Assignment 6

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. It helps the engine to operate under high dense air than a naturally aspirated engine this increases the oxygen availability so the BSFC and Efficiency of the engine are also increased. The first turbocharger is introduced in 1915 and different technologies improved time by time the different Turbocharger technology trends are:

1. Wastegate turbocharger
2. Variable geomentry Turbine
3. Two-stage turbine
4. variable geomentry compression
5. e-Turbocharger.

1)Wastegate turbocharger: A wastegate is a device integrated into a turbocharger that controls the maximum allowable boost pressure. The concept of a wastegate is somewhat simple if you recall that turbocharger boost is directly related to the mass flow rate, pressure, and temperature of exhaust gases as they pass through the turbine housing of the turbocharger. It is at this stage that the heat energy (potential energy) of the engine exhaust is translated into mechanical energy by the turbine wheel. If exhaust flow is diverted such that it does not flow across the turbine wheel of a turbocharger then its Thermal energy is not converted by the turbine. Simply put, reducing exhaust flow across the turbine reduces and/or controls boost pressure. In a nutshell, a wastegate simply diverts exhaust gas flow around the turbine wheel and straight into the exhaust downpipe once a predetermined manifold pressure (boost pressure) is reached.

2)Variable geomentry Turbine: In a fixed geomentry turbocharger there is a mismatch between the exhaust gas flow of the internal combustion engine and the radial flow of the turbocharger. If the geometry of the turbine is designed to match the full speed and load of the engine at low and medium speeds, the response of the turbocharger will be poor. If the geometry of the turbine is matched for low and medium speeds 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 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 achieving this Variable geomentry turbine include a ring of aerodynamically-shaped vanes in the turbine housing at the turbine inlet. In turbos for passenger cars and light commercial vehicles, these vanes rotate to vary the gas swirl angle and the cross-sectional area. These internal vanes alter the turbos area-to-radius (A/R) ratio to match the engines RPM, and so give peak performance. At low RPM, a low A/R ratio allows the turbo to quickly spool up by increasing exhaust gas velocity and. At higher revs the A/R ratio increases, ther4eby allowing increased airflow. This results in a low boost threshold reducing turbo lag and provides a wide and smooth torque band.

3)Two-stage Turbocharger: The Two-stage Turbocharger uses a power turbine to extract additional energy from the exhaust. The power turbine output shaft is connected to the engine’s crankshaft through a mechanical linkage, usually a gear train, the technology is commonly referred to as mechanical Turbocompounding. For applications with exhaust flow rates beyond that required to meet the turbocharger demand, the power turbine can be arranged in parallel with the turbocharger turbine it acts as an efficiency booster.

This is the layout of a two-stage Turbocharger and the graph shows the comparison of with and without two-stage Turbocharger in a 1983 RTA engine.

4) Variable geomentry compression: Variable-geometry compressor for compressing a fluid stream having at least one variable compressor inlet stator preceding a first compressor rotor stage, wherein the first compressor rotor stage is directly followed in a compressing flow direction by a first variable stator which includes separate inlet and outlet stator cascades arranged one downstream of the other in tandem construction and wherein the separate inlet stator cascade is adjusted variably and independently of the separate outlet stator cascade so that a compressed fluid stream which exits the first compressor rotor stage is adjustably deflected.

5) e-Turbocharger: Electric turbos are designed to blend the efficiency of a turbocharged system with the responsiveness of a turbocharger by eliminating turbo lag. They combine a traditional turbo with a small electric air compressor driven by a powerful motor. When the accelerator is pressed, the motor drives the air compressor to instantly force cold air into the engine. This ensures power delivery is smooth, even before the exhaust-driven turbo gets up to speed. Once the conventional turbocharger is spinning fast enough, the electric system shuts off to save energy.

Exploring tutorial 6 and 7:

Tutorial 6 and 7 is the step by step process of converting natural aspirated inline 6 cylinder into the turbocharged engine.

This is the 6-cylinder inline engine with an intercooler and dual-outlet exit. The intercooler is installed to reduce the temperature of the inlet charge this will increase the density of the air. The dual outlet is installed to support the twin entry turbocharged this will increase the smoothness in working of the turbocharger.

