FINAL TEST

Questions based on PFI Engine:

Q1. What is the Compression ratio for the engine?

The compression ratio is the ratio between the volume of the cylinder and combustion chamber in an internal combustion engine at their maximum and minimum values. 

The compression ratio can be calculated with the help of the below graph

In this graph, it can be seen that the volume of air is maximum at -520 degree crank angle which indicates that the piston is at the bottom dead center. As the piston moves forward the air inside the combustion chamber compresses and at this time the combustion starts, the air volume decreases at the top dead center. Once the energy is released after the combustion the piston again moves to BDC and the volume of air increases which indicates that the intake valve is open to increase the volume of air. At TDC the exhaust valve is open which releases the product of combustion.

The Compression ratio is given by : `frac{V_max}{V_min}`

`V_max=0.00056259868`

 `V_min=5.7029438E-05`

Therefore, the Compression ratio of this Engine is 9.865

Q2. Modify the compression ratio for this engine to 10.3 without changing geometrical parameters, just need to show the calculation.

The compression ratio is given as the ratio of total cylinder volume to the clearance volume. 

`CR=frac{V_c+V_s}{V_c}`

where the terms are having their usual meaning.

Thus the compression ratio can be changed by changing the values of clearance volume or stroke volume. 

By changing the clearance volume 

`10.3=frac{V_c+V_s}{V_c}`

`9.3V_c=V_s`

`V_s=` Maximum volume - Minimum volume

`V_s=0.0005055692`

Therefore, `V_c=frac{0.0005055692}{9.3}``=0.00005436228``m^3`

By changing the stroke volume 

`9.3V_c=V_s`

Therefore  `V_c=5.7029438E-05`

`V_s=0.00053037377m^3`

Similarly, the new stroke length can be calculated as `V_s=frac{pi**D^2}{d}**L`

Thus, by increasing the compression ratio from 9.865 to 10.3 there is an increase in the stroke volume and the clearance volume.

Q3. Calculate Volumetric Efficiency for this engine

It is defined as the ratio of the density of air-fuel mixture inside the cylinder and, the density of air inside the intake port.

Therefore, the density of air inside the cylinder region 0 is given by = mass of air inside the cylinder/maximum volume of the air 

Mass of the air inside the cylinder can be obtained by the below graph which is 0.00050051627 Kg

Maximum volume of the cylinder = 0.000574218`m^3`

Therefore `rho_(cyl i nder)=frac{0.00050051627}{0.000574218}`

`rho_(cyl i nder)=0.87164852024frac{Kg}{m^3}`

The temperature inside the intake port-1 is shown below which is 369.59627K

Pressure is given as 101325 Pa

R is the gas constant of the air = 287 J/KgK

Therefore, `rho=frac{P}{RT}`

`rho=frac{101325}{287**369.59627}`

`rho_(i n take-p o rt)=0.95522`

Therefore volumetric efficiency is given as, 91.2%

Q4. Measure the air mass flow rate for this engine in kg/s

The total mass of the (air+fuel) when the piston is at the bottom dead center is 0.00050051627Kg

Mass of fuel sprayed in 720 degrees=3e-5Kg

Mass of air entering the cylinder in 720 degrees=0.00050051627-3e-5=0.00047051627Kg

Mass of air entering the cylinder in 360 degrees=0.00023525813Kg

RPM of engine=3000

RPS of engine=50

Mass flow rate of the air=0.00023525813 * 50=0.01176Kg/s

Q5.Why is the cell count varying during the simulation?

The cell count variation in the PFI engine is shown below

The cell count variation in the region is due to adaptive mesh refinement and, fixed embedding.

Adaptive mesh refinement is a technique that is used to refine the grids automatically based on fluctuating and moving conditions such as temperature or velocity. The feature is used to refine the grid in the flow region of interest such as flame propagation or high velocity without slowing down the simulation with a globally refined grid. There are two types of AMR, mainly sub-grid scale-based and value-based.  The AMR type and criteria are chosen in such a way that the embedding is added where the flow field is most unresolved.

AMR Group-1 is velocity type AMR in which SGS is 1 m/sec. AMR Group-2 is temperature type AMR in which SGS is 2.5K. For the cell whose value is greater than the SGS criterion then AMR is going to refine the mesh in that region. 

Fixed embedding is used to refine the cells near the wall or boundary region where the flow needs to be captured more accurately. Permanent mode of embedding is used to increase the cell count but it will not cause the variation in cells during simulation whereas, cyclic embedding is used to cause the variation of cells during the simulation. 

Q6. In a real engine, valves are in contact with the cylinder head but while running simulation, there is a small gap

• WHY?

During the simulation, there is a small gap in between the cylinder and the valves to prevent the intersection of triangles, which will result in errors.

What happens if this gap is big?

The smaller the gap more is the time taken to complete the simulation but the solution is accurate. The bigger the gap, the less is the time taken and, the solution is less accurate. 

Q7. How many seconds does combustion last in this case?

The graph of isooctane indicates that the mass of fuel starts to decline from crank angle -5 deg to 45 deg 

Combustion duration in terms of degree=50 degree

RPM of engine=3000

RPS of engine=3000/60=50

Combustion duration in degree per second =50*360=18000degree/second

Duration of combustion=50/18000=0.0027777sec

Questions based on DIESEL Engine:

Q1. What is an advantage of SECTOR Simulation? 

Sector simulation is computationally cheaper, takes less time to simulate and, is required to evaluate the piston profile of the engine based on the performance and the emission of the engine. This is a closed cycle simulation and hence the cell count is also less during the simulation. It is used for simulating compression and power strokes. Moreover, only the sector of the engine is required for the simulation, not the whole engine.

Q2. When will the sector approach not work?

For closed-cycle simulation, sector profile will only work if the piston profile is axisymmetric( i.e. symmetrical around the axis).

It cannot be used for open-cycle simulation where we need to model the intake and the exhaust process.

Q3. How to choose Start Crank Angle (SCA) and End Crank Angle (ECA) for a sector simulation?

The Start Crank Angle (SCA) can be chosen as the start of compression and End Crank Angle (ECA) as the end of the power stroke respectively. If the intake stroke starts at 0 degrees then the compression will start at 180 degrees and the power stroke will end at 540 degrees.

Q4. Can we convert the PFI case into a SECTOR case? Explain your answer? 

The answer is no. In PFI, the piston profile is not axisymmetric means we have to simulate the fuel droplet vaporization in the intake port whereas, the sector simulation is a closed cycle simulation in which compression and power strokes are simulated and, the piston profile is axisymmetric in nature. It is not possible to divide the intake and the exhaust port into a sector therefore, sector simulation is not possible for PFI.