cairo 2nd petrol lecture 8

Mechanical EngineeringThermodynamics

Lecture (8) Internal Combustion Engines (ICE)

Lecturer : Dr. Esmail Bialy

2nd year Petrol dept.2011-2012

Air and Fuel will expand due to combustion, pushing the piston downwards.

If we connected the piston to a crank mechanism, we can convert the reciprocating motion into rotation.

Air and fuel need intake system to get into the cylinder.

Combustion products need exhaust system to be pushed out the cylinder.

Intake valve is oppened.

The Intake valve is closed, when the piston reached the BDC ( Bottom dead center).

Both valves are closed

Now ignition starts to begin the heat addition process.

Exhaust valve begins opening.

Engine Volume.

Vcylinder= ¼π d2l.

Where, d: cylinder diameter (bore) L: cylinder length (stroke)

Vcylinders= ¼π d2l * z where, z: number of cylinders

V˚Stroke = ¼π d2l * z * N/60ζ

Where, 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/60

2- Indicated Power: is the theoretical power of a reciprocating engine if it is completely frictionless in converting the expanding gas energy (piston pressure × displacement) in the cylinders.

IP= BP + FP

3- Friction Power FP: increases proportionally with N2

Engine Performance Parameters.

b- Thermal efficiency:

1- Brake thermal efficiency: ηb= BP/Q˚add

Where, Q˚add: rate of heat added to engine per second due

to fuel burning

2- 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 completely burned fuel (kJ/kg)

Engine Performance Parameters.

c- mean effective pressure:

1- Brake mean effective pressure: Bmep= BP/ V˚Stroke Where, BP : brake powerV˚Stroke :engine cylinders volume

2- Indicated mean effective pressure :

Imep= IP/ V˚Stroke

Mean effective pressure: a valuable measure of an engine's capacity to 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

Engine Performance Parameters.

e- Engine efficiencies:

1- Mechanical efficiency : ηm= BP/IPηm= ηb/ηI

2- Volumetric efficiency :V˚Stroke )actual = ηv ¼π d2l * z * N/60ζ

Brake power calculation:BP=ηb*Q˚add

=ηb m˚f C.V=ηb m˚a F/A C.V=ηb ρa V˚Stroke F/A C.V=ηb ηv ρa F/A C.V ¼π d2l * z * N/60ζ

Q˚add=m˚f * C.VF/A = m˚f / m˚a

m˚a= ρa V˚Stroke

BP=ηb ηv ρa F/A C.V ¼π d2l * z * N/60ζ

Example (6-3):

An eight-cylinder, four stroke diesel engine develops 900 kW

of brake power at 600 rpm. The cylinder size is 37 cm bore by 46

cm stroke and the engine uses 4.5 kg of fuel per minute. Upon

complete combustion, the fuel releases heat energy of 45 MJ/kg.

The indicated mean effective pressure is 660 kPa. Calculate the

indicated, brake and mechanical efficiencies.