cross sectional area A, to get an overall variable A/R ratio.

The most common types of variable geometry turbochargers are:

• pivoting vanes
• moving wall
• sliding ring
• variable area

Pivoting vanes variable geometry turbochargers

Pivoting (rotating) vanes turbochargers are widely used in passenger vehicles applications and they are the most common type of variable geometry turbochargers (VGT).

Image: Variable geometry turbocharger – components

1. turbine casing
2. turbine wheel
3. vanes
4. unison ring
6. lever system
7. compressor wheel
8. compressor casing
9. pneumatic actuator

The variation of the cross-sectional flow area of the turbine is achieved by the rotating vanes (3). These are mechanically linked to an adjustable ring (5), which is controlled by the pneumatic actuator (9) through a mechanical lever system (6).

Depending on the operating point of the engine, the engine control module (ECM) is adjusting the air pressure in the pneumatic actuator, which is closing or opening the pivoting vanes.

 Image: Narrow vane opening Credit: Volvo Image: VGT (narrow vane opening) Credit: BorgWarner Image: Exhaust gas flow (narrow vane opening) Credit: BorgWarner

At low engine speeds, the vanes are in a narrow position, the cross-sectional area for the exhaust gas flow is small, the A/R ratio is at its minimum value and the velocity of the exhaust gas through the turbine at its maximum. This translates into high compressor speed and high intake air boost.

 Image: Wide vane opening Credit: Volvo Image: VGT (wide vane opening) Credit: BorgWarner Image: Exhaust gas flow (wide vane opening) Credit: BorgWarner

At high engine speeds, the vanes are in a wide position, the cross-sectional area for the exhaust gas flow is large, the A/R ratio is at its maximum value and the velocity of the exhaust gas through the turbine at its minimum. The compressor speed will be slower but enough to provide the required intake air boost.

Also, the flow capacity of the turbine is increased, which will decrease the exhaust gas backpressure and allow the engine to “breathe” normally.

The position of the vanes (A/R ratio) can be controlled between a minimum (fully closed) and a maximum (fully open) position. The exact position of the vanes depends on the operating point of the internal combustion engine (speed and torque) and is regulated by the engine control module (ECM) or powertrain control module (PCM).

 Image: GT17VNT variable geometry turbocharger Credit: Honeywell Turbo Technologies Image: GT17VNT variable geometry turbocharger Credit: Honeywell Turbo Technologies

The most common design of variable geometry turbochargers are using rotating vanes (airfoils) arranged like slats in a window blind around the turbine wheel. These vanes are moved to regulate the cross-sectional area of the exhaust gas flow through the turbine. The vanes are mounted in the turbine housing with one end pinned to the housing. The other end of the vane is connected through a pin to a plate called a unison ring. Rotation of this unison ring causes the all the vanes to revolve around the fixed pivot point.

Image: Variable geometry turbocharger (VGT) – pivoting vanes assembly

The pivoting vanes assembly is also known as a nozzle ring.

At high exhaust gas temperatures, the metal-to-metal dry friction between the vanes, pivots and ring can be problematic and cause the pivoting mechanism to stick. If they get stuck in an open position, the engine performance will be poor at low speeds. If the vanes get stuck in a closed (narrow) position, at high engine speeds there will be a significant exhaust gas backpressure, which will lead to over-speed and even to turbine failure.

The pivoting vanes design is most of the time used in diesel and gasoline applications for passenger vehicles.

Moving wall variable geometry turbocharger

Another way of obtaining a variable A/R ratio is by using a moving wall inside the turbocharger. The variable cross-sectional area will be created between the moving wall and the turbine casing.

Image: Moving wall variable geometry turbocharger (VGT) – operation
Credit: Cummins Turbo Technology

1. compressor wheel
2. shaft speed sensor
3. pneumatic actuator
4. fixed shroud plate
5. turbine wheel
6. sliding nozzle ring and vanes (moving wall)
7. push rod and bushes
8. operating yoke

In this design the moving wall (6) contains the nozzle ring, with the vanes being fixed at a constant angle. The position of the nozzle ring is relative to the turbine casing, its position being adjusted by the pneumatic actuator (3). When reducing the cross-sectional area, the vanes of the nozzle ring are entering in a fixed wall (4) through radial slots.

