Indicator Diagrams and Internal Combustion Engine Performance Parameters

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Indicator Diagrams and Internal Combustion
EnginePerformance Parameters

• Much can be learned from a record of the cylinder
  pressure and volume. The results can be analyzed
  to reveal the rate at which work is being done by
  the gas on the piston, and the rate at which
  combustion is occurring. In its simplest form,
  the cylinder pressure is plotted against volume
  to give an indicator diagram.

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Pressure-Volume Graph 4-stroke SI engine One
power stroke for every two crank shaft revolutions
Pressure
Spark
Exhaust valve opens
Exhaust valve closes
Intake valve closes
1 atm
Intake valve opens
TC
BC
Cylinder volume
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Efficiency

• In general, energy conversion efficiency is the
  ratio between the useful output of a device and
  the input. For thermal efficiency, the input, to
  the device is heat, or the heat-content of a fuel
  that is consumed. The desired output is
  mechanical work, or heat, or possibly both.
  Because the input heat normally has a real
  financial cost, a memorable, generic definition
  of thermal efficiency is

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• When expressed as a percentage, the thermal
  efficiency must be between 0 and 100. Due to
  inefficiencies such as friction, heat loss, and
  other factors, thermal engines' efficiencies are
  typically much less than 100. For example, a
  typical gasoline automobile engine operates at
  around 25 efficiency. The largest diesel engine
  in the world peaks at 51.7.

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• Work done on the piston due to pressure

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• The term indicated work is used to define the net
  work done on the piston per cycle
• the indicated mean effective pressure (imep),can
  be defined by

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• The imep is a hypothetical pressure that would
  produce the same indicated work if it were to act
  on the piston throughout the expansion stroke.
  The concept of imep is useful because it
  describes the thermodynamic performance of an
  engine, in a way that is independent of engine
  size and speed and frictional losses.
• Unfortunately, not all the work done by the gas
  on the piston is available as shaft work because
  there are frictional losses in the engine. These
  losses can be quantified by the brake mean
  effective pressure (bmep,), a hypothetical
  pressure that acts on the piston during the
  expansion stroke and would lead to the same brake
  work output in a frictionless engine.

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Mechanical Efficiency
Some of the power generated in the cylinder is
used to overcome engine friction and to pump gas
into and out of the engine. The term friction
power, , is used to describe collectively
these power losses, such that
Friction power can be measured by motoring the
engine. The mechanical efficiency is defined as
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Mechanical Efficiency, contd

• Mechanical efficiency depends on pumping losses
  (throttle position) and
• frictional losses (engine design and engine
  speed).
• Typical values for automobile engines at WOT
  are
• 90 _at_2000 RPM and 75 _at_ max speed.
• Throttling increases pumping power and thus the
  mechanical efficiency
• decreases, at idle the mechanical efficiency
  approaches zero.

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• Brake Specific Fuel Consumption (BSFC) is a
  measure of fuel efficiency within a
  shaft reciprocating engine. It is the rate
  of fuel consumption divided by the power produced.
  Specific fuel consumption is based on the torque
  delivered by the engine in respect to the fuel
  mass flow delivered to the engine. Measured after
  all parasitic engine losses is brake specific
  fuel consumption BSFC and measuring specific
  fuel consumption based on the in-cylinder
  pressures (ability of the pressure to do work) is
  indicated specific fuel consumption ISFC.

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• The final parameter to be defined is the
  volumetric efficiency of the engine the ratio of
  actual air flow to that of a perfect engine is
• In general, it is quite easy to provide an engine
  with extra fuel therefore, the power output of
  an engine will be limited by the amount of air
  that is admitted to an engine.

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The Ideal Air Standard Otto Cycle
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• Systems which are thermally insulated from their
  surroundings undergo processes without any heat
  transfer such processes are called adiabatic.
  Thus during an isentropic process there are no
  dissipative effects and the system neither
  absorbs nor gives off heat.
• A reversible process, is a process that can be
  "reversed" by means of infinitesimal changes in
  some property of the system without loss
  or dissipation of energy.
• Isentropic process is a process which is a
  process is both adiabatic and reversible .

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• A closed cylinder with a locked piston contains
  air. The pressure inside is equal to the outside
  air pressure. This cylinder is heated to a
  certain target temperature. Since the piston
  cannot move, the volume is constant, while
  temperature and pressure rise. When the target
  temperature is reached, the heating is stopped.
  The piston is now freed and moves outwards,
  expanding without exchange of heat (adiabatic
  expansion). Doing this work cools the air inside
  the cylinder to below the target temperature. To
  return to the target temperature (still with a
  free piston), the air must be heated. This extra
  heat amounts to about 40 more than the previous
  amount added. In this example, the amount of heat
  added with a locked piston is proportional to CV,
  whereas the total amount of heat added is
  proportional to CP. Therefore, the heat capacity
  ratio in this example is 1.4

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The Ideal Air Standard Diesel Cycle
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Efficiencies of Real Engines

• The efficiencies of real engines are below those
  predicted by the ideal air standard cycles for
  several reasons. Most significantly, the gases in
  internal combustion engines do not behave
  perfectly with a ratio of heat capacities.

