Transcript معاملات المحرك Engine Parameters

Engine Parameters
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VC
Combustion
Chamber
Gasket
TDC
Piston
VS
Stroke
Cylinder
BDC
Connecting
Rod
Bore
Crank Radius
Crank Shaft
Stroke
Crank Radius (crank throw)
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Compression ratio (r)
Volume above piston at BDC
r
Volume above piston at TDC
VC  VS
r
VC
VS
r 1
VC
• VC = Clearance volume
• VS = Swept volume = /4 D2 L
where: L (stroke) = 2 ρ, ρ is the crankshaft radius
- Increasing the compression ration increases the thermal
efficiency, compression is limited by the knock limit.
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Engine Displacement, Swept
Volume or Engine Capacity (Ve):
TDC
Stroke
VS
VS
VS
VS
BDC
Bore
• Ve = VS n
• Ve = (/4) D2 L n
Where:
Ve = engine capacity, Vs = cylinder swept volume
n = number of cylinders, L = stroke, D = bore diameter
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Volumetric Efficiency V
Air Entering the Engine
ηV 
Engine Displacement
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Volumetric Efficiency V (cont.)
• Engines are only capable of 80% to 90%
volumetric efficiency.
• Volumetric efficiency depends upon throttle
opening and engine speed as well as induction
and exhaust system layout, port size and valve
timing and opening duration.
• High volumetric efficiency increases engine
power.
• Turbo charging is capable of increasing
volumetric efficiency up to 50%.
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Indicated mean effective pressure
(imep)
Factors affecting imep:
•
•
•
•
•
•
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Compression ratio
Air/fuel ratio
Volumetric efficiency
Ignition timing
Valve timing and lift
Air pressure and
temperature
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Factors affecting (imep)
- Retarded ignition
- Compression ratio
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- Weak mixture
- Super charged
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Pressure, Force, Work & Power
a
F (N)
A (m2)
c
p = imep (N/m2)
L (m)
b
F= P.A (N)
Work (W) = F.L (N m)
Time (t) = 60 / (Ne /k) (s)
Indicated power (Pi) cylinder = W/t = F.L .Ne/(k*60) (W)
(Pi) cylinder = (imep.A.L.Ne) / (k . 60)
k = 2 (four stroke)
k = 1 (two stoke)
(Pi) engine = imep. (A.L.n) Ne / (k . 60)
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(Pi) engine = [imep. Ve . Ne/ (k . 60)] (W)
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Engine Indicated Power (Pi)
Engine power factors:
• Engine capacity (Ve)
• Engine Speed (rpm) (Ne)
• Number of strokes “k”
k=2, four stroke engine
k=1, two stoke engine
• (imep):
volumetric efficiency,
compression ratio,
ignition quality, mixture
strength, temperature …
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Pi = imep.Ve.Ne / (60. k)
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Engine friction
Three types of frictionbearing surfaces in
automobile engines:
• Journal
• Guide
• Thrust
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Engine Brake Power (Pb)
-This is the power developed at the
crankshaft or flywheel.
-The term brake originated from the method
used to determine an engine’s power
output by measuring the torque using
some form of friction dynamometer.
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Engine Mechanical Efficiency m
• Pb = Pi - Pf
Where:
Pi = indicated power
Pb= brake power
Pf = friction power
• m = Pb / Pi
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Engine Brake Power (Pb)
• Pb = Pi m
• Pb = (imep Ve Ne / 60 k) m
• Pb = (imep m)Ve Ne / 60 k
• Pb = bemp Ve Ne / 60 k
Where:
bmep = brake mean effective pressure
bmep = imep m
* bmep is indication of engine efficiency regardless
of capacity or engine speed, 1000 kPa represent
high efficiency.
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Gross & Net Brake Power
• Gross brake power is
measured without the
following items:
Cooling fan, coolant
pump, radiator, alternator,
exhaust system. (SAE)
• Net brake power is
measured with all the
above items. (DIN)
• Gross power is 10-15%
more than net power.
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Engine Torque Te
Torque and crankshaft angle:
Work is also accomplished
when the torque is applied
through an angle.
• Distance xy = rθ
• W = F . xy = F r θ = T θ
• W per one revolution = T (2)
• P = W/t = T (2)/t = Tω/1000
Where: ω = 2 Ne/60
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Engine Torque Te (Cont.)
• Pb = T ω = Te (2 Ne/60) = Te Ne / 9550 (kW)
• bmep . Ve . Ne / k 60 = Te (2 Ne/60)
• Te = bmep . Ve / 2 . K
Where:
Pe = Engine power (kW)
Ne = Engine speed (rpm)
Te = Engine torque (N m)
bemp = brake mean effective pressure (Pa)
Ve = engine capacity (m3)
k = 2, for 4-stroke engines
1, for 2-stroke engines
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Engine Torque Te (Cont.)
- There is a direct
relationship between
BMEP and torque
output.
- The torque curve with
engine rpm is identical to
the bmep curve, with
different values.
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Engine Fuel consumption (FC)
The amount of fuel an engine consumes can
be measured by:
• volume (cm3 or liter) per (sec. or mint, or hr)
or
• mass (kg) per (sec, or mint, or hr).
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Engine Specific Fuel Consumption
(SFC)
• Specific fuel consumption represents the
mass or volume of fuel an engine
consumes per hour while it produces 1 kW
of power.
• Typical gasoline engines will have an SFC
of about 0.3 kg/(kW.h).
• SFC is an indication of the engine’s
thermal or heat efficiency.
m
SFC

