Irreversible Gas Dynamics of Intake and Exhaust Flows

1. Irreversible Gas Dynamics of Intake and Exhaust Flows P M V Subbarao Professor Mechanical Engineering Department Design Rules for Maximum Intake Flow….

2. Isentropic Specific Mass flow Rate Mass flow rate per unit area of cross section:

3. Size of Minimum Flow Area • For lower valve lift the minimum area is the valve curtain area. • For larger lifts the minimum area is the valve seat area.

4. Isentropic Compressible Flow Through Inlet Valve : low Lift Resource : The total pressure in the inlet manifold – Cylinder pressure

5. Conditions for Choked Flow Resource : The total pressure in the inlet manifold – p* When the flow is chocked:

6. Stages of Valve Lift The maximum valve lift is normally about 12% of the cylinder bore or 25% of .

7. Instantaneous Flow Area : Low Lift • For low lift valves, the minimum flow area is normal to a frustum of right circular cone. • The area perpendicular to the conical face between valve and seat is, defines the flow area. The minimum area is:

8. Most Popular Intake Poppet Valve Geometry

9. For the second stage, the minimum area is still the slant surface of a frustum of a right circular cone. However, the flow area is not perpendicular to the valve seat. The base angle of the cone increases from (900-) toward that of a cylinder. For this stage: Instantaneous Flow Area : Intermediate Lift Dm is mean diameter of seat :

10. Instantaneous Flow Area : High Lift • When the valve lift is sufficiently large, the minimum flow area is no longer between the valve head and seat. • It is the port area minus the sectional area of the valve stem. Then,

11. Available Intake manifold Pressure

12. Local vs Average Mach Number • Choking will occur at a local Mach index of 1. • The average gas speed corresponding to choked condition is less than 1. • For highest intake flow rate, the real value of highest average Mach number ≤ 0.6.

13. Irreversible Flow thru Intake Valve • The real mass flow rate is always lower than the ideal mass flow rate. • At any location in a flow passage, there are two reasons for lower values of real mass flow rate: • The first reason: Actual local velocity of the flow is always less than velocity of isentropic flow. • The ratio of real velocity to ideal velocity is called as flow coefficient, Cf.

14. Irreversible Flow thru Intake Valve • The second reason: Only at minimum area location the area of the stream tube is equal to available area. • At other locations this ratio is less than 1. • This is called as coefficient of contraction. • For a given flow passage, the discharge coefficient, Cdis defined as Real measured mass flow rate over the ideal mass flow rate.

15. Source of flow losses in the port • Source of flow loss % of loss • Wall friction = 4% • Contraction at push-rod = 2% • Bend at valve guide =11% • Expansion behind valve guide = 4% • Expansion 25 degrees = 12% • Expansion 30 degrees = 19% • Bend to exit valve = 17% • Expansion exiting valve = 3%

16. Actual Flow Characteristics of A Valve

17. Frictional Compressible Flow Through Inlet Valve • The real gas flow effects are included by means of an experimentally determined discharge coefficient CD. • The air flow rate is related to the upstream stagnation pressure p0 and stagnation temperature T0, Static pressure just down stream of the valve and a reference area AR. • AR is a characteristic of the valve design. When the flow is chocked:

18. Computational Results : Coefficient of Discharge of A Valve

19. Gas Dynamics of Flow Past A Valve

20. Four-mode model of flow separation in a valve gap

21. General Theory : Four-mode model of flow separation in a valve gap Mode I: At low lifts (approximately L/D < (0.07- 0.09)) the flow is attached to both valve seat and sealing faces, and the discharge coefficient usually has a high value. At intermediate valve lifts (approximately 0.09 <L/D < 0.12) the flow separates from the valve but remains attached to the valve seat, and the discharge coefficient decreases..

22. General Theory : Four-mode model of flow separation in a valve gap At intermediate-high lifts (approximately 0.12 < L/D < 0.15) the flow separates from both valve seat and sealing faces and forms a free jet which is hardly influenced by the valve seat geometry. At high valve lifts (approximately L/D > 0.15) the flow separation from the valve seat face is more extensive; the discharge coefficient recovers slightly and decreases linearly with increasing lift

23. Complex Fluid Mechanics due to flow Separation

24. The sensitivity of CD to differences in the flow conditions in the valve gap

25. FUEL A I R Computation of Intake Process : SI Engine SI Engine

26. Instantaneous Heat Transfer During Intake Process

27. FUEL A I R Total Mass inhaled during Intake Process

28. Volumetric Efficiency : A Global Measure of Breathing Effectiveness • Volumetric efficiency a measure of overall effectiveness of engine and its intake and exhaust system as a natural breathing system. • It is defined as: • If the air density ra,0 is evaluated at inlet manifold conditions, the volumetric efficiency is a measure of breathing performance of the cylinder, inlet port and valve. • If the air density ra,0 is evaluated at ambient conditions, the volumetric efficiency is a measure of overall intake and exhaust system and other engine features. • The full load value of volumetric efficiency is a design feature of entire engine system.

29. True mass of Intake air in Current SI Engines • The true mass of intake air is function of • Fuel type, fuel/air ratio, fraction of fuel vaporized in the intake system, and fuel heat vaporization. • Partial pressure of air in Intake mixture pa,im. • Fuel/ air ratio (F/A). • Mixture temperature as influenced by heat transfer. • Intake mixture Temperature Tim. • Compression ratio rv. • Exhaust pressure, pe. • Duration of actual intake is less than duration inlet valve opening.

30. Philosophy of Fuel Injection in SI Engine

31. Fuel Evaporation in SI Engine Intake Manifolds • The liquid fuel is completely/partially evaporated in the inlet pipe and ports of SI engines. • The presence of gaseous fuel and moisture in the intake system reduces the air partial pressure below the mixture pressure. • For a mixture: In most of the current commercial spark ignition engines, the fresh mixture contains unevaporated fuel during the inlet process. This quantity, under certain operating conditions, may be up to 40% of the total fuel supplied to the engine in one cycle.

32. Fuel Chemistry Vs Volumetric Efficiency 1.0 C8H18 CH4 H2 0.6 1.5 0 Equivalence Ratio