ci combustion chamber

7/28/2019 Ci Combustion Chamber

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Detonation in CI engines

If the rate of fuel supply is very large than the rate of fuelburned then the accumulated fuel in the combustion chamberis considerably large and the burning of fuel in suddenly startsas a bomb and a very high pressure wave is generated .It pressurises the other mixture to that pressure andtemperature which is much higher than the self ignitiontemperature and that part of mixture also suddenly ignitescreating another pressure wave and this is continued till all the

fuel in the combustion chamber burns.This creates highly pulsating combustion (1000 cps) and isknown as Detonation .



Factors Affecting the Detonation in C.I. Engines

The fuel having longer delay period and higher self ignitiontemperature lead to detonation.If the injection of fuel is too far advanced, the rate of pressurerise during auto-ignition is very high and causes detonation.Inferior fuels (having lower cetane number ) promote dieselknockFuel injection parameters (better penetration and distribution )better combustion chamber design(split type) influences lessdetonation tendency.Initial condition of the air (higher temperature or higher pressure in case of supercharged engine) influences lessdetonation tendency.



Comparison of Knocking in S.I. and C.I.Engines

In S.I. engines, the knocking occurs near the endof combustion whereas in C.I. engine, this occursat the beginning of combustion.

The knocking in S.I. engine takes place in anhomogeneous mixture, therefore, the rate of pressure rise is high. In C.I. engines, the mixture

is heterogeneous and hence rate of pressure islower.



Comparison of Knocking in S.I. and C.I. Engines

The question of pre-ignition does not arise inC.I. engines as the fuel is supplied only near theend of compression stroke.

The knocking in S.I. engines is because of auto-ignition of the last part of the charge. To avoidthis, the fuel must have long ignition lag and

high self ignition temperature. To avoid knock inC.I. engine, the delay period should be as smallas possible and fuel self –ignition temperatureshould be as low as possible.



The cetane number blends of two pure hydrocarbon referencefuels.

Iso - cetane (hepta methyl nonane, HMN) has a cetane number of 15 and cetane (n-hexadecane, C 16 H34 ) has a value of 100.

-methyl napthalene (C 11 H10 ) with a cetane number of zerorepresented the bottom of the scale. This has since been

replaced by HMN which is a more stable compound.

The higher the CN the better the ignition quality - shorter ignitiondelay.

The cetane number is given by CN = (% hexadecane) + 0.15 (%

CETANE NUMBER



Octane number reflects gasoline’s ability to resist auto-

ignition (also calledpre-ignition, self-ignitionOctane number describes how poor the ignitioncharacteristics

Cetane number is theability of diesel to auto-ignite or self-ignite

The higher the cetane

number, the shorter theignition delay time& good the ignitioncharacteristic



Both high cetane and high octane provide theability to extract more power from fuel.

Isooctane - highlyignition-resistant liquidhydrocarbon

cetane - ignites readilyunder compression



The method employed to measure CN uses a standardized single-cylinder engine with variable compression ratio

The operating condition is:

Inlet temperature ( oC) 65.6Speed (rpm) 900Start of fuel injection ( oBTC) 13Coolant temperature ( oC) 100Injection pressure (MPa) 10.3

With the engine running at these conditions on the test fuel, the compressionratio is varied until combustion starts at TC ignition delay period of 13 o.

The above procedure is repeated using blends of cetane and HMN. Theblend that gives a 13 o ignition delay with the same compression ratio isused to calculate the test fuel cetane number.

Cetane Number Measurement



Combustion chamber in SI

Obsolete one.Two cam shaft to

operate valve.Long flame travel – knock.High surface to volumeratio.

T - Head



L - Head

Flat slab - both valves are same sideLength of combustion chamber cover entire piston and valve.Easy to cast.Easy maintenance.Loss of velocity of air.Poor turbulence.Easy knock, Low power and efficiency.



Bath tub

oval-shapedchamber valves at head sideby side.Easy to cast.Less tend to knock.



Wedge shape

Spark plug –centrallocationTapering away fromthe spark plug.Valves are inclinedfrom the vertical.

Smaller surfacearea than other designs



I head

Overhead valve.

Valve are located incylinder head.Less surface to volumeratio.Less flame travel length.High volumetricefficiency.



F head

combination of theL- and I- headone valve in thecylinder block andone in the head.Inlet –low pumping

losses & increasebreathing of air.Inclined - spark plug



Diesel Combustion

Fuel sprayed in cylinder near TDC.

Atomization, vaporization & mixing delay ignition.

Ignition occurs wherever conditions right.

Combustion rate controlled by injectioncharacteristics (injection rate, spray angle,injection pressure, nozzle size and shape),chamber shape, mixture motion, & turbulence.

Glow plug may be used to aid cold starting.

Power output controlled only by amount of fuelinjected.



CI Engine Types

Two basic categories of CI engines:

• Direct-injection – have a single open combustion chamber into which fuel

is injected directly

• Indirect-injection – chamber is divided into two regions and the fuel isinjected into the “prechamber” which is connected to the main chamber via anozzle, or one or more orifices.

• For very-large engines (stationary power generation) which operate at lowengine speeds the time available for mixing is long so a direct injectionquiescent chamber type is used (open or shallow bowl in piston).

• As engine size decreases and engine speed increases, increasing amountsof swirl are used to achieve fuel-air mixing (deep bowl in piston) • For small high-speed engines used in automobiles chamber swirl is notsufficient, indirect injection is used where high swirl or turbulence is generatedin the pre-chamber during compression and products/fuel blow down and mixwith main chamber air.



Combustion Chamber Design

1. Open Chamber: (i) Wedge(ii) Hemispherical

(iii) Bowl-in-piston

(iv) Other design

2. Divided Chamber: (For CI): (i) Swirl chamber (ii) Pre-chamber



INDUCTION SWIRL

Tangential to the piston.Shrouding the inlet valve.Turning the valve around the axis.



NON TURBULENT OR OPEN OR DIRECTINJECTION

Induction Air swirl is used.



Torodial chamber



Shrouding the inlet valve.Cone angle – 150 to 160

TRUNCATED CONE



Small squish.

Large diameter -Lower squish.

RECTANGULAR

CYLINDER



COMPRESSION SWIRL

Combustion - Separate cell.Forced through small holeHigh velocity.Swirl turbulence – high heatlossLow thermal efficiency.



Pre-combustion chamber

Anti chamber.

Roger Krieger, GM R&D Center



Air Swirl in DI Engine



Bowl piston

Squish effect.The cylinder head is flat.Used on an over-square engine(where the diameter of the pistonis greater than its stroke) large

valves can be fitted.



AIR CELL COMBUSTIONCHAMBER

Air cell splits intotwo vertices =>highswirl.High turbulence andhigh temperature.



Combustion in CI Engine

In a CI engine the fuel is sprayed directly into the cylinder and the vaporisedpart of the fuel mixes with air and ignites spontaneously.

These photos are taken in a RCM under CI engine conditions with swirl

0.4 ms after ignition 3.2 ms after ignition

3.2 ms after ignition Late in combustion process

1 c m

Air flow



Direct Injectionquiescent chamber

Direct Injectionmulti-hole nozzleswirl in chamber

Direct Injectionsingle-hole nozzleswirl in chamber

Indirect injectionswirl pre-chamber

ci combustion chamber

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