Differences between fixed geometry turbo and variable geometry turbo

The turbocharger, or turbo, has been a given component in a diesel engine for decades now. It forces air into the engine to improve the combustion process. In addition, it helps to enable a reduction of NOx levels, which is a prerequisite in order for vehicles in the industrial businesses to live up to the Stage V emission legislation. This is the fourth, and final, part in our article series about engine technology. In this article, I will describe the turbo’s function in an engine as well as outline the differences between fixed geometry turbo (FGT) and variable geometry turbo (VGT).  

An air pump system that regulates the amount of air in the engine

The turbo can be described as the engine’s lungs. It is an air pump system that forces air into the engine’s combustion chambers. Air and fuel are the two most basic and necessary components that a diesel engine needs to function since oxygen molecules are required for the fuel’s combustion process. A turbo consists of mainly three parts; a centrifugal compressor, a radial turbine and a bearing housing. The centrifugal compressor is designed to compress air. It is powered by a radial turbine, which is driven by the exhaust gases produced by the engine. The centrifugal compressor and the radial turbine are linked together via an axle through the bearing housing.

Airflow and workflow must be adjusted to reduce fuel consumption

When increasing the power output of an engine the airflow also needs to increase. When adding fuel to the engine, more exhaust energy is produced that increases the speed of the turbo, which in turn produces more air. When more air is added to the engine, even more fuel can also be added. This is a circular process where the inserted amount of fuel is regulated by how much air is available in the engine. Thus, the amount of air coming from the turbo must be adjusted according to the workflow of the engine. If the turbo produces more air than the combustion process requires the engine will cool down, which indirectly may increase the fuel consumption due to countermeasures taken in order to keep the exhaust after treatment system at the right temperature.

When the compressed air leaves the turbo at a high speed and pressure, the temperature of the air increases. Since hot air takes up a lot of space, the air is cooled down in an intercooler. As the air temperature decreases the density goes up, which facilitates the process of inserting the air in the engine. The compressed air is transported through a pipe to the inlet side of the engine and enters the combustion process.

Fixed geometry turbo (FGT)

The turbo reuses the heat energy coming from the combustion process

There are several advantages of using a turbo-driven engine. The turbo offers a lot of power to the engine and the power can be controlled based on the size of the turbo. The turbo controls the air that enters the combustion process and by adding the right amount of air, the fuel consumption can decrease. Also, it is efficient to use turbos in diesel engines since they reuse the heat energy coming from the combustion process. Another advantage with the turbo is that it can be used to create the pressure that is needed to transport the Exhaust Gas Recirculation to the inlet side of the engine.    

There are many different types of turbos, where the two most common types are; fixed geometry turbo (FGT) and variable geometry turbo (VGT).

FGT – a simple design where the turbine and compressor geometries are fixed

FGT has a simple design, where the turbine and compressor geometries are fixed and the boost pressure is entirely determined by the exhaust flow. An exhaust side bypass, or wastegate, is a common means to control the boost pressure with fixed geometry turbines. The wastegate can be built into the turbine side of the turbocharger or it can be a separate valve connected to the external piping. An FGT is suitable for specific industry applications where the vehicles are often driven within a narrow air flow range. The amount of air these industry applications need in their engines can often be produced with an FGT without having to adjust the airflow. The advantages with an FGT is that it is a simple design that is robust, durable and efficient. However, since the adjustment of the airflow is limited, the design has a limited flexibility and is most efficient in a narrow work area.    

FGT is usually combined with uncooled EGR, which is mainly used for heating purposes at low load. Since an FGT is optimized to work in a small flow range it doesn’t create a pressure that is sufficient enough to enable EGR at high engine loads to function.

Volvo Penta Stage V engine, Turbo

The VGT allows you to control the flow and operate with various speed and load

Compared to FGT, the airflow can be regulated with a VGT. When the exhausts enter the VGT it passes through a circle of aerodynamically-shaped vanes that can regulate the pressure of exhausts entering the turbo. Also, as the vanes change their angles they also change the angle and speed that the exhausts enter in, which could either increase or decrease the speed of the turbine. For example, when reducing the opening of the turbo, the speed of the exhausts increase as well as the pressure in the turbo. With this, the VGT provides the flexibility that enables different emission strategies and the airflow is automatically adjusted according to the speed and load of the engine, which also helps to decrease the fuel consumption. However, using a VGT to force the turbo to run in an off-design point will reduce its efficiency. On the other hand, if an emission strategy is used which allows the turbo to run in its most efficient area the VGT function may not be needed.

VGT is suitable to combine with cooled EGR. A high pressure is needed in order to transport the gas through the EGR’s cooler, and since VGT can adjust the airflow it can create the high pressure that is required in order for the cooled EGR to function.   

Volvo Penta Stage V engine

Volvo Penta’s Stage V solution has an FGT combined with uncooled EGR

The turbo is closely connected to the exhaust aftertreatment system (EATS) and the heat management system in the engine. The turbo provides air to the engine, which also is a key ingredient in order for the EATS and heat management system to be able to function and decrease NOx levels. Volvo Penta has chosen to use FGT combined with uncooled EGR in our Stage V solution. Combined with EATS and our heat management system, we believe this combination will benefit our industrial customers in the long run, and help them maintain high uptime and efficiency in their operations.        

I hope you enjoyed reading my blog article about the turbo and its different functions and variants. This was the last part in our article series about engine technology. Hopefully, you learned something new about the subject. Feel free to contact me with questions or visit our website for more information.

Also, don’t forget to visit our Professional Power blog where you can read the other three articles in the series: A combination of systems creates a modern engine technology, Exhaust aftertreatment system – a cleaning process for exhausts and A heat management system helps decrease NOx levels.

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