combustion chambers

INTERNAL COMBUSTION ENGINES-II CI Combustion Chambers

University of Petroleum & Energy Studies

Dehradun

I C ENGINES – II : CI Combustion Chambers

CI combustion chamber :

- provide proper mixing of fuel and air in short time

- An organized air movement ;

swirl, turbulence,squish

- produce high relative velocity between fuel

droplets and air

- Shape of combustion chamber control air motion

- CI engines classified as ;

Direct- Injection (DI) & Indirect- Injection (IDI)


Direct- Injection (DI)

- entire volume of combustion chamber located in

main cylinder where fuel is injected. Also called

open combustion chamber.

Indirect- Injection (IDI)

- Combustion space divided into two parts ;

• one part main cylinder

• other part in cylinder head


Indirect- Injection (IDI)

- Classified as :

1. Swirl chamber ;

compression swirl generated

2. Precombustion chamber ;

combustion swirl induced

3. Air cell chamber ;

compression & combustion swirl induced


Direct- Injection (DI) Chambers

- open comb. Chamber, single cavity w/o

restrictions

- no large press. difference within the chamber

- induction swirl obtained by intake; passage,

valves or ports

- chambers consists of space between a flat

cylinder head and a cavity in piston crown

- fuel injected with multihole injectors at high pres



Some of the Open Combustion Chamber Designs



- Shallow Depth Chambers: depth of cavity in piston is

quite small, usually adopted to low speed - large

engine,cavity dia. being very large squish is negligible.

- Hemispherical Chamber: depth to diameter ratio can

be varied to give desired squish which is generally

small.

- Cylindrical Chamber: modification of cylindrical

chamber in the form of a truncated cone with base



angle of 30º. The swirl produced by masking the

intake valve for nearly 180º of circumference. Squish

can be varied by varying the depth.

- Toroidal Chamber: This shape provides a powerful

squish along with the air movement. Mask needed on

inlet valve is small providing spray cone angle of 150º

to 160º with better air utilization.



Advantages :

-Minimum heat loss during compression because of lower

surface area to volume ratio and hence better efficiency.

- No cold start problems.

- Fine atomization because of multihole nozzles

Drawbacks :

- Complex design of fuel injection pump due to high fuel injection

pressure requirements

- Necessity of accurate metering of fuel by injection system ,

particularly for small engines


Indirect- Injection (IDI) Chambers

- A divided combustion chamber space in

two or more distinct compartments

connected by restricted passages.

- During combustion, considerable

pressure differences are created between

the compartments.

I C ENGINES – II : CI Combustion Chambers Indirect- Injection (IDI) Chambers

- Swirl Chamber: It is a spherical-shaped chamber

separated from cylinder and located in the

cylinder head as shown


- About 50 % air is transferred during compression

stroke into the swirl chamber

- A throat connects the chamber to the cylinder

tangentially so that air gets strong rotary movement

inside the swirl chamber. After combustion the

products rush back into cylinder through same throat

at much higher velocity.

- The heat loss to the wall of passage is high

- applications; poor fuel quality, reliability under

adverse conditions

I C ENGINES – II : CI Combustion Chambers - Precombustion Chamber: A typical precombustion

chamber consists of an antichamber connected to

main chamber through a number of small holes is as

shown :

I C ENGINES – II : CI Combustion Chambers - The precombustion chamber located in cylinderhead

accounts for 40 % volume of total combustion space

- During compression piston forces air into precomb.

chamber, fuel injected and combustion initiated in it

-The flaming droplets together with air & combustion

products rush into main chamber at high velocity

through the holes

-Thus it creates turbulence and distribution of flaming

droplets all over the main chamber where bulk of

combustion take place amounting to about 80%

release of energy

I C ENGINES – II : CI Combustion Chambers - The rate of press. rise and max. press. are lower

than the open type chambers

- Initial shock of combustion limited to prechamber

and because of high temp. it has multi-fuel capability

w/o modification in injection system.

