Week-6 : Turbocharger Modelling

Modelling of Turbocharger in GT-Power

Aim

To analyse the models of different types of turbochargers using GT-Power

Introduction:

Methods of Power Boosting:

The maximum power a given engine can deliver is limited by the amount of air introduced into each cylinder during each cycle. If the induced air is compressed to a higher density than ambient, before the entry into the cylinder, the maximum power an engine of fixed dimensions can deliver will be increased [1].

As seen from the equations below, the power, torque and the mean effective pressure of an engine are proportional to the inlet air density (`rho_(a,i)`).

Power:

`P = (eta_f * eta_v * N * V_d * Q_(HV) * rho_(a,i) * (F/A))/2`

Torque:

`T = (eta_f * eta_v * V_d * Q_(HV) * rho_(a,i) * (F/A))/(4*pi)`

Mean Effective Pressure:

`"mep" = eta_f * eta_v * Q_(HV) * rho_(a,i) * (F/A)`

Thus, increasing the inlet air density increases the power, torque and mean effective pressure of an engine. The three basic methods of power-boosting include

Supercharging:

It increases the air density using a compressor which is driven by power taken from the engine

Turbocharging:

It uses a compressor-turbine configuration to increase the air density. The energy available in the exhaust is used to drive the turbine, which drives the compressor.

Pressure wave supercharging:

It uses the wave action in the intake and the exhaust systems to compress the intake mixture.

Among the basic methods, the turbocharger and its modelling are studied in detail in this case study.

Turbocharger:

“A turbocharger is a turbine-driven, forced induction device that increases the internal combustion engine’s power output by forcing extra compressed air into the combustion chamber” [2]. A schematic sketch of a turbocharger is shown in the figure below. The use of a turbocharger increases the power output of an engine by 30 – 40% [3]

Working Principle:

The turbocharger consists of a compressor and a turbine connected via a common shaft. First, the compressor takes the air from the atmosphere, compresses it and then supplies the compressed air to the cylinders. Then, after combustion, the exhaust from the cylinders expands in the turbine, rotating it before exiting into the atmosphere. As the turbine rotates, it also rotates the compressor via the common shaft.

Advantage and Disadvantages:

The key difference between the turbocharger and the conventional supercharger is that superchargers are mechanically driven by the engine. In contrast, the turbochargers are powered by a turbine driven by the engine’s exhaust. Therefore, the turbocharger reduces the mechanical load on the engine.

The primary disadvantage of a turbocharger is the lag or spool time. It is time taken by the turbocharger to change the power output in response to a throttle change when accelerating as compared to the naturally aspirated engine. It occurs because of the reliance of the turbochargers on the build-up of exhaust gas pressure. At idle, low engine speeds, the exhaust gas pressure is usually insufficient to drive the turbine [2].

Different types of Turbochargers:

The following are the common types of turbochargers used in automobiles:

Single- Turbo:

This type of turbocharger has only a single turbine with fixed geometry. Therefore, different torque characteristics can be obtained by differing the wheel size of the compressor and the turbine.

They are cost-effective and simple to install, but they have a narrow effective RPM range. The response of the single-turbo may not be as fast as the other types.

Twin-turbo:

This type of turbocharger has two turbines. For example, in a 6- or 8-cylinder engine, each turbine is connected to one cylinder bank. In addition, the two turbines can be connected in parallel or series, depending on the application. The main advantage of the twin-turbo is that it reduces the turbo-lag

Twin-scroll:

In this type of turbocharger, the turbine inlet casing and the exhaust manifold is divided such that the correct engine is connected with the right turbine inlet. For example, in a four-cylinder engine with the firing order 1-3-4-2, cylinders 1,4 are connected to one inlet and the rest to the other. This type of connection helps prevent the overlapping of the exhaust pulse and ensures that only a single pulse enters the turbine at any given time [4]. The following figure shows the distribution of the exhaust pulse in a 4 cylinder engine.

The overlapping of the exhaust pulse will lead to a decrease in the exhaust pressure and its kinetic energy. Therefore, by separating the overlapping pulse to separate inlet, this problem is avoided.

The advantages of the twin-scroll turbocharger in comparison with a single scroll include

• High turbine inlet energy
• Good performance at low-medium engine speed and load
• Good performance during transient engine operation

The disadvantages include

• Poor efficiency at high engine speed and load
• Complex and expensive exhaust manifold and turbine casing

Variable geometry turbo (VGT):

These turbos, also known as variable nozzle turbines, are a type of turbochargers, usually designed to alter the aspect ratio of the turbine to be altered as the conditions change. For example, 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, a low boost threshold, and high efficiency at higher engine speeds [6].