In the first step, The compressor is installed at the inlet and it is driven by the mechanical component at the fixed speeds. The power for this mechanical component is taken either from battery or crankshaft.

In the second step, A twin entry turbine is installed at the outlet of the engine to convert exhaust products energy into the useful energy that can be stored.

In the last step, a mechanical shaft component is arranged between the turbine and compressor so the energy produced at the outlet is used to run the compressor this will increase the overall efficiency of the engine.

3) Plot operating points on compressor and turbine maps: There are different types of compressors and turbines that can be used in the Turbocharger but in the regular automotive and truck applications the centrifugal compressor and radial flow turbine are used.

Centrifugal compressor: A centrifugal compressor is primarily used to boost inlet air or mixture density coupled with an exhaust-driven turbine in a turbocharger. It is a single-stage radial flow device, well suited to the high mass flow rates at the relatively low-pressure ratios approximately about 4.5 which is suitable for the engine. To operate efficiently it must rotate at high angular speed. It is therefore much better suited to direct coupling with the exhaust-driven turbine of the turbocharger.

The operating characteristics of the centrifugal compressor are usually described by a performance map. This shows lines of constant compressor efficiency and constant corrected speed on a plot of pressure ratio in against corrected mass flow. As shown in the figure The stable operating range is at the center of the map which is separated from an unstable region on the left by the surge line. When the mass flow is reduced at a constant pressure ratio, local flow reversal eventually occurs in the boundary layer. Further reductions in mass flow cause the flow to reverse completely, causing a drop in pressure. This relieves the adverse pressure gradient. The flow reestablishes itself, builds up again, and the process repeats. Compressors should not be operated in this unstable regime. The stable operating regime is limited on the right by choking. The velocities increase as mass flow rate increases, and eventually, the flow becomes sonic in the limiting area of the machine. Extra mass flow through the compressor can only be obtained at a higher speed. When the diffuser is choked, compressor speed may rise substantially with only a limited increase in the mass flow rate.

Radial Flow Turbine: The radial flow turbine is similar in appearance to the centrifugal compressor; however, the flow is radially inward not outward. Radial flow turbines are normally used in automotive or truck applications. It consists of an inlet casing or scroll, a set of inlet nozzles, and the turbine rotor or wheel. When the high-pressure exhaust gases enter through nozzle it accelerates the flow, with the use of pressure energy and the drops in stagnation enthalpy occurs, and hence the work transfer, occurs only in the rotor passages, and the rotor is designed for minimum kinetic energy that is at minimum velocity at the exit. The best way of defining the characteristics of a radial flow turbine is done the corrected mass flow rate against the corrected rotor speed plot.

For analyzing the turbine and compressor performances a simulation was done on a 6-cylinder fixed geomentry turbocharger Direct Injection diesel engine with three different case setup at varying speeds.

The different case setups are

These case setups are carefully selected to replicate the different operating conditions in the compressor and turbine.

The efficiency map of the compressor

The turbocharged engine which we are using is designed to run at the 3600RPM at this speed the best performance of the compressor can be achieved. In case 2 when the engine running at 2000rpm(at low speeds) there wouldn't be enough mass flow rate and pressure ratio this would lead to run the engine at the surge point which would lead to maximum vibrations and damage of the vanes may occur. In case 3 when the engine running at 5000rpm as we can see that it is running nearer to choke region where the maximum mass flow rate of the air is reached and further increase in the mass flow rate cant be done the compressor operating at this point will be running at low volume efficiencies this leads to the reduced mass flow rate and ultimately leads to mechanical failure.

The efficiency map of the Turbine:

The radial flow turbine shaft is directly connected to the compressor as the speed of the engine increases the mass flow rate also increases this will leads to the high rotation speed of the turbine and vice versa.

The efficient working of the turbine can be seen in the design speed of 3600RPM at the 2000RPM and 5000RPM speeds the efficiency of the turbine goes down.

From the engine performance table, we can see that engine is working efficiently at 3600rpm.

