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ME403 Internal Combustion Engine Theory

ME403 Internal Combustion Engine Theory. 4-Stroke Spark-Ignition Engine Cycle. Idealize Otto Cycle. Otto Cycle. Idealization of the piston-cylinder internal combustion (IC) gasoline engine cycle which uses a spark plug to ignite the combustion process.

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ME403 Internal Combustion Engine Theory

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  1. ME403 Internal Combustion Engine Theory

  2. 4-Stroke Spark-Ignition Engine Cycle

  3. Idealize Otto Cycle

  4. Otto Cycle • Idealization of the piston-cylinder internal combustion (IC) gasoline engine cycle which uses a spark plug to ignite the combustion process. • It consists of four reversible processes: • Isentropic (no heat or friction loss) Compression • Isochoric (constant volume) Heating (combustion) • Isentropic Expansion • Isochoric Cooling • The last process replaces the actual intake and exhaust processes, which do not contribute to any work done.

  5. Thermodynamics of Otto Cycle There is only one isochoric heating process (2-3) and one isochoric cooling process (4-1). Assuming air is a perfect gas (ideal gas with constant specific heats and constant specific heat ratio k), for each kg of air: The work done per cycle:

  6. Diesel Cycle • Idealization of the piston-cylinder internal combustion (IC) engine cycle utilizing the Diesel fuel, which ignites as long as the temperature reaches a critical point due to compression, and therefore does not require a spark plug. • It consists of four reversible processes: • Isentropic Compression • Isobaric (constant pressure) Heating (combustion) • Isentropic Expansion • Isochoric Cooling • The last process replaces the actual intake and exhaust processes, which do not contribute to any work done.

  7. 4-Stroke Diesel Engine Cycle

  8. Thermodynamics of Diesel Cycle There is only one isobaric heating process (2-3) and one isochoric cooling process (4-1). Assuming air is a perfect gas (ideal gas with constant specific heats and specific heat ratio k), for each kg of air: The work done per cycle:

  9. Summary: 4-Stroke Engine Cycles

  10. 2-Stroke Spark Ignition Enginehttp://science.howstuffworks.com/two-stroke1.htmCompression +Exhaust stroke, Combustion, Expansion + Exhaust + Intake stroke

  11. Internal Combustion Engines: Geometry Definitions N = Number of Cylinders S = Stroke B = Bore TDC = Top Dead Center BDC = Bottom Dead Center Vmax–Vmin = Displacement Volume per cylinder Vdisp = N S (p/4)B2 = N (Vmax – Vmin) = Engine Displacement Volume rv = Vmax/Vmin = Compression Ratio B

  12. IC Engine Performance • Engine shaft rotation speed (rev/sec): • Thermodynamic cycles per second: For 2-stroke engine (1 cycle/rev): For 4-stroke engine (1 cycle/2 rev): • Total engine power (N cylinders): • Heating rate required: • Given heating value of fuel: • Fuel consumption rate: • Specific Fuel Consumption:

  13. Example: Performance of Ideal Otto Engine

  14. Example: Performance of Ideal Diesel Engine

  15. Conclusion • Actual engine efficiency (typically 25% to 30%), is much lower than the ideal thermal efficiency, due to the following factors: • Deviation of actual cycle from the idealize Otto or Diesel cycle • Air is not a perfect gas, especially at temperatures >500K • Mechanical (friction) losses • Heat loss to surrounding • Combustion of fuel is often incomplete • Typical BSFC: 0.4 to 0.5 lb/hr-hp (0.24 to 0.3 kg/hr-kw) • Engine power output is proportional to mass of air in cylinders, which is proportional to air density. Hence engine power available decreases proportionately with air density as flight altitude increases.

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