83354809 04-combustion-si-ci

April 10, 2023Slide 1

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

April 10, 2023 I.C. Engines Laboratory Slide 1

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




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83354809 04-combustion-si-ci