ci com chamber

7/28/2019 Ci Com 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 is

known as Detonation.



Factors Affecting the Detonation in C.I. Engines

The fuel having longer delay period and higher self ignition

temperature lead to detonation.

If the injection of fuel is too far advanced, the rate of pressure

rise during auto-ignition is very high and causes detonation.

Inferior fuels (having lower cetane number ) promote diesel

knock

Fuel injection parameters (better penetration and distribution)

better combustion chamber design(split type) influences less

detonation tendency.

Initial condition of the air (higher temperature or higher

pressure in case of supercharged engine) influences less

detonation 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 reference

fuels.

Iso - cetane (hepta methyl nonane, HMN) has a cetane number

of 15 and cetane (n-hexadecane, C16H34) has a value of 100.

-methyl napthalene (C11H10) with a cetane number of zero

represented 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 ignition

delay.

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-

ignition

Octane number

describes how poor

the ignition

characteristics

Cetane number is the

ability of diesel to auto-

ignite or self-ignite

The higher the cetane

number, the shorter theignition delay time

& good the ignition

characteristic



Both high cetane and high octane provide the

ability to extract more power from fuel.

Isooctane - highly

ignition-resistant liquid

hydrocarbon

cetane - ignites readily

under compression



Diesel Combustion

Fuel sprayed in cylinder near TDC.

Atomization, vaporization & mixing delay ignition.

Ignition occurs wherever conditions right. Combustion rate controlled by injection

characteristics (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 fuel

injected.



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 is

injected into the “prechamber” which is connected to the main chamber via a

nozzle, or one or more orifices.

• For very-large engines (stationary power generation) which operate at low

engine speeds the time available for mixing is long so a direct injection

quiescent chamber type is used (open or shallow bowl in piston).

• As engine size decreases and engine speed increases, increasing amounts

of swirl are used to achieve fuel-air mixing (deep bowl in piston)

• For small high-speed engines used in automobiles chamber swirl is not

sufficient, indirect injection is used where high swirl or turbulence is generated

in the pre-chamber during compression and products/fuel blow down and mix

with main chamber air.



The method employed to measure CN uses a standardized single-cylinder

engine with variable compression ratio

The operating condition is:

Inlet temperature (oC) 65.6

Speed (rpm) 900

Start of fuel injection (oBTC) 13

Coolant temperature (oC) 100

Injection pressure (MPa) 10.3

With the engine running at these conditions on the test fuel, the compression

ratio is varied until combustion starts at TC ignition delay period of 13o.

The above procedure is repeated using blends of cetane and HMN. The

blend that gives a 13o ignition delay with the same compression ratio is

used 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 volume

ratio.

T - Head



L - Head

Flat slab - both valves are same side

Length 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.



Hemispherical

Used in high-performance engine

valves inclined at 90 need two overhead

camshafts Intake valve is on one

side of thecombustion chamber and an exhaust valveis on the other side.

spark plug - center location



Bath tub

oval-shaped

chamber

valves at head side

by side.

Easy to cast.

Less tend to knock.



Wedge shape

Spark plug –central

location

Tapering away from

the spark plug. Valves are inclined

from the vertical.

Smaller surfacearea than other

designs



I head

Overhead valve.

Valve are located incylinder head.

Less surface to volume

ratio.

Less flame travel length.

High volumetric

efficiency.



F head

combination of the

L- and I- head

one valve in the

cylinder block andone in the head.

Inlet –low pumping

losses & increasebreathing of air.

Inclined - spark plug



objective

Fine atomization of fuel.

Proper mixing of fuel and air.

Turbulence.

minimum surface to volume ratio.

Short delay period.



Combustion Chamber Design

1. Open Chamber: (i) Disc type

(ii) Wedge

(iii) Hemispherical

(iv) Bowl-in-piston

(v) Other design

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

chamber

(For SI) (i) CVCC

(ii) Other



INDUCTION SWIRL

Tangential to the piston.

Shrouding the inlet valve.

Turning the valve around the axis.



NON TURBULENT OR OPEN OR DIRECT

INJECTION

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

Roger Krieger, GM R&D Center



COMPRESSION SWIRL

Combustion - Separate

cell.

Forced through small

hole High velocity.

Swirl turbulence – high

heat loss Low thermal efficiency.




Pre-combustion chamber

Anti chamber.




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 piston

is greater than its stroke) large

valves can be fitted.

AIR CELL COMBUSTION



AIR CELL COMBUSTION

CHAMBER

Air cell splits into

two vertices =>high

swirl.

High turbulence and

high temperature.

C b ti i CI E i



Combustion in CI Engine

In a CI engine the fuel is sprayed directly into the cylinder and the vaporised

part 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 Injection

quiescent chamber

Direct Injection

multi-hole nozzle

swirl in chamber

Direct Injection

single-hole nozzle

swirl in chamber

Indirect injection

swirl pre-chamber

ci com chamber

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