CI Engine Knocking

INTERNAL COMBUSTION ENGINES-II Combustion
University of Petroleum & Energy Studies
Dehradun

I C ENGINES - II Air Motion & Combustion
•Fuel - Air Mixing and Combustion :
-The air motion so organized inside the combustion
chamber and the turbulence of the air passes across the fuel jet tears it into fine particles.
- A mixture of air and fuel forms at some location in the spray envelope and oxidation starts
- The liquid fuel droplets initially evaporate and then get heated up , and A/F ratio ( local ) within combustible range , autoignite.

•Combustion in CI Engines
- Stages/ Phases of combustion
1. Ignition delay period ;
Physical delay and chemical delay
2. Rapid combustion period / rapid pressure rise
3. Controlled combustion period / mechanically
controlled period
4. After-Burning period

•Combustion in CI Engines

Stages/ Phases of combustion
1. Ignition delay period ;
- Preparatory phase:some fuel admitted,not ignited
- Period : start of injection to start of combustion
(p-t curve separates from motoring curve) - Important : combustion, design, performance,
emissions
- Fuel does not ignite immediately and total delay period can be explained as; Physical delay and Chemical delay. Details are shown in the figure.

•Combustion in CI Engines : Ignition Delay

Physical delay :
- period ; from beginning of injection to attainment
of chemical reactions.
- process; atomization, vaporization, mixing of fuel
& air, raising to self-ignition temperature
- depends on; fuel,air motion,combustion chamber

Chemical delay :
- Period ; from beginning of chemical reactions to
auto-ignition
- process ; pre-reactions in the mixture taking place
up to auto-ignition that occurs at a local
excess-air factor of 0.5 < λ < 0.7
- depends ; chemical reactions are faster at higher temperature of the surroundings and accordingly the chemical delay becomes shorter.

2. Rapid combustion period / rapid pressure rise
- This is the phase of combustion when the rate of pressure rise is maximum and is also called uncontrolled combustion phase.The rate of heat release is maximum during this phase.
- period ; from end of delay or beginning of
combustion to the point of maximum pressure on
indicator diagram
- process ; most of the fuel would have formed combustible mixture with air and preflame reactions

completed so that the layers of fuel vapours burn as fast as they can find fresh oxygen to continue the combustion process
- depends ; It may be noted that the maximum pressure reached during this phase depends upon the delay period. Longer the delay the higher is the rate of pressure rise as more fuel gets accumulated
during the delay period.

3. Controlled combustion period / mechanically
controlled period
- This is the phase of combustion when very steep rate of pressure rise has changed to much slower rate of pressure rise.
- process ; This is the second phase of fuel injection when injected fuel finds more difficult to seek out and react with fresh oxygen in the remaining unburnt air mass.
- period ; from the point of maximum pressure to the

point of maximum cycle temperature.
- depends ; crank-angle duration of fuel injection as per load requirements (higher and full load power out-put ). Therefore, this is mechanically controlled by the injection pump out put characteristics. Prolongation of this variable period is determined by the amount of additional fuel supplied above that needed for no- load running.

4. After-Burning period : fuel with the air
- Combustion does not cease with completion of fuel
injection.The unburnt and partially burnt fuel particles
start burning as soon as they come in contact with
oxygen. This continue for a certain duration called
after- burning period.
- period ; starts from the point of max.cycle temp. and
continue over the part of expansion process.
- depends ; rate of after burning depends upon velocity
of diffusion and turbulent mixing of unburnt and
partially burnt fuel with the air

•Factors Affecting the Delay Period :
Many design and Operating factors affect the delay period. Some of the significant factors are;
- Compression ratio
- Engine speed
- Output
- Atomization of fuel and duration of injection
- Injection timing
- Quality of fuel
- Intake temperature and pressure

- Compression ratio (CR )
The effect of CR on compression temperature of air and the minimum auto ignition temperature of fuel at the end of compression is shown below

- It can be seen that the comp. temp. increases and the auto ignition temp.decreases due to increased density of air resulting in closer contact between molecules of air and fuel, thereby reducing the time of reaction and consequently the delay period.
- The peak pressure is only marginally affected due to lower rate of pressure rise whereas mechanical efficiency decreases.
- Optimum CR required considering cold starting &
light loads at high speeds.

