04 combustion si ci

April 11, 2023 I.C. Engines Laboratory Slide 1April 11, 2023Slide 1

Dr. J. M. MallikarjunaInternal Combustion Engines Laboratory, Indian Institute of Technology Madras, Chennai - 600036

ME 323 : I.C EnginesME 323 : I.C Engines

Combustion in I.C EnginesCombustion in I.C Engines



INTRODUCTIONINTRODUCTION



Combustion

Conditions for combustion

Ignition Limits

Combustion Equations

Stoichiometric Combustion

Air requirement

Composition of air



Air-fuel ratio

Fuel-air ratio

Relative air-fuel ratio

Equivalence ratio

Pre-mixed and Diffusion combustion

Swirl/Squish/Turbulence



For all calculation : O2 = 21% ; N2 = 79% and ratio of N2/O2 is 3.76

Average molecular mass of air is 28.967 or 29 kg/kmol.



Swirl :

The orderly motion of the air particularly almost parallel to the axis of the engine.

Very much required for diesel engines.

Squish/Squash :

The radial inward motion of the air-fuel mixture towards (squish) and away from the axis of the engine (squash).

Very much required for the gasoline engines.

Turbulence :

Random mixing of the burned and unburned gases

very much required for both Otto and Diesel engines.



Normal Combustion Normal Combustion in SI Enginesin SI Engines



Combustion in SI engine



Flames detected between

Type of cycle

- 450 and TDC early burn cycle

TDC and 45° Fast burning cycles

450 and 900 slow burn cycles

90° and BDC Late burn cycles

BDC and TDC delayed burn cycles

Flames not detected misfires and partial burn cycles

Good combustion is almost entirely made up of fast burn cycles. poor combustion consists of a high proportion of late and delayed burn cycles.

COMBUSTION QUALITY



EFFECT OF ENGINE VARIABLES ON IGNITION LAG

1. Fuel

2. Mixture ratio:



3. Initial temperature and pressure

4. Electrode gap :



5. Turbulence:



EFFECT OF ENGINE VARIABLES ON FLAME PROPAGATION

1. Fuel-air ratio :



Rate of Burning



2. Compression Ratio



3. Intake temperature and pressure

4. Engine load

5. Turbulence

6. Engine Speed

7. Engine size



Combustion Chambers Combustion Chambers for for

Four Stroke SI EnginesFour Stroke SI Engines



Early Ricardo turbulent combustion chamber (1968)



Old type Combustion Chambers for SI engines



Combustion Chambers Combustion Chambers for for

Two Stroke SI EnginesTwo Stroke SI Engines



Abnormal Combustion Abnormal Combustion in SI Enginesin SI Engines



p-t diagram of a normal cycle



p-t diagram of a knocking cycle



Knock Limited Compression Ratio

Knock Limited Inlet Pressure

Knock Limited Indicated Mean Effective Pressure

Performance number is defined as the ratio of Klimep with the fuel in question to Klimep with isooctane when the inlet pressure is kept constant.

Relative performance number, rpn :

Actual Performance number rpn

Performance number corresponding to the imep of 100

Knock Limited Parameters



Density factors

Time factors

Composition factors

EFFECT OF ENGINE VARIABLES ON KNOCK



Compression Ratio

Mass of Inducted Charge

Inlet Temperature of the Mixture

Temperature of the Combustion Chamber Walls

Retarding the Spark Timing

Power Output of the Engine

Density Factors:



Turbulence

Engine Speed

Flame Travel Distance

Engine Size

Combustion Chamber Shape

Location of Spark Plug

Time Factors:



Fuel-Air Ratio:

Octane Value of the Fuel:

Composition Factors:



KNOCK RATING OF SI FUELS



Normal Combustion Normal Combustion in CI Enginesin CI Engines



Schematic representation of a fuel jet



Disintegration of a fuel jet



Effect of fuel-air ratio on power output of CI engine



Stages of combustion in CI engines



P-t diagram showing delay period



Steps of combustion process in CI engine



Combustion Rate (CI)



Effect of varying the amount of fuel

injected on P-θ diagram



Compression ratio Engine speed Power output Atomization of fuel and duration of

injection Injection timing Quality of the fuel Intake temperature Intake pressure

FACTORS AFFECTING THE DELAY PERIOD



Effect of compression ratio on maximum air temperature and minimum autoignition temperature



Effect of speed on ignition delay in a diesel engine



Effect of injection timing on indicator diagram



Effect of engine variables on ignition delay



Abnormal Combustion in CI Engines



Combustion Chambers for Four Stroke CI Engines



CI Engine Combustion chambers

• Open chamber or DI engine

• Divided chamber or IDI engine



Phases of Combustion in Toroidal open combustion

chamber



M-Combustion chamber



IDI Combustion chambers

Swirl generation: Induction induced swirl

Compression induced swirl

Combustion induced swirl



Combustion in Ricardo swirl Chamber - Comet



Swirl chambers



Pre-combustion chambers



Mercedes Benz Pre-combustion System



Advantages of DI engines

Fuels of poorer ignition quality can be used.

Single-hole injection nozzles and moderate injection pressures can be used and can tolerate greater degrees of nozzle fouling.

Higher fuel-air ratios can be used without smoke.



Disadvantages of IDI Engines

More expensive cylinder construction.

More difficult cold starting because of greater heat loss through the throat.

Poorer fuel economy due to greater heat losses and pressure losses through the throat, which result in lower thermal efficiency and higher pumping loss.



Knock Rating of CI Engine Fuel

• Cetane number

• Variable compression ratio engine

• CFR Engine



THE END

04 combustion si ci

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