 Image: Sliding nozzle turbocharger – narrow Credit: Cummins Turbo Technology Image: Sliding nozzle turbocharger – wide Credit: Cummins Turbo Technology

At low engine speed, the nozzle ring is pushed to the right, reducing the cross-sectional area and the A/R ratio. This will force the increase of the exhaust gas speed, the turbocharger will spin faster and the intake air boost will increase.

When the nozzle ring (moving wall) is at its maximum left position, the cross-sectional area for the exhaust gas flow is at its maximum. The A/R ratio is also at its maximum value, with the engine operating at high speed.

Compared to the pivoting vanes desing, the moving wall variable geometry turbochargers have the advantage of less moving parts, which means less wear points and better reliability (less chances to fail). Moving wall design has the potential of a better efficiency at high exhaust flow. Not having multiple pivoting points, the exhaust gas leakage is reduced and the overall efficiency improved. The main disadvantage of the moving wall design is high manufacturing costs, mainly due to tight clearance and minimum contact between the nozzle ring vanes and the shroud plate openings.

The moving wall design is most of the time used in diesel applications for commercial vehicles. For example, Scania is using on its diesel engines applications a sliding nozzle variable geometry turbocharger (VGT).

Image: Variable geometry turbocharger (VGT) – slidevane
Credit: Scania

1. air intake
2. compressor wheel
3. charge air outlet
4. speed sensor
5. actuator
6. sliding nozzle-ring
7. turbine wheel
8. exhaust gas inlet
9. exhaust gas outlet

The geometry and gas flow in the variable geometry turbocharger is regulated by the sliding nozzle-ring, which is controlled by an electric actuator. This allows precise control of both charge-air to the engine and the flow of EGR.

The flow of intake air can be optimized throughout the working speed range of the engine. This means that the VGT can be used to improve engine response and low-end torque. It is also used to speed up gear changes with Scania Opticruise, by maintaining the turbine speed during gear changes.

Sliding ring variable geometry turbocharger

The siding ring design is similar to the moving wall architecture. The main difference is that the vanes are fixed in a static nozzle plate. The variation of the cross-sectional exhaust gas flow area is done by a moving (axial) ring.

 Image: GT17 sliding ring turbocharger Credit: Honeywell Turbo Technologies Image: GT17 sliding ring turbocharger Credit: Honeywell Turbo Technologies

In closed (narrow) position the sliding ring is close to the nozzle plate and all the exhaust gas flow is forced through the vanes. This is the position with the smallest A/R ratio, high shaft speed and high intake air boost.

When the sliding ring moves away from the nozzle plate, the exhaust gas partially bypasses the vanes assembly and enters the turbine directly. In this position the turbine has a higher A/R ratio, lower shaft speed and the compressor provides a lower air boost.

Variable area turbocharger

The pivoting vanes variable geometry turbocharger obtains a variable A/R ratio by rotating the vanes around their pivoting point. The main disadvantage of this technology is the complicated and high cost mechanical system.

Aisin Seiki designed a variable geometry turbocharger which has a much simpler mechanical system, therefore lowering the manufacturing cost and increasing reliability. The variable flow turbocharger (VFT) developed by Aisin Seiki is based on a variable area principle. The turbine casing has two scrolls, an inner scroll and a outer scroll. A central pivoting valve guides the exhaust gas flow through the inner vane, outer vane or both, depending on the operating point of the engine (speed and torque).

Along the turbocharger wall, between the inner scroll and the outer scroll, there are also some stationary vanes which help redirecting the exhaust gas flow in the turbine wheel.

Compared to a pivoting vanes variable geometry turbocharger, the number of components in a variable flow turbocharger is smaller. Also, there is only one moving part, the central valve, which allows the engine control module (ECM) to employ a simple control algorithm, similar to the one used for fixed geometry turbochargers with wastegate.

 Image: Variable flow turbocharger (VFT) – low flow rate Credit: Aisin Seiki Image: Variable flow turbocharger (VFT) – high flow rate Credit: Aisin Seiki
1. inner scroll
2. outer scroll
3. central flow control valve
4. stationary vanes

At low engine speed (low exhaust gas flow rate), the central valve (3) is fully closed and the exhaust gas is forced through the inner scroll (1), which has a smaller cross-sectional area and A/R ratio. In this state, there is no exhaust gas flow into the outer scroll although there are passages between the outer and inner scrolls, as the outer scroll (2) is regarded as a statically pressurized chamber.

