ae 1005-automotive engines - wordpress.com. ignition delay or delay period 2. uncontrolled...

AE 1005- AUTOMOTIVE ENGINES

COMBUSTION IN CI ENGINES

UNIT IV

Importance of air motion - Swirl, squish and turbulence - Swirl ratio. Fuel air mixing -Stages of combustion - Delay period -Factors affecting delay period, Knock in CI Factors affecting delay period, Knock in CI engines - methods of controlling diesel knock. CI engine combustion chambers -Combustion chamber design objectives -open and divided. Induction swirl, turbulent combustion chambers. - Air cell chamber - M Combustion chamber.


1. Ignition delay or Delay Period

2. Uncontrolled combustion

STAGES OF COMBUSTION IN CI ENGINES

3. Rapid or Controlled Combustion

4. After Burning

COMBUSTION IN CI ENGINES1.Ignition delay - fuel is injected directly into the cylinder

towards the end of the compression stroke. The liquid fuel

atomizes into small drops /droplets and penetrates into the

combustion chamber. The fuel vaporizes and mixes with

the high-temperature, high-pressure air.

2.Uncontrolled combustion [Premixed combustion phase

]– combustion of the fuel which has mixed with the air to

within the flammability limits (air at high-temperature and

high-pressure) during the ignition delay period occurs

rapidly in a few crank angles.

3.Rapid or Controlled Combustion [Mixing controlled

combustion phase ] – Burning rate is controlled by the rate

at which mixture becomes available for burning. The

burning rate is controlled primarily by the fuel-air mixing


burning rate is controlled primarily by the fuel-air mixing

process.

4.After Burning [Late combustion phase] – heat release

may proceed at a lower rate well into the expansion stroke

(no additional fuel injected during this phase).

Combustion of any unburned liquid fuel and soot is

responsible for this.

Stages of combustion (P-θ curve)

b

cd

e

a

(θ)

Four Stages of Combustion in CI Engines

(Heat Release)

Start of

injection

End of

injection

Delay period1. Physical Delay

� The period of physical delay is the time between the beginning

of injection and the attainment of chemical reaction conditions.

� During this time, the fuel is atomized, vapourised, mixed with air

and the temperature is raised.and the temperature is raised.

� Physical delay depends on type of fuel.

� High viscuous fuel means high physical delay. It is reduced by

high injection pressures.

2. Chemical Delay

� During this period, preflame reaction start slowly and then

accelerate until the local inflammation or ignition takes place.

� The delay period refers to the sum of physical and chemical

delay.

� In most CI engines, the ignition delay is shorter than the

duration of injection.

� Ignition lag in SI engines is equivalent to the chemical delay in CI

Delay period

� Ignition lag in SI engines is equivalent to the chemical delay in CI

engines.

� The delay period in the CI engines affects the rate of pressure

rise and hence, knocking results.

� It also affects the engine startability.

� The pressure reached during the combustion depends up

on delay period.

� The longer the delay period, the more rapid is the pressure

rise, since more fuel will be present in the cylinder before

the rate of burning comes under control.

Delay period

the rate of burning comes under control.

� Therefore the diesel engine designers aim to keep the delay

period as short as possible.

� But, some delay period is necessary, otherwise the droplets

would not be dispersed in the air for complete combustion.

Factors Affecting the Delay period� Compression ratio

� Engine speed

�Output

�Atomization of fuel�Atomization of fuel

� Injection timing

�Quality of fuel

� Intake temperature & pressure

Variables affecting delay period1. Fuel

� Self ignition temperature

� Cetane number

� Viscosity

2. Injection pressure or size of droplet2. Injection pressure or size of droplet

� Fuel should be injected at the smallest size to obtain

largest surface to volume ratio

�Rate of burning depends upon the rate at which the

products of combustion can be removed from the

surface and replaced by fresh oxygen.

� Smaller droplets will have lesser momentum and hence lesser

relative velocity and once its initial velocity is lost it will travel

in air resulting in partial suffocation by its own products of

combustion.

� As the pressure rise after ignition depends on the area of

Variables affecting delay period

� As the pressure rise after ignition depends on the area of

inflammation, if the droplet size is small, more aggregate area

of inflammation results in greater uncontrolled pressure rise.

� As the size of the droplet depends upon the injection pressure,

lower the rate of pressure rise during the uncontrolled phase

and smoother engine running.

3. Injection advance :

� The delay period increases with increase in injection advance.

This is due to the lower pressure and temperature when the

injection begins.

