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Geometry Vs Engine Breathing

Geometry Vs Engine Breathing. P M V Subbarao Professor Mechanical Engineering Department. Its not engine Volume, but Mass of air Decides the Power Output …. Improper Breathing in an Engine Creates A Pumping Cycle. Anatomy of Intake System.

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Geometry Vs Engine Breathing

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  1. Geometry Vs Engine Breathing P M V Subbarao Professor Mechanical Engineering Department Its not engine Volume, but Mass of air Decides the Power Output ….

  2. Improper Breathing in an Engine Creates A Pumping Cycle

  3. Anatomy of Intake System

  4. The Intake System and Instantaneous Mass of air in Engine Cylinder

  5. Piston Speed Piston Displacement : • The speed of the piston

  6. Mean Piston Speed • Better than using rotational speed (RPM) • Gas flow velocities in the intake manifold and cylinder all scale with mean piston speed. • Should be between 8 and 16 m/sec. • Lower end is typical of large industrial engines • High end is typical of automotive engines

  7. Cylinder Geometry Vs Breathing Issues • The smaller bore reduces the area available for valves in the cylinder head, requiring them to be smaller or fewer in number. • These factors favor lower engine speeds, under-square engines are most often tuned to develop peak torque at relatively low speeds. • An under-square engine will typically be more compact in the directions perpendicular to piston travel but larger in the direction parallel to piston travel. • An over-square engine allows for more and larger valves in the head of the cylinder.

  8. Effect of Rod Ratio on Piston Speed

  9. Rod Ratio vs. Intake Efficiency • A “R” value of 1.75 is considered “ideal” by some respected engine builders, if the breathing is optimized for the design. • Except for purpose-built racing engines, most other projects are compromises where 1.75 may not produce the best results. • The “R” value can be used as a correction factor to better “match” the intake to the manifold. • Low “R” numbers (1.45 - 1.75) are produced by short rods in relation to the stroke. • High “R” numbers (1.75 - 2.1) are produced by long rods in relation to the stroke.

  10. SHORT ROD : Intake Stroke • Intake Stroke -- Short rod spends less time near TDC and will suck harder on the cylinder head from 10o ATDC to 90o ATDC the early part of the stroke. • Will not suck as hard from 90o ATDC to BDC as a long rod. • Will require a better cylinder head than long rod to produce same peak HP. • Will require stronger wrist pins, piston pin bosses, and connecting rods than a long rod. • Short rod spends more time at the bottom which may reduce intake charge being pumped back out intake tract as valve closes. • May permit longer intake lobe and/or later intake closing than a long rod.

  11. LONG ROD • Intake Stroke -- will draw harder on cylinder head from 90o ATDC to BDC. • Compression Stroke -- Piston travels from BDC to 90o BTDC faster than short rod. • Goes slower from 90o BTDC to TDC--may change ignition timing requirement versus short rod as piston spends more time at top.

  12. http://victorylibrary.com/mopar/rod-tech-c.htm

  13. The Valve and its Inlet Conditions

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