Control of Heat Release Rate in S.I. Engines

P M V SubbaraoProfessor

Mechanical Engineering Department

Development of Models to Design & Select Hardware…...

Mass of Fuel per Cycle

• Accurate thermodynamic analysis of engine cycle determines the fuel to be inducted per cycle.

• This analysis cannot advocate the rate of consumption of this fuel mass with in the cycle.

• The pace and duration of fuel consumption is an essential knowledge in developing low emission and high fuel economy vehicular engines.

• Many hardware (mechatronic ) solutions along with software solutions are being continuously developed.

• Development of these solutions need a knowledge of fuel consumption vs crank angle.

Quasi-static Approach to Rapid Combustion

For an infinitesimal displacement of crank, d

Cumulative Fuel Consumption Model

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

LHVm

dd

Q

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

The Actual Release of Net Heat Rate in SI Engines

Phases of Combustion in Homogeneous SI Engine

Crank Angle,

Sparking

Start of Combustion

End of Combustion

Mixture Burn Time vs Engine Speed

The time for an overall burn is:

36060

%90%90

revkN

to

General acceptable value is 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.

Phases of Combustion Vs Generation of Air-fuel Mixture : SI Engines

• For spark ignition engines, Mixture preparation precedes all the other engine processes.

• Metering the ambient air and fuel and forming a mixture that satisfies the requirements of the engine over its entire operating regime is pre-decided.

• The structures of fuel spray dominates the mixture preparation process.

• This also strongly affect the durations of combustion phases over a wide range of operating conditions.