•
(kg/h)/kW or kg/(kW h)
.
Pb
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Engine Thermal Efficiency (th)
• The efficiency of an engine in converting the
heat energy contained in the liquid fuel into
mechanical energy is termed its thermal
efficiency.
• The petrol engine is particularly inefficient and
at its best may reach 25% efficiency.
• The thermal efficiency of a diesel engine can
reach 35% due to its higher compression
ratio.
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Thermal Efficiency (Cont.)
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Thermal Efficiency (th) (Cont.)
brake thermal efficiency (η th ) 
Pb . 60 .60
brake thermal efficiency (η th ) 
3600 Pb
.
m . CV
.
V . ρ . CV
where:
.
m is the fuel consumption (kg/h)
V is the fuel consumption (L/h)
CV is the calorific or heat value of 1 kg of the fuel
(kJ/kg or MJ/kg). (CV for gasoline is 40000 kJ/kg)
ρ is the relative density (kg/L) of the fuel.
.
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Specific Fuel Consumption (SFC)
& Thermal efficiency (th)
3600 Pb
3600
3600
η th  .
 .

m . CV (m /Pb ) . CV SFC. CV
Where:
th = thermal efficiency
.
m = fuel consumption (kg/h)
Pb = brake power (kW)
CV = calorific value (kJ)
SFC = specific fuel consumption (kg/(kW.h))
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Specific Fuel Consumption (SFC)
& Thermal efficiency (th)
• A mirror reflection of
the SFC curve shows
the shape of the
engine’s thermal
efficiency curve.
• The lowest point on
the SFC curve
becomes the highest
point on the thermal
efficiency curve.
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Power Units
• BHP (bhp) = 550 ft lb/s
• PS = 75 kg m/s
• kW = 1000 (N m/s)
BHP = British and American “horse power”
PS ="PferdeStärke“ is "horse power“ in
German
• PS = 0.986 bhp, BHP = 1.0142 PS
• kW = 1.36 PS, PS = 0.73529 kW
• kW = 1.341 bhp, BHP = 0.7457 kW
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Engine Performance Curves
1.
2.
3.
4.
5.
Imep
Bemp and torque
Indicated power
Brake power
Indicated thermal
efficiency
6. Brake thermal
efficiency
7. Specific fuel
consumption
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