� If the injection advance is small, the delay period reduces and


� If the injection advance is small, the delay period reduces and

operation of the engine is smoother, but the power produced

is reduced as large amount of fuel burns during expansion.

� The optimum angle of injection advance depends upon many

factors, but generally it varies between 12° to 20° before TDC.

This causes the peak pressure to occur 10° to 15° after TDC.

4. Compression ratio

� Increase in compression ratio reduces delay period as it

rises both temperature and density.

1200

1400


0

200

400

600

800

1000

1200

0 5 10 15 20 25

Te

mp

era

ture

(K

)

Compression Ratio

Max Air Temperature

Min Auto ignition Temp

5. Intake temperature

� Increasing the intake temperature results in increase in the

compressed air temperature, which reduces delay period.

� Increasing the inlet temperature reduces density of air

entering the cylinder, hence volumetric efficiency and power


entering the cylinder, hence volumetric efficiency and power

output.

6. Jacket water temperature

� Increase in jacket water temperature increases compressed

air temperature and hence delay period is reduced.

7. Fuel temperature

� Increase in fuel temperature reduces both physical and

chemical delay period.

2

57 Cetane Fuel

9. Speed

8. Intake pressure and supercharging

� Increase in intake pressure or supercharging reduces the

auto ignition temperature and hence reduces delay period.


0.5

1

1.5

500 700 900 1100 1300 1500 1700 1900 2100

Ign

itio

n D

ela

y (

ms)

Engine Speed (RPM)

57 Cetane Fuel

10. Air fuel ratio

With increase in air-fuel ratio (lean mixture), the combustion

temperatures are less and cylinder wall temperatures are

lowered and hence delay period increases. The rate of pressure

rise is unaffected but the maximum pressure is reduced.


rise is unaffected but the maximum pressure is reduced.

11. Engine size

The engine size has little effect on the delay period in

milliseconds. As large engines operate at low rpm, the delay

period in terms of crank angle is smaller and hence less fuel

enters the cylinder during delay period.

12. Type of combustion chamber.

Pre combustion chambers give shorter delay period compared to

open type of combustion chamber.

Sl no Increase in Variable Effect on Delay

Period

Reason

1 Cetane No of Fuel Reduces Reduces SI Temperature

2 Injection Pressure Reduces Greater S/V ratio, hence less

physical delay

Factors affecting delay period in CI Engines

physical delay

3 Injection timing

Advance

Reduces Pressures and temperature

lower when injection begins

4 Compression Ratio Reduces Increases air temperature

and pressure

5 Intake Temperature Reduces Increases air temperature

6 Jacket Water

Temperature

Reduces Increases wall temperature,

hence air temperature

Sl no Increase in Variable Effect on Delay

Period

Reason

7 Fuel Temperature Reduces Better vaporation and

increases chemical reaction

8 Intake Pressure

(Supercharging)

Reduces Increases in density, reach

auto ignition temperature fast

9 Speed Reduces in time, Less loss of heat.9 Speed Reduces in time,

increases in crank

angle

Less loss of heat.

10 Load (Fuel-Air

Ratio)

Decreases Operating temperature

increases

11 Engine Size Little effect on time

but crank angle

decreases

Low RPM

12 Type of Combustion

Chamber

Lower for pre-

combustion

chamber

Due to compactness of

chamber.

Diesel Knock� If the delay period is long, a large amount of fuel

will be injected and accumulated in the chamber.

�The auto ignition of this large amount of fuel cause

high rate of pressure rise and high maximum high rate of pressure rise and high maximum

pressure which causes knocking in a diesel engine

�A long delay period not only increases the amount

of fuel injected by the moment of ignition, it also

improves the homogeneity of the fuel-air mixture

and its chemical preparedness for explosion type of

self ignition

Difference between SI and CI knock1. In SI engines, detonation occurs near the end of

combustion, whereas in the CI engine, detonation occurs near the beginning of combustion

Difference between SI and CI knock2. The detonation of SI engine is of a homogeneous

charge causing very high rate of pressure rise and

very high maximum pressure. In the CI engine, the

fuel and air are imperfectly mixed and hence the fuel and air are imperfectly mixed and hence the

rate of pressure rise is normally lower than that in

the detonating part of the charge in the SI engine.

3. In CI engine, fuel is injected into the cylinder only at the end of the compression stroke - there is no pre-ignition as in the case of a SI engine

4. In SI engine, it is relatively easy to distinguish between knocking and non-knocking operation

Difference between SI and CI knock

4. In SI engine, it is relatively easy to distinguish between knocking and non-knocking operation as the human ear can easily find the distinction. In the case of CI engine, the normal ignition itself is by auto-ignition and most CI engines have sufficiently high rate of pressure rise per degree of crank angle to cause audible noise.