The figure below shows a schematic representation of the variable geometry turbine. The image coloured in blue represents the changing vanes, which alter the aspect ratio.

It provides a wide and flat torque curve, but it is expensive.

Variable Twin Scroll Turbo:

This type of turbocharger is a combination of the twin-scroll and variable geometry turbos [5].

Electric Turbo:

These types of turbochargers are the latest improvements in the turbocharger industry. They help reduce the turbo-lag and assist a normal turbocharger at low engine speeds. This is accomplished by connecting an electric motor, which rotates the compressor from the start and lower engine speeds until the exhaust volume is sufficient to make the turbocharger work [5]. The energy consumed by the compressor at low engine speeds can be recovered by connecting an electric motor to the turbine. Its major drawback is its cost.

In addition to the major turbocharger types, some external technologies can be installed with the turbocharger to improve its performance [2]. Two of these technologies are

Intercooling:

As the inlet air is compressed, its temperature also increases, which will reduce the density of the compressed air. Hence an additional intercooler is installed downstream of the compressor to cool the compressed air, increasing its density further.

Wastegate Turbocharger:

A wastegate is a device used to regulate the amount of exhaust gas entering the turbine. Thus, it helps prevent over-boosting in the turbocharger. When the inlet pressure overshoots a certain limit, the wastegate is opened (usually by a pneumatic controller), and the turbine power is altered.

Turbocharger Modelling:

Models in GT-Power:

GT-Power has in-built examples of some of the turbocharger types.

Single fixed geometry Turbocharger:

Variable Geometry Turbocharger

Power Turbo:

Wastegate controller Turbocharger

Important Parameters:

When selecting a turbocharger for a given application, care should be taken such that the operating point lies within the operating range of a compressor and turbine map. A compressor (turbine) map is a chart that shows the compressor's performance (turbine).

If the operating point lies beyond the surge line, then it means that the given compressor is too big for a given mass flow. Therefore, the required pressure ratio cannot be achieved. On the other hand, if the operating point lies outside the choke line, then it means that the compressor is too big for the given mass flow. As a result, the compressor will reach its maximum speed without achieving the required pressure ratio. Hence, a compressor should be selected such that all the operating points in a given application lie within the surge and the choke lines, as shown in the figure below.

Single Turbo fixed geometry Turbocharger:

A simple model of a 6 cylinder compression ignition engine was modelled with a single turbine fixed geometry turbocharger in GT-Power.

Compressor Input

Compressor configuration:

Shaft initial conditions:

Turbine configurations:

The above model was simulated for 3 cases where the influence of altitude on the turbochargers was studied. At the higher altitudes, the density of the gas is lower. Hence a turbocharger operating at the same speed cannot achieve the required compression. This will, in turn, lead to a lower Air-fuel ratio and hence lower power output and high soot emissions. To avoid this, the fuel injection should be lowered so that the air-fuel ratio remains the same.

Case setup:

Results:

The compressor and the turbine efficiency maps are plotted to ensure that all the operating conditions are inside the efficiency islands. As they are inside the efficiency island, this configuration of the compressor and the turbine can be used for our application. If not, then we need to change the configuration with either a larger or smaller compressor depending on the operating point's position.

As the airflow rate is lower at higher altitudes, the power at higher altitudes decreases, as seen from the table above.

4 cylinder CI VGT:

Model:

In this model, the rack position is controlled by a controller, which depends on the injection rate and the engine speed.

The top row is the engine speed, and the first column represents the injected mass. Depending on these two values, the target boost pressure is determined using a table as shown below:

The model was simulated for the following cases.

Results:

Efficiency Map:

Applications of VGT:

The VGT is best suited for passenger car applications where driving comfort has a high priority. This is because the VGT helps avoid the lag, thereby providing better driving comfort. Other than that, turbochargers are also used in trucks, motorcycles and aircraft, where the cost of VGT can be justified.

VGTs tend to be much more common on diesel engines because of the lower exhaust temperatures. However, recently VGTs are also used in gasoline engines as the VGTs have improved their resistance to higher temperature exhausts, thanks to recent technological advancement [6].

Conclusion:

Different types of turbochargers were studied and a single turbine fixed TC was successfully modelled and simulated.

References:

1. Heywood, John B. Internal combustion engine fundamentals. McGraw-Hill Education, 2018.
2. Turbocharger - Wikipedia
3. How Turbochargers Work | HowStuffWorks
4. Twin-scroll turbochargers – x-engineer.org
5. Which Types of Turbochargers is Best? with Pros & Cons [PDF] (theengineerspost.com)
6. Variable Geometry Turbocharger (VGT) – x-engineer.org