Nature of Heat Release Rate in an Engine

P M V SubbaraoProfessor

Mechanical Engineering Department

The Pace of Net Heat Addition Influence the Area of the

Engine Cycle …..

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:

HVactfcomb

EoC

SoCgen QmQ ,

productsjjf

o

iif

oambpambR hhmTHTH

,,j

reactants,,i

~y~x

The combustion Efficiency: HVactf

ambpambRcomb Qm

THTH

,

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.

• 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.

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

soc

ddX sin.

2• and the Wiebe function

1

exp1m

socm

soc amad

dX

the rate at which unburned mass is consumed by the flame

ddX

• The most common method of defining cumulative combustion is with a mass fraction burned curve.

LHVm

ddQ

MFBcombtotalf

gen

soc

,

•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

The Wiebe function

• θ is crank angle degrees with θsoc corresponding to the initialization of heat release and

• Δθ 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.

1

exp1m

socaMFB

Measurement of Mass Burn Rate

ddpp m

d

m

dd

d

d

m

p

m

pp

p

pgen amQaamQ

adQ

expexp11

ddRTm

ddpV

RC

ddVp

RC

dQ

dQ mixmixmix

mix

v

mix

vlossgen21

1

•The parameters θp and θd 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.

Real MFB Curve in an Engine

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.

Dual Phase MFB Curves

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.

Variation in Mass Fraction Burned CoefficientsSI Engine

a1

a2

m1

m2

x

The Actual Release of Net Heat Rate

Mixture Burn Time vs Engine Speed

The time for an overall burn is:

360

60min

%90%90

revsN

to

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.

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.

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.

Phases of Combustion in Homogeneous SI Engine

Crank Angle,

Ignition

Start of Combustion

End of Combustion

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.

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.

The Carburetor: A Natural Fuel Induction System

1