• At 2000RPM the A/F ratio is 17.54 which is not very much suitable for working conditions of the diesel engine.
• At the 5000RPM the brake efficiency is decreased to 29% and BSFC is also increasing this will eventually affect the emissions.

• At 2000RPM the efficiencies of both compressor and turbine are very low and the inlet temperatures of the turbine are very high this will affect the mechanical components of the turbine.
• At 5000RPM the efficiencies of both the components are low when compared to the ideal working conditions and the maximum RPM is exceeded in both compressor and turbine.

Variable Geometry Turbine: When variable Geomentry turbine Compared to a fixed geometry turbine, the variable geometry turbine allows significant flexibility over the pressure ratio/flow relationship across the turbine and by extension, the engine pressure. This flexibility can be used for improving low-speed torque characteristics, reducing turbocharger lag in diesel engines, and driving EGR flow.

APPLICATION:

• no throttling loss of the wastegate valve.
• higher air-fuel ratio and higher peak torque at low engine speeds.
• improved vehicle accelerations without the need to resort to turbines with high pumping loss at high engine speeds.
• potential for the lower engine that has pressure difference between the exhaust manifold and intake manifold.
• control over engine Pressure difference that can be used to drive EGR flow in diesel engines with High-Pressure Loop (HPL) EGR systems.
• a better ability to cover a wider region of low BSFC in the engine speed–load domain.
• ability to provide engine braking.
• ability to raise exhaust temperature for aftertreatment system management.

Simulation on a Diesel engine in the combination of both VGT and EGR:

In this challenge, we are working on a 4 cylinder diesel engine including variable geometry turbine, boost control, intercooler, EGR circuit with cooler and EGR rate control, injection limiting for smoke control, and exhaust after-treatment device modeling. In exploring the working of engine simulation was done on 3 different operating conditions of full load, Part load, and idle conditions.

TURBOCHARGER: The Variable Geomentry turbocharger consists of 6 different sets of turbine map data entered at each of 5 VGT rack positions. The VGT rack position is determined by a control system to achieve a target boost pressure upstream of the intake manifold. The boost pressure is sensed and is then filtered using a 'MovingAverage' part to produce an average pressure signal. The model-based controller compares the sensed boost pressure to the target boost pressure and then adjusts the VGT rack position accordingly.

The above picture shows the different rack positions of the VGT, the Rack array variables give the information about the percentage of the opening of fixed vanes in the VGT. The adjust of a rack position is controlled by the VGT rack controller which is fed with a set of target boost pressure values based on the variables of engine speed and fuel injection rate.

EGR: The model includes an exhaust gas recirculation (EGR) system to transport exhaust gases from the exhaust manifold back to the intake manifold to control the combustion rate for achieving emission standards. The mass flow rate of exhaust gas into the intake is controlled by the EGR valve and the valve diameter can be varied and it is controlled by an EGR controller. To adjust the valve diameter the EGR controller is fed with a set of percentages of valve opening values based on the variables of engine speed and targeted BMEP values.

CASE SETUP: The case setup is done in such a way that the engine should operate at three different operating conditions full load, part load, and idle conditions.

RESULTS: The operating conditions and Engine performance are

As mentioned above the case 1 is for full load operating conditions capable of producing 79.9kW, case 2 is a part load condition capable of producing 15kW, and case 3 is an idle condition, The Airflow rate is maximum in case 1 so the maximum opening of the fixed vanes in VGT occurred here.

The above two figures show the rack position, mass flow rate, and working efficiencies of the turbine in different cases, The Rack position is maximum for full load conditions.

The above figure shows the three different efficiency maps of the same turbine this is because for each case the rack position in the turbine changes according to it the efficiency map also changes. From the plot, the maximum efficiency of the turbine can be seen at full load conditions.

The EGR is maximum at idle conditions to control the combustion rate for minimum production of power. At full load conditions, EGR can't be used because the maximum amount of fresh charge is needed to the energy to produce the maximum torque.

The above table shows the different EGR values for all the cases.

The efficiency map of the compressor

These are the operating points in the compressor for different cases, full load and part load conditions will fall under the maximum efficiency zone, the idle condition will fall under the low-efficiency zone.