Mechanical Engineering Thermodynamics 2nd year Petrol dept. 2011-2012 Lecture (8) Internal Combustion Engines (ICE) Lecturer : Dr. Esmail Bialy
Air and Fuel will expand due to combustion, pushing thepiston downwards.
If we connected the piston to a crank mechanism, we canconvert the reciprocating motion into rotation.
Air and fuel need intakesystem to get into thecylinder.
Combustion productsneed exhaust system to bepushed out the cylinder.
Intake valve is oppened.
The Intake valve is closed,when the piston reachedthe BDC ( Bottom deadcenter).
Both valves are closed
Now ignition starts tobegin the heat additionprocess.
Exhaust valve beginsopening.
Engine Volume.Vcylinder= ¼π d2l.Where, d: cylinder diameter (bore) L: cylinder length (stroke)Vcylinders= ¼π d2l * z where, z: number of cylindersV˚Stroke = ¼π d2l * z * N/60ζWhere, N: number of crank shaft revolutions per minute (rpm) ζ: =2 for 4-stroke engines = 1 for 2-stroke engines
Engine Performance Parameters.a- Power:1- Brake Power: the measured power output of the engine BP= T * ωWhere,T: Torque (N.m)ω: radial speed (rad/s) and ω= 2πN/602- Indicated Power: is the theoretical power of a reciprocatingengine if it is completely frictionless in converting the expandinggas energy (piston pressure × displacement) in the cylinders. IP= BP + FP3- Friction Power FP: increases proportionally with N2
Engine Performance Parameters.b- Thermal efficiency:1- Brake thermal efficiency: ηb= BP/Q˚addWhere,Q˚add: rate of heat added to engine per second due to fuel burning2- Indicated thermal efficiency : ηI= IP/Q˚add Q˚add=m˚f * C.VWhere,m˚f : consumed fuel flowrate (kg/s)C.V: fuel heating value: heat released per each kg of completelyburned fuel (kJ/kg)
Engine Performance Parameters.c- mean effective pressure:1- Brake mean effective pressure: Bmep= BP/ V˚StrokeWhere,BP : brake powerV˚Stroke :engine cylinders volume2- Indicated mean effective pressure : Imep= IP/ V˚Stroke Mean effective pressure: a valuable measure of an engines capacityto do work that is independent of engine displacement.
Engine Performance Parameters.d- specific fuel consumption:1- Brake specific fuel consumption: Bsfc= m˚f /BPWhere,BP : brake powerm˚f : consumed fuel flowrate (kg/s)2- Indicated specific fuel consumption : Isfc= m˚f /IP
Example (6-3): An eight-cylinder, four stroke diesel engine develops 900 kWof brake power at 600 rpm. The cylinder size is 37 cm bore by 46cm stroke and the engine uses 4.5 kg of fuel per minute. Uponcomplete combustion, the fuel releases heat energy of 45 MJ/kg.The indicated mean effective pressure is 660 kPa. Calculate theindicated, brake and mechanical efficiencies.