-Air-Cell Chamber : The clearence volume is divided

into the main and the other called the energy cell

which is further divided into two parts; major & minor

connected to each other & main chamber by orifices


Air-Cell Chamber :


- A pintle type of nozzle injects fuel across main chamber towards the

energy (air ) cell.

- During compression, press. in main is higher than

energy cell due to restricted passage. At TDC, air will

be forced into the cell and fuel injection also begins

-Combustion initially begins in main where temp. is

high but rate of burning low due to absence of air

motion. In energy cell fuel is well mixed with air and

high pressure forces hot burning gases blow out in

main .

-The high velocity jet produces swirling motion in main thereby

thoroughly mixes fuel with air resulting in complete combustion

I C ENGINES – II : CI Combustion Chambers - Design is not suitable for variable speeds due to

combustion induced swirl .

- Energy cell is designed to run hot to reduce ignition lag.

Advantages :

- Low injection pressure requirements

- Injection spray direction is not important

Drawbacks :

- Poor cold starting performance require heater plugs

- High specific fuel consumption because of pressure loss due to air

motion through the ducts and heat loss due to large heat transfer

area.

I C ENGINES – II : CI Combustion Chambers Methods of Mixture Formation in DI Engines

Volumetric Method

- Fuel injected directly into combustion chamber space

- Fuel composition is non-uniform distribution in spray

- Macro and Micro mixing attained using;

- Energy of fuel jet (atomization)

- Energy of moving air charge (swirl)


- Drop in load or speed reduces fuel delivery per cycle

and also the atomizing energy

- Leads to poorer combustion & higher fuel consumption

- It may be noted that when mixture is formed in DI

engines with combustion chamber in piston, some of

the fuel gets onto the chamber walls

•

I C ENGINES – II : CI Combustion Chambers Film Method of Mixture Formation

- Underlying idea is to allow the minimum amount of

fuel to evaporate and mix with air during ignition delay

- With this aim the fuel is directed to the wall of

Combustion chamber at an acute angle so that the

droplets are not reflected but spread over the surface

in the form of thin film


- Injection pressure should be sufficiently high for

# film to be formed rapidly, and

# fine droplets and fuel vapors formed move towards

hottest place – centre of chamber where mixture

ignites and burns

- Heat needed to evaporate fuel is supplied by piston

- Intensive motion of air promote rapid evaporation of

fuel film and carries away the vaporized fuel


- Improved performance ;

• minimum SFC lower

• high bmep (air utilization),

• high power with lower Pmax and Rate of Press.

Rise ( smooth engine operation),

• wider range of fuels including gasoline may be

used

- Such a combustion ( M- process ) employed in

MAN combustion chamber

I C ENGINES – II : CI Combustion Chambers MAN Type Combustion Chamber

- Combustion Chamber developed for small high

speed engine

- Direct injection with single-hole nozzle, in spherical

bowl-in-piston chamber with high swirl

- It differs DI open comb. chambers in respect to ;

fuel spray impinges tangentially and spreads over

the surface of spherical space in piston


- Basic theory of MAN system is that ;

# enough spray will ignite before impingement so that

delay remains normal while most of spray evaporate

prior to combustion

# Thus second stage of combustion is slowed down

avoiding excessive rate of pressure rise

# Shrouded inlet valve used to give air swirl


Advantages

- Low peak pressure

- Low rate of pressure rise

- Low smoke

Disadvantages

- Low volumetric efficiency

- Cold starting ( piston surface temp. )

- High HC emissions at starting

IC ENGINES-II

Course Outlines ADEG-222 LTP-3 1 1

I: BASIC THEORY

II: FUEL INJECTION SYSTEM

III: AIR MOTION, COBUSTION & COMBUSTION

CHAMBERS

IV: SUPERCHARGING and TUBOCHARGING

V: DIESEL ENGINE TESTING &

PERFORMAMCE

combustion chambers

Engineering

open combustion chamber

internal combustion

ports chambers

hemispherical chamber

toroidal chamber

chamber induction swirl

small engines

base i c engines