FINAL TEST

Questions based on PFI Engine:

Q1. What is the Compression ratio for the engine?

• The compression ratio is defined as the ratio between the volume of the cylinder with the piston in the bottom position, V_bottom (largest volume), and in the top position, V_top (smallest volume). The higher this ratio, the greater will be the power output from a given engine. It is generally in the 6–10 range.

Q2. Modify the compression ratio for this engine to 10.3 without changing geometrical parameters.

• To change compression ratio without changing geometrical parameters is possible only by changing clearance or swept volume.

  maximum volume is clearance volume plus displacement volume clearance volume is the minimum volume and equals to
  5.70e^−5.

  
  Vmax =Vc +Vd where - Vc is the clearance volume and Vd is the displacement volume.

  The compression ratio should be 10.3 therefore ratio = (Vd+Vc)/Vc=10.3

therefore Vc is replaced to be 5.55e^−5, Furthermore we can change Vd which will be 5.336e^−3 to get the compression ratio of 10.3.

Q3. Calculate Volumetric Efficiency for this engine.

• Volumetric efficiency(η) = volume of air-fuel mix sucked into the cylinder/displacement volume

  displacement volume is the maximum volume of the cylinder substituted to clearance volume therefore in our case displacement volume(Vd)=5.17e^−4 m^3

  According to the ideal gas equation, we can determine the volume of the sucked air. According to the output files sucked mass is approximately 0.0005kg.

  According to the ideal gas equation
  V=(m⋅R⋅T)/P where -m is mass , Ris the universal gas constant ,T is the temperature in Kelvins and Pis the ambient pressure

  η=(4.149e^−4 / 5.17e^−4)⋅100 = 80.26≈80%

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

• Mass of air during the 360 degrees of the crank angle is 0.0005kg. RPM in our case is 3000, therefore RPS will be 3000/60=50.

  50 revolution per second in other words equals 50*60=18000degrees per second.

  mass per 18000degrees will be 18000*0.0005/360 = 0.025kg/s.

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

• During the combustion process or fuel injection process we have temperature and velocity second derivative curvatures therefore we need refined mesh within some specific areas to appropriately predict the flow structure or temperature development, therefore we use AMR for proper calculations.

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

(ii)What happens if this gap is big ?

• In real live valves are in contact with the cylinder head in order to seal the combustion cylinder to prevent excessive heat transfer and therefore thermal losses.

  In the simulation, we have a gap because we are using the disconnect triangle concept which ensures sealing while we are using the "VALVE" option in the "EVENT" section in the "Converge CFD".

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

• The engine is running power stroke through 240degrees, thus we should determine the time needed for 240degrees of the crank angle.

  As we know engine covers 18000degrees within 1 second, therefore to cover 240degrees the time
  t=240/18000=0.013sec

Questions based on DIESEL Engine:

Q1. What is an advantage of SECTOR Simulation?

• Sector simulation is used while having closed geometry with the symmetrical 360degree geometry. Main benefit of using this method is dramatically reducing the computational time.

Q2. When will the sector approach not work?

• The sector approach will not work if we don't have a symmetrical case or having port injection valve openings which we faced in the PFI case.

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

• The start angle should be when the volume is maximum and the end angle should be when the piston reaches TDC in other words the clearance volume.

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

• We can not convert the PFI case into a Sector case because PFI is opened cycle case, where we have intake and exhaust phases, fuel injection via the port therefore flow structure in the combustion cylinder will be severely unsymmetrical leading Sector case simulation to unrealistic results.