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• Consider a spark ignition engine with a
  compression ratio of 10, for which the Otto cycle
  efficiency predicts an efficiency of 60 and the
  fuel-air cycle predicts an efficiency of 47 for
  stoichiometric operation. In reality, such an
  engine might have a full throttle brake
  efficiency of 30, and this means 17 percentage
  points must be accounted

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Ignition and Combustion in Spark Ignitionand
Diesel Engines

• Spark ignition (SI) engines usually have
  pre-mixed combustion, in which a flame front
  initiated by a spark propagates across the
  combustion chamber through the unburned mixture.
  Compression ignition (CI) engines normally inject
  their fuel toward the end of the compression
  stroke, and the combustion is controlled
  primarily by diffusion.
• Whether combustion is pre-mixed (as in SI
  engines) or diffusion controlled (as in CI
  engines) has a major influence on the range of
  air-fuel ratios (AFRs) that will burn.
• In pre-mixed combustion, the AFR must be close to
  stoichiometric-the AFR value that is chemically
  correct for complete combustion. In practice,
  dissociation and the limited time available for
  combustion will mean that even with the
  stoichiometric AFR, complete combustion will not
  occur.
• In diffusion combustion, much weaker AFRs can be
  used (i.e., an excess of air) because around each
  fuel droplet will be a range of flammable AFRs.
• Typical ranges for the (gravimetric) air-fuel
  ratio are as follows

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• Diesel engines have a higher maximum
  efficiency than spark ignition engines for three
  reasons
• 1. The compression ratio is higher.
• 2. During the initial part of compression, only
  air is present.
• 3. The air-fuel mixture is always weak of
  stoichiometric.

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Simple Combustion Equilibrium

• For a given combustion device, say a piston
  engine, how much fuel and air should be injected
  in order to completely burn both? This question
  can be answered by balancing the combustion
  reaction equation for a particular fuel. A
  stoichiometric mixture contains the exact amount
  of fuel and oxidizer such that after combustion
  is completed, all the fuel and oxidizer are
  consumed to form products.

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• Combustion stoichiometry for a general
  hydrocarbon fuel, with air can be expressed as
• The amount of air required for combusting a
  stoichiometric mixture is called stoichiometric
  or theoretical air.

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Methods of Quantifying Fuel and Air Contentof
Combustible Mixtures

• In practice, fuels are often combusted with an
  amount of air different from the stoichiometric
  ratio. If less air than the stoichiometric amount
  is used, the mixture is described as fuel rich.
  If excess air is used, the mixture is described
  as fuel lean. For this reason, it is convenient
  to quantify the combustible mixture using one of
  the following commonly used methods
• Fuel-Air Ratio (FAR) The fuel-air ratio, f, is
  given by

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• Equivalence Ratio Normalizing the actual
  fuel-air ratio by the stoichiometric fuel air
  ratio gives the equivalence ratio,
• The subscript s indicates a value at the
  stoichiometric condition. f lt1 is a lean mixture
  , f¼1 is a stoichiometric mixture, and f gt1 is a
  rich mixture
• Lambda is the ratio of the actual air-fuel ratio
  to the stoichiometric air-fuel ratio defined as

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Fuel Requirements

• Gasoline is a mixture of hydrocarbons (with 4 to
  approximately 12 carbon atoms) and a boiling
  point range of approximately 30-200C. Diesel
  fuel is a mixture of higher molarmass
  hydrocarbons (typically 12 to 22 carbon atoms),
  with a boiling point range of approximately180-380
  C. Fuels for spark ignition engines should
  vaporize readily and be resistant to
  self-ignition, as indicated by a high octane
  rating. In contrast, fuels for compression
  ignition engines should self-ignite readily, as
  indicated by a high cetane number.

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• Octane number is a standard measure of the
  anti-knock properties (i.e. the performance) of a
  motor or aviation fuel. The higher the octane
  number, the more compression the fuel can
  withstand before detonating. In broad terms,
  fuels with a higher octane rating are used in
  high-compression engines that generally have
  higher performance.
• Knocking (also called knock, detonation, spark
  knock, pinging or pinking) in spark-ignition
  internal combustion engines occurs when
  combustion of the air/fuel mixture in the
  cylinder starts off correctly in response to
  ignition by the spark plug, Effects of engine
  knocking range from inconsequential to completely
  destructive.
• .

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• Cetane number or CN is a measurement of the
  combustion quality of diesel fuel during
  compression ignition. It is a significant
  expression of diesel fuel quality among a number
  of other measurements that determine overall
  diesel fuel quality.

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• The octane or cetane rating of a fuel is
  established by comparing its ignition quality
  with respect to reference fuels in CFR
  (Co-operative Fuel Research) engines, according
  to internationally agreed standards. The most
  common type of octane rating worldwide is the
  Research Octane Number (RON). RON is determined
  by running the fuel in a test engine with a
  variable compression ratio under controlled
  conditions, and comparing the results with those
  for mixtures of iso-octane and n-heptane.

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Engine Configuration

• After the type and size of engine have been
  determined, the number and disposition of the
  cylinders must be decided. The main constraints
  influencing the number and disposition of the
  cylinders are as follows
• 1. The number of cylinders needed to produce a
  steady output
• 2. The minimum swept volume for efficient
  combustion
• 3. The number and disposition of cylinders for
  satisfactory balancing
• 4. The number of cylinders needed for an
  acceptable variation in the torque output

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• The "V" engines form a very compact power unit. A
  more compact arrangement is the "H" configuration
  (in effect, two horizontally opposed engines with
  the crankshafts geared together), but this is an
  expensive and complicated arrangement that has
  had limited use. Whatever the arrangement, it is
  unusual to have more than six or eight cylinders
  in a row because torsional vibrations in the
  crankshaft then become much more troublesome.
  Nonetheless, the final decision on the engine
  configuration also will be influenced by
  marketing, packaging, and manufacturing
  constraints.

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