The simulation lasts a time interval corresponding to a 360o crank revolution. During it, the valve lifts follow the curves represented on the graphic below.
The first 20
degrees, the exhaust valve opens itself. It stays open until 160o
and then, and closes linearly by 180o. At this moment, the intake
valve opens itself until 200o and stay wide open from 200o
to 340o. Then, it closes by 360o.
The piston motion is prescribed by setting the standard kinematic parameters for the connecting rod lenght, the crank radius and the top dead centre position.
During the simulation, the solution runs for 360 time steps and dump flow field data to file tut.pstt every 5 degrees of crank revolution (each time step corresponds to 1o of crank revolution). The vertex coordinates per time step are also stored in this file. However, it is necessary to reconstruct by PROSTAR the cell status at any stage using the events file.
For the studied case, it was possible to observe two caracteristics of the flow: the velocity vectors and the scalar concentrations.
IV.1 The velocity vectors field
As one has
previously seen, it is possible to access at the flow field, so at the
velocity vectors field every 5 degrees of crank revolution. Therefore,
it is possible to visualise the velocity vectors field for 72 different
crank angles, which correspond to 72 different time. However, it exists
a more powerful means of visualising the results by using PROSTAR' animation
command. Indeed, a serie of static frame can be save in a binary neutral
plot file and then be animated.
The animation below presents you what you can have on your screan by using the animation command of PROSTAR.
At the begining,
the velocity is weak in the whole cylinder (around 2 m/s) and is quite
important in the exhaust port (between 15 and 20 m/s). This is due to the
difference of between the cylinder section and the exhaust port section.
If one considers that the density of the fluid remains constant (there
is no compression stroke), this is a consequence of the mass flow rate
In the second part of the simulation, when the intake valve opens itself, the fresh melange enters in the cylinder with a velocity magnitude comparable with the exhaust port one during the exhaust stage. One can see that a tumble air motion appears in the cylinder during this stage. In fact, there are two tumble which appear in the cylinder. Generally, this phenomenon does not occur: this is probably due to the intake port position and the form of the valve.
This tumble air motion creation play a very important role in the combustion, because it allows to obtain a good mixture between gasoline and air. That is why cars manufacturers try to increase the intensity of this tumble, especially in the case of direct injection where the mixture between gasoline and air is done in the cylinder.
IV.2 The concentration field
it is possible to visualise the concentration field for the 72 time step
of the simulation. Of course, it is possible too to access at the animation,
which gives the evolution of the concentration in function of the time.
As one has seen it in part II, the fluid originally in the intake port is a different fluid species than the in-cylinder one. It is the same thing for the fluid originally in the exhaust port. By tracking the concentration of these species, it is possible to deduce the distribution of the fresh charge and exhaust gas, and global parameters such as volumetric efficiency can be computed.
The picture below shows the concentration field at time 0.06s, which corresponds to the end of the simulation.
One can see that the fresh gas has fulled the whole cylinder in a non-homogenous way: there is high concentration near the left partition, and a quite weak concentration in the right down corner. A better repartition would be suitable to assure a good combustion.