Nature of Heat Release Rate in an Engine

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Nature of Heat Release Rate in an Engine

• P M V Subbarao
• Professor
• Mechanical Engineering Department

The Pace of Net Heat Addition Influence the Area
of the Engine Cycle ..
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Net Heat Addition in an Engine

• m number of reactants convert into n number of
  products, in order to release the required heat
  in an engine cycle.
• Net available air will influence the number and
  quantity of products.
• The fraction of fuel chemical energy available
  due incomplete combustion is quantified using
  combustion efficiency.
• The net chemical energy release due to actual
  combustion with in the engine is

The combustion Efficiency
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The Approach to Estimate Optimal Designs

• At a time when optimum designs and systems need
  to be developed as quickly as possible and at
  minimum cost, the following advantages are
  particularly attractive.
• The development of a more complete understanding
  of the physical system that emerges during
  formulation of the model.
• The identification of key controlling variables
  which provides guidelines for rational and less
  costly experimental test programs.
• The ability to predict behavior over a wide range
  of design and operating variables.

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• These variables can be used
• to screen concepts prior to major hardware
  programs,
• to determine trends and tradeoffs and,
• if the model is sufficiently accurate, to
  optimize design and control.
• One need to strengthen commitment to continuous
  development of IC engine models that will be
  applicable to the problems at hand.

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Development of Phenomenological Combustion Model

• Combustion rate is generally specified by some
  functional relationship where the start of
  combustion,?soc, the combustion duration, ??, and
  the current crank angle are related.
• Two examples of such functions are the cosine
  burning law

• and the Wiebe function

the rate at which unburned mass is consumed by
the flame
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• The most common method of defining cumulative
  combustion is with a mass fraction burned curve.

• Mass fraction burned is the ratio of the
  cumulative heat release to the total heat
  release.
• Therefore if the mass fraction burned is known
  as a function of crank angle, then the apparent
  heat release can be approximated.
• When the cumulative mass fraction burned is
  graphed against crank angle degrees, the curve
  can be described using a Wiebe function

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The Wiebe function

• ? is crank angle degrees with ?soc corresponding
  to the initialization of heat release and
• ?? corresponding to the duration of burn.
• The equation is also defined by two constants a
  and m which have typical values of 5 and 3
  respectively.

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Measurement of Mass Burn Rate

• The parameters ?p and ?d represent the duration
  of the premixed and diffusion combustion phases.
• Qp and Qd represent the integrated energy
  release for premixed and diffusion phases
  respectively.
• The constants a, mp and md are selected to match
  experimental data.

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Real MFB Curve in an Engine
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Nature of Real Combustion

• The combustion has two modes.
• A dual mode combustion mass fraction burned
  curve is required to predict actual combustion
  process.
• To determine the best way to model the real
  combustion, the MFB curve is separated into two
  parts.
• The first section burns with an abundance of
  oxygen and should resemble a Wiebe curve.
• For the second half of the curve, the combustion
  is very slow and is nearly linear in many cases.

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Dual Phase MFB Curves
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Model Constants

• The values of a1 and a2 in the two Wiebe
  approximation ranged from 10 to 2000 and 3 to 37
  respectively.
• The values of the m constants also changed
  between the two types of approximations.
• The values of m1 and m2 ranged from 2 to 9 and 2
  to 8 instead of about 3.
• The value of x, the scale factor, ranges from 0.5
  to 0.9.

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Variation in Mass Fraction Burned CoefficientsSI
Engine
a1
m1
a2
m2
x
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The Actual Release of Net Heat Rate
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Mixture Burn Time vs Engine Speed
The time for an overall burn is
If we take a typical value of 50o crank angle
for the overall burn N (rpm)
t90(ms) Standard car at idle 500
16.7 Standard car at max power
4,000 2.1 Formula car at max power
19,000 0.4 Note To
achieve such high engine speeds a formula car
engine has a very short stroke and large bore.
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Design Centered Burn Rate Model

• One of the major criticisms of the
  phenomenological models has been the a priori
  specification of the burn rate.
• This has resulted in the use of engineering
  judgment for interpreting parameter studies
  produced by these models.
• In an attempt to alleviate this inadequacy,
  several investigators have proposed models to
  predict the rate of burning in spark-ignited
  engines.
• Ideally, such a model should be based on
  fundamental physical quantities such as
• The turbulent intensity urms
• The turbulent integral length scale, l
• The turbulent micro-scale, ?
• The specification of the kinetics.

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The model should be able to predict the ignition
delay time and combustion duration for variations
in engine operating conditions, i.e. spark
timing, EGR, equivalence ratio, load and engine
speed.
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Phases of Combustion in Homogeneous SI Engine
Crank Angle,q
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Air-fuel Mixture Formation

• For spark ignition engines, the fuel-air mixture
  preparation process is known to have a
  significant influence on engine performance and
  exhaust emissions.
• Mixture preparation precedes all the other engine
  processes by metering the ambient air and fuel
  and forming a mixture that satisfies the
  requirements of the engine over its entire
  operating regime.
• This has a dominant effect on the subsequent
  combustion process and control the engine fuel
  consumption, power output, exhaust emissions and
  other operating performance.
• The structures of port injector spray dominates
  the mixture preparation process and strongly
  affect the subsequent engine combustion
  characteristics over a wide range of operating
  conditions.

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Induction of Fuel in SI Engine

• The task of the engine induction and fuel systems
  is to prepare from ambient air and fuel in the
  tank an air-fuel mixture that satisfies the
  requirement of the engine.
• This preparation is to be carried out over entire
  engine operating regime.
• In principle, the optimum air-fuel ratio for an
  engine is that which give the required power
  output with the lowest fuel consumption.
• It should also ensure smooth and reliable
  operation.
• The fuel Induction systems for SI engine are
  classified as
• Carburetors.
• Throttle body Fuel Injection Systems.
• Multi Point Fuel Injection Systems.

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The Carburetor A Natural Fuel Induction System
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