To understand turbocharging in IC engines and modelling of VGT turbocharger configuration in GT-Power

Objective:

To understand the turbocharging in IC engines and modelling of VGT turbocharger configuration in GT-Power.

Turbocharging :

A turbocharger, or turbo, is a gas compressor. It is used to force air into an IC engine. A turbocharger is a form of forced induction. It increases the amount of air entering the engine to create more power. A turbocharger has the compressor powered by a turbine. The turbine is driven by the exhaust gas from the engine. It does not use a direct mechanical drive. This helps to improve the performance of the turbocharger.

A turbocharger consists of a compressor wheel and exhaust gas turbine wheel coupled together by a solid shaft and that is used to boost the intake air pressure of an internal combustion engine. The exhaust gas turbine extracts energy from the exhaust gas and uses it to drive the compressor and overcome friction. In most automotive-type applications, both the compressor and turbine wheel are of the radial flow type. Some applications, such as medium- and low- speed diesel engines, can use an axial flow turbine wheel instead of a radial flow turbine. The flow of gases through a typical turbocharger with radial flow compressor and turbine wheels is shown below.

Supercharging :

Supercharging is the process of improving the volumetric efficiency of the engine by using the power of engine. In other words it is the process of improving the breathing ability of the engine. An supercharged engine gives better power for the same amount of fuel that a non-supercharged engine does. A turbine which acts as an compressor is connected to the crankshaft through a belt.When the combustion begins and the crankshaft rotates,the belt connecting the compressor rotates sucking in more air and improving the air fuel mixture.Thus improving the power output for the same fuel quantity.

Types of Turbocharging :

1. Fixed Geometry Turbocharger :

The simplest turbocharger design from a control perspective is one whose turbine and compressor geometry are fixed and that uses no means to control boost pressure. The boost pressure provided by this type of turbocharger is entirely determined by the engine exhaust flow and the characteristics of the turbocharger. The turbocharger is optimized for a particular operating condition. Turbocharger turbine size and/or A/R ratio tend to be relatively large for a given application because of the need to size the turbocharger so that at the highest flow conditions, the turbocharger does not overspeed or provide excessive boost pressure. While boost pressure near rated conditions can be selected via turbocharger sizing, transient response and boost pressure at lower engine speeds can suffer. Also, at high altitudes, turbocharger speeds would tend to increase which could lead to problems with surge and/or turbocharger overspeeding unless accounted for by oversizing the turbocharger. However, for some engine applications operating primarily at a limited number of steady-state conditions, an uncontrolled turbocharger with a fixed geometry turbine can prove entirely satisfactory.

2. Waste Gate Turbo Charger :

Adding a bypass valve that allows some of the exhaust gas to bypass the turbine is the more common means of achieving better boost pressure control with fixed geometry turbines. In most applications, this allows a smaller size or smaller A/R ratio fixed geometry turbine that is able to provide more power to the compressor at lower exhaust flows to be used for a given application.

3. Variable Geometry turbine :

Variable-geometry turbochargers use moveable vanes to adjust the air-flow to the turbine, imitating a turbocharger of the optimal size throughout the power curve. The vanes are placed just in front of the turbine like a set of slightly overlapping walls. Their angle is adjusted by an actuator to block or increase airf low to the turbine. This variability maintains a comparable exhaust velocity and back press ure throughout the engine's rev range. The result is that the turbocharger improves fuel effi ciency without a noticeable level of turbocharger lag.

4. 2-stage turbochargers:

2-Stage or Twin-turbo designs have two separate turbochargers operating in either a sequence or in parallel. In a parallel configuration, both turbochargers are fed onehalf of the engine's exhaust. In a sequential setup, one turbocharger runs at low speeds an d the second turns on at predetermined engine speed or load. Sequential turbochargers fu rther reduce turbo lag but require an intricate set of pipes to properly feed both turbochargers.

2.1. 6-cyclinder Turbocharged Diesel engine :

In the above example, a turbocharged 6-cylinder engine has been shown, instead of modelling the compressor and the turbine, the compressor output and turbine input pressures are defined.

The copressed air from the compressor is forced into a heat exchanger or inter cooler, where it is cooled inorder to increase the volumetric efficiency of the engine, more cooler the air, more will be the density making it compressed and more air will be forced in the chamber. The air is then divided into various manifolds of each cyclinder, where it is further supplied to the intake runner and introduced in the combustion chamber. After the combustion exhaust gases are pushed via the exhaust port to the entry of turbine. And finally are pushed to the enironment via turbocharger.

2.2 Modelling of 6-Cylinder turbocharged Diesel Engine :

In the tutorial number 7 from GT-Power, COmpressor and Turbine are added to the above model step by step and finally they are connected together as follows

After running the simualtion at 3600 RPM, we get the following speed and efficiency maps of compressor and turbine.

Here, half the speed is operated in the surge zone, hence stalling the compressor ultimately, either the speed of the compressor needs to be optimised or other compressor of higher operating points needs to be selected to sove this issue.

The un-favourable speed of compressor ultimately affects the efficiency of the compressor.

The Turbine for this application has been selected properly, as the opearting points lie in the speed map of the turbine, hence performing an efficient operation.

According to the graph, upto 80% of the efficiency can be reached by operating in the following region which is favourable for the turbine.

3. Modelling of Variable Geometry Turbine :

1. Turbine casing
2. Turbine wheel
3. Vanes
4. Unison ring
5. Adjustable ring
6. Lever system
7. Compressor wheel
8. Compressor casing
9. Pneumatic actuator

Above is the construction of a typical VGT,

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.

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.

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.

Typically, in the applications where we need low end torque, with optimal cost of production, this geometry is suitable. During low speeds, turbocharger suffers from turbo lag, which can be eliminated implementing a smaller turbo or twin turbo, but which also increases the cost factor, hence VGT can efficiently decrease the cost, maintaing the torque at both high and low ends such as in Sedans, SUVs and commercial cars.

Advantages of variable geometry turbochargers

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

Diesel VGT EGR model in GT-Power :

Here, the engine used is a 4-cylinder, four-stroke, direct injection, 2litre diesel engine. The intake system consists of the air piping to the the compressor inlet and contains a simple
model of an airbox. The airbox is modeled as a group of interconnected flow splits that
represent the total volume of the airbox. At the inlet and outlet of the air box, orifice connections are used to model smooth transitions. The intercooler is found
downstream of the compressor outlet. This intercooler is an air-to-air intercooler that is
modeled using a "semi-predictive" method based on heat exchanger effectiveness.
The intake manifold is modeled for 4 different ccylinders separately. The fuel is injected directly into the cylinders. The model includes an exhaust gas recirculation (EGR) system to transport exhaust gases from the exhaust manifold back to the intake manifold. The turbocharger in this model contains a fixed geometry compressor and a variable geometry turbine (VGT). An exhaust system is modeled downstream of the turbine outlet. The geometry of a Diesel Oxidation Catalyst (DOC) is modeled in the aftertreatment components.

Case setup was done for 4 cases as follows :

Following are the operating points of the compressor and turbine maps :