Performance Factors

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Performance Factors

• Volumetric Efficiency

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1a. Indicated Power.

• Indicated Power (IP) Power obtained at the
  cylinder. Obtained from the indicator diagram.
  Given by
• IP PiLANn/60x in Watts
• where Pi is the indicated mean effective
  pressure, in N/m2,
• L is the stroke length, in m
• A is the area of cross section
  of the piston, m2,
• N is the engine speed in rev/min,
• n is the number of cylinders and
• x 1 for 2 stroke and 2 for 4 stroke
  engine.

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1b. Brake Power

• Brake Power (BP) Power obtained at the shaft.
  Obtained from the engine dynamometer.
• Given by
• BP 2?NT/60 in Watts
• where T is the brake torque, in Nm, given by
• T W.L
• where W is the load applied on the shaft by the
  dynamometer, in N and
• L is the length of the arm where the
  load is applied, in m
• N is the engine speed, in rev/min

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1c. Friction Power

• Friction Power (FP) Power dissipated as
  friction. Obtained by various methods like Morse
  test for multi-cylinder engine, Willans line
  method for a diesel engine, and Retardation test
  and Motoring test for all types of engines. Given
  in terms of IP and BP by
• FP IP BP in Watts

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2. Mean Effective Pressure.

• Indicated Mean Effective Pressure (IMEP). This is
  also denoted by Pi and is given by
• Pi (Net work of cycle)/Swept Volume in N/m2
• The net work of cycle is the area under the P-V
  diagram.
• Brake Mean Effective Pressure (BMEP). This is
  also denoted by Pb and is given by
• Pb 60.BPx/(LANn) N/m2
• This is also the brake power per unit swept
  volume of the engine.
• Friction Mean Effective Pressure (FMEP). This is
  also denoted by Pf and is given by
• Pf Pi - Pb N/m2

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3. Efficiencies.

• Indicated Thermal Efficiency (?i) given by
• ?i IP/(mf . Qcv)
• mf is the mass of fuel taken into the engine in
  kg/s
• Qcv is the calorific value of the fuel in J/kg
• Brake Thermal Efficiency (?b) given by
• ?b BP/(mf . Qcv)
• Indicated Relative Efficiency (?i,r) given by
• ?i,r ?i/ASE
• ASE is the efficiency of the corresponding air
  standard cycle
• Brake Relative Efficiency (?b,r) given by
• ?b,r ?b/ASE
• Mechanical Efficiency (?m) given by
• ?m BP/IP Pb/Pi ?b/?i
  ?b,r/?I,r

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Specific Fuel Consumption (sfc or SFC)

• This is the fuel consumed per unit power.
• Brake Specific Fuel Consumption (bsfc). This is
  given by
• bsfc mf/BP kg/J
• if BP is in W and mf is in kg/s
• bsfc is usually quoted in kg/kWh. This is
  possible if BP is in kW and mf is in kg/h.
• Indicated Specific Fuel Consumption (isfc). This
  is given by
• isfc mf/IP kg/J
• if IP is in W and mf is in kg/s
• isfc is also usually quoted in kg/kWh. This is
  possible if IP is in kW and mf is in kg/h.
• Mechanical Efficiency in terms of the sfc values
  is given by
• ?m isfc/bsfc

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Specific Energy Consumption (sec or SEC).

• This is the energy consumed per unit power.
• Brake Specific Energy Consumption (bsec). This is
  given by
• bsec bsfc.Qcv
• We can similarly define indicated specific energy
  consumption (isec) and based on the two
  quantities also we can define mechanical
  efficiency.

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Air Capacity of Four-stroke cycle Engines

• The power, P, developed by an engine is given by
• Power will depend on air capacity if the quantity
  in the bracket is maximized.
• Plot of power versus air flow rate is normally a
  straight line.

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Volumetric Efficiency

• Indicates air capacity of a 4 stroke engine.
  Given by
• Mi is the mass flow rate of fresh mixture.
• N is the engine speed in rev/unit
  time.
• Vs is the piston displacement (swept
  volume).
• ?i is the inlet density.

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Volumetric Efficiency

• Can be measured
• At the inlet port
• Intake of the engine
• Any suitable location in the intake manifold
• If measured at the intake of the engine, it is
  also called the overall volumetric efficiency.

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Volumetric Efficiency Based on Dry Air

• Since there is a linear relationship between
  indicated output (power) and air capacity
  (airflow rate), it is more appropriate to express
  volumetric efficiency in terms of airflow rate
  (which is the mass of dry air per unit time).
• Since fuel, air and water vapor occupy the same
  volume
• Va Vf Vw Vi
• Thus we have

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Here ?a is the density of dry air or the mass of
dry air per unit volume of fresh mixture. Thus,
since
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Also Vd ApL s 2LN
L is the piston stroke and s is the piston speed.
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Measurement of Volumetric Efficiency in Engines

• The volumetric efficiency of an engine can be
  evaluated at any given set of operating
  conditions provided and ?a can be
  accurately measured.
• Measurement of Air Flow
• Airflow into the engine can be measured with the
  help of a suitable airflow meter. The
  fluctuations in the airflow can be reduced with
  the help of surge tanks placed between the engine
  and the airflow meter.

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Measurement of Inlet Air Density

• By Daltons Law of partial pressures
• pi pa pf pw
• In this case pi is the total pressure of the
  fresh mixture,
• pa is the partial pressure of
  air in the mixture,
• pf is the partial pressure of
  fuel in the mixture,
• pw is the partial pressure of
  water vapor in the air.
• Since each constituent is assumed to behave as a
  perfect gas, we can write

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(No Transcript)
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M indicates mass of the substance, 29 is the
molecular weight of air, mf is the molecular
weight of the fuel, and 18 is the molecular
weight of water vapor.
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Fi is the ratio of mass of fuel vapor to that of
dry air and h is the ratio of mass of water vapor
to that of dry air at the point where pi and Ti
are measured.
This indicates that the density of air in the
mixture is equal to the density of air at pi and
Ti multiplied by a correction factor, that is,
the quantity in the parentheses.
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• The value of h depends on the humidity ratio of
  the air and is obtained from psychrometric
  charts.
• For conventional hydrocarbon fuels, the
  correction factor is usually around 0.98, which
  is within experimental error. For diesel engines
  and GDI engines, Fi is zero.
• In practice, with spark ignition engines using
  gasoline and with diesel engines the volumetric
  efficiency, neglecting the terms in the
  parentheses, is given by

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If we do not neglect the terms in the parentheses
we get the following relation for volumetric
efficiency
If the humidity is high or a low molecular weight
fuel is used in a carbureted engine, the
correction factor cannot be ignored. For
example, with methanol at stoichiometric
conditions and h 0.02, the correction factor
is 0.85.
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Volumetric Efficiency, Power and Mean Effective
Pressure

• Since
• and

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For an engine, the mean effective pressure, mep,
is given by
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Ways to increase power and mep

• The mean effective pressure may be indicated or
  brake, depending on whether ? is indicated or
  brake thermal efficiency. Thus, the mean
  effective pressure is proportional to the product
  of the inlet density and volumetric efficiency
  when the product of the thermal efficiency, the
  fuel-air ratio, and the heat of combustion of the
  fuel is constant.
• From the preceding two expressions we can figure
  out ways to increase the power and mep of an
  engine.