- Engine speed :
The effect of engine speed on ID ( time ) is shown below

- It can be seen that delay period in milliseconds decreases with increase in speed. With increase in speed, the heat loss during compression decreases
resulting in rise in temp. & press. and therefore reducing the delay period.
- However, in ºCA the delay period increases as engine operate at higher rpm. The amount of fuel injected during delay period increases (as fuel pump is geared to engine) resulting in high rate of press. rise during second stage of combustion.

- Output :
With increase load or output, the amount of fuel injected ( at constant rpm ) increases and therefore the air-fuel decreases and results in increase in temperature thereby reducing delay period.
- The rate of pressure rise is unaffected but the peak
pressure may be high.

- Atomization of fuel and duration of injection :
Higher fuel-injection press. increases the degree (fineness) of atomization which reduces ignition delay due to higher surface to volume ratio.
- Smaller droplets have low depth of penetration due to less momentum. Such droplets have less velocity relative to air and shorter fuel spray path, where it has to find oxygen after vaporization affecting air utilization factor. Also the aggregate area of inflammation after ignition will increase resulting in

higher pressure rise during second stage of combustion.
- On the other hand, lower injection pressure giving larger droplets give lower pressure rise and soother
running.
- An optimum group mean diameter of the droplet should be attempted with appropriate fuel delivery characteristics.

- Injection timing :
The effect of injection advance timings ( 9º,18º,27º )
for constant injected quantity of fuel on cylinder pressure is shown below :

- As pressure and temperature at the begning of injection are lower for higher injection advance, the
delay period increases with increase in injection advance.
- The optimum angle of advance depends on many factors. Generally it is about 20º bTDC.

- Quality of fuel
Self ignition temperature is the most important property of fuel which affects the delay period. A lower self-ignition temperature results in lower delay period.
- Fuels with higher cetane number give lower delay.
Other properties of fuel which affect delay period include volatility, latent heat, viscosity and surface tension.

- Intake temperature and pressure :
Increase in intake temp. increases compressed air temp. resulting in reduced delay period. Preheating
may not be desirable as it reduces density of air affecting volumetric efficiency and power.
- Increase in intake pressure reduces the auto - ignition temp. and hence the delay period. The peak pressure will be higher as compression pressure will be higher with higher intake pressure.

•Knock in CI Engines :
- Fuel Injection takes definite interval
- Injection & Combustion take place simultaneously
- Initial droplets while undergoing ignition delay, additional
droplets are being injected
- If ignition delay of fuel is short,
initial few droplets will commence
actual burning
- As a result, mass rate of mixture
burned produce rate of press. rise
for soother combustion as shown

- As ignition delay is longer , the actual burning of first few droplets is delayed and accumulation of greater quantity of fuel take place. When actual burning take place under such conditions the rate of pressure rise increases as shown in fig.

- If ignition delay is quite large and the actual burning is substantially delayed and the accumulation of fuel is high when actual burning
take place the rate of pressure rise is almost instantaneous the knocking begins as shown in fig.

- Knocking is characterized by extreme pressure differentials and violent gas vibrations evidenced by audible knock.
- In CI engines knocking occurs in the beginning of combustion.
- In order to decrease knocking the actual burning should start as early as possible after the fuel injection begins.
- It has been found that provided the rate of press. rise does not exceed 3 bar per ºCA combustion is

- smooth. Between 3 to 4 bar there is tendency to knock, above this rate of pressure rise the diesel knock will be prominent.
- Normally audible knock is always present in CI engines, when it becomes sever and cause heavy vibrations in the engine , it is said to be knocking. It
is, therefore, a matter of judgment.
- Low ignition temp., ignition delay & speed reduce knock as also higher CR, Inlet Temp. & press., comb. wall temp. cylinder size

Combustion Chambers for C.I. Engines
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)

CI combustion chamber :
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 cyl. & 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

IC ENGINES-II Air Motion & Combustion
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
VI: DIESEL ENGINE Emissions AND their Control Technology

CI Engine Knocking

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