FINAL TEST

Question - 1. What is the Compression ratio for the engine?

• Compression ratio = Total Volume / Clearance Volume = (Vs+Vc)/Vc

where,

Vs - Swept Volume,

Vc - Clearance Volume

• Values are imported from the converged studio's Engine calculation simulation.

• Stroke Volume:
• Clearance Volume:
• Using above values, CR = 11.08

Question - 2. Modify the compression ratio for this engine to 10.3 without changing geometrical parameters.

• To change the CR without changing the geometric parameters, the total volume (cylinder geometry) must be kept constant.
• Changing the clearance volume would change the CR without causing any geometric changes.

• 10.3=(0.0005742195+Vc)/Vc
• Changing Vc to 6.1744032258064516129032258064516e-5m would result in a CR of 10.3 for this engine.

Question - 3. Calculate Volumetric Efficiency for this engine.

• The volumetric efficiency v is defined as the ratio of the actual (measured) volume of intake air Va [m3] drawn into the cylinder/engine during the intake engine cycle to the theoretical volume of the engine/cylinder Vt [m3].

• Volumetric efficiency can also be defined as the internal combustion engine's ability to fill the cylinders with intake air. The higher the volumetric efficiency, the more intake air there is in the engine.
• The intake air is mixed with fuel in indirect fuel injection engines (mostly gasoline). Because the amount of fuel is relatively small (ratio 1:14.7) in comparison to the amount of air, we can ignore the fuel mass when calculating volumetric efficiency.

- m3 = 0.000579723248 m3 Vt = 0.00052269381 (theoretical Vs) + 0.000057029438 (Vc from plot)

- The ideal gas law equation is used to calculate Va.

- PVa=mRT

- P= 1atm=101325 N/m2 in this case.

- 287 R = ideal gas constant

- T=STP temperature = 298.15K

• According to the above plot, the mass of air in kg is approximately 0.000534 kg (excluding fuel particles).
• As a result, Va= 0.00045 m3.

where,

ηv = Va / Vt = 77.78%

Question - 4. Measure the air mass flow rate for this engine in kg/s.

• RPM = 3000; RPS = 3000/60=50.
• The total air mass flow rate per second is 50*0.00010733 = 0.0053665 kg/s.

Question - 5. Why is the cell count varying during the simulation?

• In convergence, a baseline mesh is generated along three axes in either a uniform or non-uniform pattern. The cell count in this case does not change during the simulation. However, most engineering problems, such as port fuel injection and shock wave simulation, necessitate dynamic or region-specific grids in order to accurately capture interesting physics and reduce computational power requirements.
• Apart from the base grid being uniform along three axes with a cell size of 0.004m, two approaches are used in this PFI simulation. Fixed embedding and adaptive mesh refinement are the two approaches.
• Fixed embedding reduces the cell size within the user-defined boundaries. This aids in accurately capturing the desirable physics at a specific location and the rest of the regions where the physics behaves benignly are set with a baseline grid. A fixed embedding grid is assigned to a region around the spark plug in the PFI simulation.
• Adaptive Mesh Refinement: This reduces the cell size based on the expression below;

• This is used in the PFI Cylinder region, Intake ports 1 and 2, where temperature and velocity gradients are high.
  A velocity and temperature-based AMR is used with a Sub-grid scale embedding level of 3 and a criterion of 2.5K difference in temperature curvature () and 1m/s in velocity.
• AMR based on velocity is applied permanently, whereas AMR based on temperature is applied for a specific crank angle period.
• During the simulation, the above two techniques cause cell count variation.

Question- 6. In a real engine, valves are in contact with the cylinder head but while running a simulation, there is a small gap. Why? What happens if this gap is big?

• The intersection of surface triangles is not permitted in CONVERGE and results in an error. A small gap is provided in the simulation geometry to prevent the intersection. CONVERGE, on the other hand, generates disconnect triangles to close the gaps (and thus cut off the flow) between regions. A concentric circle disconnection exists between the valve (a circular loop) and the valve seat in an engine's poppet valves (another circular loop). The image below depicts a cross-section of an engine valve and valve seat. In this example, the purple lines represent region 1 boundaries, while the red lines represent region 2 boundaries.
• CONVERGE will detect two circular loops where regions 1 and 2 meet at the start of the simulation: one loop around the base of the valve. CONVERGE will generate a ring of disconnect triangles between the two loops (i.e., between the valve and the valve seat) because there are two loops. You can control the flow between regions by activating or deactivating these disconnect triangles.
• If the distance between regions is too great, either heat transfer will be disrupted, resulting in knocking, or fuel and exhaust gas will leak.

Question - 7. How many seconds does combustion last in this case?

• Duration = 240.199 degrees
• RPM = 3000;
• RPS=3000/60=50
• Total angular rotation per second = 50*360 degrees = 18000 degrees
• Combustion 240.199/18000=0.01335 seconds (duration in seconds)

Questions on DIESEL Engine:

Question - 1. What is an advantage of SECTOR Simulation?

• The primary benefit of a SECTOR simulation is the reduction in computation time due to minimal geometry, which results in a smaller number of cells and equations to solve. The time required to preprocess the CAD is also reduced.

Question - 2. When will the sector approach not work?

• The SECTOR approach will not work in the following circumstances:
• In this case, valves are modelled and timing is set to initiate specific actions. This cannot be accomplished through a sector approach.
• In real-time scenarios, injectors are asymmetric. As a result, open cycle analysis is the best option because it can be modelled as needed. This is set to be symmetric by default in sector simulation.

Question - 3. How to choose Start Crank Angle (SCA) and End Crank Angle (ECA) for a sector simulation?

• The Start Crank Angle must be at the start of the Compression stroke and the End Crank Angle must be at the end of the Expansion stroke.

Question - 4. Can we convert the PFI case into a SECTOR case? Explain your answer.

• No, the PFI case cannot be converted into a SECTOR case for the reasons listed below.
• Sector simulation only simulates a portion of the piston profile and does not account for other factors such as valve profiles, injectors, and so on.
• Even if injectors are added, they will not be symmetric, and extrapolating a sector's profile may not yield an accurate result.