Factors reducing knock in SI and CI engines

S. No Factors SI Engine CI Engine

1 SI Temperature of Fuel High Low

2 Delay period of fuel Long Short

3 Compression Ratio Low High

4 Inlet Temperature Low High

5 Inlet Pressure Low High

6 Combustion Chamber wall

Temperature

Low High

7 Speed High Low

8 Cylinder Size Small Large

Methods of controlling diesel knocka) Design and operating factors for reducing delay

period

The delay period can be reduced by reducing the degree of turbulence as it will reduce heat loss. degree of turbulence as it will reduce heat loss. However, it will increase the combustion period.

b) High rate of pressure rise and high maximum pressure in the second stage if large amount of fuel collects during the delay period.

It can be reduced by arranging the injector so that only a small amount of fuel is injected at first.

c) The delay angle is reduced (cetane number is

increased) by adding chemical dopes.

� The two chemical dopes added are ethyl nitrate

and amyl nitrate in concentrations of 8.8gm/litre

and 7.7gm/litre respectively

Methods of controlling diesel knock

and 7.7gm/litre respectively

� The chemical dopes increase the cetane number

and accelerate the pre-flame reactions and

reduce the flash point.

� NOx emissions will be a problem.

CI ENGINE

COMBUSTION CHAMBERCOMBUSTION CHAMBER

COMBUSTION CHAMBER

�It is the space within the cylinder when the piston

is at the top dead centre.

�It is formed by the top of the piston and a cavity�It is formed by the top of the piston and a cavity

in the cylinder head.

�Since the air-fuel mixture burns in this space, its

design and shape greatly affect the power, fuel

efficiency and emissions of the engine.

Factors to be considered while designing a

diesel engine combustion chamber• High thermal efficiency

• Fuel requirement – ability to use less expensive fuel; multi fuel capability

• Ease of starting• Ease of starting

• Variable speed operation

• Smoothness of operation, without knock

• Low exhaust emissions

• Simple nozzle design

• High volumetric efficiency

• High brake mean effective pressure

Classification of CI Engine combustion Chambers

CI Engine Combustion Chambers

Open Chamber Divided Chamber

OrOr

Turbulent Chamber

Swirl Chamber Pre-Combustion Chamber Air Cell Chamber

Combustion Induced Swirl

Induction Swirl

Compression Swirl

CI Engine combustion chambers

Air motion - Proper mixing of fuel in a short timeThree parameters are used to characterize large-

scale in-cylinder fluid motion: swirl, squish, and

tumble.

Swirl is the rotational flow about the cylinder axis.Swirl is the rotational flow about the cylinder axis.

Swirl is used to:

Rapidly mix fuel and air in direct injection engines

The swirl is generated during air induction into the

cylinder by either:

i) tangentially directing the flow into the cylinder,

or

ii) pre-swirling the incoming flow by the use of

helical ports.

Induction Swirl and Open Combustion Chambers

Induction Swirl can be achieved by

�Careful formation of the air intake passages

�Making or shrouding the intake valve

Squish

• Induction swirl is augmented by secondary air

movement called squish

• Squish is the radial inward movement of air

towards the combustion recess by squeezing it towards the combustion recess by squeezing it

out from between the piston and cylinder

head as they approach each other at the end

of the stroke.

Cylinder Swirl and its Generation

Helical port

Tangential injectionSwirl motion

Contoured valve

Squish

Important point to be noted

• With a weak swirl, single hole nozzle cannot

provide desired air-fuel mixing.

• Hence, always multi hole nozzle (4 to 8 @ 1.2

to 1.5mm) is preferred for open combustion to 1.5mm) is preferred for open combustion

chambers

Direct and indirect injection systems

Direct Injection (DI) System. In this system, fuel is injected

directly into a combustion chamber formed in the cylinder itself,i.e. between a suitably shaped non-stationary piston crown and afixed cylinder head in which is mounted the fuel injector with itssingle or multiple spray orifices or nozzles.

Quiescent combustion system. Application-Four-stroke and two-

stroke engines mostly above 150 mm bore

Quiescent chamber for a truck Dl engine and a

swirl assisted HSDI engine chamber

Fuel Atomization - multi hole nozzle

Direct injection combustion systems

The wide flat chambers are associated with high pressure injectionsystems and the deeper bowls are used with high swirl, lowinjection pressure systems. Direct injection engines dependprimarily on the kinetic energy of the fuel spray to mix the air andfuel. This dependence increases the importance of the fuelinjection system for optimizing the combustion system in DIengines. Increased air swirl can enhance the fuel-air mixing andextend the smoke limiting fuel-air ratio, but it increases NOx at thesame time.

