Assignment 6

Aim

To list down different Turbochargers and to study the Diesel VGT EGR and understand the modeling part.

Different Types of Turbocharger

• Wastegate Turbocharger
• Variable GeometryTurbocharger
• Fixed Geometry Turbocharger
• E-Turbocharger
• Twin Turbocharger

Wastegate Turbocharger

The Wastegate turbocharger consists of a bypass valve. It circumvents some part of the exhaust gas going to the turbine and releases them into the outlet. Furthermore, the waste-gate turbocharger has a bypass valve built into the turbine housing. It diverts some of the exhaust gases away from the turbine wheel through this valve. In addition, the wastegate valve prevents the turbocharger from over-running.

The waste-gate regulates the pressure of the relief valve. It, in turn, limits the boost pressure in the turbocharger system. This is helpful in preventing the engine from potential mechanical damages caused by the high pressure. Furthermore, the system automatically opens the waste-gate valve when the pressure reaches the pre-set levels. Then, it allows all the high-pressure exhaust gases to escape the turbine wheel and enter into the downstream/outlet. Thus, it prevents the exhaust gas pressure from rising more than required.

Advantage

1. Needs a smaller space to fit.
2. Reduces turbo lag to some extent.
3. Installation of compact and simple external exhaust pipe system. Thus, reducing the engine weight.
4. Delivers optimum engine performance at all times.
5. Avoids mechanical damage to engine parts.

Variable Geometry Turbocharger and its Beneficials

A Variable Turbine Geometry turbocharger is also known as a variable geometry turbocharger (VGT), or a Variable Nozzle Turbine (VNT). A turbocharger equipped with Variable Turbine Geometry has little movable vanes which can direct exhaust flow onto the turbine blades. The vane angles are adjusted via an actuator. The angle of the vanes vary throughout the engine RPM range to optimize turbine behaviour.

At low speeds, the vanes close, which:

• Restricts exhaust airflow through the turbine
• Increases turbine power
• Increases boost pressure

At high speeds, the vanes open, which:

• Maximizes exhaust gas flow
• Avoids turbo over-speed
• Maintains required boost pressure

Advantages

• Wide, flat torque curve. Effective turbocharging at a very wide RPM range.
• Requires just a single turbo, simplifying a sequential turbo setup into something more compact.
• Reduces the fuel consumption.
• Reduces exhaust smoke emission and hence air pollution.
• Lower’s CO₂ , Nox, and Sox emission

Fixed Geometry Turbocharger

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.

Advantages 

• Simple design
• Robust
• Durable and
• Efficient

E-Turbocharger

In the E-Turbo, the electric motor is placed on the shaft between the impeller and the turbine. Exhaust gases still also power the turbo, but the electric motor is active at low speeds when the flow of exhaust gas is not sufficient to bring the turbine up to speed quickly enough to prevent turbo lag.

Advantages

• By directly connecting an electric motor to the compressor wheel, turbo lag and insufficient exhaust gases can be virtually eliminated by spinning the compressor with electric power when needed.
• By connecting an electric motor to the exhaust turbine, wasted energy can be recovered (as is done in Formula 1).
• A very wide effective RPM range with even torque throughout.

Twin Turbocharger

Twin-turbo refers to an engine in which has two turbochargers to compress the intake fuel/air mixture.It is divided into 2 types Parallel twin turbo, Sequential twin turbo and Staged turbocharging

• Parallel Twin Turbo

This is the standard  twin-turbocharging which uses two turbochargers of the same size to work together to force air as quickly as possible into the cylinders. The exhaust gasses recycled to the turbos are split equally between the two but usually combine again in a common inlet before entering the cylinders.The benefit of this simplistic system is the potential for much less turbo lag than from one large turbocharger doing all the work.

In V-engines, each turbocharger is generally assigned its own bank of cylinders, instead of one large turbocharger having to force air through convoluted plumbing to make its way around the engine bay to the required cylinders. The lack of lag also occurs due to the convention to use slightly smaller turbochargers when parallel twin-turbocharging, replacing one large turbo that will have larger vanes. This makes the spooling process much easier for the incoming air.

