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Engine Parameters


Combustion Chamber

Crank Shaft

Piston

Connecting Rod

TDC

BDC

Gasket

VC

VS Stroke

Stroke

Bore

Crank Radius (crank throw)

Crank Radius

Cylinder


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


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


Volumetric Efficiency V

ntDisplacemeEngine

EnginetheEnteringAirη

V


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%.


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


Factors affecting (imep)

- Retarded ignition - Weak mixture - Compression ratio - Super charged


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)


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)


Engine friction

Three types of friction-bearing surfaces in automobile engines:

• Journal• Guide• Thrust


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.


Engine Mechanical Efficiency m

• Pb = Pi - Pf

Where:

Pi = indicated power

Pb= brake power

Pf = friction power

• m = Pb / Pi


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.


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.


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


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


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.


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).


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


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.


Thermal Efficiency (Cont.)


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.


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.


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.


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


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

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