At high engine speed (high exhaust gas flow rate), the central valve controls the amount of the exhaust gas entering the outer scroll. The gas entering the outer scroll is fed to the inner scroll through the stationary vanes and merged with the flow in the inner scroll. The direction of the flow to the turbine rotor is a combination of the vectors of the two flows. Varying the flow angle to the turbine rotor can control the turbine speed, and hence control the turbine inlet pressure (exhaust back pressure of the engine).

The variable flow turbocharger (VFT) is a much simpler and lower cost option compared to a pivoting vane or moving wall turbine variable geometry turbocharger. Japanese automotive manufacturers (Honda) have integrated the VFT in both gasoline and diesel engines.

In terms of actuation systems, variable geometry turbochargers have a pneumatic actuator or an electrical actuator. Despite the higher cost, electrical actuated turbochargers have faster response time and more precise actuation of the moving elements.

Image: Variable geometry turbocharger (VGT) – electric actuation
Credit: Audi

Compared with a fixed geometry turbocharger, a variable geometry turbocharger has the following advantages:

• higher low-end maximum torque: a variable geometry turbocharger can improve the maximum torque of the engine in the low-end area due to the ability of the turbocharger to provide a higher air mass quantity; this translates into more fuel being injected, hence a higher mean effective pressure and torque
• faster engine torque response: especially in the low speed area, the torque lag of the engine is minimized due to ability of the turbocharger to accelerate faster and provide the required intake air boost
• higher air-fuel ratio at low engine speed: the extra intake air boost gives a higher air-fuel ratio (more air available for combustion) which could help reducing exhaust gas emissions
• reduced throttling losses in the exhaust manifold: a variable geometry turbocharger doesn’t need a wastegate, since the exhaust gas flow is regulated by the pivoting vanes, sliding ring or central valve; therefore the throttling losses of the exhaust manifold are reduced, which increases the ability of the engine do “breathe” (perform gas exchange) with fewer losses
• improves exhaust gas recirculation (EGR) rates: for high pressure EGR systems, when the EGR valve is open, it’s important that the exhaust gas pressure is higher that the intake air pressure in order to have a gas flow; being able to increase the backpressure in the exhaust manifold, a variable geometry turbocharger improves the efficiency of an EGR system
• improves engine braking performance: when the engine is in overrun state (engine braking), if the A/R ratio of the turbine is small, the backpressure in the exhaust manifold will be higher; in this case, the engine braking torque will be higher since it would need to compress the air in the exhaust at a higher level
 Image: Boost pressure comparison Credit: BorgWarner Turbo Systems Image: Engine torque comparison Credit: Garrett Engine Boosting Systems Image: Engine power comparison Credit: Garrett Engine Boosting Systems Image: Fuel consumption comparison Credit: Garrett Engine Boosting Systems

AVNTTM – Advanced Variable Nozzle Turbocharger (Trade Mark: Garrett Engine Boosting Systems)

Studies performed by Garrett Engine Boosting Systems shows significant improvements in the torque curve of the engine, thanks to an improved control of the air-fuel ratio. For a given powertrain, the clutch engagement torque increased up to 45 % and the peak torque with over 30 %. These two improvements are directly related to the increased intake air flow generated by the AVNTTM at low engine speeds.

In addition, higher power ratings of up to 6 % have also been evaluated due the ability of the AVNTTM to reduce the boost levels at high engine speeds, thus reducing the engine cylinder firing pressure and thermal loading of the charge air cooler.

Fuel economy improvements, on the dynamometer, have also been demonstrated. The ability to optimize air-fuel ratio, minimize pumping losses and operate at higher efficiencies, all influence break specific fuel consumption in a positive way.

On diesel engines, at low engine speeds, smoke emissions can be reduced significantly due to the ability of the turbocharger to adjust the air-fuel ratio. NOx emissions can also be reduced thanks to the increased backpressure in the exhaust manifold. Negative pressure difference across the engine (exhaust manifold pressure higher than intake manifold pressure) increases the exhaust gas flow into the intake manifold.

Depending on the manufacturer, the variable geometry turbochargers have different acronyms, but all of them are achieving the same thing: a variable A/R ratio of the turbine:

• VGT – Variable Geometry Turbocharger (Cummins, Holset)
• VNT – Variable Nozzle Turbine (Honeywell Garrett Turbo Systems)
• VFT – Variable Flow Turbocharger (Aisin Seiki)
• VTG – Variable Turbine Geometry (BorgWarner Turbo Systems and ABB)
• VGS – Variable Geometry System turbocharger (IHI Turbo)
• VTA – Variable Turbine Area (MAN Diesel Turbo Systems)