• Sequential Twin Turbo

This setup uses two different sizes of turbochargers; a small-vaned turbo for low exhaust gas flow at lower engine speeds and then a much larger second turbo to take over once it’s had a chance to spool up.

A compression valve sits in front of the large turbo, making sure that all of the lower energy exhaust gasses produced at the bottom end of the rev range are isolated to the smaller turbocharger to maximise power delivery at a rev range once useless to most single turbocharger setups. As the engine speed rises, the compression valve is opened slightly, allowing the larger turbine to begin to spool. The valve is then triggered to open fully at a set volume of airflow, allowing the secondary turbo to maximise its efficiency.

• Staged Turbocharging

Using the same principles as a sequential setup, staged turbocharging uses a ‘stepped’ process to build up air compression to extremely high levels before entering the engine’s cylinders. Starting with a small turbocharger, the air is passed directly to a slightly larger turbo which compresses the air further. The final boost pressure in a staged system can be much larger than a normal twin-turbo system but is fairly catastrophic when it comes to lag. This is why it is generally used in diesel engines with high compression ratios and low rev ranges.

Advantages

• For parallel twin turbos on ‘V’ shaped engines, the benefits (and drawbacks) are very similar to single turbo setups.
• For sequential turbos or using one turbo at low RPM and both at high RPM, this allows for a much wider, flatter torque curve. Better low-end torque, but the power won’t taper at high RPM like with a small single turbo.

Tutorial 6 

The tutorial 6 has no turbocharger maps but the inlet environment is defined for certain conditions which replicates an compressor environment.The outlet environment is similiar to the inlet enviornment where the exhaust gases from the cylinders are send to the two outlet environment which feed to the turbine.   

Tutorial 7

The tutorial 7 shows the procedure for the turbocharger construction from the turtorial 6 in step by step process.

In step 2 the compressor is placed in the in between the cooler inlet and the inlet environment. In step 3 the the turbine is connected from the two outlet environment,where the exhaust gases are sent. In step 4 the the turbine and compressor are connected by a shaft to rotate the compressor to compress the air making a high dense and high pressure air from STP condtitons.    

Compressor Map Data

Above is the compressor data map,where the controls the mass flow rate of air at certain speed of the compressor. The data is given by manufacturer. It takes in account of speed, mass flow rate,pressure ratio and efficiency.   

Turbine Map Data

Above is the turbine data map where the data is given by manufacturer. It takes in account of speed, mass flow rate,pressure ratio and efficiency.

Diesel VGT EGR  

It is a 4 cylinder 2L diesel-EGR-VGT engine.The turbocharger consist of a fixed geometry rack compressor and 5 variable geometry rack in the turbine.The VGT rack position is determined by a control system to achieve a target boost pressure upstream of the intake manifold.The model includes an exhaust gas recirculation (EGR) system to transport exhaust gases from the exhaust manifold back to the intake manifold which is contolled by a EGR controller.The Exhaust system consist of diesel particulate filter and a muffer. 

Compressor Data Map

Above is the compressor data map,where the controls the mass flow rate of air at certain speed of the compressor. The data is given by manufacturer. It takes in account of speed, mass flow rate,pressure ratio and efficiency. 

Compressor Efficiency Map

 Here we can see the operating points in the compressor with respect to the pressure ratio and mass flow rate.The efficiency of the compressor at the peak rpm of  247700 to 260000 with a efficiency of around 57% to 65% at the pressure ratio of 2.4 to 2.6. 

Turbine Data Map

This is the Turbine Data of 1 of the variable 5 racks in the turbine,where rack changes with respect to pressure boost.  

Turbine Efficiency Map

The efficieny of the turbine at reduced rpm of 8072 with a pressure ratio of 2.95 is around 57% to 61%.