Turbocharger Modelling
1. List down different TC and locate examples from GT Power
The turbocharging and supercharging are two of the most important inventions applied to the internal combustion engine. These techniques were born for aviation applications, and so for the aircraft moved by ICE, that presented a loss of power flying a certain altitude. This fact was due to the less density of the air, and a very low volumetric efficiency for the engine. To avoid this problem, a compressor was integrated inside the engine, that could compress the inlet air, increasing density and so volumetric efficiency. This technique was developed even for road cars, at first for the CI engine, and later for the SI engine. Let's analyze what happens for both applications:
A) Supercharger
The supercharger configuration is provided by a compressor directly linked to the crankshaft. In this way, the speed range of the compressor is quite limited, and it's directly dependent on rpm of the engine. One of the main aspects of this configuration is that turbo-lag is reduced to the minimum, and so it's possible to use immediately, even at low loads, the power given by the compressor. For this application generally, rotary positive displacement compressors are used, such as screw and lobe. These types of compressors have the attitude to work with a smaller airflow rate but can pressurize the fluid till a very high level of pressure. The worst aspect of the supercharger is that it is directly coupled with the crankshaft, and for this reason, it's moved using a bit of the total power delivered by the engine. As an example: considering a naturally aspirated engine that could deliver 100kW of power. Using a supercharger the power delivered increases till 150kW because the compressor uses 25kW to compress the inlet air. This problem is totally avoided with a turbocharger.
B) Turbocharger
A turbocharger (or colloquially called "turbo") is a compact element consisting of a compressor and a turbine, coupled on the same shaft. The main aspect of this configuration is that the couple compressor-turbine is not linked to the crankshaft, but it's moved by exhaust gases. In fact, the gases outside the exhaust manifold, have a lot of energy inside them, and for this reason, they expand into the turbine that rotating can aliment the compressor, which compresses the inlet air. For this application are generally used dynamic compressors, that can work with a bigger airflow rate than the compressor of the supercharger, but with an inferior compression ratio. One of the biggest problems of this configuration is the turbo-lag that increases with the increase in the size of the compressor.
There are different technologies used to improve turbocharging.
- The first one is "twin-scroll", generally used for multi-cylinder engines (more than 4). Let's consider a six-cylinder engine: if the exhaust gases are linked to the compressor through one duct, the power obtainable is inferior because while an engine is during the exhaust phase, another one could be in a different phase, and this can create interferences between exhaust flows. For this reason, to avoid this problem, the "twin-scroll" is used. In fact, it consists of two different ducts for two different couples of cylinders, without creating flow interferences.
- Another one is "variable geometry". This innovation derivates from the velocity triangles of the turbine. To reduce the turbo-lag at different speeds the exhaust gases should enter inside the compressor with different directions of the absolute velocity. In fact, the relation between velocities is given by the following equation:
C = U + W
where:
C is the absolute velocity of the fluid
U is the circumferential speed of the compressor
W is the relative velocity of the fluid, so the velocity of the exhaust gases compared to the compressor speed
It's obvious that changing the rpm of the engine should change the C of the fluid because of changes in W and U. For this reason, the variable geometry allows using variable vanes that can change the direction with which the exhaust gases enter inside the turbine (so the direction of the absolute velocity).
-One of the most recent innovations is E-BOOST: the compressor always moved by exhaust gases, but it's even linked to an electric machine, that activates the compressor to reduce the turbo-lag.
In the end to increase the efficiency of a supercharged configuration, more than one "turbo" could be used. There are different and different configurations, such as series, parallel, and even a mixed configuration, using a turbocharger and a supercharger together.
2. Explore tutorial number 6 and 7
Plot operating points on compressor and turbine maps
After the exploration of tutorial 6 and 7, the operating points on the compressor and turbine maps were plotted for tutorial 7, shown below:
To visualize how the operating point changes on the compressor and turbine maps, different simulations were run, as visible in the case setup below:
Case 1) 3600 rpm and 80mg of injected mass:
Compressor
Turbine
Case 2) 6400 rpm and 20mg of injected mass:
Compressor
Turbine
Observing the maps is clearly visible that increasing the rpm of the engine is necessary to decrease the injected mass to do not work in critical zones fo the maps.ù
3. In which application Variable Geometry Turbine is beneficial?
Explore example- Diesel VGT EGR and understand the modeling part
The variable geometry turbine is particularly beneficial when applied to common road vehicles. As explained in the first question, this technology allows reducing turbo-lag, obtaining higher peak torque at low loads, working on a wide range of low BSFC at low loads... It consists to use variable vanes, controlled by an electronic actuator, that at different rpm of the engine permit to define correctly the absolute velocity direction of the exhaust gases entering into the turbine.
The example "Diesel VGT EGR" of the software, was very interesting, in which was modeled a variable geometry turbine. Let's analyze the model:
A 4-cylinders turbocharged engine, with direct injection, was modeled. The turbocharger is composed of a single scroll, variable geometry turbine, coupled on the same shaft with a compressor. The variable vanes of the VGT are controlled by the VGT-Rack that receives the signal of the boost pressure of the inlet air (the air that's entering inside cylinders). Let's see how variable geometry was modeled for the turbine.
There are two different ways to model a VGT:
the first one is to define different a different turbine map for every rack position.
The second one was used for this model. It consists to define a turbine map for only one rack position but observing the "map Object or File":
is visible that a rack array was defined, and for everyone corresponds a turbine map.
In the model is even present: an intercooler, that permits to cool the compressed air before entering inside cylinders; an "EGR cooler" that permits to recirculate a part of the exhaust gases inside the cylinders, to reduce maximum temperature and to avoid the formation of NOx; a "controllerDinj" that allows controlling the quantity of fuel injected in every cycle, to obtain a target, and in this model the bmep was chosen as the target.
Finally is possible to say that both methods are equivalents, are the same thing but presented in two different ways.