FINAL TEST

PFI Engine

Q1. What is the Compression Ratio for the Engine?

The compression ratio (CR) of an engine is the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity. It's given by:

$$\text{CR} = \frac{V_{max}}{V_{min}} = \frac{V_{cyl} + V_{clearance}}{V_{clearance}}$$

Q2. Modify the Compression Ratio 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 5.70e-5

Vmax = Vc + Vd V. 1 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 the Engine

Volumetric Efficiency (VE) is defined as the ratio of the volume of air inducted into the cylinder to the volume displaced by the piston.

$$\text{VE} = \frac{\text{Actual Volume of Air Entered}}{\text{Theoretical Volume of Cylinder}} \times 100$$

Q4. Measure the Air Mass Flow Rate in kg/s

Air mass flow rate ($\dot{m}_{air}$) can be calculated using the intake conditions (density and volumetric flow rate). For an ideal gas:

$$\dot{m}_{air} = \rho \times Q_{vol}$$​

where:

• $\rho$ is the air density, and
• $Q_{vol}$​ is the volumetric flow rate.

Q5. Why is the Cell Count Varying During the Simulation?

In simulations, adaptive meshing changes the cell count to provide finer resolution in regions of high gradients (e.g., near moving boundaries or heat exchange zones) to capture complex flow and thermal characteristics accurately.

Q6. Valve Contact with the Cylinder Head in Simulation

1. Small Gap: This gap exists to avoid mesh interference or numerical instabilities from direct contact, which helps simulate valve sealing without inaccuracies.
2. Big Gap: A large gap could lead to significant inaccuracies, as unintentional gas leakage or unrealistic pressure gradients may develop.

Q7. Duration of Combustion

Combustion duration can be calculated from start and end crank angles, divided by the engine's speed. It typically lasts a few milliseconds in an engine cycle, depending on fuel type and mixture quality.

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Diesel Engine

Q1. What is an Advantage of Sector Simulation?

Sector simulation enables a significant reduction in computational cost by only simulating a radial sector (often 1/6 or 1/8) instead of the full cylinder, which is possible in symmetrical diesel combustion chambers.

Q2. When Will the Sector Approach Not Work?

Sector simulation won’t work effectively for non-symmetric combustion chambers or for situations where asymmetric phenomena (like non-uniform injection) need to be captured, as these require the full cylinder’s model.

Q3. Choosing Start Crank Angle (SCA) and End Crank Angle (ECA) for Sector Simulation

SCA and ECA should be chosen to capture the entire combustion process and pressure build-up in the sector. Typically, this range includes intake valve closure and extends beyond top dead center, where combustion completes.

Q4. Can We Convert the PFI Case into a Sector Case?

PFI engines typically have non-symmetric flow characteristics due to intake air-fuel mixing and therefore require full geometry. Sector modeling is usually unfit for PFI cases unless a symmetrical aspect of the cycle can be isolated (e.g., combustion phase).