Assignment 6

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.

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 engines rev range. The result is that the turbocharger improves fuel efficiency without a noticeable level of turbocharger lag.

4. Twin scroll turbo-It requires divided inlet housing and exhaust manifold that pairs the correct engine cylinder with each scroll. This layout provides more efficient delivery of exhaust gas energy to the turbo, and results and helps provide denser, purer air into each cylinder. More energy is sent to the exhaust turbine, meaning more power. Again, the cost involves more for addressing the complexity of a system requiring complicated turbine housings, exhaust manifolds, and turbos.

5. 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 engines exhaust. In a sequential setup, one turbocharger runs at low speeds and the second turns on at predetermined engine speed or load. Sequential turbochargers further reduce turbo lag but require an intricate set of pipes to properly feed both turbochargers. 

II. Tutorial 6:

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.

Tutorial 7:

The above case is run at 3600RPM and the results are as below:

Variable geometry turbine:

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.

Diesel VGT EGR :

Case study:

Results: