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(3) Noise / Vibration
Noise :- measured directly - subject to noise regulations.
Vibration :- can be detrimental, leading to failure of major and
auxiliary
engine parts; engine must be dynamically balanced anti -
vibration mounts prevent vibration transmission ; field of study in
it's own right
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(4) Engine Efficiencies
Mechanical efficiency ( as above )
Thermal efficiency (h th) =m x LCV fuel
In practical terms an engine's fuel efficiency is measured
byit's Specific Fuel Consumption (SFC), also calledBRAKE Specific
Fuel Consumption (BSFC), recognising thatthe engines power output
is measured using a 'brake'.
Fuel flow (kg/h)Brake Power (kW)
W
Volumetric efficiency (a measure of how well the
engine'breathes')
Vol. eff. ( h vol) =Induced air volume flow rate (m /min)
Engine capacity x RPM 2-stroke
3
[kg/kWh]
RPM/2 4-stroke
BSFC =
NB: Engine capacity = total swept volume of theengine = L x A x
n (n = number of cylinders)
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(5) Exhaust Emissions
The nature of the exhaust emissions is dependent upon the
fuelused, the quality of the combustion process, and the presenceof
contaminant substances such as sulphur or oil additives.
Most IC engine fuel molecules consist of approximately
equalnumbers of Carbon and Hydrogen atoms - and can be repre-sented
by the chemical formula C 6H6
The combustion process consists of these fuel molecules
breaking down and combining with the oxygen of the air to
giveenergy (in the form of the heat of combustion) and the
productsof combustion.
If the combustion process were ideal or complete all of
thecarbon would oxidise to CO 2, and all of the hydrogen
wouldoxidise to give water (in the form of steam) H 2O.As a
chemical equation we would write:
C6H6 + y (0.21O 2 + 0.79N 2) > a CO 2 + b H 2O + c O 2 + d N
2
moleculeof fuel
moleculeof air
air/fuel ratio(molecular)
excessoxygen
The value of y is critical to the combustion process.The
chemically correct value which results in completecombustion and no
excess oxygen is known as thestoichiometric air/fuel ratio
(AFR).Above this value the peak combustion temperature reduces;
below this value partially oxidised products are formed -in
particular pure carbon (soot) and carbon monoxide CO.
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(6) Stability / Controlability
Usually has to be evaluated in conjunction with the load it
isdriving
Engine / Load Matching
t o r q u e
speed
torque/speedcharacteristicof a ship
torque/speed characteristics ofthe engine at different
throttlesettings (or gears)
Stable operating points
unstable operating point(stall!)
