Turbocharger Modelling using GT-POWER

Objective

To study and model different types of turbocharger using GT-POWER

Descreption

Forced Induction vs naturally aspirated engines

If more air is available then more fuel can be burned producing more power.

In naturally aspirated engines, the atmospheric air enters the engine at atmospheric pressure and due to throttling losses 100 % volumetric efficiency cannot be achieved. The image shown below shows a volumetric efficiency of 80 %.

Fig.(1)- Volumetric efficiency visualization

However, in forced induction we can achieve volumetric efficiency greater than 100% because of high pressure air generated by the compressor.

In IC Engines, the forced Induction can be achieved in two ways as shown below:

• Supercharger

• Turbocharger

In superchargers, a compressor is driven by connecting it with a belt to the engine crankshaft. The compressor compresses the incoming air and hence increasing its density and therfore increasing the engine's volumetric efficiency.

Fig.(2)- Turbocharger

The above image shows a turbocharger. A Turbocharger contains a turbine mechanically connected with the compressor. During exhaust stroke the high pressure exhaust gases runs the turbine wheel which in turn runs the compressor.

The turbocharger also contains an intercooler which decreases the air temperature and hence increasing the density.

The major difference between a supercharger and a turbocharger is the source of power to drive the compressor. Superchargers directly uses the engine output and hence affects the engine power output while turbochargers uses the exhaust gases which if not is simply wasted through the exhaust outlet. However turbo lag (the amount of time it takes a turbo to spool up to the minimum RPM to provide boost to the engine) is the problem with a turbocharged engine.

Types of turbochargers:

1) Single turbo

In this type we either have a single small turbo or a single big turbo. A single turbo can't be effective across the whole revolution range.

2) Sequential turbo

One small turbo fitted with one large turbo results in a wide and flat torque curve. However the engine setup becomes expensive, heavy and complex.

3) Twin scroll turbo

For a multicylinder engine, say a four cylinder engine with firing order 1-3-4-2, the cylinders 1 and 4 have a common exhaust pipe and cylinders 2 and 3 have a second exhaust pipe. Both pipes transport the the exhaust gas up to the turbine inlet. Since it uses two pipes for the exhaust gas, the system is called twin-scroll turbocharging.

4) VGT Turbo

The variable geometry turbine(VGT) allows significant flexibility over the pressure ratio across the turbine. A VGT has a ring of aerodynamically shaped vanes in the turbine housing that can alter their area-to-radius ratio to match the revolutions of the engine.

5) Variable twin scroll turbo

A variable twin-scroll turbo combines a VGT with a twin-scroll setup.

6) Electric turbo

A very recent development is the introduction of turbos with electric compressors. The compressor provides instant boost to the engine, until the turbocharger has spooled up enough. However it again makes the engine expensive and complex.

Turbocharger Modelling

Tutorial-6

Fig.(3)- 4 cylinder Turbocharged Diesel engine

In this tutorial, the complete turbocharger is not modelled, However the Turbine inlet and Compressor exit boundary conditions are defined.

Turbine inlet pressure=2.5 bar

Compressor outlet pressure=2.4 bar

Tutorial-7

Fig.(4)- 6 cylinder Turbocharged Diesel engine

In this tutorial the entire turbocharger setup is modelled. The turbine is connected to the compressor through the shaft.

Engine Geometry:

Compressor Map:

The compressor map takes compressor map data, typically supplied by the compressor's manufacturer. This compressor map will actually give us the operating point of our compressor. Hence we can find out whether the compressor is giving the required performance and within the desirable operating conditions.

Fig.(5)- Operating point of the Compressor at Engine speed=3600 RPM

It is mandatory that the compressor has to operate within the specified range of operating points given in the compressor map. The blue dot in the above image shows the operating point of the compressor.

Fig.(6)- Engine speed=3600 RPM

Turbine map:

The turbine map contains operating points of the turbine taken as user input, which is interpolated to find the operating point of the turbine. Using the turbine map, power output, mass flow rate and gas outlet temperature can be found.

Fig.(7)- Operating point of the Turbine at Engine speed=3600 RPM

The black dot in the above image indicates the operating point of the turbine.

These maps are always desirable for us to find out whether the compressor and turbine are operating within the desired operating conditions. The operating point is also helpful in carefully choosing the type of turbocharger required to get the required pressure ratio.

Fig.(8)- Compressor and Turbine operating conditions at Engine speed=3600 RPM

Variable Geometry Turbine:

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).

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.

Fig.(9)- A/R ratio- Turbine

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).

Fig.(10)- Variable Geometry Turbine

The variation of the cross-sectional flow area of the turbine is achieved by the rotating vanes (3), which in turn is helpful in achieving different pressure ratios.

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).

Fig.(11)- 4 cylinder Turbocharged (VGT) Diesel engine

Fig.(12)- Engine speed=1500 RPM, rack position=0.15

Fig.(13)- Engine speed=2000 RPM, rack position=0.5

Fig.(14)- Engine speed=3000 RPM, rack position=0.8

Fig.(15)- Engine speed=4500 RPM, rack position=0.95

The above tables show the pressure ratio at different engine speed at different rack positions. It can be seen that as engine speed increases the rack position increases and pressure ratio also increases.