www.thecartech.com 1 engine parameters. 2 combustion chamber crank shaft piston connecting rod tdc...
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Engine Parameters
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Combustion Chamber
Crank Shaft
Piston
Connecting Rod
TDC
BDC
Gasket
VC
VS Stroke
Stroke
Bore
Crank Radius (crank throw)
Crank Radius
Cylinder
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Compression ratio (r)
• 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.
C
SC
V
VVr
C
S
V
V1r
TDCatpistonaboveVolume
BDCatpistonaboveVolumer
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Engine Displacement, Swept Volume or Engine Capacity (Ve):
• 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
Stroke VS
Bore
VS VS VS
TDC
BDC
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Volumetric Efficiency V
ntDisplacemeEngine
EnginetheEnteringAirη
V
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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:
• 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 - Weak mixture - Compression ratio - Super charged
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Pressure, Force, Work & Power
p = imep (N/m2)A (m2)
F= P.A (N)
L (m)
F (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)
(Pi) engine = imep. (A.L.n) Ne / (k . 60)
(Pi) engine = [imep. Ve . Ne/ (k . 60)] (W)
a
b
c
k = 2 (four stroke)k = 1 (two stoke)
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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 …
Pi = imep.Ve.Ne / (60. k)
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Engine friction
Three types of friction-bearing 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 kWhere:bmep = brake mean effective pressurebmep = 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 = Tω =Te(2 Ne/60x1000) = 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 (Nm)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.
• (kg/h)/kw or kg/(kw h)b
.
PSFC m
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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.)
where: is the fuel consumption (kg/h)
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.
CV.ρ.
P3600)(ηefficiencythermalbrake
CV.
.6060.P)(ηefficiencythermalbrake
V
m
.b
th
.b
th
m.
V.
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Specific Fuel Consumption (SFC) & Thermal efficiency (th)
Where:th = thermal efficiency = fuel consumption (kg/h)Pb = brake power (kW)CV = calorific value (kJ)SFC = specific fuel consumption (kg/(kW.h))
CVSFC.
3600
CV.)/P(
3600
CV.
P3600η
b
..b
th
mm
m.
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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 Curves1. Imep
2. Bemp and torque
3. Indicated power
4. Brake power
5. Indicated thermal efficiency
6. Brake thermal efficiency
7. Specific fuel consumption