FINAL TEST

Questions based on PFI engine:

1. Compression Ratio:

The formula for finding out the compression ratio is as below:

rc=Vmax/Vmin=Vd+Vc/Vc

Where Vd is the displacement volume and Vc is the clearance volume.

The maximum and minimum value of volume can be found out from the graph.

From the graph:

Vmax=0.000574216Vmin=5.70301e−5 max=0.000574216 min=5.70301e-5

Thus, the compression ratio is:

rc=Vmax/Vmin=10.7

2.Modifying the compression to 10.3:

Two methods can be adopted for modifying the compression ratio of the engine.

1. Keep the Displacement volume constant and vary the clearance volume.
2. Keep the clearance volume constant and vary the displacement volume.

First method:

rc=Vmax/Vmin=Vd+Vc/Vc

Now Vd is constant. First, we need to find out the actual displacement volume of the engine.

WeknowVmax=Vd+Vc/vc=0.000574216

So, Vmax-Vc should give Vd.

Therefore, Vd= 0.000519 m³

Now let us keep Vd constant and change Vc so that Compression ratio becomes 10.3

Vd+〖Vc〗′〖Vc〗'=10.3′=10.3

0.000519+〖Vc〗′〖Vc〗′=10.30.000519+′=10.3

From the above equation the new clearance volume, Vc' is 5.58 e−5-5 m³

Second Method:

Now we keep the clearance volume constant and vary the displacement volume so that the compression ratio becomes 10.3

〖Vd〗′+VcVc=10.3=10.3

From the above equation we get 〖Vd〗′=0.0005303〖′=0.0005303 m³

Hence to increase the clearance volume to 10.3 we need to increase the displacement volume by altering the bore and stroke of the engine or decrease the clearance volume.

3.Volumetric Efficiency:

The volumetric efficiency of an IC Engine is given by:

ηv=VaVdη which is Volume of air inducted during intake stroke/ Displacement Volume

The displacement volume was calculated earlier and it was found to be 0.000519 m³

Now we need to find out the volume of air inducted into the cylinder.

Assuming the air that is inducted into the cylinder is ideal, the ideal gas equation can be applied to find out the volume of air inducted

PoVa=maRTo

Where P_o= atmospheric pressure of air

m_a = total mass of air inducted

R = Universal Gas Constant

T_o = Atmospheric temperature of Air

Therefore,

Va=maRToPo00534203 kg

R= 287 J/kg-k

To = 293 K

Po = 101325 N/m²

Substituting these values into the equation we get, Va= 4.43342e−4m3-43

Now substituting Va in volumetric efficiency equation:

ηv=VaVd=4.43342e−40.000519=0.8542η=4.43342-40.000519=0.8542

Therefore, the volumetric efficiency of the engine is 85.42%.

4.Mass flow rate per second:

From the graph, we found that the total mass flow rate into the cylinder is 0.0005342 kg. This mass is the mass flow rate into the cylinder per cycle. The engine speed is 3000 RPM which is nothing but 50 rotations per second. Thus, the angle covered in one second is 50*360 = 18000 degrees. Thus, we need to find out the mass entering into the cylinder for 18000 degrees. For 360 degrees mass entering is 0.0005342 kg, therefore for 18000 degrees mass flow is 0.0005342*50 = 0.02671kg/s.

Hence the mass flow rate into the engine is 0.02671 kg/s.

5.Variation of Cell Count:

The cell count varies during the simulation due to adaptive mesh refinement and fixed embedding provided. AMR and fixed embedding are provided to achieve better accuracy. In the PFI engine setup, AMR has been given at two levels: Velocity and temperature. In both the cases, the scale was given as 3 and subgrid scaling was 1.0 for velocity and 2.5 for temperature. Two levels of fixed embedding were provided for the cylinder and the effect can be seen from the reduced cell size inside the cylinder. During the simulation as the subgrid criterion changes the AMR kicks in and the cell count is varied. And the fixed embedding provided to the spark and injector also contributes to change in cell size.

6.Valve contact with cylinder head:

In a real engine, the valve seat is made to press against the cylinder head using a valve spring and retainer. This is done to ensure that the heat inside the cylinder/combustion chamber dissipates via the walls to the cylinder head.

1. In the PFI simulation, valve spring and retainer assembly were not designed and hence the gap can be noticed between the valve and cylinder head.
2. If the gap maintained becomes large, heat will not be dissipated to the cylinder head thus causing the air inside the cylinder/combustion chamber to be at a higher temperature. This might lead to knock/detonation and other unwanted phenomena like emission of NOx.

7.Duration of Combustion:

Duration of Combustion in seconds:

From the engine performance calculator, we found out that the duration of combustion is 240 degrees. Our aim is to convert it into seconds. We know that engine speed is 3000 RPM or 50 RPS.

And previously we saw that 18000 degrees is covered by the engine in a second.

Therefore,

18000 degrees ----------------- 1 second

240 degrees ------------------- x seconds

x=24018000=0.01333sec=24018000=0.01333sec

Therefore, combustion lasts for 0.01333 seconds in this case.

DIESEL ENGINE:

1. The primary advantage of Sector Simulation is computational efficiency. In a sector simulation, only a portion(sector) of the combustion chamber is simulated and hence the number of cells required will be less. Since it Is just a closed cycle simulation the number of constraints and boundary conditions required will also be minimized. Hence leading to faster computing time and accurate results.

2. The sector model cannot be adopted for an open cycle engine simulation. In an open cycle simulation, the intake and exhaust processes are also required to be modeled. In such a case a sector cannot be used as the position of intake and exhaust ports will vary and they cannot be accommodated in a sector. Also, for a multi-injector simulation, a sector model cannot be used as the position of injectors might not be symmetric and thus a sector approach doesn’t work out.

3. Since only a closed cycle simulation is to be modeled using a sector simulation, the start of compression stroke can be taken as the beginning of the simulation and the end of expansion stroke can be taken as the end of the simulation. Assuming we start injection from 0 CAD, 180 CAD can be taken as start crank angle and 540 CAD can be taken as end crank angle.

4. The PFI case cannot be converted into a sector simulation. The sector simulation simulates only a portion of the 360-degree combustion chamber for computational efficiency. The PFI simulation is an open cycle simulation and as discussed before the sector model will not be able to model an open cycle case. Also, the injector positions for each sector might vary and thus modeling one sector and extrapolating it to the entire engine might not work. Also, the combustion in the PFI case occurs due to a spark and the spark plug cannot be modeled using a sector model. Thus, owing to all these reasons it will not be feasible to convert PFI case into a sector model.