Title: Nature of Heat Release Rate in an Engine
1Nature 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 ..
2Net 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
3The 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.
5Development 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
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
7The 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.
8Measurement 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.
9Real MFB Curve in an Engine
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12Nature 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.
13Dual Phase MFB Curves
14Model 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.
15Variation in Mass Fraction Burned CoefficientsSI
Engine
a1
m1
a2
m2
x
16The Actual Release of Net Heat Rate
17Mixture 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.
18Design 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.
19The 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.
20Phases of Combustion in Homogeneous SI Engine
Crank Angle,q
21Air-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.
22Induction 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.
23The Carburetor A Natural Fuel Induction System
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