Direct Injection

High swirl system

Application to all truck and bus engines, but increasingly to the high speed

passenger car engine

Indirect InjectionIndirect Injection (IDI) Systems as used in IDI engines in which

fuel is injected into a prechamber which communicates with thecylinder through a narrow passage. The rapid transfer of air from themain cylinder into the prechamber towards top dead centre (TDC) ofthe firing stroke promotes a very high degree of air motion in theprechamber which is particularly conducive to rapid fuel-air mixing.

Prechamber system-compression swirl. Application traditionally to high speed passenger car engines but now increasingly replaced by direct

injection engine

Classification of indirect combustion

chambers

Swirl chamber� It consists of a spherical chamber located in the

cylinder head and separated from the engine cylinderby a tangential throat. About 30 to 50% of the airenters the swirl chamber during the compressionstroke of the engine, producing a swirl.enters the swirl chamber during the compressionstroke of the engine, producing a swirl.

� After combustion, the products return through thesame throat to the main cylinder at much highervelocity. So more heat loss to walls of the passagetakes place. This type of chamber finds application inengines in which fuel control and engine stability aremore important than fuel economy.

Pre-combustion chamber� This chamber is located at the cylinder head and is

connected to the engine cylinder by small holes. Itoccupies 40% of the total cylinder volume. During thecompression stroke, air from the main cylinder entersthe precombustion chamber. At this moment, fuel is


chambers

the precombustion chamber. At this moment, fuel isinjected into the precombustion chamber andcombustion begins. Pressure increases and the fueldroplets are forced through the small holes into themain cylinder, resulting in a very good mix of the fueland air. The bulk of the combustion actually takes placein the main cylinder. This type of combustion chamberhas multi-fuel capability because the temperature ofthe prechamber vaporizes the fuel before the maincombustion event occurs

Pre-combustion chamber

• Air cell chamber• The air cell is a small cylindrical chamber with a hole in one

end. It is mounted more or less coaxially with the injector,on opposite sides, axis being parallel to the piston crown,with the injector firing across a small cavity which is opento the cylinder into the hole in the end of the air cell. Theair cell is mounted so as to minimise thermal contact with


chambers

air cell is mounted so as to minimise thermal contact withthe mass of the head. A pintle injector with a narrow spraypattern is used. At TDC the majority of the charge mass iscontained in the cavity and air cell.

• Air cell injection is considered as a sort of half way stagebetween fully indirect and fully direct injection, gainingsome of the efficiency advantages of direct injection whileretaining the simplicity and ease of indirect injection

Air cell chamber

Advantages of Induction swirl

1. High excess air allows low average combustion

temperature. Low turbulence and low heat losses permits

ηthermal to approach ideal cycle efficiency

2. Intensity of swirl is low – heat losses to chamber wall is less

– easy cold starting

3. Swirl is obtained during suction stroke – no additional work

for creating swirl. Hence better ηbrake thermal and fuel economy.for creating swirl. Hence better ηbrake thermal and fuel economy.

4. When used in low speed engines, poor quality fuel can be

used as injection spreads for a long duration of time.

5. When used in high speed engines, high cetane fuel

produces good thermal efficiency with better swirl results

in better economy.

6. Very simple in construction

1. As the swirl is of low intensity, multi hole nozzles with high

injection pressure is required – problems like clogging, low

injection quantity etc

2. Use of shrouded valves lowers ηVol. Injector has to be located at

the centre of the CC, restricting the size of the valve in multi valve

Disadvantages of Induction swirl

the centre of the CC, restricting the size of the valve in multi valve

engines

3. Mixing of fuel and air and fuel is not easy at low speeds and high

loads

4. Weak swirl necessitates excess air ie low air utilisation, hence low

mean effective pressure

5. Swirl is not proportional to engine speed, hence efficiency is not

maintained over a wide speed range in a variable speed engine.

1. Due to strong swirl, single orifice injector with low injection pressure can be used. Pintle type nozzle with self cleaning capacity can be used

2. Due to strong swirl, better utilization of air. Hence higher mean effective pressure.

3. As the injector is located inside the swirl chamber, valves of

Advantages of Compression swirl

3. As the injector is located inside the swirl chamber, valves of larger diameter can be conveniently located, hence better volumetric efficiency.

