Nature of Heat Release Rate in an Engine - PowerPoint PPT Presentation

1 / 23
About This Presentation
Title:

Nature of Heat Release Rate in an Engine

Description:

One need to strengthen commitment to continuous development of IC engine models ... air-fuel ratio for an engine is that ... SI Engine Air-fuel Mixture ... – PowerPoint PPT presentation

Number of Views:114
Avg rating:3.0/5.0
Slides: 24
Provided by: abc95
Category:

less

Transcript and Presenter's Notes

Title: Nature of Heat Release Rate in an Engine


1
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 ..
2
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
3
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.

4
  • 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.

5
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
6
  • 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

7
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.

8
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.

9
Real MFB Curve in an Engine
10
(No Transcript)
11
(No Transcript)
12
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.

13
Dual Phase MFB Curves
14
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.

15
Variation in Mass Fraction Burned CoefficientsSI
Engine
a1
m1
a2
m2
x
16
The Actual Release of Net Heat Rate
17
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.
18
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.

19
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.
20
Phases of Combustion in Homogeneous SI Engine
Crank Angle,q
21
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.

22
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.

23
The Carburetor A Natural Fuel Induction System
1
Write a Comment
User Comments (0)
About PowerShow.com