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I.C. ENGINES

I.C. ENGINES. LECTURE NO: 13 ( 28 Apr 2014). ENGINE PERFORMANCE PARAMETERS. Indicated Thermal Efficiency Brake Thermal Efficiency Mechanical Efficiency Volumetric Effic iency Relative Efficiency Mean Effective Pressure Mean Piston Speed Specific Power Output

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I.C. ENGINES

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  1. I.C. ENGINES LECTURE NO: 13 (28 Apr 2014)

  2. ENGINE PERFORMANCE PARAMETERS • Indicated Thermal Efficiency • Brake Thermal Efficiency • Mechanical Efficiency • Volumetric Efficiency • Relative Efficiency • Mean Effective Pressure • Mean Piston Speed • Specific Power Output • Specific Fuel Consumption

  3. ENGINE PERFORMANCE PARAMETERS • Inlet Value Mach Index • Fuel Air Ratio • Calorific Value of the Fuel

  4. Atmospheric Pressure

  5. Work • Work is the movement of a body against an opposing force.

  6. Torque

  7. Conversion of Work into Torque

  8. Power • Rate of doing Work is called power • Through testing, it was found that the average horse could lift a 200-lb. weight to a height of 165 ft in 1 minute. The equivalent of one horsepower can be reached by multiplying 165 ft by 200 lb. (work formula) for a total of 33,000 ft lb. per minute

  9. Power

  10. Dynamometer • Pony Brake

  11. Dynamometer

  12. Classic Dynamometer

  13. Indicated Thermal Efficiency • Indicated thermal efficiency is the ratio of energy in the indicated power ip, to the input fuel energy in appropriate units ηith= ip[kJ/s] energy in fuel per second [kJ/s] ηith= ip[kJ/s] mass of fuel/s x calorfic value of fuel

  14. Brake Thermal Efficiency • Brake thermal efficiency is the ratio of energy in the brake power bp, to the input fuel energy in appropriate units ηbth= bp[kJ/s] energy in fuel per second [kJ/s] ηith= ip[kJ/s] mass of fuel/s x calorific value of fuel

  15. Mechanical Efficiency • Mechanical efficiency is the ratio of bp (delivered power) to indicated power ( power provided to the piston) ηm = bp[kJ/s]= bp ip[kJ/s] bp+fp fp = ip - bp

  16. Volumetric Efficiency • Volumetric efficiency is the ratio actual volume flow rate of air into the intake system to the rate at which the volume is displaced by the system . ηv = ma / ρa V dispN/2 ρa = Inlet density SI = 80 to 85 % CI = 85 to 90 %

  17. Volumetric Efficiency • Alternative equivalent definition of Volumetric • = is atmospheric air density • = the pumping performance of the entire inlet system • And if it is taken as the air density in the manifold then is the pumping performance of the inlet port and valve only

  18. Relative Efficiency or Efficiency Ratio • Relative efficiency is the ratio thermal efficiency of actual cycle to ideal cycle . ηrel= Actual Thermal Efficiency Air Standard Efficiency

  19. Mean Effective Pressure ip = Indicated power [kW] pim= Indicated mean effective pressure [N/m2 ] L = Length of the stock [m] A = Area of the piston [m2] N = Speed in revolutions per minute [rpm] n = Number of stock N/2 & N K = Number of cylinder

  20. Mean Effective Pressure • indicated mean effective pressure is • Break mean effective pressure is ip = Indicated power [kW] pim= Indicated mean effective pressure [N/m2 ] L = Length of the stock [m] A = Area of the piston [m2] N = Speed in revolutions per minute [rpm] n = Number of stock N/2 & N K = Number of cylinder

  21. Mean Piston Speed • Sp= 2LN L = Length of the stock [m] N = Crank Speed in revolutions per minute [rpm]

  22. Specific Power Output • Ps = bp/A = constant x pbm x Sp

  23. Specific Fuel Consumption • sfc= Fuel Consumption per unit Time Power

  24. Inlet – Valve Mach Index (Z) • u = Gas velocity through the inlet valve • Ap= piston area • Ai = nominal intake valve operating area • Ci= inlet valve flow coefficient

  25. Inlet – Valve Mach Index (Z) • Where • b = cylinder diameter • Di = inlet valve diameter • V = mean piston speed • 𝛼 = inlet sonic velocity • C = inlet valve average flow coefficient • Z = inlet valve Mach index

  26. Fuel – Air ( F/A) or Air –Fuel Ratio (A/F) • Where • = 1 means stoichiometric ( chemically correct) mixture • ˂ 1 means lean mixture • ˃1 means rich mixture

  27. Calorific Value ( CV) • Calorific value of a is the thermal energy released per unit quantity of the fuel when the fuel is burning completely and the products of combustion are cooled back to the initial temperature of the combustible mixture.

  28. PROBLEMS

  29. PROBLEM NO 1 • The Mechanical effencicy of a single cylinder four stroke engine is 80 %. The frictional power is esitmated to be 25 kW. Calculate:- Ip bp

  30. SOLUTION • ηm = bp/ip • fp = ip - bp

  31. PROBLEM NO 2 • A 42.5 kW has a mechanical efficiency of 85%. Find the ip and fp. If the frictional power is assumed to be constant with load, What will be mechanical efficiency at 60 % of the load?

  32. SOLUTION • ηm = bp / ip • fp = ip – bp • ηm = bp / (bp + fp)

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