4. Swirl is proportional to speed, hence more suitable for variable speed engines.

5. Transfer of air from the main chamber to swirl chamber heats air, hence delay period can be reduced.

6. Engine operation is smooth because, the initial shock of peak pressure is absorbed by the swirl chamber.

1. The work done during compression and expansion are

considerable. Hence ηmech is lower. ηIThermal is also lower due

to low excess air. 5 to 8% excess fuel consumption.

2. Due to the high intensity of swirl, heat losses to the CC walls is

high. S/V ratio is also high as the CC is not compact.

Disadvantages of Compression swirl

high. S/V ratio is also high as the CC is not compact.

3. Cold starting is a very serious problem – glow plug is a must.

4. More heat is lost in the exhaust gases

5. Cylinder construction is more expensive

Comparison of induced and compression swirl

Induction Swirl Compression Swirl

Advantages Disadvantages

1. Indicated thermal efficiency

high due to high excess air

and low turbulence

2. Easier cold starting due to

1. Less excess air, low indicated

thermal efficiency. 5 to 8%

more fuel consumption.

Decreased exhaust valve life2. Easier cold starting due to

low intensity of swirl

3. No additional work required

for producing swirl

4. High mechanical and brake

thermal efficiency

5. Low quality of fuel can be

used when used for low

speed engines

Decreased exhaust valve life

2. Cold starting trouble due to

high heat loss due to strong

swirl, greater S/V ratio.

3. Work loss during

compression results in lower

mechanical efficiency

4. Cylinder complicated and

more expensive.

Induction Swirl Compression Swirl

Disadvantages Advantages

1. Weak swirl, multi orifice nozzle, high

injection pressure, clogging of holes.

High maintenance

2. Idling and high load complications due

to fuel mixing problems.

3. Shrouded valves, smaller valves,

1. Single hole injector can be used, less

maintenance

2. Large valves, higher volumetric

efficiency

3. Greater air utilisation due to strong

swirl

Comparison of induced and compression swirl

3. Shrouded valves, smaller valves,

restriction in inlet passage reduces

volumetric efficiency

4. Weak swirl, low air utilization, low

mean effective pressure

5. Swirl not proportional to speed.

Efficiency is not maintained in a

variable speed engine

swirl

4. Swirl proportional to speed, suitable

for variable speed operation

5. Smooth engine operation

• Dr. Meurer of MAN (Germany) in 1954 developed the

“M-Process” engine

• This engine was very silent, hence he named it as

“whisper” engine

M Combustion Chamber

“whisper” engine

• This belongs to the open combustion chamber category.

• Fuel spray impinges and spreads over the surface of the

spherical cavity in the piston.

M Combustion Chamber

M Combustion Chamber1.Fuel injected tangentially from a multi hole nozzle on the surface of

the chamber in the direction of the swirl.

2.Injected fuel spreads on the piston surface and forms a film of about 0.15mm thick.

3.As the film evaporates due to the heat of the piston, it starts burning. burning.

4.Combustion of the fuel is initiated by auto ignition of a small portion of fuel which is air borne.

5.The quantity of air borne fuel is controlled by selecting a proper distance between the nozzle tip and the combustion chamber wall.

6.The heat of the piston has to be within a temperature range to achieve fuel evaporation without causing thermal decomposition and carburizing of the fuel.

7. The fuel vapour rise from the hot surface and mix with the

swirling air in successive layers and combustion takes place in a

near homogeneous air-fuel mixture at the desired rate.

8. The rate of heat release is almost equal to the rate of evaporation

of fuel

M Combustion Chamber cond..

of fuel

9. Even though, the engine works on diesel cycle, its combustion

characteristics is almost like Otto cycle.

10. As thermal decomposition is prevented, soot formation is very

less.

Advantages of M Combustion Chamber

• Low rate of pressure rise

• Low smoke level

• Ability to operate on a wide range of liquid fuels

• Very less combustion noise• Very less combustion noise

Disadvantages of M Combustion Chamber

• Cold starting problem

• White smoke, diesel odour and high HC

emission when the engine is cold

• Low volumetric efficiency

CONCLUSION

• Both IDI and DI engines require small

clearances between the piston and the

cylinder head. This clearance restricts the

timing of exhaust valve closing and intaketiming of exhaust valve closing and intake

valve opening unless valve cutouts are

provided in the piston. These cutouts increase

piston cost and adversely affect the in-cylinder

flow field.

Thank You

ae 1005-automotive engines - wordpress.com. ignition delay or delay period 2. uncontrolled...

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